Asteroid impact craters

Impact craters are geological structures that are formed when an extraterrestrial object, most often an asteroid, hits the solid surface of a planet or a satellite. On Earth, these events have been responsible for creating dramatic land features like Arizona's Meteor Crater. In addition, some evidence links impact events with large-scale planetary effects, such as causing sudden changes in the earth's climate and triggering mass extinctions such as the one thought to have wiped out the dinosaurs.

Scientific Importance of Impact Events

Asteroid impact craters are formed when an extraterrestrial object hits the solid surface of a planet or a satellite. The vast majority of these objects are asteroids; a few are comets (which are made of ice and dust rather than rock). Although geologists have been identifying and studying terrestrial impact craters since the early twentieth century, the scientific community has only recently begun to explore their true geological significance in terms of understanding the Earth's past and predicting its future.

Critically, the results of space exploration have made it clear that almost every planet in our solar system has a surface that is pockmarked with signs of ancient impacts. (Other planets, including Mars, Mercury, and the moon, tend to retain many more ancient asteroid impact craters than Earth, whose surface is constantly renewing itself—burying old craters through processes such as volcanic activity, shifting tectonic plates, and erosion.) What has also become clear, largely through data gleaned from dating samples of lunar rock, is that the rate at which interplanetary bodies like asteroids smashed into planets was much higher about four billion years ago, during the formation of the solar system, than it is today. Most scientists now believe that this period of an unusually vigorous rain of rocky bodies, sometimes known as the late heavy bombardment, represented one of the most significant processes, shaping the solar system as we know it, including Earth.

For instance, one prevailing theory about the origin of the moon holds that it came into being as a result of an enormous celestial object, about the size of Mars, smashing into the still-forming Earth. This event is thought to have broken through the planet's crust, causing volatile vaporized gases, molten rock, and other debris to be ejected into space. This ejecta then clustered together and condensed, eventually forming a satellite planet that entered into orbit around Earth. It has also been suggested that meteorite impacts may have played a critical role in delivering to the proto-Earth a number of elements that were necessary for the formation of life on our planet—including both water molecules and simple organic compounds such as amino acids, proteins, and nucleotide bases.

In other words, asteroid impact craters represent physical remnants of the kinds of events that took place at the birth of our planet. Studying these structures is one way of gaining a deeper insight into Earth's complex geological history. Other asteroid researchers are engaged in the task of figuring out how to accurately predict and possibly deflect future impact events, since a large meteor on a collision course with our planet could have catastrophic effects. (It is worth noting that some popular literature and films have suggested that an asteroid hitting the earth could actually alter its mass enough to change its orbit—the shape of its path around the sun. However, the impact energy required to expel enough of the planet's mass to shift its orbit even in the most minor way is so tremendous that it would have to be generated by an asteroid that was many times larger than any scientists have ever observed; such an impactor would certainly destroy all life on the planet if it ever collided with the earth.)

Formation and Types of Impact Craters

Scientists have identified three main stages in the formation of an asteroid impact crater. During the first, known as compression, the impactor strikes its target, creates a small break in its surface, and delivers a shock wave that begins to flow through the impact site. This compresses the target and produces shock metamorphism effects (changes in the structure of the target rocks such as melting, vaporizing, or crushing). This stage lasts only a few microseconds, and very little material is yet being thrown up from the developing crater. During the second stage, known as excavation, the initial shock wave spreads both outward and upward from the impactor itself. This rapidly expands the size of the crater, and also shoots up a stream of vaporized and molten rock and other debris (the ejecta) that will land in the area around the crater. This period is also when the rim of the crater begins to fold over to form a lip. During the final stage, known as modification, loose or molten debris begins to collapse and fall back down into the crater; this has the potential to change the overall shape of the structure by forming shelf-like formations on the walls of the depression or even a high central peak made up of material that rebounds up from the crater floor. The entire process happens extremely quickly, especially the first two phases, which together may last only a few seconds.

Although the same three stages are seen in the formation of every asteroid impact crater, not all craters end up looking the same. Geologists generally categorize impact craters into two types: simple and complex. Simple craters typically have a bowl-shaped depression with a diameter that is about five to seven times as wide as the pit is deep. Complex craters are often shallower (the diameter of a complex crater may be up to twenty times as wide as the pit is deep), and though they may have started out with relatively steep walls, these are likely to have partially collapsed to form either a single peak or a ring of peaks within the depression. Complex craters are also generally larger in size than simple craters. Both types of craters are typically found partially filled with breccia, rock composed of broken fragments that have been cemented together.

