United States Meteorite Impact Craters

Earth's meteorite impact craters result from high-speed impacts by meteoroids of substantial mass upon the planet's surface.  Meteorite impact craters are some of the most unique and amazing structures on the planet.  They are also among the most rare. 

The vast majority of meteoroids that enter the atmosphere burn up during their initial impact with the atmosphere itself, and thus never reach the ground.  These impactors are reduced to gas or to dust, which slowly settles to the ground over time.  Meteors enter the atmosphere at velocities ranging from about 11 to 72 kilometers per second, speeds that are defined by the fundamental physics of our particular solar system.  The average velocity is about 20km/second.  This initial velocity at entry into the earth system is known as the meteors 'cosmic velocity.'  The term cosmic velocity has a fairly complex (and multiple) specific definition, but may be approximated in this context as the speed at which a meteor was traveling through space, relative to earth, prior to its interaction with the atmosphere.  From the moment they enter the atmophere, meteors begin decelerating.  The energy lost (converted) during deceleration results in an extremely violent environment around the object as it passes through the atmosphere.  This is visible from the ground as a meteor - a meteoroid's bright trail.  The violence and heat that a space rock is subjected to during deceleration results in the ablation, or burning away, of the stones surface.  If a rock is going to survive to reach the surface, it will tend to stall out at about 8 to 12 miles above the earth's surface, the altitude where it finally sheds the last of its cosmic velocity.  Below this altitude, in the earth's very dense lower atmosphere, the stone will tend to fall at about 300 miles per hour, or terminal velocity.  Most of the relatively few inbound objects that survive passage through the atmosphere and become meteorites follow approximately this pattern of deceleration.  If these types of objects leave a hole in the ground when they impact, it will be relatively small (meters at most), and will tend to weather away quickly.

A very small percentage of the meteoroids that enter the atmosphere experience a very different process.  If an object is sufficiently large when it enters the atmosphere, the density and thickness of the atmosphere will be inadequate to slow the object in any meaningful sense.  In these cases, which typically involve objects 10s of meters or more in diameter, the meteor decelerates slightly during its passage through the atosphere and then extremely abruptly when it impacts the ground.  The remnant orbital speed (cosmic velocity), far in excess of terminal velocity, makes this type of impact a 'hypervelocity impact.'  It is from this type of even that impact craters are formed.  Instead of a conventional impact of the sort we envision happening when a rock splashes into water or muddy ground, the meteor and the ground it impacts both explode.  The tens to thousands of cubic meters of impacting and impacted rock immediately transition to thousands to millions of cubic meters of rapidly and forcefully expanding plasma and vapor which combines with the directly transmitted kinetic energy (shock wave) of impact to accelerate the surrounding rocks and soil in a manner that is almost unimagineable in terms of normal events on Earth's surface.  Even the most violent earthquakes and volcanoes on the planet do not produce the levels of compressive shock found in large scale meteorite impact craters.  The resulting impact scars can range from several kilometers to well over 100 kilometers in diameter and can leave traces that last millions to billions of years.

The Moment of Crater Formation

Humanity has never witnessed a large hypervelocity impact take place on earth's surface or any other solid surface in the solar system at close range.  We have created small scale hypervelocity impacts in laboratry settings, we have seen the flashes of small hypervelocity impacts on the moon's surface, and we have witnessed at least one very large scale impact into the dense, gaseous surface of Jupiter.  All of these experiences provide us with clues to the details of what happens in the moments after a crater forming impact, but they leave much to the imagination.  What we actually know about the seconds and minutes following a high-speed meteorite impact comes from a combination of detailed mapping and interpretation of impact sites and from computer modelling of meteorite impact energies and target material behaviour.  Nuclear detonations have also offered some insights into hypervelocity impact crater formation and into the associated geomorphological features and metamorphic changes to impacted rocks.  The energy released in even the largest nuclear detonation, however, doesn't come close to comparing to the impact energies associated with some of earth's larger impact scars.

When an impactor strikes the earth with remnant cosmic velocity, the crater forming process is extremely fast.  The actual process of crater formation takes only seconds to minutes regardless of scale.  The impact event is described in 3 stages, detailed below.  In reality, these stages blend seamlessly into each other.

1. Contact and Compression - Kinetic energy is transferred on contact between the impactor and target rock.  Rock at the point of impact is compressed downward and outward, sometimes by hundreds of feet or more.  A shock wave propagates outward from the contact point. 

2. Excavation - Outward compression of rock is relieved by the upward and outward acceleration of near surface rock that becomes airborne or shifts beneath the ground.  This combines with an expanding cloud of hot gas and fluids to form a plume above ground.  Within the earth, far more energy is in motion.  Shock waves (compaction and rarefication) pass through materials as energy moves outward from the contact point.  Explosive decompression occurs immediately behind the shock front.  As the shock decays, and in the turbulence behind the front, coherent acceleration of large volumes of rock, outward and upward, takes place.  Blocks in excess of hundreds of meters may be displaced in the near surface.  This forms a roughly bowl shaped 'transient' cavity.  Deeper rock buckles and metamorphoses from compression as energy is absorbed and then heats violently with subsequent rarefication.  The wave passes and absorbed kinetic energy becomes heat.  Impact melts form in the wake of the shock.  Everything that is very near the center of a large impact is converted to an expanding cloud of vapor and plasma.  Much mixing of rock occurs during this stage as breccias are forcefully injected into intact bedding, strata are buckled, folded, or crushed, and rocks are launched into the air to fall back to the earth in a mixed mass.  These few seconds represent the most violent events known in earth's geological record.

