Dike
Dikes are tabular or sheet-like bodies of magma that cut through and across the layering of adjacent rocks and then solidify. They form when magma rises into a fracture or creates a new crack by forcing its way through rock. Hundreds of dikes can invade the cone and inner core of a volcano, often along zones of structural weakness.

Left: An exposed dike, approximately five feet wide, at the caldera of Mauna Loa Volcano in Hawaii.

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