What this page contains: This is one of a group of several pages dedicated to communicating the nature of diagnostic evidence for impact crater identification and the specific tools and techniques used in this science. If you are new to impact crater science, you might want to start by reading [Crater Identification] and [What Makes a Confirmed Crater?] before returning to this and other specific topic pages. Please note that this website is perpetually under construction in an ongoing effort to make it more understandable and more useful.
This image shows examples of well preserved simple and complex craters on Mars.
The study of 'crater morphology' is the study of the shape of impact craters, the 3-dimensional structures that are left behind following hypervelocity impacts. Many factors affect the size and shape of these stuctures, and these factors may be different on different planets. The size, velocity and angle of entry of an impactor, the composition of the impacted surface, the gravity at the point of impact, the density of the overlying atmosphere, and subsequent weathering of the impacted surface all affect crater morphology. This page will offer brief reviews of how each of these factors (and several others) relate to the current impact craters that we observe.
Simple Crater Morphologies versus Complex Crater Morphologies
The modification stage of the impact process will produce the crater morphology that we later see based on a combination of impact environment and impact energy. Smaller impacts produce simple craters and larger impacts produce complex craters. Terrestrial (Earth) craters with a 'simple' morphology tend to be 4 km or less in diameter and have a bowl shaped depression with a raised rim. Craters with a complex morphology, depending on impacted rock type, tend to be a minimum of about 4 kilometers in diameter, and may be any size above this. Complex morphologies of larger impacts vary, but generally express either an uplifted centers from rebounding of impacted material and a surrounding raised crater rim at some distance, or a raised ring surrounding the relatively flat center surrounded by the larger raised ring of the outer crater wall. Both complex crater morphologies look as if a wave were frozen as it propagated outward from a stone splashing in water. The outer edge of a complex crater is marked, like a simple crater, by an inwardly sloping crater wall and a raised rim.
Craters between 5 and 15 km in diameter characteristically express complex crater morphologies of the first type mentioned above, a central peak and surrounding bowl that rises again to a raised outer crater rim. All of the primary deformation and faulting in craters of this scale occurs within 15 to 60 seconds. (Kenkmann, 2002)
The majority of the world's known impact craters exhibit complex crater morphologies, though they differ in their particulars.
Factors Affecting Evolution of Complexity on Earth
Impact Energy: Size and Velocity of Impactor
Composition of Impacted Surface
Target rock type (sedimentary = complex at smaller diameters), volatiles, stratigraphy. Larger craters have a lower depth to diameter ratio. Angle of impact.
Excavation Craters (<<1/2 km)
The smallest impact structures on earth, typically 100 meters or less in diameter, are excavation pits. (How many total, and what is the smallest hypervelocity example?) Such structures are excavated by the transferred kinetic energy of low speed projectiles that are typically travelling no more than a few hundred km per hour. Excellent examples of this type of structure include the Haviland and Sikhote Alin impacts. The principle distinction between this type of structure and larger ones, is the velocity of the projectile. Objects that have been slowed to terminal velocity (maximum speed when wind resistance is balanced by acceleration due to gravity), and have lost all remnant cosmic velocity, strike the ground at low enough speeds that the energy can propagate outward in an ordinary manner. This is essentially identical (to a first order of approximation) to what happens when you jump into a swimming pool or stomp your foot in sand or mud.
Simple Craters (~1/2 km up to 2.5 to 4 km Bowl shaped depressions with a raised rim.
Central Peak Craters (Complex Craters with a Central Uplift) and Peak-Ring Craters (Complex Craters with a Raised Central Ring)
Simple craters do not simple replace excavation pits above a certain energy level, and complex craters with central peaks do not abruptly replace simple craters in the case of larger impacts, and then proceed to be replaced by mulit-ringed structures in turn. These commonly recognized types are points in a continuum that includes, and is influenced by, a wide range of transitional features and structures, as well as a host of impact specific variables, such as fluid cover, gravity of the target body, and even type of target rock.
Weathering and Erosion of Craters
Differences from Planet to Planet
Differences in gravity, from planet to planet, result in differences in crater morphology and in the size at which transitions between crater morphologies take place. In general, everything is bigger in lower gravity. This means simple bowls can be found at diameters well above the range within which they occur on earth. The transition to complex craters, similarly, happens at larger diameters.
Lower gravity also typically, but not always, goes hand-in-hand with a thinner atmosphere. The thicker the atmosphere, the larger an object must be in order to reach the ground at hypervelocity. On the moon, an object the size of a grain of dust can reach the ground at many km/second. This means that smaller hypervelocity impact craters can be found. Also, average speed of impact varies form body to body, depending upon location within the solar system. In general, bodies farther from the sun are struck more slowly, but this is modifed by the fact that a body adds velocity to an in-bound impactor in accordance with its gravity.
What craters on earth really look like
While having an understanding of idealized impact crater morphology, as described on this page, is invaluable in locating and interpreting the evidence necessary in order to establish whether a geological structure is an impact crater, it is equally important to recognize that very few terrestrial impact craters actually 'look-like' impact craters. Most, if they are exposed above the eath's surface at all, look like vaguely circular areas of slightly subdued topogoraphy, lakes, oddly deformed or positioned mountains, or like nothing at all. Looking closely at the pictures of specific craters in this website is likely to help the researcher get a sense of this. Out of all of the United States' meteorite impact craters, only one, Barringer Crater, strongly resembles the unweathered, idealized structures that we see on the moon or Mars. Nevertheless, topographraphical clues can and do assist researchers.
References and additional resources:
French, B. M., 1998, Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. Houston, Texas: Lunar and Planetary Institute. pp. 120. LPI Contribution No. 954. http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html.
Melosh, H. J. and Ivanov, B. A., 1999, Impact Crater Collapse, Annu. Rev. Earth Planet Sci. 27 pp. 385-415
Pike, R. J., 1980, Control of crater morphology by gravity and target type: Mars, Earth, Moon, Proc. Lunar Planet. Sci. Conf. 11th pp. 2159-2189
Page development notes to self: draw graphics of the various morphologies, label parts, then expand to include transitions. Fix the lack of citations.