In many respects, impact craters form in a similar manner to explosion craters where explosives are buried at some depth and then ignited. But, the energy needed to form an impact crater begins to act at the point of contact at the surface. As it burrows into the target (rock or water), it has an effective (not real) center of energy release. An impact crater involves energies on the order of thousands to millions of megatons (equivalent nuclear detonation energy). A series of panel cartoons, and accompanying descriptions, shows the sequence of formation of an impact crater (and is also described in the text). The final result is shown as a cross-section.

### Cratering Mechanics

No other natural event is as powerful, devastating, or potentially catastrophic as an 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. To approximate a valid diagram, simply consider two points: Meteor Crater at 0.8 mile and Sudbury at 134 miles. The scaling formula relating energy to diameter can be approximated by D = 0.1 times the cube root of the energy E in kilotons).

18-3: The Zhamanshin crater, in Asia, is 13.5 km (8.3 miles) in diameter. From the above graph, are there enough nuclear warheads in the arsenals of all nations combined to make a crater of this size if they are exploded simultaneously underground at one place? Are there enough atomic bombs to bring about nuclear winter? Roughly, what is the time likelihood of an impact of the size needed to have something like a nuclear winter forced on the Earth? (And, did you see "Armageddon" or "Deep Impact" in 1998? Does this potentiality for an impact catastrophe worry you?) ANSWER

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.

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. 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, until we found its age and disqualified it. This 74 million year old, 35 km (22 mi) wide crater, 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.

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) overlain by as much as 52 m (171 ft) of young glacial till and 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) 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 fragmentaton tension. 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 angles. Generally, particles deeper and farther out from the impact center left later and usually fell on top of particles that left earlier and remained near the surface. Ejecta layers tended to deposit in reverse order of initial position, 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. Especially 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.

About a minute after the impact started, the central peak began to subside into its final position and the walls slid and tumbled 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 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).

18-4: To recapitulate, specify the time or time interval at which each of these stages in the Manson crater formation was important: a) Maximum melting of rock; b) Maximum depth of transient crater; c) Moment when shock wave had decayed to about 20 kilobars (roughly the lower limit at which signficant shock features are produced in the rocks; d) Maximum excavation of fragmented rocks; e) Inward failure of crater walls; f) Start of central peak rise; g) Collapse of central peak; h) Deposition of fallout. ANSWER

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 spawned huge waves, whose initial heights may have exceeded 325 m (1,066 ft). If this is so, 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 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.

18-5: Drillers at the surface above Manson cannot see what they are "aiming" for because of the glacial cover. But, suppose geophysical surveys have outlined the main elements of the crater, so the drill team knows where the center and the rim are located underneath. What would they encounter, as evident in the recovered drill core, if they drilled a) at the center; b) half way out (in the "moat"); and c) into the rim? ANSWER

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.

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.

Primary Author: Nicholas M. Short, Sr. email: [email protected]