Work in Progress …

A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts).

A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material can vary significantly. Many meteorites categorized as breccias were produced due to collisional processes like hypervelocity impacts. An excellent paper on the subject can be found by clicking this link where the excerpt originates:

Impacts at speeds above ~20 m/s broke rock, while hypervelocity impacts left shock damage and formed breccias from fragments of earlier rocks. The study of meteoritic breccias contributes significantly to our understanding of early solar system processes of accretion, differentiation, and surface (regolith) evolution, and also provides unique information about the primordial, chemical, and mineralogical characteristics of the accreted components themselves (Bischoff and Stöffler, 1992).

Note that the definition for a breccia used by meteoriticists is only loosely correlated to the definition used by terrestrial geologists.

There are other geologic processes beyond collisional processes that produce breccias including  tectonic, volcanic, sedimentary, and collapse. On Earth, breccias and conglomerates can appear similar but in breccias the large clasts are mostly angular in shape, while in conglomerates the clasts are nearly all rounded, evidence of prolonged tumbling in running water or moving waves in the past. In the case of “lunar” meteorwrongs, a clear indicator of its terrestrial origin is the rounded clasts.

Breccias other than impact-melt breccias are lithifiied by impacts that caused limited shock induced grain boundary melting cementing the rock fragments together (Kieffer, 1975; Ashworth and Barber, 1976; Bischoff et al., 1983). Ashworth and Barber (1976) and Bischoff et al. (1983) showed that ordinary chondritic regolith breccias experienced limited shock-induced grainboundary melting. This melt is important for consolidating loose debris into brecciated rock. Some porous lunar regolith breccias may have been lithified by a thermal welding process (McKay et al., 1989), and carbonaceous chondrites by growth of secondary phases. Thermal annealing after mixing of fragments and lithification may lead to a recrystallized matrix between large fragments as in the case of LL chondritic fragmental breccias (Jäckel and Bischoff, 1998).


This entry was posted in . Bookmark the permalink.