Impact craters are formed when a large meteoroid (asteroid or comet) crashes into a larger planetary body that has a solid surface. All the bodies in our solar system have been heavily bombarded by meteoroids throughout their history. The landscapes of the Moon, Mars, and Mercury have conspicuously preserved this bombardment record because the surfaces of these relatively small planetary bodies have remained unchanged over hundreds of millions of years.Compared to the Moon, the Earth has been even more heavily bombarded over the course of its history due to its stronger gravitational attraction. However, impact craters are not immediately obvious on the surface of Earth because our planet is geologically active; the surface is in a constant state of change from erosion, infilling, volcanism, and tectonic activity. These processes have led to the rapid removal or burial of Earth's impact structures. Thus, only about 160 terrestrial impact craters have been recognized to date. The majority of them are located within the geologically stable cratons of North America, Europe, southern Africa, and Australia; this is also where most of the crater searches have taken place. Spacecraft orbital imagery and geophysical surveys for resource exploration have helped to identify structures in more remote locations.
Meteor Crater (also known as Barringer Crater), Arizona, with a diameter of approximately 1.2 kilometers, was the first terrestrial impact crater (slides #10 and #11) to be recognized as such. Its impact origin was first suspected late in the nineteenth century, when abundant iron meteorite fragments were discovered in the immediate vicinity of the crater. This finding led the mining engineer Daniel Moreau Barringer to embark, between about 1905 and 1928, on a drilling project to find a suspected large iron meteorite body underneath the crater floor. At this time, however, researchers did not yet have a clear understanding of the immense energy that is liberated when an extraterrestrial body hits the surface of the Earth with cosmic velocity. It was only in the 1920s that the first quantitative studies revealed the explosive nature of meteorite impact. Under impact conditions, tremendous amounts of energy are released instantaneously, completely destroying the cosmic projectile and generating a crater that is many times larger than the original meteoroid. In the case of Meteor Crater, an iron meteorite body only about 30–50 meters in diameter was sufficient to create a crater 1.2 kilometers in diameter.
The key to understanding the explosive nature of an impact event is the high velocity with which a meteoroid hits the Earth. These velocities range between 11.2 kilometers per second (the escape velocity of the Earth-Moon system) and 72 kilometers per second (the orbital velocity of the Earth plus the escape velocity of the solar system at the distance of the Earth from the Sun). Because the kinetic energy liberated on impact of an object is proportional to the square of its velocity, these high-speed meteoroids can be, gram for gram, more than 100 times as explosive as TNT!
After the first studies on Meteor Crater, several other relatively small craters were also found to contain impactor fragments, and for many years these remnants were the only accepted evidence for impact origin. But because the projectile does not survive intact in large impact events, scientists have developed more sophisticated means of detecting the signatures of meteorite impact.
In some cases, nonterrestrial relative abundance of siderophile elements can be detected in the impact melt rocks within large craters (or in impact ejecta, as at the K-T boundary sediments mentioned above); this provides a chemical signature of the meteorite impactor. The most commonly used chemical elements for such studies are the platinum group elements (e.g., iridium, osmium, and platinum). This is based on the fact that almost all meteorites have abundances of these elements that are higher by factors of 20,000 to 100,000 than those of average terrestrial crustal rocks. The addition of even a small meteoritic component (less than 1%) results in distinctly elevated platinum group element contents in the impact breccias or melt rocks.
Since the 1960s, numerous studies have documented another physical marker of meteorite impact: shock metamorphism. This refers to metastable or irreversible effects produced in various target rocks and minerals as the strong shock wave passes through them. As hypervelocity impact is the only naturally occurring process capable of generating strong shocks in crustal rocks, certain shock-metamorphic effects are unambiguous signatures of meteorite impact. Diagnostic shock effects include shatter cones, multiple sets of microscopic planar deformation features (PDFs) in quartz, feldspar, and most other rock-forming and accessory minerals, diaplectic glass, and high-pressure mineral phases, such as stishovite (a high-pressure form of quartz). Even diamonds are formed by high-pressure conversion of graphite in target rocks. Researchers have recognized that the presence of shock-metamorphic effects is a much better indicator of the impact origin of a geologic structure than the presence of meteorite fragments (which are rapidly destroyed by erosion anyway). Experimental studies over the past three to four decades have provided a good database that shows which types of shock features form at which pressures. It was also recognized that the effects resulting from shock (nonequilibrium processes) are different from those resulting from static high pressures (an equilibrium process). Today, terrestrial impact structures are confirmed based on the presence of some or all of these shock effects.
