IDENTIFYING IMPACT STRUCTURES
by: Charles O'Dale
PART I - Crater Formation and Classification
(AstroNotes December 2009;) [Updated 20 January 2010]
Impact Structure: n. A large geologic structure, such as a crater or astrobleme, created by the violent collision between a planet and a space projectile such as a comet or meteor.
Impact cratering is one of the most common geological processes that have happened on planetary objects with solid surfaces (our home planet Earth included).
Why is this research important to Astrobiology? Studying the effects of impact events on the ancient biosphere of Earth can help astrobiologists understand how similar events in the future could affect the habitability of our planet. Understanding the "principles that will shape the future of life, both on Earth and beyond" is one of the primary goals defined by the NASA Astrobiology Roadmap. Another goal of the Astrobiology Program is to determine how "past life on Earth interacted with its changing planetary and Solar System environment."
In this and following articles, I will document the geology used to identify impact structures. I will also describe my amateur observations of various craters that I have visited (on this planet for the time being) and how these observations can and cannot be used as evidence of an impact. Before I get into the recently discovered methods that are used for identification of impact structures, I will first describe how they are created.
Impact crater formation is unlike any other known terrestrial geological process. Impact involves the transfer of massive amounts of energy to a relatively small area of the Earth’s surface, in an extremely short period of time (Kinetic energy). The kinetic energy of an object of mass m traveling at a speed v is mv2/2, provided v is much less than the speed of light.
The cratering process has been divided into three distinct stages, each dominated by different forces and mechanisms:
- Contact & Compression
The cratering process begins when the impactor, travelling at 10 to 75 km/s, makes initial contact with the target body. This starts the contact and compression process which lasts only fractions of a second. During this time the impactor will penetrate up to 2 times its diameter. At this point the kinetic energy of the impactor is transformed into shock waves that radiate into the target body and back into the impactor itself. The energy of the impact produces a spherical expanding envelope of hot gas regardless of the angle of impact. The impactor and the immediate area of the impact are completely vaporized . With few exceptions, the impact explosion almost always produces a circular crater. Elliptical impact craters are known elsewhere in the solar system and are caused from impactors at a very low-angle or obliquity of 10°–15° (Kenkmann 2009). After only a fraction of a micro-second, the compression stage ends with the complete transfer of energy in the form of a shock wave and latent heat.
The crater excavation stage (Melosh, 1980) overlaps somewhat with the compression stage and involves two processes:
- upward ejection (spalling) of large near-surface fragments and smaller ejecta (ejecta curtain);
- subsurface flow of target material to form the transient crater.
In seconds the resulting kinetic energy release creates an explosion that forms the initial transient crater. This energy release is transferred into complex interactions within the resulting shockwaves. The crater now consists of an evacuated zone (forming impact ejecta) and a lower displaced zone (forming crater-fill impactites). The excavation process is complete when the energy in the shock waves can no longer displace target rocks. This process could last up to 90 seconds for a crater of up to a 200 km diameter.
The initial transient crater is unstable and the modification stage commences. Small craters of <4 km (on Earth) are relatively stable after the excavation stage. For larger craters, the impact structure is gravitationally unstable and its modification stage will include uplift of the crater floor and collapse of the unstable steep walls (slumping). These movements will be completed in a few minutes and could result in a complex or multi-ring crater. Minor faulting, mass movement and/or hydrothermal activity in the larger craters could last indefinitely.
 In some rare cases the projectile survives the cratering process, at least in the form of meteorite fragments (apart from geochemical traces in melts at larger structures). At Barringer crater and at small simple impact structures (<< 2km) like Wolfe Creek or Odessa, the meteorite can survive. A large fragment of the impacting meteorite was found beneath the melt sheet at the ≥70 km Morokweng impact structure, South Africa (Schmieder 2010).
On this planet, impact craters are divided into three basic morphologic subdivisions: simple craters, complex craters, and peak ring craters. The transition size between simple to complex craters is 2km in sediments and 4km in crystalline rocks (Dence 1972). The transition size between complex to ringed basin craters is 10 to 50 km (Osinski, G. 2008). With increasing diameter, impact structures become proportionately shallower and develop more complicated rims and floors, including the appearance of central peaks and interior rings. While a single interior ring is required to define a basin, basins have been subdivided, with increasing diameter, on other planetary bodies, into central-peak basins, with both a peak and ring; peak ring basins, with only a ring; and multi-ring basins, with two or more interior rings (Wood and Head 1976). It is not known if there are examples of true multi-ring basins equivalent to those observed on the moon on Earth (Grieve 2006). A possible exception to this may be the Hudson Bay Arc, also known as the Nastapoka Arc, which I describe in a later article.
- Simple Crater
- Complex Crater
The central peak of the complex crater is formed as a result of uplift of material beneath the crater. Complex craters on Earth first occur at diameters greater than 2 km in layered sedimentary target rocks but not until diameters of 4 km or greater in stronger, more coherent, igneous or metamorphic, crystalline target rocks (Dence 1972).
- Peak Ring Crater
Anther peak ring basin on Earth is Ries crater (crater diameter 24 km, diameter of the crystalline ring 12 km), the ring is not visible very well due to the Ries lake sediments that cover large parts of the crater. It is a major structural feature that outcrops, e.g., beneath Nördlingen city.(Schmieder 2010);
Why are impact craters always round?
Most incoming objects must strike at some angle from vertical, so why don't the majority of impact sites have elongated, teardrop shapes?
Gregory A. Lyzenga, associate professor of physics at Harvey Mudd College, replies:
"When geologists and astronomers first recognized that lunar and terrestrial craters were produced by impacts, they surmised that much of the impacting body might be found still buried beneath the surface of the crater floor. (Much wasted effort was expended to locate a huge, buried nickel-iron meteorite believed to rest under the famous Barringer meteor crater near Winslow, Ariz.) Much later, however, scientists realized that at typical solar system velocities--several to tens of kilometers per second--any impacting body must be completely vaporized when it hits.
"At the moment an asteroid collides with a planet, there is an explosive release of the asteroid's huge kinetic energy. The energy is very abruptly deposited at what amounts to a single point in the planet's crust. This sudden, focused release resembles more than anything else the detonation of an extremely powerful bomb. As in the case of a bomb explosion, the shape of the resulting crater is round: ejecta is thrown equally in all directions regardless of the direction from which the bomb may have arrived.
"This behavior may seem at odds with our daily experience of throwing rocks into a sandbox or mud, because in those cases the shape and size of the 'crater' is dominated by the physical dimensions of the rigid impactor. In the case of astronomical impacts, though, the physical shape and direction of approach of the meteorite is insignificant compared with the tremendous kinetic energy that it carries.
"An exception to this rule occurs only if the impact occurs at an extremely shallow, grazing angle. If the angle of impact is quite close to horizontal, the bottom, middle and top parts of the impacting asteroid will strike the surface at separate points spread out along a line. In this case, instead of the energy being deposited at a point, it will be released in an elongated zone--as if our 'bomb' had the shape of a long rod.
"Hence, a crater will end up having an elongated or elliptical appearance only if the angle of impact is so shallow that different parts of the impactor strike the surface over a range of distances that is appreciable in comparison with the final size of the crater as a whole. Because the final crater may be as much as 100 times greater than the diameter of the impactor, this requires an impact at an angle of no more than a few degrees from horizontal. For this reason, the vast majority of impacts produce round or nearly round craters, just as is observed."
PLANET VS CRATER SIZE
|SOLAR SYSTEM BODY||DIAMETER km||GRAVITY||ESCAPE VELOSITY||BASINS km||PEAK RING km||COMPLEX km||SIMPLE km||REFERENCE|
|MERCURY||4,880||0.38||4.2 km/s||>~400||~110 - ~400||10 - 110||<~10||Greeley 2011|
|VENUS||12,104||0.91||10.4 km/s||-||>50 - 60||10 - ~50||<10 - 20||Greeley 2011|
|EARTH||12,756||1||11.2 km/s||>100||~30 - ~100||5 - ~30||<5||Grieve 2006|
|EARTH'S MOON||3,476||0.17||2.38 km/s||>220||175 - 220||30 - 175||<30||Wood 2003|
|MARS||6,788||0.38||5.0 km/s||~200||~20||~2.6||Greeley 2011|
Greeley, R. 2011, Planetary Geomorphology, Cambridge.
