The Wetumpka Impact Structure is believed by some to be a meteorite impact crater.

What features of the structure lead us to believe a meteor collided with the Earth millions of years ago in the vicinity of what is now Wetumpka, Alabama?

First, some basics...

What is an impact crater and what causes one?

An impact crater is "an approximately circular depression, sometimes surrounded by a raised rim. Craters are typically formed by explosion during meteorite impact." (Welcome to the Planets Version 1.5 Glossary)

Impact craters are usually caused by meteors that strike the surface of a planet or planetary body. A meteor is a mass of extraterrestrial material passing through the atmosphere of a planetary body. Because meteors are thought to have originated during the formation of the solar system, they could supply information about its origin (Monroe and Wicander 1992).

"Meteorite" is the term given to a meteor once it has impacted the surface of a planetary body. Therefore impact craters are also referred to as meteorite craters.

There are three categories of meteorites (click on these to learn more about the types of meteorites):
stony meteorites
iron meteorites
stony-iron meteorites

What can impact craters tell us about a planet's history?

By studying impact craters we can learn about the geologic history of a planet. If a planetary body is riddled with impact craters--for example our moon--we assume that the body is geologically "dead." That is to say, surface-shaping processes such as erosion, plate tectonics, volcanism, etc., are not "erasing" the craters. On the other hand, if a planet shows little evidence of impacts--Earth for example--we might assume the body is geologically "alive." The Earth was once probably as pock-marked as the moon, but because the Earth is constantly being modified by surface-shaping processes, the majority of craters have been removed. By studying craters on other planetary bodies, we learn to recognize similar structures on our own planet, thus enabling us to understand better the geologic processes that shape our world.

Examples of impact craters

What general criteria classify a depression as an impact crater?

Koeberl and Anderson (1996) outline the scientific criteria by which to recognize and confirm an extraterrestrial origin for a "suspect" impact structure. The most important criteria fall into the following four categories:

morphology
evidence of shock metamorphism
geochemical evidence
presence of geophysical anomalies

Morphology

According to Webster 1985, morphology is a study of structure and form. Applied to geology, this means a study of landforms or topographic features.

Impact craters can be classifed by their shape as either simple or complex. Simple impact craters are circular, bowl-shaped depressions with well defined, raised rims and an interior slope that is steepest near the rim and smoothly decreases toward the crater's center. Ejecta deposits are often seen radiating from crater rims (Melosh, 1989). Two examples of simple craters are Isidorus on Earth's moon and Meteor Crater in Arizona.

Complex impact craters central peaks and flat floors (Melosh, 1989). These large craters have collapsed causing material from the wall of the crater to slide or "slump" to the crater floor. The central peaks are not piles of debris that slid down the crater walls and came to rest in the center of the floor but are rocks from beneath the crater floor that were uplifted during the impact of the meteorite. Two examples of complex craters are Theophilus on Earth's moon and Yuty on Mars.

(For further information about the morphological differences between simple and complex craters see Melosh, 1989.)

Evidence of Shock Metamorphism

Although craters can be formed by meteor impacts, the can also result form volcanic processes. Careful analysis of minerals in rocks can help determine whether a depression is the result of a meteorite impact or not. The presence of two shock indicators, coesite and stishovite, in quartz-rich rocks have aided in the positive identification of impact craters. Coesite and stishovite are mineral pseudomorphs of quartz that form at extremely high pressures. Volcanic explosions cannot reach the pressures needed to form coesite and stishovite (greater than 15 and 30 GPa, respectively) (Melosh, 1989).

Another indicator of shock metamorphism is the presence of shatter cones. The Dictionary of Geological Terms defines a shatter cone as

"A distinctive striated conical fragment of rock along which fracturing has occurred, ranging in length from less than a centimeter to several meters, generally found in nested or composite groups in the rocks of cryptoexplosion structures, and generally believed to have been formed by shock waves generated by meteorite impact. Shatter cones superficially resemble cone-in-cone structure in sedimentary rocks; they are most common in fine-grained homogeneous rocks such as limestone and dolomite, but are also know in shale, sandstone, quartzite, and granite. The striated surfaces radiate outward from the apex in horsetail fashion; the apical angle varies but is close to 90 degrees."

Geochemical Evidence

According to Koeberl and Anderson 1996, the presence of meteorites or meteorite fragments at an impact crater is possible, although rare. Generally, the impactor vaporizes almost immediately upon impact. However, minor amounts of a meteor can sometimes survive an impact because atmospheric drag can slow the object down.

Meteorites contain some chemical elements that are very rare in terrestrial (Earth) rocks. During an impact, elemets contained in the meteorite can chemically interact with elements in terrestrial rocks. These chemical changes result in "geochemical anomalies" and provide strong evidence of an impact event.

Presence of Geophysical Anomalies

According to the Dictionary of Geological Terms (1984), an anomaly is a departure from the expected or normal. A geophysical anomaly can refer to an unusual measurement or measured value of such things as the Earth's gravitational or magnetic fields, or the speed at which seismic waves travel through the interior of the Earth. Gravity, magnetic and seismic anomalies are often associated with an impact crater. For an example of a geophysical anomaly associated with the Chicxulub crater in Mexico, click here. Geophysical anomalies can help identify impact craters in areas that we cannot "see" at the Earth's surface, such as those that are hidden beneath the oceans or ice. However, geophysical anomalies, by themselves, are not proof of an impact origin. They must be combined with other evidence.