How Calderas Form Calderas form when volcanoes collapse. These caldera collapses are generally not violent. Collapse of a volcano in a catastrophic landslide, now recognized as a common event in the lives of volcanoes. Tabular and Irregular Intrusions Tabular intrusions are sheetlike, and consist of dikes and sills. All others are irregular.
Sills are thin sheets of magma between layers.
Laccoliths are blister-like intrusions, usually near the surface, that form when magma pushes overlying layers upward. Lopoliths are mega-sills, usually of gabbro or diorite, that may cover hundreds of square kilometers and be kilometers thick. They often have a concave structure and are differentiated. That is, they take so long to harden that heavy minerals have a chance to sink and light minerals can rise. Discordant intrusions cut across layers.