Grieve, R.A.F. 2006, Impact Structures in Canada, Geological Association of Canada.
Morrison, D. 2007 The Impact Hazard: Advanced NEO Surveys and Societal Responses, Comet/Asteroid Impacts and Human Society 2007, pp 163-173
Wood, C.A. 2003, The Modern Moon, Sky Publishing
CRATER URL References
For the complete physics of impact crater formation I recommend the following references:
- United States Meteorite Impact Craters
- Ernstson Claudin Impact Structures – Meteorite Craters
- Meteor/Meteorite News
- A CATASTROPHE OF COMETS
- The recognition of terrestrial impact structures,
- The Cambridge Encyclopedia of Meteorites,
- Large Meteorite Impacts and Planetary Evolution II,
- Traces of Catastrophe,
- Earth Impact Database,
- Meteorite and Impacts Advisory Committee,
- Impact Cratering Tutorial,
- About Cosmic Impacts
- Wonders of Astronomy
- Phil Plait: How to defend Earth from asteroids.
- B612 Foundation
PART II - Circular Geologic Structures
(AstroNotes January/February 2010;) [Updated 15 January 2010]
Nearly all meteorite impact craters on Earth are circular. However, ~4% of craters should be formed by impacts at angles lower than 12° from the horizontal, which should result in elongated crater structures. Illustrated above is an "elongated" crater imaged on Mars (upper right in the image). An elliptical crater on Earth, the Matt Wilson structure in Northern Australia, has recently been documented. It contains a central uplift and provides insights into the mechanisms of crater formation at a critical threshold angle of 10°–15° (Kenkmann 2009).
In this article I will document seven of many circular structures that I have explored on this planet; one confirmed impact structure (Pingualuit), one possible impact structure (Merewether) and five "suspicious" circular structures (Parry Sound, Goose Islands, Lake Skootamatta, Dauphin and Hudson Bay Arc). Specifically I will demonstrate that some (but not all) "circular" geological structures may have natural "non-impact" geological explanations.
Pingualuit impact structure is a classic example of the difficulties for firm impact identification. Pingualuit Crater has a diameter of 3.44 km and a depth of 400 metres. It had kept its original shape over eons of erosion. Geologists were sure that this structure was the result of an impact, mainly because there are not any natural geological events that could explain how this structure formed. BUT, there was no firm evidence for impact. E.M. Shoemaker explored the area in 1961, and in his view, there remained little doubt of a meteoritic origin. He stated that obtaining critical evidence probably would require drilling through the crater floor (Shoemaker 1962).
J. Boulger found a rounded vesicular pebble 1.75 cm across that was totally unlike any of the country rocks. The sample was sent to the Harvard-Smithsonian Centre for Astrophysics for petrographic examination. A thin section proved to be rich in quartz grains with multiple sets of planar features (Marvin, Kring, and Boulger 1988). Planar deformation features in quartz confirm an impact event (Grieve 2006).
The infilling lake is approximately two to three times deeper relative to its diameter than other lakes nearby and is symmetrical and bowl-shaped with respect to depth. (Grieve 2006)
Irrefutable evidence for a meteorite impact at the Merewether structures is still lacking. Drilling in the craters for evidence of planar deformation features has not been performed and no shatter cones, impact melt or meteorite fragments have been found at this site. This may be explained by the existence and movement of glaciers over the structure at the time of impact causing a smoothing of the rims and removal of any fragments of the impactor.
The study revealed two "semi-circular" lakes, Lakes McGruther and Dell near Parry Sound, the largest of which forms a circle approximately 600 metres wide and is 4 to 10 metres deep. The "rib" of rock that divides the two lakes dips sharply to the south. Under a stereoscope the dished nature of the "crater" is striking. (McKean, 1964)
Further geological study revealed that this structure is NOT a meteorite impact crater; rather, it is the result of multiple folding episodes of the Grenville gneiss bedrock with subsequent erosion and glacial modification. It is classified as a "Stratified Circular Feature". There are many features like this still visible in the Canadian Shield (The Mecatina structure in eastern Quebec is a similar feature and will be a future exploration destination).
The Skootamatta Syenite is centrally located in the Elzevir Terrane. The body is slightly elongate to the south-southwest with a length of 7.5 km and a width of 4 to 6 km. The Skootamatta Syenite has not been studied in detail. Mapping of Anglesea Township was undertaken by Meen (1944) and was later followed up by the work of Moore and Morton (1986) who cover only the far eastern part of the pluton. These two sources provide all the known information on the Skootamatta body.
Alsever Lake compared to Brent impact crater
Alsever Lake (image left) is located at the southern boundary of Algonquin Park. It is similar in appearance to the Brent impact crater (image right) with its two distinct bodies of water forming a circular pattern. Alsever has a central "land mass" like Brent and what appears to be circular outline. This is best viewed on some topographical maps.
Dauphin Manitoba "Circular Structure"
The feature is definitely "circular", although it most probably has a natural non-impact explanation to explain its formation. BUT, a ground exploration to the area just might reveal a surprise (what to look for to determine if an impact caused a feature like this will be a subject of future articles).
The present state of the studies of the Hudson Bay Arc is described here in M.E. Brookfield's ABSTRACT:
Over 40 years ago, Beals (1968) proposed an impact origin for the great eastern arc of Hudson Bay, which extends for 650 kilometres through an angle of 155 degrees and has a coherent circular raised rim on its landward side. A rift extends at right angles outwards on the southeastern side and within the arc, the basin is filled with Proterozoic sediments.
The best fit circle has a radius of 230 kilometres and the arc deviates from this circle by less than 10 km along its entire length. More recently, Goodings and Brookfield (1992) noted that closing the James Bay rift aligns the Sutton ridge to form an arc of 240 degrees, or two-thirds of a circle. The remainder is cut by the younger circular northern Hudson Bay cratonic basin. Apart from impact, no other plausible explanation has been proposed for this great ring fracture (and another ring fracture may exist outside this one). But, because no definitive evidence of impact was found, little has been published on the Hudson Bay arc since 1968. Recent studies of multi-ringed basins on other planets, and of other old multi-ringed basin on Earth (e.g. Vredefort), provide criteria for re-investigation and re- interpretation of published reports. Along the Hudson Bay arc, bodies of pseudotachylite, monomict and exotic breccias are associated with faults, and overlying sediments may show evidence of re-worked impact melts. If investigations are positive, Hudson Bay arc would form part of the largest identified multi-ringed impact on Earth, with a minimum diameter of 450 kilometres. Beals, C. S., 1968. On the possibility of a catastrophic origin for the great arc of eastern Hudson Bay. In: Beals, C.S.(editor), Science, History and Hudson Bay, volume 2. Department of Energy Mines and Resources, Ottawa. p.985- 999. Goodings, C.R. & Brookfield, M.E., 1992. Proterozoic transcurrent movements along the Kapuskasing lineament (Superior Province, Canada) and their relationship to surrounding structures. Earth-Science Reviews, 32: 147-185. (M.E. Brookfield 2006)
PART III - Crater Ejecta & Crater Rims
As described in Part I of this series, there are three stages to the impact cratering process, contact & compression, excavation and modification. The excavation stage is further subdivided into two distinct processes:
- upward ejection of large near-surface fragments and smaller ejecta (ejecta curtain);
- subsurface flow of target material to form the transient crater.
In this Part III of "Identifying Impact Structures" series I will give examples of this excavation process that happened elsewhere in our solar system and document the data I have collected in my explorations here on this planet that illustrates these two excavation processes.
