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.
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 well under 1/2 km 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 Transition to Simple Craters (~1/2 km up to 2.5 to 4 km)
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.
Bowl shaped depressions with a raised rim.
Central Peak Craters (Complex Craters with a Central Uplift)
Peak-Ring Craters (Complex Craters with a Raised Central Ring)
Multi-Ringed Basins
Transitional Forms
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.
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. .
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.
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