Understanding the Impact Cratering Process: a Simple Approach
We are sure, every girl and boy, and many also in adulthood have sometime thrown pebbles and cobbles into mud, and they saw nice little craters with a rimmed wall and ejected mud around had been formed (Fig. 1). Later, when they saw images of the famous bowl-shaped Barringer meteorite crater in Arizona (Fig. 2) they might have thought both had been formed by the same mechanism.
Fig. 1. Pebble-into-mud craters.
Fig. 2. The Barringer Meteorite Crater.
Now on harder ground, hailstones and even raindrops may produce smallish craters as well (Fig. 3), and the central mound in the craters of Fig. 3 strongly reminds of meteorite impact craters exhibiting a central uplift like the Tycho impact crater on Earth's moon (Fig. 4). And again, one might suspect a similar formation process.
Fig. 3. Hailstone craters. The diameter of the miniature craters is about 5 - 10 mm.
Fig. 4. Tycho meteorite crater on the Moon. Tycho's diameter is 85 km.
HOWEVER: METEORITE CRATERS DO NOT FORM LIKE THIS.
Fig. 5. Not a model for impact crater formation.
WHAT IS THE DIFFERENCE?
But before focusing on this important question, we have first to clarify the meaning of the term meteorite impact crater or impact structure. It is true meteorites crashing as stones from the sky at free-fall velocity may produce small craters in the ground similar to the mud or hailstone craters. These craters are, however, no impact structures.
Impact structures are formed by a cosmic body at a velocity exceeding the sound velocity (velocity of elastic [seismic] waves) in the impacted target rocks (commonly around 5 km/s) leading to the spreading of shock waves. These conditions are only met with larger projectiles (a few hundred tons and more) that are not significantly slowed down by friction in the atmosphere and that impact the ground at cosmic velocities (10 - 70 km/s).
The term impact structure is often used synonymous with impact crater, and sometimes, impact structures are distinguished from impact craters by their weak morphological signature compared with a true crater. In both cases, the conditions of a hypervelocity impact and propagation of shock waves are crucial for the formation of these geologically extraordinary structures.
The three stages of impact crater formation
In impact research, the subdivision of the crater-forming process into three main stages has generally been accepted. These stages are:
-- the contact and compression stage (Fig. 6)
-- the excavation stage (Fig. 7)
-- the modification stage (Fig. 8)
In the following, these three stages are illustrated and described in a somewhat simplifying manner.
Fig. 6. The contact and compression stage of impact cratering.
On impact of a cosmic body, shock waves are starting from the contact point and propagating into both the underground target rocks and into the impactor. Impact shock waves are characterized by an instantaneous onset of extreme pressures (up to the order of megabars) and extreme temperatures (up to 10,000 degrees or more) on release of the pressure. These temperatures are enough to more or less completely vaporize the impactor and a volume of the target rocks roughly comparable to the volume of the impactor resulting in a giant expanding impact vapor plume.
On propagating roughly hemispherically into the underground target rocks, shock wave energy diminishes and so do pressures and temperatures. Correspondingly, a zone of rock melt follows the vaporized zone, and when shock energy is further lowered rocks will only be heavily damaged (fractured, brecciated) with decreasing intensity. Driven by the hypervelocity impact deformation, melted and fractured rocks will be accelerated behind the shock front initiating - in the second stage - the excavation mass flow.
Since impactors can attain to practically arbitrary size, the (kinetic) energy brought to Earth by impact and turned into geological processes may practically be unlimited. This is clearly different from "normal", endogenetic geological processes like volcanism or tectonics and may contribute to the situation that impact remains inconceivable for many geologists all the more as this energy is set free in extremely short time.
Fig. 7. The excavation stage of impact cratering.
Different from the cobble-into-mud craters, the excavation in impact cratering is inextricably linked with the propagation of the shock waves. Shock waves behave like other waves, they can interfere, and they may be reflected and refracted. Spreading outwards from the point of contact, the compressive shock waves are permanently reflected from the free target surface as tensile rarefaction waves of comparable intensities and, like the shock waves, are propagating downwards. In this way, all rock particles behind the expanding shock front are "captured" by both the compressive shock and the tensile rarefaction, and both combine into a vector of acceleration. Computing these vectors (direction and magnitude) for each point in the subsurface, a field of excavation flow with arcuate trajectories as shown in the sketch above (Fig. 7) will result. This flow field is growing with time, and the rock mass flow is directed upwards, sideward and downwards. In the upper part, the flow field enables the rock masses to escape as ejecta from the growing excavation cavity. Below a trajectory defining floor and walls of the expanding cavity, the rock material cannot leave and is compressed sideward and downwards. The excavation stage ends on release from shock and when the displacements by excavation cavity formation and downwards/sideward compression are maximum. The now existing bowl-shaped structure surrounded by an uplifted rim and a blanket of ejected material is termed the transient crater obviously indicating a continuation of the impact cratering process arriving in the modification stage.