Ejecta Curtain (#1 listed excavation process)
When the crater formation process ends, the resulting circular structure and the surrounding area is covered by an ejecta blanket. The factors affecting the appearance of impact crater ejecta are the geology of the target surface and the size and velocity of the impactor. Another factor, the impact angle, will modify the pattern of the ejecta blanket. A study at the Ames Vertical Gun Ballistic Range confirmed this effect.
The studies found that the ejecta pattern remains more or less linear around the impact site until the impact angle is <45° (measured from horizontal). At shallower angles the crater becomes increasingly elongated in the direction of projectile travel (documented in Part II of this series), and the ejecta patterns undergo even more pronounced changes. When the impact angle is <15°, the ejecta pattern becomes elongated in the downrange direction and an exclusion zone, where no ejecta appears, develops in the uprange direction. Exotic ejecta patterns like this can be found on the Moon, as well as elsewhere in the solar system (Wood, 2003).
A projectile, roughly 1 to 1.5 km wide travelling in a north-easterly direction low over Mare Tranquillitatis, impacted just west of Mare Crisium. It excavated the 28 km diameter crater Proclus. The ejecta exploded sideways and forward leaving an exclusion zone to the south west (pointing to the approach direction of impacting projectile).
To a lesser extent, the ejecta pattern around the Tycho Crater demonstrates the same type of pattern.
Another relatively fresh low angle impact on Mars shows the same ejecta pattern. The exclusion zone implies an impact direction from the lower right of the image.
On our planet, erosion will quickly remove this blanket and destroy any surviving meteorite fragments, with the result that crater ejecta remains in only the youngest and best-preserved impact structures. This ASTER Infra Red image documents the pattern of the ejecta blanket around the relatively young Barringer Crater. The pattern of the ejecta blanket, although it has been modified by 49K years of erosion, implies an impact from the south west. The majority of the ejecta blanket forms in the north east, downrange, direction.
Some of the Barringer Crater ejecta curtain can been seen here in visible light as illustrated in this image. I took this image of the crater from about 1000' above the ground. The "lighter" coloured sand is the remnants of the ejecta curtain. The pattern of the ejecta implies that the direction of the impactor was from the upper left of this image. If I was flying here at the time of impact, 49 thousand years ago, I would not know what hit me!!
Transient Crater - Rim (#2 listed excavation process)
At deeper levels of the impact where the material is not ejected, tensional stresses in the release waves are lower. As a result, fracturing is less pronounced, excavation flow velocities are lower, and the excavation flow lines themselves are not oriented to eject material beyond the crater rim. This region forms a displaced zone in which material is driven downward and outward more or less coherently. Both zones in the transient crater continue to expand, accompanied by the uplift of near-surface rocks to form the transient crater rim. However, these waves continually lose energy by deforming and ejecting the target rocks through which they pass. Eventually, a point is reached at which the shock and release waves can no longer excavate or displace target rock. At that point the growth of the transient crater ceases. (French, 1998).
It is estimated that it was a 1-3 km wide asteroid that impacted at the Moon's northeast limb to form the 22 km diameter Giordano Bruno Crater. When viewed from orbit, Giordano Bruno is at the center of a symmetrical ray system of ejecta that has a higher albedo than the surrounding surface, implying a high angle impact. The ray material extends for over 150 kilometers and has not been significantly darkened by space erosion.
As illustrated at the Giordano Bruno Crater, what remains when the growth of the transient crater stops is a depression with an upraised rim, and the modification stage begins. The exposed rim, walls, and floor define the so-called apparent crater. At the rim, there is an overturned flap of ejected target materials, which displays inverted stratigraphy, with respect to the original target materials (Grieve, 2002). An overturned rim sequence is now recognized as one of the hallmarks of an impact crater.
The rim of the 1.19 kilometre diameter Barringer Crater is still well defined, even after approximately 49 thousand years of erosion. It has been estimated that the first two stages of the cratering process (time from initial contact of the impactor until the end of the excavation stage) here at Barringer took approximately 6 seconds! Almost 63 million cubic metres were evacuated from this area in that time to form the crater. The height of the rim over the surrounding plain is 36 - 61 metres. Investigations around this rim confirmed an "overturned rim sequence".
My 11 kilometre exploration hike around the rim of the 3.44 kilometre diameter Pingualuit Impact Structure took most of a day and was not one of the easiest of hikes that I have experienced. Along the lip of the rim there were frequent gullies that we had to traverse. This image gives you a good size perspective, as the people leading the hike are just visible on the rim in the far distance.
The rim of the St. Martin complex crater is buried by over 100 metres of Jurassic red beds and glacial drift. It is my hypothesis that the cause of this extreme diversion of the Dauphin River at the rim of the St. Martin crater is the differential sagging of the outlaying bedrock compared to the breccia within the impact structure. To my knowledge, there is no published report that explains the cause of this river's diversion at this specific location. The Dauphin River then follows this rim to the East and flows into Lake Winnipeg.
PART IV - Impact Shock Wave Effects
In Part I of this series I documented the three stages of impact cratering; contact compression, excavation, and modification.
Furthermore, the excavation stage has two distinct simultaneous processes:
- upward ejection of large near-surface fragments and smaller ejecta (ejecta curtain - described in Part III);
- subsurface flow (lower displaced zone) of target material to form the transient crater.
In the excavation stage, the rocks in the impact zone are changed geologically as a result of the energy released by the subsurface flow of target material. The change process is complete when the energy of the impact shock wave can no longer displace or "modify" the target rocks (French, 1998). In this article of my "Identifying Impact Structures" series, I will show the geologic change in country rocks that are fractured by the impact "subsurface flow" shock wave.
One of the impact shock effects I noticed in my crater explorations was the fractured rocks around the perimeter of the structure where there should have been solid rock outcrops. Though this phenomenon IS NOT firm evidence of an impact but is a feature of impacting and would indicate that further investigation of the structure is warranted.
The Barringer Crater is one of the youngest impact sites on this planet and the effects of the impact still remain in situ. On the rim of the crater I noted fracturing of this country rock by the impact shock wave. Note that the country rock at this point was uplifted approximately 45 metres from its original position over the surrounding plain. Erosion has not yet exposed the fractured rocks buried outside of the crater.
An overturned rim sequence is also present at the rim of the Barringer Crater and is now recognized as one of the hallmarks of an impact crater.
In 2008 I was very fortunate to have had the opportunity to explore the Pingualuit Impact Structure in Northern Quebec. While hiking outside the crater rim I noted the effects of the impact on the exposed bedrock. The target rock surrounding the Pingualuit Crater consists of a mélange of metamorphosed, Archean plutonic rocks cut by rare basic dykes (Shoemaker, 1962). This image shows an in situ sample of this bedrock that was completely fractured by the impact. The rim of the crater is visible as the small hill 6 kilometres distant on the horizon. The distance gives you an appreciation of the energy required to fracture this rock from that distance.
This block of exposed fractured bedrock is on the rim of the Brent Impact Structure. It has the characteristics of rock exposed to the shock of an impactor 396 million years ago! The force of the explosion is estimated to have been equivalent to the explosion of 250 megatons of TNT. I was fascinated to see the effect first hand, a wall of bedrock with this amount of damage!
When approaching the Manicouagan Impact Structure from the south, I was specifically on the lookout for any changes in the local geology that may have been caused by the impact. Upon entering the inner fracture zone of the crater I noted that some of the rock cuts along the highway changed from solid granite faces to fractured walls. The rocks in this image were fractured by the energy release from a large meteorite impact, approximately 40 kilometres from this spot.
The magnitude of fracturing of the country rocks in the Manicouagan structure increased towards the centre of the crater, the point of maximum shock effect. The fragmentation increased to where the energy from the impact caused the rocks to melt. These melted rocks remain today as the central peak of the crater (the island in the image). The "smaller" fragmented rocks surrounding the "melt rock" central peak were easily evacuated by glaciation and erosion. The annular moat around the Manicouagan central peak (the circular lake in the image) is what remains after the country rocks that experienced "maximum" fracturing were removed. This circular moat is an impact indicator.
