lag breccia (co-ignimbrite breccia) A coarse, lithic-rich deposit accumulated synchronously with the formation of ignimbrite, but confined to a proximal, near-vent location. As the outer margins of a pyroclastic eruption column collapse, forming a pyroclastic flow moving under gravity away from the vent area, clasts which are too large to be supported by the column or flow accumulate as a lag breccia around the vent. Because they are deposited as the ignimbrite forms, they are now called ‘co-ignimbrite’ breccias to distinguish them from other types of breccias.
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