Pseudotachylite

Rock formed by frictional melting of rocks during faulting and impact cratering as a result of high rates of deformation and nearly total transformation of kinetic energy to heat. After deformation ceases, rapid quenching of the frictional melt leads to the formation of glass. The term was first applied to veins in the Vredefort impact structure, which are found in rocks of widely varying composition. The compositions of associated pseudotachylites are the same as those of host rocks, with some systematic variations, thus indicating in situ formation. One mystery of pseudotachylite formation is how frictional processes can form large amounts of melt, because thick masses ought to preclude melting by reducing the friction between sliding rock masses. However, J. Melosh (U. Arizona) has suggested that if melt produced by sliding friction in narrow shear zones is extruded into adjacent country rock, the shearing zone can be kept narrow and will continue to produce melt.


Some or all content above used with permission from J. H. Wittke.


Vredefort structure pseudotachylite. Image source: www.geology.uct.ac.za
Impact Breccia from Vredefort (southwest of Johannesburg, South Africa). Image Source: Pierre Thomas (2008), planet-terre.ens-lyon.fr
About 2 billion years ago, an asteroid measuring more than 10 km in diameter impacted Earth and formed the world’s oldest and at ~90 km across, the largest known impact structure. Modified from Image Source: earthobservatory.nasa.gov