During our exploration of the Presqu'ile impact structure, we climbed a series of rapids to arrive at this site where the first indicator of an impact was realized, the discovery of shatter cones. These shatter cones and fractured rock surfaces occur within meta-basalt and rhyodacite rocks 5 km east of the crater, Lac de la Presqu’ile. The cones have angles of ~90° and their apparent position is vertical (Grieve). The discovery of these shatter cones at this site confirmed that an impact formed the Presqu’ile structure.
This Sudbury Crater fractured bedrock is outside of the Sudbury Igneous Complex (SIC), north-west of Windy Lake on highway 144 approximately 30 Km from the centre of the crater. At the time of impact this fractured rock was several kilometres underground. It has since been exposed by 1.8 billion years of erosion. When driving into the SIC from the north this is the first indicator of an impact event. The fracturing of these footwall rocks illustrates the deformation of the local bedrock that immediately followed impact. Shatter cones (an item I will describe in a later article) are also found in this area.
Pseudotachylite is a breccia having the aspect and the black color of a volcanic rock (a tachylite). It is formed when a high pressure from an impact is applied to country rocks and then abruptly released. This causes the rock along and within fracture lines or faults to partly melt. The fractures or faults containing the pseudotachylite are welded shut as soon as the motion created by the impact stops. Microscopic shock metamorphic features, shatter cones, impact glasses and pseudotachylites were formed during the contact and compression phase of the impact process. Polymict, clastic matrix breccia dikes, suevite, and bunte breccia contain fragments that were formed during the excavation and central uplift stage of the impact process when target rocks were in a cohesionless state allowing long-range fragment mixing. Subsequent stress is supported by the pseudotachylite as though it had never been active. The entire period of activity of a fracture or fault filled with pseudotachylite may be measured in minutes. (e.g., Pseudotachylite is a rock type formed by friction-induced melting, during very rapid deformation) Philpotts 1964; Maddock 1983.
Around the "outskirts" of the Sudbury impact structure are many examples of pseudotachylite deposits. Pseudotachylite like this is also found around the large Vredefort impact structure in Africa. At Sudbury, the pulverized and melted country rock injected into the pink gneiss country rocks has similar chemistry to the derived gneisses. The pseudotachylite zones fall along structures inferred as super faults or fault-controlled structures.
In this image the pseudotachylite is illustrated as the black pulverized (by the impact) country rock injected into the pink gneiss country rock (the toe of my boot is for scale). Sudbury pseudotachylite dikes range from veins less than 1 mm thick to massive zones measuring up to 1 km thick and extending for approximately 45 km. Formations of Sudbury pseudotachylite are found up to 100 km north of the SIC.
Planar Deformation Features (PDF)
In this article, the fractured bedrock and pseudotachylite that I documented at the "outskirts" of impact structures is NOT firm evidence of an impact. But these finds would indicate that further investigation to gather evidence of in impact may be warranted. One of the evidences used to confirm an impact is the discovery of shatter cones. In Part V of my "Identifying Impact Structures" series, I will document the formation of shatter cones and the shatter cone discoveries I have made at various impact structures.
PART V - Shatter Cones
Crater formation from a meteoroid impact is one of the most common geological processes occurring on planetary objects with solid surfaces. Impact at cosmic or hypervelocities involves the transfer of massive amounts of energy in the form of high pressure and temperature shock waves to a relatively small area in an extremely short period of time. A smaller meteoroid on contact with the atmosphere may slow from its hypervelocity of >11.2 km/sec to a terminal velocity of ~0.04 km/sec. (The escape velocity of our planet is 11.2 km/sec). If the velocity of the meteoroid decreased to terminal velocity, it will contact the planet as a meteorite. One confirmation of impact is meteorites found in situ within the impact crater. A curious observation is that the smaller the crater the larger the meteorites that are found in it. The dividing line between finding whole, unbroken meteorite specimens and only broken fragments seems to be related to crater size, with approximately 3-metre diameter craters as the dividing line (Norton 1998).
For reference, the confirmed Carancas impact crater in Peru was formed by an impactor making contact at greater than its terminal velocity but significantly less than its cosmic velocity. The Carancas impactor at contact had a diameter of 0.6 to 1.1m (0.3 to 3 ton), a velocity of >3 km/sec to <6 km/sec and formed a crater of 13.5 m rim-rim diameter. There are no definitive indications of large remnants of the meteorite inside the crater. Levels of shock metamorphism in ejecta minerals were compatible with peak pressures at impact of over several GPa (Tancredi et al, 2009).
If a meteoroid is large enough and makes ground contact at hypervelocity, the resulting explosion vaporizes the meteoroid and, in addition to the crater, produces various levels of metamorphism in the country rocks.
Rock Impact Metamorphism
As the science of impact crater identification progressed in the 1950's, it was soon realized that fractured and brecciated rocks in combination with a circular structure could not be used as absolute proof of an impact crater. Even though deformation of this type is consistent with meteorite impact, the equivalent deformation can also be produced by tectonic and volcanic activity (Grieve, Robertson 1975). It is then necessary to establish an alternative set of criteria to confirm identification of old impact craters. Shock-metamorphic effects in rocks have been established as the distinctive mark of an impact crater.
The passage of the high-pressure shock wave results in shock metamorphism, the progressive breakdown in the structural order of minerals and rocks. The pressures and temperatures in the shock wave are well above the magnitudes of pressures and temperatures occurring naturally on this planet.
 Hypervelocity - 11.2 km/sec to 70 km/sec.
 The crater:impactor size ratio ranges from 20:1 to 50:1 (Shoemaker 1963, Baldwin 1963).
 The standard unit of pressure is the Pascal, abbreviated Pa, which is equivalent to 1 kilogram per square meter. A GPa is a gigapascal (giga means billion), a measurement of pressure, and is equal to 10,000 times the atmospheric pressure at the Earth's surface.
Research has established that near the hypervelocity impact point, initial shock pressures can exceed 100 GPa, resulting in the total melting and vaporization of a large volume of target rock together with virtually all of the impactor. Passing outward, the lower shock pressures produce a series of distinctive effects in the target rocks (Koeberl, French, 2009):
- rock melting (≥60 GPa);
- selective mineral melting (40–60 GPa),
- diaplectic glass phases (30–45 GPa);
- high-pressure minerals - coesite and stishovite (12–30 GPa),
- planar deformation features (PDFs) in quartz (10–25 GPa);
- multiple fracturing (cleavage) and basal Brazil twinning in quartz (5–10 GPa);
- rock fracturing (2–5 GPa);
- shatter cones (≥2-30 GPa).
Of these shock metamorphic effects only shatter cones can be easily identified with the naked eye. The remainder of these effects tends to be microscopic in size. Shatter cones are found exclusively in two places on Earth:
- in nuclear (or very LARGE non-nuclear) explosion test sites, and
- cosmic velocity meteorite impact structures.
Curious cone-shaped rocks called shatter cones were first found in 1905 at the Steinheim Basin in southern Germany, an eroded meteorite crater about 3 km in diameter. In 1947, R.S. Dietz established that the recognition of shatter cones was a reliable geologic criterion for the recognition of impact structures (in the absence of meteorites). Dietz was the first to suggest that shatter cones only occur in impact (or explosion) craters. By 1964 shatter cones were found around the Sudbury ore body in Ontario, providing the first evidence for the meteoritic origin for the structure (Dietz 1947).
Shatter Cone Formation
Shatter cones have been observed in rocks shocked in nuclear test explosions and produced experimentally in the laboratory. The required shock pressure to produce a shatter cone is estimated between roughly 2 and 20 GPa. In general, the apex of the cones points to the shock source, but irregular orientations and even counter orientation are frequent. Research proposes that shatter cones form from the passage of an impact pressure shock-wave.
In impact structures, the size of the cones varies between centimeters to meters and fully developed cones are rare. In the extreme, shatter cones may degenerate into shatter cleavage.
