Lab 3_ImpactVolcanic - Lab 3: Impact and Volcanic Features...

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Lab 3: Impact and Volcanic Features Impact Craters Impact cratering is a very dynamic process, more like an underground explosion than a surface impact at low velocity. This somewhat unexpected behavior is because the meteors and comets that make the impacts travel at extremely high velocities, typically much greater than the speed of sound. When such impactors initially penetrate the planetary surface, they are still traveling faster than the speed of sound. Only as the projectile slows through the sound barrier is the energy of the projectile dissipated into the surrounding medium in the form of an explosive energy release at the current depth of the projectile. Thus, impact craters are almost always initially circular in shape as a result of this explosive energy release, even if the impactor hits the surface at a large angle from the vertical. Only when the angle of impact is very close to horizontal, is a non-circular crater formed. Most non-circular impact craters on the planets were formed by post- impact modification of an originally circular crater. The primary diagnostic feature of an impact crater on a planetary surface is that there is no significant addition of new material to the surface . Material is simply redistributed from the crater to the impact ejecta blanket, the discontinuous layer of debris thrown out of the crater and usually centered on the crater. Thus, the volume of the basin below the original planetary surface should be approximately equal to the volume of material in the ejecta blanket above the original planetary surface. Cratering Process The series of cross-sections below shows the progressive development of a small, bowl- shaped impact structure. The most similar man-made process is the underground detonation of a very large explosion, such as a nuclear weapon test. The actual process of impact cratering with reference to the figure may briefly be described as follows: (a) Penetration of the projectile into the planetary surface; (b) , (c) and (d) Explosive release of the kinetic energy (that energy which the projectile possesses by virtue of its mass and velocity) of the projectile throwing ejecta upward and outward from the crater, and forming a relatively deep transient crater; (e) Collapse of the transient crater by rebound and slumping (gravitational flow) of material into the crater; (f) Re-deposition of ejecta into and around the crater forming an ejecta blanket (not shown) Post-impact modification of the crater by erosion, volcanism, tectonic activity, or further impact activity.
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Crater Morphology and Complexity During the collapse of the transient crater, if the crater is large, there may be some rebound of the central portion of the floor of the crater forming a central peak . In even larger craters, rebound may form two or more circular ridges as the energy radiates out from the crater, and these ridges may be preserved to form a multi-ring basin . On the Moon, the transition between simple bowl-shaped craters, in which the depth of the
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Lab 3_ImpactVolcanic - Lab 3: Impact and Volcanic Features...

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