Cratering Mechanics¶
Contents
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¶
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 <Sect18_answers.html#18-3>`__
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 mv:sup:`2`, 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.
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).
|Panel 2 - Sequence of steps involved in the development of the Manson structure.|
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.
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 <Sect18_answers.html#18-4>`__
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 <Sect18_answers.html#18-5>`__
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.