Shatter Cone Identification
The identification of shatter cones (especially poorly-developed ones) depends considerably on the experience and the eye of the beholder. They have several characteristics that distinguish them from non-impact features (Koeberl, French, 2009):
- they can form in all the rock types present in an impact structure: carbonates, shales, clastic sediments, granites, gabbros, and other crystalline rocks;
- they consist of penetrating fracture surfaces, along which the rock can be broken to reveal new cones or partial cones;
- the surfaces have positive and negative relief, and concave negative surfaces (“casts”) of the convex cone surfaces can commonly be found; and,
- the striations on cone surfaces are distinctive and directional; they consist of alternating positive and negative grooves that radiate downward and outward from the apex of the cone. Secondary radiating striations commonly develop along the primary ones, forming a distinctive structure.
Meteoroid impacts on this planet have formed many impact craters, the contact speeds varying from terminal velocity to hypervelocities. The extreme pressures and temperatures at hypervelocity impacts have caused shock metamorphic effects on the target rocks. One of the features that are formed by these impacts is shatter cones, conical striated fractures on rock surfaces. Shatter cones have been detected in many meteorite impact structures and are widely regarded as a diagnostic macroscopic (can be seen with the naked eye) recognition feature for impact. They are a dead give-away for the amateur crater hunter (like me) to confirm that the structure being explored is the result of an impact (if a LARGE man-made explosion can be eliminated). In many cases, the initial discovery of shatter cones has spurred successful searches for other shock effects. In my next article I will document the shatter cones I have found in various geologic structures that have led to their identification as impact craters.
Finally, a bit of hypothesizing on my part. In recent years the Hayabusa space mission imaged and possibly took samples from the Itokawa S-type (siliceous composition) asteroid. The Hayabusa probe is scheduled to return to our planet in June 2010, and I sincerely hope it is bringing the asteroid sample with it!
It is suggested that Itokawa may be a contact binary formed by two or more smaller asteroids that have gravitated toward each other and stuck together (Rayl 2008).
Looking closely at the Itokawa asteroid's boulders, are there some shatter cones in that rubble pile?
PART VI - in situ Shatter Cones
As of May 2010, there are 174 confirmed impact craters found on our home planet. Of 57 discovered in North America, 29 are in Canada, 27 are in the United States and 1 is in Mexico (Spray 2010). I would venture that there are several more remaining to be discovered, just look at the moon! Finding a formerly undiscovered impact crater here on our planet would be good science and a heck of a lot of fun!!
As amateur crater hunters tromping through the bush hoping to find the results of a large meteorite impact, we should realize that: (1) most rocks on Earth have never been involved in a meteorite impact; (2) even in impact structures, most of the rocks will not look shocked; (3) no matter how "circular" a structure may look, definite and unquestionable impact-produced features will only be found in the rocks (French 2005).
In Part V of my impact structure identification series, I described shatter cones that ARE firm evidence of a hypervelocity impact (or a very large man made explosion). Some of the craters discovered over the years were first identified after finding shatter cones in the immediate area. So, as amateur impact crater hunters, to find shatter cones with these unique features we have to get down on the ground and root around.
Craters & Shatter Cones
Shatter cones have unique features visible to the naked eye and can be identified with the following characteristics: (1) shatter cones form in all rock types, the best ones in fine-grained rocks; (2) the orientation of shatter cones are relative to the centre of the impact structure; (3) freshly-broken samples will show shatter-coned interior surfaces; and (4) shatter-cones show grooves with positive and negative relief (French 2005).
In this article I will show the shatter cones I have found in situ that led to the respective structure being identified as an impact crater. For detailed documentation of these impact structures, please refer to my web site (O'Dale 2010).
Charlevoix, a complex impact structure, is visible as a heavily eroded semicircular area located on the north-shore of the St.Lawrence River. Shatter cones were discovered in the course of regional mapping of the area (Rondot, 1966). They are widespread and abundant in the gneisses and limestone of the central uplift at La Malbaie (Charlevoix) crater. Virtually all rocks within a radius of 12 km from Mont des Eboulements are shatter-coned, with the exception of those on Isle aux Coudres (Robertson 1968).
Lac de la Presqu'ile impact structure is locate in Central Quebec and includes a roughly annular lake ~6km in diameter with a large promontory on its eastern shore. The structure was recognized as impact related with the discovery of shatter cones during the course of regional mapping (Higgins and Tate, 1990).
Our exploration team was exploring in the south end of west Presqu'ile lake when a fairly substantial line squall almost caught our canoe in the open. We high-tailed it to the lee of an island and waited the storm out. We sat there for awhile discussing life, the universe and everything but the rain didn't stop. So I thought, what the heck, I'm getting out of the canoe and have a look around at these rocks. The first rock I picked up was a shatter cone! I don't think this area has ever been explored and these shatter cones had not been disturbed since the last ice age.
The Slate Islands impact structure is visible as a circular series of islands almost totally submerged in Lake Superior. The Discovery of extensive breccias and shatter cones during geological mapping of these islands in 1974 led to their recognition as the central peak of an impact structure (Sage, 1974).
The Manicouagan impact structure in central Quebec contains a large ~65km diameter annular moat (filled by a water reservoir) which is very obvious in satellite imagery. The crater itself extends outside of the reservoir to a diameter of over 150 km. A geophysical survey was carried out in 1963 to confirm the structure as an impact crater (Grieve 2006).
The Isle Rouleau impact structure is in Lac Mistassini in central Quebec. Regional mapping in 1973 confirmed the structure as impact related (Caty et al., 1976). It is assumed that the island represents the eroded remnant of a central peak.
The Sudbury Impact Structure comprises a 200-250 km multi ring impact basin formed 1.85 billion years ago. The core of the structure is elliptical, 60 x 30 km, containing a layered 2.5 km thick impact melt sheet, referred to as the Sudbury Igneous Complex (SIC). The SIC was formed by differentiation of the impact melt pool at the probable main contact point of the impactor.
Shatter cones have been reported up to 15 kms away from the periphery of the SIC. The cones commonly point toward the centre of the Sudbury basin, indicating that the Sudbury crater structure has undergone considerable erosion since the impact occurred 1.85 billion years ago.
The identification of shatter cones (especially poorly-developed ones) depends considerably on the experience and the eye of the beholder. They have several characteristics that distinguish them from non-impact features (French, Koeberl, 2009):
- they can form in all the rock types present in an impact structure: carbonates, shales, clastic sediments, granites, gabbros, and other crystalline rocks;
- they consist of penetrating fracture surfaces, along which the rock can be broken to reveal new cones or partial cones;
- the surfaces have positive and negative relief, and concave negative surfaces (“casts”) of the convex cone surfaces can commonly be found; and
- the striations on cone surfaces are distinctive and directional; they consist of alternating positive and negative grooves that radiate downward and outward from the apex of the cone. Secondary radiating striations commonly develop along the primary ones, forming a distinctive structure
Shatter cones have been described from many meteorite impact structures and are widely regarded as a diagnostic macroscopic (can be seen with the naked eye) recognition feature for impact. They are a dead give-a-way for the amateur crater hunter (like us) to confirm that the structure being explored is the result of an impact (if a LARGE man-made explosion can be eliminated). In many cases, the initial discovery of shatter cones has spurred successful searches for other shock effects.
Part VII - Shock Metamorphism
An obvious craterform is an excellent indicator of a possible impact origin; particularly, if it has the appropriate morphometry as illustrated in Part II of this series. But as noted, such features are rare and short-lived in the terrestrial environment. The burden of proof for an impact origin generally lies with the documentation of the occurrence of shock-metamorphic effects. Impacts produce distinctive "shock-metamorphic" effects that are found in situ within the crater and allow impact sites to be distinctively identified. Such shock-metamorphic effects, in addition to the shatter cones (documented in Parts V & VI), include brecciated rocks, suevites, impact melts and pseudotachylites. They attest to the destructive power of the impact event.
