CRATER FORMATION

by: Charles O’Dale

Impact Structure: n. A large geologic structure, such as a crater or astrobleme, created by the violent collision between a planet and a space projectile such as a comet or meteor.

Cratering Process

Impact cratering is one of the most common geological processes that have happened on planetary objects with solid surfaces (our home planet Earth included) and is unlike any other known natural geological process. Impact involves the transfer of massive amounts of energy to a relatively small area of the Earth’s surface, in an extremely short period of time (Kinetic energy).

On earth the lowest impact velocity with an object from space is the gravitational escape velocity of 11 km / sec. The highest velocity impact is more than 70 km/s:
earth escape velocity [11 km/s] + sun escape at the earth’s orbit [30 km/s] + earth’s orbit velocity [30 km/s] = ~70 + km/sec.
The average earth impact velocity is 17 km/s.
At these speeds, impacts produce shock waves in solid materials. Both meteorite and target rocks are compressed to high density, then rapidly depressurized and exploding violently. Since craters are caused by explosion they are always circular with only very low angle impacts causing elliptical craters.

The kinetic energy of an object of mass m traveling at a speed v is mv²/2, provided v is much less than the speed of light. The pressures and temperatures in the shock wave after impact are well above the magnitudes of pressures and temperatures occurring naturally on this planet.

The cratering process has been divided into three distinct stages:

1 -Contact & Compression

CONTACT AND COMPRESSION; Meteorite impact is a process in which a large object strikes an even larger one at hypervelocity^a, which locally releases a huge amount of energy producing an impact crater^b. This diagram documents that the magnitude of the shock^c from an impactor is inversely proportional to the distance from the point of impact. The shock metamorphic effects in the country rock will then vary with shock magnitude (French 1998).

^a Hypervelocity – 11.2 km/sec to 70 km/sec.

^b Crater: impactor size ratio ranges from 20:1 to 50:1 (Shoemaker 1963, Baldwin 1963).

^c The standard unit of pressure is the Pascal, abbreviated Pa, which is equivalent to 1 kilogram per square meter. A GPa is a gigapascal (giga means billion), a measurement of pressure, and is equal to 10,000 times the atmospheric pressure at the Earth’s surface.

The cratering process begins when the impactor, travelling at 10 to 75 km/s, makes initial contact with the target body. This starts the contact and compression process which lasts only fractions of a second. During this time the impactor will penetrate up to 2 times its diameter. At this point the kinetic energy of the impactor is transformed into shock waves that radiate into the target body and back into the impactor itself. The energy of the impact produces a spherical expanding envelope of hot gas regardless of the angle of impact. The impactor and the immediate area of the impact are completely vaporized^*. With few exceptions, the impact explosion almost always produces a circular crater. Elliptical impact craters are known elsewhere in the solar system and are caused from impactors at a very low-angle or obliquity of 10°–15° (Kenkmann 2009). After only a fraction of a micro-second, the compression stage ends with the complete transfer of energy in the form of a shock wave and latent heat.

Zircon grains that contain reidite forms at >30 GPa during the crater compression stage,

^* In some rare cases the projectile survives the cratering process, at least in the form of meteorite fragments (apart from geochemical traces in melts at larger structures). At Barringer crater and at small simple impact structures (<< 2km) like Wolfe Creek or Odessa, the meteorite can survive. A large fragment of the impacting meteorite was found beneath the melt sheet at the ≥70 km Morokweng impact structure, South Africa (Schmieder 2010).

2 – Excavation

The crater excavation stage (Melosh, 1980) overlaps somewhat with the compression stage and involves two processes:

upward ejection (spalling) of large near-surface fragments and smaller ejecta (ejecta curtain);
subsurface flow of target material to form the transient crater.

In seconds the resulting kinetic energy release creates an explosion that forms the initial transient crater. This energy release is transferred into complex interactions within the resulting shockwaves. The crater now consists of an evacuated zone (forming impact ejecta) and a lower displaced zone (forming crater-fill impactites). The excavation process is complete when the energy in the shock waves can no longer displace target rocks. This process could last up to 90 seconds for a crater of up to a 200 km diameter.

3 – Modification

The initial transient crater is unstable and the modification stage commences. Small craters of <4 km (on Earth) are relatively stable after the excavation stage. For larger craters, the impact structure is gravitationally unstable and its modification stage will include uplift of the crater floor and collapse of the unstable steep walls (slumping). These movements will be completed in a few minutes and could result in a complex or multi-ring crater. Minor faulting, mass movement and/or hydrothermal activity in the larger craters could last indefinitely.

Crater modification by gravity – A mechanical analysis of slumping

TRANSIENT CRATER
The crater that exists at the end of the excavation stage of impact cratering. The transient crater undergoes only slight modification in the case of a small, bowl-shaped crater. Large transient craters exhibit a gravity-dependent instability which leads to its collapse by elastic rebound and slumping of the walls and, to a large extent, to filling up of the cavity. Consequently, these complex impact structures/craters show a much smaller depth-to-diameter ratio compared with simple, bowl-shaped craters.

The form of collapse is governed almost completely by the dimensionless parameter (pgh)/c, when <5.5 = stable, 5.5.to 10 = slope failure, ~>20 = failure (where p is the density of the excavated rock, g is the acceleration of gravity, c is the yield strength of the substance), [from Melosh 1977 – The Role of Slumping in Crater Modification, Melosh, H. J. 1977]

ELASTIC REBOUND
Part of a theory explaining the forces that cause earthquakes. In impact cratering , elastic rebound describes the readjustment of the highly compressed floor of the transient cavity in the modification stage.

Charlevoix impact structure Digital Elevation Model with Earthquake Epicentres.

Charlevoix crater. Simplified map showing shock isobars, distribution of shattercones and pseudotachylites, and main topographic and structural features (modified after Robertson 1975; Rondot 1989). The topographic central peak corresponds closely to the 20 GPa isobar. The peripheral trough is a topographic low underlain by down-dropped, strongly faulted and folded pre-impact lower Paleozoic sedimentary rocks. The rim is defined by the margin of a regional plateau with average elevation about 900 m above the river. The Appalachian Front (Logan’s line) is the trace of a thrust dipping 20° SE; the St. Lawrence (S-L) fault is a zone of late (still active) normal faulting. (Dence 2004)

Proposed Charlevoix crater model. Three stages in the development of a peak ring crater. Stage 1 the transient crater stage, represents the postulated maximum development of the central uplift. Stage 3 depicts the final stage with the partial collapse of the central peak to its present state and modest uplift of the intermediate ring at the margin of the subsiding central peak. By analogy with craters where a melt sheet is preserved, a crater lining of melt and attendant breccias is shown forming in stage 1 then sliding off the central peak at stage 2 and consolidating in stage 3. Pseudotachylites formed by frictional melting and crushing along shear surfaces in stages 1 and 2 move into cracks that dilate as the central peak overshoots (stage 2) and act as a sealant as the peak subsides in stage 3. It should be noted that the central peak rocks move as large blocks, coherent over hundreds of meters, lubricated by generally thin shear zones. The total vertical motion of the center at Charlevoix is estimated to have been about 25 km. (Dence 2004)

Dence, Michael R. Structural evidence from shock metamorphism in simple and complex impact craters: Linking observations to theory. Meteoritics & Planetary Science 39. Nr 2, 267-286 (2004).