The amount of silica determines how fluid the magma is, as well as the final chemical composition of the rock formed from the magma. The amount of water, which is in a gaseous state within the magma, determines the force or explosiveness of the eruption. The magma is thrown high into the atmosphere where it cools in the air and falls to earth as a blanket of ash. Over time, the ash consolidates and forms a highly porous rock called ash-fall tuff. A smaller amount of silica and water, in which less magma mixes with air, causes a less explosive eruption.
More magma flows out at ground level and the temperature remains high over a longer period of time.
When the magma has a low silica content, it s called basaltic. 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.