The rocks at the target site are melted, shattered, and mixed during the impact explosion. When the site finally settles and cools, a new composite rock, impact breccia in bodies tens to hundreds of meters in size, is the result. Lithologies showing these unique diagnostic shock effects, formed at pressures ≥10 GPa, tend to be restricted to two locations: (1) crater-fill materials (suevites, melt breccias, and fragmental impact breccias) deposited in the crater; and (2) brecciated basement rocks, often containing shatter cones, near the center of the structure. The magnitudes of the impact shock relative to the point of impact that form the shock metamorphic effects were quantified for reference in Part V.
BRECCIA - (from a Latin word meaning “broken”) is a rock that is composed of angular fragments of other rocks surrounded by a fine-grained "matrix" that may be of a similar or a different material. Breccias are extremely common in the central uplift, in crater-fill deposits, and in the ejecta blanket of meteorite impact craters.
IMPACT MELT - rock that has been made temporarily molten as a result of the energy released by the impact of a large colliding body. Impact melts include small particles, known as impact melt spherules, that are splashed out of the impact crater, and larger pools and sheets of melt that coalesce in low areas within the crater. They are composed predominantly of the target rocks, but can contain a small but measurable amount of the impactor
PSEUDOTACHYLITE - is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It may contain clasts of the country rock and occasionally crystals with quench textures that began to crystallize from the melt.
SUEVITE - a rock consisting partly of melted material, typically forming a breccia containing glass and crystal or lithic fragments, formed during an impact event. It forms part of a group of rock types and structures that are known as impactites.
Manicouagan - breccia, melt rock
Approximately 214 million years ago an estimated 10 kilometre wide hypervelocity meteorite impacted at between 12 and 30 kilometres per second and formed the Manicouagan Impact Structure.
The resultant 100 kilometre diameter crater (image left - Courtesy NASA/LPI) is one of the largest impact craters still preserved on the surface of our planet. The Copernicus crater on the moon (image right - Courtesy NASA - A12) has a diameter of 93 kilometres. For comparison, the Manicouagan's annular moat would fit comfortably within the rim of the Copernicus Crater.
Morphological elements of the Manicouagan structure are based on topographical expression and are:
- outer circumferential depression - ~150-km outer diameter;
- outer disturbed zone - ~150 km diameter;
- inner fractured zone - ~100 km outer diameter;
- annular moat - ~65 km outer diameter;
- inner plateau - ~55 km outer diameter;
- central region - ~25 km outer diameter. (Grieve, Head 1983)
In the summer of 2006, Eric Kujala and I explored the interior of the Manicouagan impact structure by canoe and on foot. We entered the structure from the east, crossing the all these morphological elements and concluding in the Memory Bay inlet. This inlet is on the east portion of the island forming the central peak of the structure. To read about our harrowing experience while in the crater please see my web site about the trip (O'Dale 2006).
At the point of impact, the country rocks were instantaneously evaporated/melted/shattered by the energy released leaving a 200 to 600 cubic kilometre sheet of impact melt directly on basement rocks. We observed changes in these impactite textures as a progressively increasing proportion of superheated melt and a decreasing fraction of cold fragmented country rock material toward the interior of the crater (Simonds 1976). The following images will illustrate these observations.
Outer Circumferential Depression, Outer Disturbed Zone and Inner Fractured Zone
In Part IV of this series I documented the shattered rock we encountered at approximately 40km from the central peak as we entered the outskirts of the crater. We did could not identify the outer circumferential depression.
The water filled circular annular moat that is prominent in space images is only one third of the size of the original crater. The water in the annular moat fills a ring where impact-brecciated rock was eroded away by glaciation. Before flooding of the reservoir, isolated outcrops of tilted and deformed limestone, siltstone and shale were found on the inner edges of the moat (Murtaugh, 1975).
This rock formation is found at the extreme eastern portion of the annular moat on one of the small islands. Note the rock structure is breccia free gneiss. The central peak of the structure is visible over 10 km in the distance.
The Inner Plateau of the Manicouagan structure is bounded by the annular moat, overlain by melt sheet, underlain by shocked basement rock (Orphal, Schultz 1978). We found a "lunar landscape" here containing various breccia types. The astronauts exploring the moon found that impact-melt breccias, similar to what we found here, were the most common rock types at the Apollo highland sites (Apollos 14, 15, 16 and 17) (Haskin 1998). We documented impact breccias formed by similar and very different country rocks like those found on the moon!
Impact breccias were melted, mixed, crushed and compressed by shock waves at various stages in the cratering process: (1) during the initial shock-wave expansion and transient crater formation; (2) during the subsequent modification of the transient crater. Even within the brief formation time of an impact crater, it is possible for the multiple generations of breccia to develop and to produce distinctive differences, even though the time between one breccia generation and the next may be measured in seconds or minutes (French 1998). The extremely small size of the grains within the matrix between the country rock fragments were formed by the very high pressure of the gas generated when the bolide impacted.
This photo illustrates a breccia outcrop found within the inner plateau area of the Manicouagan Impact Structure. Note the different types of rock fragments forming the breccia within the fine grained matrix impact melt.
This images documents "uniformly white" shattered country rocks imbedded in a fine grained matrix impact melt. This breccia outcrop is found in an inlet, cut into the central peak of the impact structure, known as Memory Bay.
Further west into Memory Bay I noticed a possible shock pseudotachylite vein within a breccia outcrop. The pseudotachylite veins associated with impacts are much larger than those associated with faults and are thought to have formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. In Part IV of this series I illustrated the pseudotachylite found in the Sudbury Impact Structure. Impact related pseudotachylite was first recognized at the Vredefort crater in Africa.
The Central Region of the Manicouagan Structure is a complex zone of uplifted, shocked and metamorphosed basement rocks with small tabular bodies of impact melt and pseudotachylite veins (Orphal, Schultz 1978). Recent U-Pb zircon dating of the impact melt gave an age of 214 ± 1 million years.
The illustrated impact melt cliff and talus (debris at the base of the cliff) is found in the central region area of the Manicouagan Impact Structure. It is composed of target rock that was made temporarily molten from the energy released during impact. There are not any detectable meteorite components in the Manicouagan structure melt rock (Palme et al., 1978).
I tried to climb the talus slope up to the cliff face but it became very unstable the higher I climbed. I got to the point that I was creating dangerous rock slides without making any progress. I stopped to take this picture; looked down and found the "Manicouagan shatter cone" I documented in Part VI of this series. Serendipity at its best!
While we explored the impact melt cliffs on the north shore of Memory Bay we noticed an odd feature in one of the cliffs. Eric took this image of the feature. It shows a 10 m block of mafic gneiss (indicated in the image) suspended about 20 m above the base of the melt sheet. Such a block is 0.3g/cm³ denser than the melt and should settle at a minimum of 5 cm/sec (Stokes Law ) through a Manicouagan composition melt with 2% H2O (water) if it were still liquid at 1000°C. In order for that block to remain suspended, the melt must have begun to crystallize rapidly enough to trap the block before it settled to the bottom of the sheet (Simonds 1976).
 STOKES LAW: If the particles are falling in the viscous fluid by their own weight due to the Earth's gravity, then a terminal velocity, also known as the settling velocity, is reached when this frictional force combined with the buoyant force exactly balance the gravitational force. The resulting settling velocity (or terminal velocity) is given by:
Vs = ( 2 (ρp - ρf ) / 9 η ) g R2 where:
- Vs is the particles' settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf ),
- R is the radius of the spherical object (in metres),
- g is the Earth's gravitational acceleration (m/s2),
- ρp is the mass density of the particles (kg/m3),
- ρf is the mass density of the fluid (kg/m3), and
- η is the fluid's viscosity (in [kg m-1 s-1]).
Sudbury - breccia, pseudotachylite
The Sudbury Structure comprises a 200-250 km multi ring impact basin formed at 1.85 Ga. The core of the structure is elliptical, 60 x 30 km, containing a layered 2.5 km thick impact melt sheet, referred to as the Sudbury Igneous Complex (SIC). The SIC was formed by differentiation of the impact melt pool at the probable main contact point of the impactor.