3. Modification - As the energy accelerating materials outward from the impact point is dispursed, things flow, slide, or collapse back into a stable configuration.  The 'transient crater' bowl formed by excavation has impossibly steep walls and the rocks, already in motion, tend to behave in a fluid manner, as in a rock fall.  The crater collapses back inward until energy is dispersed and stable slopes are formed.  The size of the excavation, which reflects the total energy of the impact, will determine the architecture of the subsequent crater that is formed.  At the same time, material that was launched into the air falls back to earth in and on the crater, as well as around it.  The center of the crater, which was compressed downward by potentially hundreds of meters or more, rebounds again, under the force of both the expanding rock below and the inwardly moving rock surrounding it.  The force again finds relief upward, forcing rocks at the crater's center far higher than they were found before the impact.  This stage is extended, with slow cooling and leveling of the land decreasing unstable slopes over time.  The process grades into weathering, background erosion rates, and other changes.

The Search for Evidence

The pressure and temperature changes associated with impact crater formation produce changes in target rocks.  Some of these changes, such as the formation of shatter cones and specific types of fractures in individual mineral grains (both of which will be discussed in detail later) are completely unique to impact crater environments.  Other types of changes that are produced, such as melted or crushed rocks or a circular explosion scar, are produced by a wide variety of other terrestrial geological processes.  Impact craters are very uncommon, and the vast majority of the planet's crater-like objects (meaning round, bowl-shaped structures) are not the result of meteorite impacts.  Most of the objects that look like craters are actually volcanoes, sink-holes, and so on.  But, because impact craters are incredibly important sources of scientific information, it is worth going to the trouble to figure out which objects are which. 

The scientific process of investigating a geological structure in order to interpret its origin depends upon understanding these various types of changes in rocks and placing them within a geological context within the structure being investigated.  Establishing, with certainty, that a geological structure results from a meteorite impact depends upon finding and describing those few types of changes that are uniquely produced by impacts.  After this is done, examination of all of the other types of altered rocks contributes to building an understanding of the crater as a whole.

Large, complex impact craters, shown above, offer more potential evidence of a hypervelocity origin, and they offer the evidence in larger quantities.  Evidence is concentrated towards the center of the crater, and some classes, such as shatter cones, are unique to this province.  The diagnostic signatures of high-pressure shock alteration of rocks can be found, however, throughout the crater environment - at least to some extent. 

Smaller craters with a 'simple' morphology offer limited potential evidence of an impact origin.  Among the largest of these craters, PDFs, diaplectic glass and high pressure polymorphs may be present, while impactor fragments are destroyed.  Among the smallest, meteorites or melted blebs of meteorite may be present around the crater or in associated glasses, while impact shock may have produced little to no diagnostic grain scale alteration in target rocks.  Throughout the range of crater morphologies, the quantity and nature of available evidence will scale with crater dimension.

A timeline of impact crater discovery

Short and Bunch (1968) list about 50 total in the world.

References and Recommended reading:

One of the best introductory books on the subject of impact craters, 'Traces of Catastrophe' by Bevan M. French, is free online and inexpensive in printed form.  For anyone with an even remotely serious interest in the subject of impact craters, impact dynamics, or related rocks, minerals, or processes, I recommend reading this book from cover to cover.  The link to the online version is below.

French B. M. 1998. Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. LPI Contribution No. 954, Lunar and Planetary Institute, Houston. 120 pp.

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Another key reference for gaining an overview of the impact process is Melosh, 1989, 'Impact cratering: A geologic process,' though this reference can be very difficult to find.

Melosh H. J. 1989. Impact Cratering. A Geologic Process. Oxford Monographs on Geology and Geophysics Series no. 11. Oxford: Clarendon Press. 245 pp.

(Hard to find. Many hope for a reprint to come out.)

Norton, O. R., 2002, The Cambridge encyclopedia of meteorites. Cambridge University Press, Cambridge, United Kingdom, 354 p.

(An accesible and understandably written overview. A reference that belongs in the library of every person interested in meteorites and impact craters.)

Rajmon D. 2006. Suspected Earth Impact Sites. 37th Lunar and Planetary Science Conference (LPSC) abstract #2372.

Rajmon, D. (2009) Impact database 2010.1.

(The Impact Database, located and explained in the above two references, lists both suspected and confirmed impact craters, as well as rejected sites, along with citations to the associated literature.)

Short and Bunch 1968 A worldwide inventory of features characteristic...

Turtle E. P., Pierazzo E., Collins G. S., Osinski G. R., Melosh H. J., Morgan J. V., and Reimold W. U. 2005. Impact structures: What does crater diameter mean? In Large Meteorite Impacts III: Geological Society of America Special Paper 384. Editors: Kenkmann T., Hrz F., and Deutsch A. Geological Society of America: Boulder. pp. 124.

(This article identifies potential sources of confusion regarding crater measurements and provides a common language for use in quantifying crater dimensions and morphology.) 

KENKMANN, T. (2002). Folding within seconds. Geology , 30(3), 231-234.

(This article looks at crater formation timing and at grain-scale compensation for movement in the rapid deformation environment unique to crater formation.)