We saw shock wave and rarefaction wave propagation are essential in the formation of a meteorite impact crater. Apart from the distinct role rarefactions waves play in the formation of the excavation flow field, they are especially relevant geologically. A compressive shock pulse is not only reflected at the free target surface but also always when it impinges on a boundary of material with reduced impedance (= the product of density and sound velocity) where part of the energy is reflected as a rarefaction pulse. The reflected tensile stresses are insofar crucial as the tensile strength of rocks is much lower than the compressive strength. Hence in an impact process much more damage is in general done by the rarefaction waves and not by the compressive shock waves, and many peculiar structural features that are observed in impact structures and that may puzzling geologists are the result of strong tensile forces acting on all scales (also see the term spallation in the SEARCH function of our website).
Fig. 8. The modification stage of impact cratering.
The term "transient crater" means the cratering process continues after the excavation flow came to rest. What happens to the transient crater? This depends especially on size. In the case of small transient craters, modifications are moderate. On relieving of pressure, there is an elastic rebound at the crater floor now hosting a layer of brecciated rocks. The structure of the transient crater is widely preserved, and we are speaking of a simple or bowl-shaped impact crater (Fig. 9).
For larger transient craters the modifications may take on a dramatic scale. Elastic rebound and collapse cause the excavation trajectories to go into reverse in a way, and the rock masses tend to move upwards and centripetally thus, accompanied by large-scale downfaulting, largely backfilling the transient crater. This will result in the formation of central uplifts and ring systems, and hence we are speaking of central-uplift or central-peak craters, peak-ring craters or multi-ring craters establishing the group of the so-called complex impact craters or complex impact structures (Fig. 9).
The transition from simple to complex craters occurs at about 1.5 to 4 km (depending on the target rocks) final diameter for terrestrial craters and is much larger (c. 15 km) for craters on the Moon. This suggests that the transient crater collapse in the modification stage is largely driven by gravity (gMoon ~ 1/6 gEarth).
For very large impact craters, the excavation and modification stages are not as discrete as previously written. Computer simulations show that the modification process may already begin before standstill of the excavation leading to large-scale countermovement of rock masses. In the large 35-40 km-diameter Azuara impact structure (Spain) there is stratigraphic evidence for such a coincidence of excavation and collapse [click here].
Fig. 9. Cross sections of simple and complex impact craters. Figs. 10 - 12 exemplify typical terrrestrial structures.
After all, large impact craters are morphologically flat structures although the impact signature - rock deformations, shock metamorphism - may extend to considerable depths. They are typically filled with impact rocks (impactites) in the form of impact melt rocks, suevites and different kinds of breccias.
Fig. 10. A bowl-shaped simple crater (Wolfe Creek, Australia, 900 m diameter).
Fig. 11. A central-uplift crater (Gow, Canada, 4 km diameter).
Fig. 12. A peak ring crater (Clearwater West, Canada, 32 km diameter). Images of Figs. 10-12 slightly modified from NASA.
Quite a few interesting questions may remain open. How big, e.g. was the impactor to have produced an impact structure of a given size? This of course is primarily a matter of energy related with the projectile's mass (and therefore density) and impact velocity, and, subordinately, also a matter of the target lithology. A very rough rule of thumb amounts to the order of a one-to-ten diameter ratio. Here again, the difference is obvious: In the case of cobble-into-mud or raindrop "impacts" the craters are not much larger than the projectile.
So far we have considered the impact of a solid object like a stony or iron meteorite. What happens if a comet or a very low-density, loosely bound asteroid (like Mathilde asteroid, Fig. 13) hits the earth? For the roughly 200 established terrestrial impact structures neither a comet nor a rubble pile asteroid could definitely be shown to have been the impactor. Computations suggest craters that were formed by a low-density projectile are flatter and have distinctly larger diameter ratios (compared to the above-mentioned one-to-ten ratio of projectile and crater diameter).
Fig. 13. The 50 km-diameter Mathilde asteroid has a mean density of 1.3 g/cm³ only and is considered a kind of rubble pile. What happens on crash of such a loosely bound impactor? Image source: NASA.
How looks an impact crater that was produced by an oblique impact? Statistically, impact trajectories most abundantly form an oblique 45° angle with the target surface. Nevertheless, the resulting impact crater is more or less circular unless the angle of incidence is very low, less than 10°. Then, elongated craters may be formed, and the ejecta blanket may considerably deviate from a circular symmetry.
In the beginning debate about meteorite craters (about one hundred years ago), astronomers believed the many craters on the Moon were volcanic. They concluded this from the already mentioned prevailing frequency of oblique impact trajectories leading in the majority, in their opinion, to craters of elliptical shape. This was obviously not the case, and so most of the Moon craters could not be meteoritic. At that time, however, the physics of impact cratering implying shock physics was not yet understood. This especially concerned also the vaporization of the impactor by shock-induced temperatures and, at that time, let the mining engineers helpless when they did not encounter the expected 50 m-diameter iron meteorite beneath the floor of the Barringer (Meteorite) crater.
Suggested reading
Kathleen Mark: Meteorite Craters. How scientists solved the riddle of these mysterious landforms. 288 pp, The University of Arizona Press, Tucson, 1986.
and for advanced persons
Melosh, H.J.: Impact cratering. A geologic process. 245 pp, Oxford Univ. Press, Oxford, 1989.