Immediately interfacing the upper SIC is the grey Whitewater breccia that contains many large angular rock fragments floating in a glass like amorphous rock. These fragments are the fallback particles from the surrounding Huronion supergroup country rock that were deposited immediately after the impact. These rock fragments were ejected ballistically hundreds of km into the atmosphere and then minutes to hours later “plopped” into this still molten rock. Note the large fragment in the lower right of the image that is hydrothermally altered and surrounded by a “chilled margin” (a mineralized area around the fragment caused by a hydrothermal vent). The size of the Sudbury structure implies that the hydrothermal venting continued for thousands of years after the impact.
Further into the structure is the darker Whitewater breccia containing smaller rock fall-back fragments originating from the igneous quarts granite north range footwall. Here the breccia indicates the introduction of carbon. A biogenic origin of the carbonaceous material (soot) found in the black Whitewater Group is theoretically caused by the evaporation/condensation from the hot impact fireball and/or from a later global cloud. The colour of the rock is not uniform indicating that the carbon is not uniformly distributed. Bucky balls (soccer-ball-shaped molecules of 60 carbon atoms) possibly of extraterrestrial origin and with traces of helium and argon gas trapped inside were found in this breccia.
The impact probably occurred in a shallow sea as there is evidence of water flow-back in the top layers of the Whitewater Group. The quantity of “breccia fall-back” specifies that the fall-back segment of the impact lasted a substantial amount of time (perhaps hours) before the appearance of the returning tsunami.
In this image the Pseudotachylite Sudbury Breccia (SB), the black pulverized (by the impact) country rock injected into the pink gneiss, was formed when the high pressure from the meteorite impact was applied to these rocks and then abruptly released. This caused the rock along and within these dykes to partly melt. The dykes containing the pseudotachylite were welded shut as soon as the motion created by the impact stopped. Subsequent stress was supported by the fault as though it had never been active. The entire period of activity of a fault filled with pseudotachylite may be measured in minutes. (e.g., Pseudotachylite is a rock type formed by friction-induced melting, during very rapid deformation) (Philpotts 1964; Maddock 1983).
SB pseudotachylite dikes range from veins less than 1 mm thick to massive zones measuring up to 1 km thick and extending for approximately 45 km. Formations of SB are found up to 100 km north of the SIC.
Charlevoix - impact melt
The Charlevoix Meteorite Crater is a multi ringed impact basin with a central uplift. It is located in southern Quebec on the north shore of the St. Lawrence River, 105 km NE of Quebec City. One feature that supports an impact origin of a structure is impact melt rock.
This expamle of impact melt rock was extracted from within the Charlevoix impact structure. At pressures in excess of about 60 GPa, rocks undergo complete (bulk) melting to form impact melts. The melts can reach very high temperatures due to the passage of shock waves that generate temperatures far beyond those commonly encountered in normal crustal processes or in volcanic eruptions. Each mineral grain is instantaneously raised to a post-shock temperature that depends on the shock-wave pressure and on the density and compressibility of the mineral itself. If the postshock temperature produced in a mineral exceeds its normal melting temperature, each grain of that mineral in the rock will melt, immediately and independently, after the shock wave has passed. The melt will have approximately the same composition as the original mineral before any flow or mixing takes place, and the melt regions will initially be distributed through the rock in the same pattern as the original mineral grains. Note the country rock fragment in the inclusion.
Skelton Lake - breccia
Skeleton Lake is a generally circular lake provisionally classified as a simple meteorite crater. It is located in the Muskoka District of Ontario on the Canadian Shield slightly east of Georgian Bay. It is the largest open body of water within the Muskoka Lakes.
These images illustrate a breccia deposit on Opal Island situated in the area of the crater rim. This breccia may have been formed as the result of a hypervelocity impact.
These are the breccia samples I had recovered from Opal Island within the Skelton Lake structure. The breccia deposit on Opal Island stands in stark contrast to the surrounding target rock in the Central Gneiss Belt. The country rocks comprise of parautochthonous gneisses overlain by northwest-transported allochthonous terraines originating from the pre-Grenvillian Laurentian margin or from farther outboard as inferred for the Parry Sound allochthon.
As of this date (Feb. 2011), firm evidence of an impact has not been found in the Skelton Lake structure.
Slate Islands - impact breccia
The ~7-km-wide Slate Islands group was created by a bolide impact 436 million years ago in northern Lake Superior. They represent the heavily eroded central peak of a ~32 km diameter (from bathymetric data) complex impact crater. It is not known if the present height of the central peak island is the result of stratigraphic uplift only or of uplift followed by partial collapse of the central peak and erosion. Target rocks consist of three main groups of Archean and Proterozoic supracrustal and intrusive rocks, about 2.7 Ga and 1.8 Ga and 1.1 Ga old respectively. Heterogeneous melt bodies are located within heavily brecciated units of the Slate Islands central uplift peak (Dressler et al, 1995). Specific impact breccia types in the target rocks are related to the various phases of the impact process.
Specific impact breccia types in the target rocks are related to the various phases of the impact process and are made up of fragments of the target rocks, containing various ratios of impact melt and shocked mineral inclusions. The breccia deposits illustrated here are typical for almost all the other breccia deposits I found on the islands.
Wanapitei - suevite
The Wanapitei structure was confirmed as an impact crater by presence of coesite, which can be formed at pressures of 425-500 kilobars and temperatures near 1000°C (Dence et al. 1974). Lake Wanapitei is classified as a simple crater with an estimated diameter of 3 km (E. L’Heureux et al, 2003) to ~7-8 km. There is no evidence of a central uplift in the submerged crater (Dence and Popelar, 1972).
The suevite samples illustrated in the above images were found on the south shore of Lake Wanapitei crater. It was apparently scooped up from the lakebed by glacial activity and deposited in places along the southern shore of the lake. Suevite is an impact fallback breccia, formed when a meteorite strikes the earth and blasts "target rock" high into the atmosphere. Some target rock falls back into the newly formed crater, and is compacted to form suevite. Suevite typically contains fragments of shock-metamorphosed rocks and glass set in a matrix of fine-grained minerals, rock, and glass fragments.
Isle Rouleau - breccia
The day after exploring the Isle Rouleau impact structure we had an interview with the government geologist posted at Baie-du-Poste. After an excellent conversation regarding the local geology (and we did brag a bit about our explorations!), she was gracious enough to give us a sample of breccia recovered from the Isle Rouleau site. Thank you!
Brent - breccia
During one of the ground explorations within the Brent impact structure, we were very fortunate to spot these breccia examples. They WERE NOT found as in situ deposits but were most probably placed here by the glaciers (glacial erratics). So, scientifically, without material analysis we cannot absolutely claim that this is breccia from the Brent structure, BUT, the circumstantial evidence is almost conclusive. The other explanation is that these deposits were from another impact site further to the north and just “happened” to be dropped off here within the Brent structure.
Impact melt is the “greyish” material between and cementing the country rock fragments. K-Ar dating of the recrystallized melt-bearing breccia gave ages of 310-365 Ma (Shafiquallah et al., 1968). Geochemical analyses show that the “melt” rocks are in fact melted target rock with ~1% contamination by chondritic material (Grieve, 2006).
Barringer - comminution and facturing
|Barringer impact structure. Coconino sandstone layers are typically buff to white in color. It consists primarily of sand deposited by eolian processes (wind-deposited) approximately 260 million years ago.|
Part VIII - Radiometric Dating Of Meteorite Craters
Dating Methods Used For Ages >100,000 Years
The potassium-argon (K-Ar) ratio is “reset” to zero by impacts when the argon gas generated by the decay of potassium is diffused out of material heated by shock. The K-Ar age will then date the impact which affected the target bedrock.
Potassium is an abundant element in the Earth's crust. One isotope, potassium-40, is radioactive and decays to two different daughter products, calcium-40 and argon-40, by two different decay methods. The production ratio of these two daughter products is precisely known, and is always constant: 11.2% becomes argon-40 and 88.8% becomes calcium-40. It is possible to date some rocks by the potassium-calcium method, but this is not often done because it is hard to determine how much calcium was initially present. Argon, on the other hand, is a gas. Whenever rock is melted to become impact melt, magma or lava, the argon tends to escape. Once the molten material hardens, it begins to trap the new argon produced since the hardening took place. In this way the potassium-argon clock is clearly reset when an igneous rock is formed.