The mass of an impactor and its velocity as it moves toward the earth are the two factors that determine its kinetic energy and, therefore, are largely responsible for the size of the crater that will be formed when it makes contact with the planet's surface. (Other slightly less critical factors that affect crater size and shape include the impactor's composition—and therefore its density—and the angle at which it strikes.) For example, if a meteorite of 30 meters (98 feet) in diameter that weighed 200,000 metric tons (about 440 million pounds) were to strike the earth at a velocity of about 30 kilometers (19 miles) per second, the kinetic energy it generated upon impact would be about 20 megatons of force, and the crater that formed would be well over a kilometer (or almost one mile) in diameter. These are approximately the forces that created Meteor Crater in Arizona some 50,000 years ago.

Arizona's Meteor Crater

Meteor Crater is an enormous pit that measures approximately 1,200 meters (almost 4,000 feet) in diameter and about 180 meters (almost 600 feet) in depth; it has an uneven rim that rises between 30 to 60 meters (100 to 200 feet) above the surrounding desert landscape. Unlike most craters formed as a result of volcanic activity, the crater floor is not found on top of a volcanic peak, but dips far below ground level. Located near the town of Winslow, Arizona, Meteor Crater is roughly bowl-shaped; however, it has four “corners” arising from tear faults in the earth's crust. These tear faults make it appear more rectangular than circular when viewed from above. Meteor Crater is also known as the Barringer Meteorite Crater, after the engineer who was the first to correctly hypothesize about its probable origin. In fact, it was the very first impact crater on the earth to be recognized for what it was.

The impact event that formed Arizona's Meteor Crater is generally agreed to have been a massive extraterrestrial object smashing into the surface of the earth. The object was probably a fragment that had broken off from the asteroid belt between Mars and Jupiter about half a billion years ago and set off on a collision course with our planet.

For many years, the pit was believed to be the site of an extinct volcano, despite evidence pointing toward its extraterrestrial origin. In 1891, a mineralogist named A. E. Foote collected a large number of rock fragments from the crater. When analyzed, the rocks were found to be allochthonous: formed somewhere other than the location where they presently appear. Specifically, they were composed of nickel-iron alloys: a material that is extremely rare on the surface of the earth, but found in virtually all stony meteorites. But a series of incorrect observations and calculations led US Geological Survey researcher G. K. Gilbert to discount the possibility of a falling space mass having created the pit; instead, Gilbert ended up backing the volcanic theory.

It was not until the early 1900s, when Philadelphia silver mining engineer Daniel Moreau Barringer bought the land containing the crater and conducted a set of independent drilling surveys at the site, that any serious evidence was collected to prove its origin. Among many other things, Barringer's experiments found that the ground beneath the crater contained millions of tons of silica that had been crushed to a powder, presumably by a tremendous pressure; that there were numerous spherules of iron meteorite found around the rim of the crater; and that there was no volcanic rock to be found anywhere near the site.

Despite Barringer's efforts, the final scientific confirmation that Meteor Crater was in fact formed by an ancient impact event would arrive only in the 1960s. That was when US Geological Survey researchers Eugene Shoemaker, Ed Chow, and Don Milton collected samples from the site and discovered two crystallized forms of silica—coesite and stichovite—that are formed only at pressures of more than 200,000 kilograms per square meter (more than 300,000 pounds per square inch). These minerals had never been found in nature.

The research associated with the quest to clarify the true nature of Meteor Crater has had wide-ranging effects on the study of asteroid impact craters in general. The presence of coesite and stichovite, for instance, is now frequently used as a diagnostic marker of a historic impact event, along with other characteristic signs like shatter cones and scattered tektites. According to the National Aeronautics and Space Administration (NASA) in 2024, nearly 200 confirmed impact craters have been identified across the world. These impact craters are listed in the Earth Impact Database. In addition to continuing to discover new impact craters, scientists conducted further research to learn more about craters that had already been identified, particularly in terms of their age. By late 2018, it had been confirmed that a research team had discovered, for the first time, the existence of a large impact crater under Greenland's Hiawatha Glacier. After initially noticing the depression on a topographical map in 2015, researchers conducted expeditions over subsequent years that provided the evidence needed to verify the crater and estimate when and how it formed. In 2020, a scientist studying the Yarrabubba impact crater in Western Australia was able to determine that it was the oldest impact crater known at that point with an age of 2.229 billion years. Two years later, a scientist from Scotland's Heriot-Watt University used seismic data to discover an impact crater 300 meters under the floor of the Atlantic Ocean. Named the Nadir Crater, it is located off the coast of Guinea in West Africa. The crater is nine kilometers in diameter and is believed to have been formed 66 million years old during the Cretaceous period.