Impact craters are divided into two main groups, based on their morphology: simple craters and complex craters. Simple craters are relatively small, with depth-to-diameter ratios of about 1:5 to 1:7 and a smooth bowl shape (slide #1, top). In larger craters, however, gravity causes the initially steep crater walls to collapse downward and inward, forming a complex structure with a central peak or peak ring and a shallower depth compared to diameter (1:10 to 1:20) (slide #1, bottom). The diameter at which craters become complex depends on the surface gravity of the planet: The greater the gravity, the smaller the diameter that will produce a complex structure. On Earth, this transition diameter is 2–4 kilometers (depending on target rock properties); on the Moon, at one-sixth Earth's gravity, the transition diameter is 15–20 kilometers.
The cratering process is traditionally divided into three stages: The contact and compression stage begins when the impactor hits the ground and initiates a shock wave that travels into the target and into the impactor, compressing the target and generating shock metamorphic effects. This is followed by the excavation stage, wherein the release of the shock compression leads to mass flow that opens up the crater and ends with a relatively deep transient cavity. Finally, the modification stage involves collapse of the steep walls of the transient crater and infilling of the crater by fall-back debris. The complete crater-forming sequence takes less time than it would to free fall from a height equivalent to the final crater diameter. In the case of Meteor Crater, the compression and excavation phases of the crater formation were over in a few seconds. Thus, impact cratering has the distinction of being the geologic process that releases the greatest amount of energy in the shortest amount of time.
The final crater expression depends on the magnitude of the event. The central peak or peak ring of the complex crater is formed as the initial (transient) deep crater floor rebounds from the compressional shock of impact. Slumping of the rim further modifies and enlarges the final crater. Complex structures in crystalline rock targets may also contain coherent sheets of impact melt overlying the shocked and fragmented rocks of the crater floor. On the geologically inactive lunar surface, this complex crater form will be preserved until subsequent impact events alter it. On Earth, weathering and erosion of the target rocks quickly alter the surface expression of the structure; despite the crater's initial morphology, crater rims and ejecta blankets are quickly eroded and concentric ring structures can be produced or enhanced as weaker rocks of the crater floor are removed. More resistant rocks of the melt sheet may be left as plateaus overlooking the surrounding structure.
Large terrestrial impacts are of greater importance for the geologic history of the Earth than the number and size of preserved structures might suggest. Currently, about 160 structures of impact origin have been confirmed on the Earth's surface (slide #2). Precise ages are known for only about one-third of these structures. Crater ages can be determined by a variety of methods. The more precise ones involve radiometric dating of impact melt rocks or impact glasses or biostratigraphic dating of related impact ejecta within a well-defined stratigraphic sequence. The paucity of age data reflects not only the lack of detailed studies, but, in many cases, the lack of datable material, especially for deeply eroded or subsurface structures.
This slide set presents orbital and aerial photographic views of a selection of proven or suspected terrestrial impact structures that represent the variety in appearance of impact structures at different erosional stages on Earth. Several examples of impact structures on Earth's planetary neighbors are included to show the fundamental role impact plays in shaping planetary surfaces. These relatively well-preserved extraterrestrial craters provide an important reference for understanding the more eroded impact features on Earth. The terrestrial structures we have chosen represent a compromise between those with the best surface expression and those that represent a diversity of age, size, and appearance as the craters are reworked by geologic processes. The freshest craters are presented first, followed by those that exhibit more ambiguity and complexity. We have included two examples of features that have the appearance of impact structures, but have not yet been shown to contain any of the diagnostic chemicals or physical markers of impact events. However, we would like to caution against just using images or remote sensing to identify impact craters on Earth without corroborating petrographic and geochemical studies on crater rocks. Only such studies can provide confirming evidence that a geological structure is of impact origin.
In the orbital photographs of terrestrial craters, north is up, unless