In its simplest form, the geologist simply needs to measure the relative amounts of potassium-40 and argon-40 to date the rock. The age is given by a relatively simple equation:
where t is the time in years, h is the half-life, also in years, and ln is the natural logarithm.
However, in reality there is often a small amount of argon remaining in a rock when it hardens. This is usually trapped in the form of very tiny air bubbles in the rock. One percent of the air we breathe is argon. Any extra argon from air bubbles may need to be taken into account if it is significant relative to the amount of radiogenic argon (that is, argon produced by radioactive decays). This would most likely be the case in either young rocks that have not had time to produce much radiogenic argon, or in rocks that are low in the parent potassium. One must have a way to determine how much air-argon is in the rock. This is rather easily done because air-argon has a couple of other isotopes, the most abundant of which is argon-36. The ratio of argon-40 to argon-36 in air is well known, at 295. Thus, if one measures argon-36 as well as argon-40, one can calculate and subtract off the air-argon-40 to get an accurate age.
One of the best ways of showing that an age-date is correct is to confirm it with one or more different dating method(s). Although potassium-argon is one of the simplest dating methods, there are still some cases where it does not agree with other methods. When this does happen, it is usually because the gas within bubbles in the rock is from deep underground rather than from the air. This gas can have a higher concentration of argon-40 escaping from the melting of older rocks. This is called parentless argon-40 because its parent potassium is not in the rock being dated, and is also not from the air. In these slightly unusual cases, the date given by the normal potassium-argon method is too old. However, scientists in the mid-1960s came up with a way around this problem, the argon-argon method.
This method uses exactly the same parent and daughter isotopes as the potassium-argon method. In effect, it is a different way of telling time from the same clock. Instead of simply comparing the total potassium with the non-air argon in the rock, this method has a way of telling exactly what and how much argon is directly related to the potassium in the rock.
In the argon-argon method the rock is placed near the center of a nuclear reactor for a period of hours. A nuclear reactor emits a very large number of neutrons, which are capable of changing a small amount of the potassium-39 into argon-39. Argon-39 is not found in nature because it has only a 269-year half-life. (This half-life doesn't affect the argon-argon dating method as long as the measurements are made within about five years of the neutron dose). The rock is then heated in a furnace to release both the argon-40 and the argon-39 (representing the potassium) for analysis. The heating is done at incrementally higher temperatures and at each step the ratio of argon-40 to argon-39 is measured. If the argon-40 is from decay of potassium within the rock, it will come out at the same temperatures as the potassium-derived argon-39 and in a constant proportion. On the other hand, if there is some excess argon-40 in the rock it will cause a different ratio of argon-40 to argon-39 for some or many of the heating steps, so the different heating steps will not agree with each other.
In nearly all of the dating methods, except potassium-argon and the associated argon-argon method, there is always some amount of the daughter product already in the rock when it cools. The Rubidium-Strontium dating method reveals how much of the daughter product was already in the rock when it cooled and hardened.
The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87. Strontium-87 is a stable element; it does not undergo further radioactive decay. (Do not confuse with the highly radioactive isotope, strontium-90.) Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium-86. If three different strontium-containing minerals form at the same time in the same magma, each strontium containing mineral will have the same ratios of the different strontium nuclides, since all strontium nuclides chemically behave the same. (Note that this does not mean that the ratios are the same everywhere on earth. It merely means that the ratios are the same in the particular magma from which the test sample was later taken.) As strontium-87 forms, its ratio to strontium-86 will increase. Strontium-86 is a stable element that does not undergo radioactive change. In addition, it is not formed as the result of a radioactive decay process. The amount of strontium-86 in a given mineral sample will not change. Therefore the relative amounts of rubidium-87 and strontium-87 can be determined by expressing their ratios to strontium-86: Rb-87/Sr-86 and Sr87/Sr-86 We measure the amounts of rubidium-87 and strontium-87 as ratios to an unchanging content of strontium-86.
The uranium-lead method is the longest-used dating method. It was first used in 1907, about a century ago. The uranium-lead system is more complicated than other parent-daughter systems; it is actually several dating methods put together. Natural uranium consists primarily of two isotopes, U-235 and U-238, and these isotopes decay with different half-lives to produce lead-207 and lead-206, respectively. In addition, lead-208 is produced by thorium-232. Only one isotope of lead, lead-204, is not radiogenic. The uranium-lead system has an interesting complication: none of the lead isotopes is produced directly from the uranium and thorium. Each decays through a series of relatively short-lived radioactive elements that each decay to a lighter element, finally ending up at lead. Since these half-lives are so short compared to U-238, U-235, and thorium-232, they generally do not affect the overall dating scheme. The result is that one can obtain three independent estimates of the age of a rock by measuring the lead isotopes and their parent isotopes.
The uranium-lead system in its simpler forms, using U-238, U-235, and thorium-232, has proved to be less reliable than many of the other dating systems. This is because both uranium and lead are less easily retained in many of the minerals in which they are found. Yet the fact that there are three dating systems all in one allows scientists to easily determine whether the system has been disturbed or not. Using slightly more complicated mathematics, different combinations of the lead isotopes and parent isotopes can be plotted in such a way as to minimize the effects of lead loss. One of these techniques is called the lead-lead technique because it determines the ages from the lead isotopes alone. Some of these techniques allow scientists to chart at what points in time metamorphic heating events have occurred, which is also of significant interest to geologists.
Dating Methods Used For The Recent 100,000 Years
Thermoluminescence (TL) Dating
A method of dating minerals and pottery. Rather than relying on a half-life, this method relies instead on the total amount of radiation experienced by the mineral since the time it was formed. This radiation causes disorder in the crystals, resulting in electrons dwelling in higher orbits than they originally did. When the sample is heated in the laboratory in the presence of a sensitive light detector, these electrons return to their original orbits, emitting light and allowing an age to be determined by comparison of the amount of light to the radioactivity rate experienced by the mineral. Variations on this method include optically-stimulated luminescence (OSL) and infrared-stimulated luminescence (IRSL) dating.
Chlorine-36 is produced naturally in the stratosphere when atoms are bombarded by cosmic rays, high-energy particles that streak through space from beyond our solar system. This so-called cosmogenic production of chlorine-36 has varied over time due to fluctuations in the strength of Earth's magnetic field. When the field is weak, more cosmic rays can reach the upper atmosphere and more chlorine-36 is produced. Moreover, because Earth's magnetic field deflects cosmic rays toward the North and South Poles, chlorine-36 production rates decrease with distance away from the poles.
Researchers can determine the rate at which chlorine-36 was deposited in the past by determining the ratio of chlorine-36 to regular chlorine atoms in materials that contain chlorine and then comparing that ratio to the age of the material.
Beryllium-10 (10Be) Dating
Beryllium-10 is rare and forms from oxygen-16 and iodine-129 by cosmic bombardment of xenon. These isotopes can tell the ages of the length of time rock has been exposed on the Earth’s surface.
Aluminium-26 (26Al) Dating
Aluminium-26 decays to magnesium-26 (26Mg ) with a half life of 7.2 × 105 years. If this decay product is found in a solid, the 26Al/26Mg ratio will determine its age.
The breccia and pseudotachylite that I documented at the "outskirts" of impact structures illustrate shock metamorphism in an impact crater. Separately, each of these shock metamorphic features could be explained by naturalistic means (tectonic, volcanic, sedimentary - other than impact), but taken together they strongly suggest evidence for an impact. These findings would suggest that further investigation to gather evidence of in impact may be warranted. Therefore, crater identification cannot rely soly on the discovery of breccias, there must be other impact evidence (IE shatter cones and/or planar deformation features) before the structure is identified as impact related.
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