The K/T Extinction Event

The geological history of the earth has been marked by dramatic shifts in its climate and its life forms. At least one catastrophic change may have been related to an asteroid impact event that some scientists believe took place about 66 million years ago, between the time period that geologists call the Cretaceous and the Tertiary periods. This time of transition corresponds with the time at which the dinosaurs are believed to have gone extinct. Although there are conflicting theories about what actually happened during the K/T boundary, most scientists agree on a few basic facts.

One is that there was a change in the overall climate of the planet. During the Mesozoic era (an era is a unit of geologic time that is divided into periods), the climate on the earth was relatively warm and consistent. But the Cenozoic era, which followed immediately afterward, was much colder and also subject to greater fluctuations in temperature and rainfall. In addition to these long-term changes, this era appears to have been a time of some unusual short-term weather phenomena that were unfavorable to life, such as toxic gases being emitted into the atmosphere and the falling of acid rain. These significant changes in climate are believed to have been responsible for the mass extinction that is known to have taken place at around this time. An enormous variety of organisms on land and sea, including the dinosaurs, completely disappeared from the face of the earth.

Some scientists believe that these changes in climate took place gradually, and were the result of intrinsic events on the earth's surface, such as volcanic activity and a shifting of tectonic plates that caused the oceans to recede from the land. Other scientists believe that they took place suddenly, and were the result of some extrinsic, or extraterrestrial event. The most well-accepted proposal within this school of thought holds that the event was, in fact, the collision of a large space object into Earth—in other words, an asteroid impact event. This is sometimes known as the Alvarez Hypothesis, after the University of California, Berkeley scientists Luis and Walter Alvarez who outlined it in its original form.

The Alvarez Hypothesis proposes that the widespread and catastrophic climate change that caused the extinction of the dinosaurs happened in the wake of a massive meteor, most likely an asteroid, colliding with Earth. This impact, the theory goes, resulted in the almost complete vaporization of the meteor itself—throwing up a thick cloud of dust over the planet and triggered the dramatic shifts in climate. One major piece of evidence for this theory is the fact that at many places across the world, a layer of clay containing a high level of the rare metal iridium has been found near the geological stratum, or sedimentation layer, that has been dated as having been formed during the transition between the Cretaceous and Tertiary periods. (Although iridium is rare on Earth, it is not an uncommon element in asteroids.) The same layer of sediment also contains soot, which may have been produced as a result of firestorms setting alight large swathes of forest; pieces of tektite, which may have been part of the impactor's ejecta; and quartz that showed signs of having undergone shock metamorphism. For a long time, researchers could not find a crater associated with this hypothetical impactor—but in the 1970s, a crater of plausible size was found on Mexico's Yucatán Peninsula. The crater, called Chicxulub, is considered to be the most likely site of the hypothesized impact event.

Principal Terms

allochthonous: rock or sediment that was not originally formed in its present location, but some distance away

asteroid: a small, rocky body in orbit around the sun; the majority exist in a belt between Mars and Jupiter

breccia: broken fragments of rock or mineral that have been fused in a matrix of sand or clay; often produced by impact events

ejecta: the material that is thrown out of an impact crater during its formation

impactor: any object, such as a meteorite, that collides with another body; the collision itself is known as an “impact event”

megaton: a unit of force equivalent to the force produced by one million tons of the high-explosive TNT; used to measure the power of both impact events and nuclear weapons

meteorite: a small extraterrestrial body, such as an asteroid or a comet, that has struck the surface of the earth; known as a meteor before impact and as meteoroid before it enters the earth's atmosphere

shatter cone: a conical fracture in the surface of the earth, caused by an impact event and marked by distinct lines or ridges radiating outward from the apex

shock metamorphism: permanent physical or chemical changes caused in rocks by a shock wave that is either generated by an impact event, or by an explosive or nuclear device

siderophile: literally, “iron-loving;” refers to elements, such as platinum, palladium, osmium, and iridium, which are readily soluble in molten iron and found commonly in meteorites but extremely rare on the earth's surface

target rocks: existing rocks on the surface of a planet that are smashed during a meteorite impact event

tektite: a dark, glassy object, typically sphere-shaped, that is formed when molten debris flies out of an impact crater upon impact and cools in the air

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