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in geology, sedimentary rock composed largely of pebbles or other rounded particles whose diameter is larger than 2 mm (.08 in.). Essentially a cemented gravel, conglomerates are formed along beaches, as glacial drift, and in river deposits.
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cemented rock composed of angular fragments (sized 1 cm or more). Breccias may be composed of fragments from one rock (monomictic breccia) or several different rocks (polymictic breccia).
Breccias are volcanic (volcanogenic), sedimentary, or tectonic in origin. Volcanogenic breccias originate when the more liquid lava cements together the lava fragments which have formed during the crushing and breakdown of flowing viscous lava (lava breccia), during the agglomeration of tuff and other volcanogenic elastic materials (tuff breccia), and during the diagenesis of mud flow deposits in volcanic regions (lahars). Sedimentary breccias are formed in continental conditions during the cementation of talus deposits and mud-flow deposits from lahars, and during the collapse of the roofs of karstic cavities (karst breccia). A specific type of breccia is bone breccia, which originates in places where the mass destruction and burial of vertebrates has occurred. In maritime areas breccia originates as a result of the destruction of littoral rocks or reefs. Tectonic breccias (friction breccia) are formed in a rock stratum during tectonic movements when rock is crushed along the fractures. Large fragments in this case are usually buried in a finely granulated mass.
V. P. PETROV