Basaltic volcanoes have relatively nonexplosive eruptions that throw little magma or material into the atmosphere. Instead, the magma usually flows quietly out of the volcano and onto the ground. When this magma cools, it forms lava or cinders, depending on how it was erupted. The cinder cones near Yucca Mountain are basaltic. Yucca Mountain itself was formed from multiple eruptions of a special type of composite volcano called a caldera.
Calderas form when large volumes of magma erupt rapidly.
The underground support for the volcano is removed, the surface then collapses and forms a circular depression, or caldera. Calderas can range in size from a few miles to tens of miles across. The layers alternate from relatively non-porous rock to highly porous, depending on the extent of welding, and how fast the layers cooled. This ancient layer is being studied as a possible site in which to build a repository. The last eruption from an explosive composite volcano occurred eleven million years ago.