Scoria is more often a product of basaltic, low-silica lavas than of felsic, high-silica lavas. This is because basalt is usually more fluid than felsite, allowing bubbles to grow larger before the rock freezes. Scoria often forms as a frothy crust on lava flows that crumbles off as the flow moves. It also is blown out of the crater during eruptions. Unlike pumice, scoria usually has broken, connected bubbles and does not float in water.
This example of scoria is from a cinder cone in northeastern California, at the edge of the Cascade Range.
For photos of related rocks, see the volcanic rocks gallery.