Extraterrestrial metamorphism

Abstract

The types of metamorphism so far described in this book have been dominated by deformation processes due to tectonic movements within the Earth due to the relative motions of lithospheric plates. There is a different type of metamorphism, which is due to intense, rapid deformation following the impact of extraterrestrial bodies onto the Earth’s surface. Particles of extraterrestrial matter (meteoroids) arrive regularly in the vicinity of the Earth. They enter the atmosphere with velocities in the range 5–20 km s⁻¹, due to the effects of relative orbital velocities, plus acceleration in the Earth’s gravitatiopal field. Interaction with the atmosphere sorts the particles of extraterrestrial matter according to their size — small particles are burned up by friction before they strike the ground (producing the incandescent trails in the night sky called meteors) particles of intermediate size are slowed down so that fragments fall to the surface relatively undamaged (as meteorites) while the largest particles are only slightly slowed down, and strike the surface at high speed. It is the last category, fortunately the rarest, which produces shock metamorphism.