Cratering Mechanics - Remoe Sensing Application - Completely Remote Sensing, GPS, and GPS Tutorial
Cratering Mechanics

No other natural event is as powerful, devastating, or potentially catastrophic as a major impact. Consider one capable of producing a 50 km (31 mi) wide crater, excavated to a depth of 5 km (3 mi): the energy expended is thousands of times greater than the simultaneous detonation at one point of all the nuclear explosive devices (euphemism for bombs) manufactured to date. We gain some idea of these magnitudes from this logarithmic (log-log) plot of the crater frequency as a function of energy at impact (or detonation) given in joules.

Below is a similar diagram with some additional information. The word "Siberia" equates to "Tunguska" in the upper diagram.

To appreciate the magnitudes of large impact events, keep in mind that the 12.5 kiloton device exploded at Hiroshima was equivalent to about 1014 J (Joules), the Mount Saint Helens volcanic eruption involved 6 x 1016 J, and the largest earthquakes release up to 1018 J (note: the relation between energy in Joules and in kilotons[kt] of explosive TNT is given by 1 kt = 4.186 x 1012 J). In this context, the impact that produced the Sudbury structure (215 km [134 mi] initial diameter) in Canada released about 1023 J, roughly 100,000 times greater than earthquakes of magnitude 9.0 on the Richter scale (Sudbury, then, could have generated an earthquake-like response on the order of magnitude 14). Slightly larger is the Chicxulub crater in the Mexican Yucatan, reputed to be evidence for the catastrophic impact event that hastened the demise of the remaining dinosaurs (many families and types had already diminished or reached extinction before this event). In common, both earthquakes and impacts are the fastest known large geologic phenomena, each causing ground disturbances that last only a few minutes at most after their initiation times.

(Note: there is a log linear relation also between crater size [diameter] and energy release, not shown on the above diagram. The scaling formula relating energy to diameter can be approximated by D = 0.1 times the cube root of the energy E in kilotons. By way of example: the cube root of 1000 kilotons [a megaton] is 10, so the diameter of the crater from an event of that magnitude is 0.1 x 10 or 1 kilometer.)

The source of this tremendous impact energy is the direct consequence of a great solid mass moving at high velocity. Remember from physics that kinetic energy (K.E.) = 1/2 mv2, where m is the moving body's mass, and v is its velocity. To gain a sense of the magnitude involved, consider this calculation. Let a 30 m (98 ft) diameter iron body (in effect, a large meteorite) weighing about 200,000 metric tons (around 440 million pounds) strike Earth at a typical, in-space velocity of 30 km (19 mi) per second (not hours!) (20 mps corresponds to 72,000 mph). This impact would generate about 20 megatons (TNT-equivalent) of energy (about 1017 joules) that would cut out a crater about a kilometer and a half (almost a mile) wide and 185 m (607 ft) deep. This is the size of Meteor (Barringer) Crater, which we will examine later. The ejection process would scatter most of the excavated rocks to a radius of at least 10 km.

The very large kinetic energies (K.E) owing to the great masses (m) and high velocities (v)(remember from physics: K.E. = 1/2 mv2) involved in impact cratering are converted to dynamic, fast moving transient shock waves that diverge hemispherically from the line of impact. These are compressional waves that have high amplitudes. The pressures generated are given in either of two units: Gpa (Gigapascals) or Kb (Kilobars) (a bar is equivalent to 0.971 of an atmosphere of pressure, namely 14.7 pounds per square inch). A kilobar is 1000 kb; a megabar is a million kb; 1 GPa = 10 kb. The shock waves decay (decrease in value) as they spread out from the impact point. When a shock wave meets a free surface, it is in part reflected as a tensional wave.

The first truly modern explanation of how an impact crater is formed was presented by Eugene M. Shoemaker in a 1963 paper. The key illustration from this paper is reproduced here:

This next diagram (adapted from H. Melosh) summarizes some of the major aspects of the pressure distribution and shock wave-produced phenomena associated with an incoming bolide (fancy term that applies to the causative projectile, such as a meteorite or an asteroid/comet):

The incoming bolide penetrates to a distance approximately twice its diameter. As it does, it is itself destroyed by tensional waves, as it vaporizes and breaks up into melt drops and fragments. The ground target is impinged by the generated shock waves. At pressures around 70 GPa, the target material (usually rock) is vaporized; rocks subjected to waves between 50 and 70 GPa are melted. The hemispherical volume of rocks that experience shock pressures between about 5 and 50 GPa is affected by processes involved in shock metamorphism - whose effects are considered in the next two pages. Beyond, to a level around 1 GPa, the rocks are fractured and brokened into pieces that accumulate as breccias (rock fragments [clasts] held in a matrix of finer particles and melt that is quenched into glass) which are deposited both within and beyond the resulting crater. The shock waves thus break up the target rock, setting it into motion along trajectories (the curved lines with arrows; only a few such lines are shown) that carry the now disrupted pieces (some as large as houses but grading in size downward to pea and dust sizes) upward and outward. In this way the crater is excavated by the combination of vaporization, melting, and fragmentation that removes nearly all rock that underwent pressures down to a few GPa and relocates it as ejecta. Rock beyond the final transient crater is displaced and disrupted, with fractures that open receiving injected debris.

