Igneous rocks crystallize from magma. Sedimentary rocks form by weathering and erosion of preexisting rock to make sediment, which is lithified into rock. Most rocks in Hawaii are igneous. If magma solidifies within the crust, the rock is called an intrusive igneous rock. Intrusive igneous rocks have been exposed by erosion along rift zones of some of the older volcanoes in Hawaii.
Intrusive rocks help to built the volcanoes from the inside.
This photo shows the Uwekahuna laccolith that is exposed in the wall of Kilauea caldera. A laccolith is a pocket of magma that became trapped in the rocks layers before it reached the surface. Laccoliths have flat bottoms and convex tops. Photo by Steve Mattox, July 1990. If the magma erupts at the surface, it is called an extrusive igneous , or volcanic rock .