![]() ![]() Some clasts of caldera megabreccias can be over a kilometer in length. They are distinguished from mesobreccias whose clasts are less than a meter in size and which form layers in the caldera floor. These are instead blocks of precaldera rock, often coming from the unstable oversteepened rim of the caldera. Ĭaldera collapse leads to the formation of megabreccias, which are sometimes mistaken for outcrops of the caldera floor. The resulting breccia is uniform in rock type and chemical composition. This occurs when the thick, nearly solid lava breaks up into blocks and these blocks are then reincorporated into the lava flow again and mixed in with the remaining liquid magma. Lavas, especially rhyolite and dacite flows, tend to form clastic volcanic rocks by a process known as autobrecciation. This may include rocks plucked off the wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge. Volcanic pyroclastic rocks are formed by explosive eruption of lava and any rocks which are entrained within the eruptive column.
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