Craters are round or circular (in plan view) and bowl-shaped (as seen in a cross-section) for this reason: the shock wave can be considered as starting from a point (actually a narrow zone of some length related to the penetration of the bolide). This waves travels radially from the point of contact at a uniform velocity. Thus the effects, including excavation, are equal in all horizontal directions (hence, a circle) and approximately so in the directions that define the hemisphere whose outer limit is the true crater.

Impact cratering is unique among natural geologic processes in that very high pressures are attained almost instantly; these decay in minutes or less so that the resulting crater is produced in a very short time. The shock metamorphic phenomena so developed are also unique. Volcanic processes can also produce craters but the processes generally occur over much longer time spans and do not achieve pressures above about 2-3 GPa and hence do not impose the shock metamorphic features in impactites (general term for any rock that is affected by shock waves from an impact) that are diagnostic of impact events. Pressures within the crust and mantle can reach well above 5 GPa with increasing depths but the processes involved (such as the weight of the overburden and superimposed tectonic stresses) are applied much more slowly, so that there are no shock wave features imposed.

Let us now follow, second by second, the formation of a large or complex crater (one greater than about 5 km [3 mi] wide that has a central peak and concentric slump walls). We use a series of sideview panels created by Dr. Raymond Anderson of the Iowa Geological Survey Bureau (and used here with his permission) to show the steps in developing the Manson structure (see page 20-3a for an in-depth survey of this Iowa crater). The writer, during the 1960s, when he was working primarily on impact structures, is generally credited with "proving" the impact origin of this very large crater, which, at one time, many thought was the "smoking gun" that killed the dinosaurs about 65 million years ago. But a new age date found it to be 74 million years old, which disqualified it as the culprit. This circular structure, 32 km (20 mi) in diameter, whose centerpoint is some 130 km (81 mi) northwest of Des Moines, Iowa, is largely intact but now buried under 30 m (98 ft) of glacial debris.

Each of the following schematic diagrams represents a stage in the sequence of mechanics of formation of a large (complex) crater; read text for description; the number in the upper right circle indicates the time in seconds or minutes after the initial moment of contact between the incoming bolide (probably an asteroid) and the ground surface. While these diagrams (prepared by R. Anderson) apply specifically to the Manson structure, they apply to the cratering process in general.

At the instant of impact (0.0 sec), the target consisted of an average of 90 m (295 ft) of Mesozoic sedimentary rocks (mainly Cretaceous in age) (in green) underlain by 495 m (1624 ft) of Paleozoic sedimentary rocks (light blue). These rocks lie unconformably on top of Proterozoic sandstones and other red clastics (yellow), whose thickness increased to nearly 3 km (1.9 mi) to the southwest. This entire section rests on top of Precambrian crystalline (granites and metamorphic) rocks (red) buried at depths to almost 4,600 m (15,088 ft).

As the incoming impactor (or bolide, a general term that includes both comets and asteroids) impressed onto this late Cretaceous surface, at 0.15 seconds, it was totally fragmented and vaporized. At it penetrated into the rock, it imparted its energy (about 2 x 1023 J) in the form of supersonic shock waves that generated compressive pressures ranging up to a megabar (1,000,000 atmospheres). We usually find such pressures only at depths well into the Earth (100s of km). Rocks just beyond the point of impact vaporized. An initial curtain of ejecta, consisting of gases and melted rock, streamed upwards in a steep cone, within which is a momentary partial vacuum caused by the projectile passage. The energy released also generated electromagnetic waves that extended into the atmosphere.

At 0.6 sec the shock wave had progressed along an enlarging hemispherical front well into the target, severely transforming rocks at pressures ranging to about 600 kilobars (kb) (or 60 Gigapascals [Ga], a fashionable new pressure unit) close to the line of penetration. A fraction of the target (up to 10% of the total that the impactor eventually displaces) melted. Some of that molten rock carried downward along with the now-compressed and mobilized rock underwent fragmentation. Some of it pushed out of the crater and fell back nearby, and some literally squirted out as tiny blebs that might have traveled hundreds of miles out of the atmosphere, and then returned to Earth as tektites (glass "pebbles"). A fireball, similar to that caused by atmospheric burning at surface detonations of chemical or nuclear explosions, started to form. Within a few seconds, the excavation phase of the crater commenced, where the shock wave first compressed the rock and then a trailing wave (rarefaction wave) moved through, causing tensile fragmentation. As the waves spread outward and down, decreasing in intensity, peak pressures dropped to a few 10s of kilobars.

By 6.9 seconds, the initial or transient crater, arising from vaporization, melting, and direct ejection and from centrifugal "shoving" of the target matter outward under compression, had reached its maximum depth. At Manson, this rapidly growing crater front cut down through the Mesozoic, Paleozoic, and Proterozoic sedimentary overburden, well into the Precambrian crystalline rocks. Most of this earthen material received shocks to varying degrees and the effects of these pressure waves were permanently imposed on the rocks. Trailing tension (rarefaction) waves continued to decompress these rocks and break them into fragments ranging from microscopic in size to objects bigger than a house that eventually came to rest as deposits called breccias.

By 11.0 sec, as excavation continued, the peak shock pressures at the wave front had now decayed to under 20 kb (2 Ga). As the pressure waves advanced outward from ground zero (point of impact), they kept breaking the rocks into fragments and blocks, launching them into ballistic trajectories that started particles downward and then swung them up, above the still growing crater along arcuate paths. Most of the material left the site along low to moderate angle paths. Generally, particles deeper and farther out from the impact center left later and usually fell on top of particles that left earlier from near-surface positions. Thus, the ejecta layers tended to deposit in reverse order relative to their initial position in the stratigraphic sequence, with ones lower in the target falling on top of upper ones (although some mixing occurred). Beyond the edges of the crater walls, rock units experienced faulting and folding. Along the upper walls, sedimentary (layered) rocks pealed back so that the layers might even overturn.

For Manson, the crater reached its maximum excavation diameter around 25 sec, as the last voluminous ejecta emerged. Its upper walls were especially unstable and began to fail along steep concentric fractures and faults. At the central bottom of the crater, the rock below began to rebound upward.

At the transitional 26 second mark, the last ejecta were well into flight. Slices of rock just past the walls now began an inward sliding along faults. The crater base had started an upward movement that soon led to a central peak. This rock material probably behaved plastically as it almost flows upward (a good analogy is the inner blob of water that shoots up into a momentary "crater" forming by dropping a stone into a pond ). The collapse of the upper walls may have aided in this effect by pushing downward toward the center.

By 35 seconds, the central peak had attained its maximum height (overshoots) and began to founder in collapse.

By about a minute after the impact started, the central peak was well into subsidence towards its final position and the crater walls had commenced to slide and tumble inward to form nested or terraced rings (see the Tycho image for an overhead view of these conditions). By this time, some of the material that left at high angles directly above the crater began to descend. The heavier, larger particles settled first, because they passed through the atmosphere more quickly, due to their momentum.

Over the next 30 minutes or so, this fallout piled up in a continuous blanket, inside and outside the crater. Other materials expelled at lower angles formed a wider apron of ejecta that these later fallout deposits covered. Small particles and dust from the event carried hundreds of miles. Manson material has been found in a thin layer at sites in South Dakota, up to 500 km (311 mi) away and the finest sizes traveled in the stratosphere probably well beyond North America (likely global in extent).

At the time of impact, 74 million years ago, the Manson area was almost certainly under water, because the region lay within a shallow sea. This impact should have produced a tsunami-like disturbance (steep fronted waves that travel at velocities >800 kph [about 500 mph]). Large-body impacts into the open oceans probably spawn huge waves, whose initial heights may exceed 325 m (1,066 ft). If this was so at Manson, the various ejecta deposits would not have formed in the usual sense, because they would have entered disturbed waters and would be irregularly deposited or stirred up by waves moving back into the crater area. The seas retreated a few million years later, leaving the land exposed to erosion. About 10% of the crater's upper structures and deposits eroded. Now glacial deposits of the Pleistocene Age cover the crater, which has no surface expression at all. We show a cross-section (side view) of the Manson crater, as it remains today (glacial cover is a thin gray line):

The dashed yellow line marks the boundary of the final transient crater, modified upwards centrally by the rise of its surface along the central peak. The curved concentric maroon or black lines are fault planes, bounding slides of bedrock that dropped downward to help create terraces. Their outer limits define the maximum (apparent) crater diameter.

Much of what has been shown in the above panel cartoons that follow the sequence of events during impact cratering can be reproduced at laboratory scales. This next set of sequential panels are photographs taken by a high speed camera of the development of the ejecta curtain from an impact of a small (centimeter-sized) projectile fired into loose sand from a gas-gun that accelerates the projectile to high velocities.

In the above experiment, vertical black-painted sand was inserted as columns (using thin celluloid tubes to contain this sand) in the target material. After the impact the target sand was sealed by a liquid glue and then sectioned, one of which is shown here:

The black sand markers just below the crater base show an abrupt bending towards the rims on either side. This confirms that the shock waves induce motions in the sand that roughly parallel the growing surface of the forming crater. Thus, transport of ejected sand is outward at angles; below the final crater base the deformation broadly follows this motion.

For anyone seeking a comprehensive and technical treatment of cratering mechanics, we recommend the book "Impact Cratering: A Geologic Process," by H.J. Melosh, 1989, Oxford Monographs on Geology and Geophysics No. 11, Oxford University Press.

Having now grasped some idea of what happens when impact craters are produced, it could prove interesting to you to run your own calculations in determining Impact Cratering Effects. This Web site brings up a page that allows you to enter various parameters to determine what is predicted to happen when an incoming asteroid or comet strikes the Earth.

Source: http://rst.gsfc.nasa.gov