Effusive Rock

a magmatic rock formed with the cooling of lava on the earth’s surface or within the crust under near-surface conditions. A significant proportion of the effusive rocks was formed during the eruption of underwater volcanoes. Typical characteristics of effusive rocks, associated with the rapid cooling of lava, are the presence of volcanic glass in their composition and, often, a unique porphyritic texture. The composition of effusive rocks varies widely. Basalts, andesites, and rocks intermediate between basalts and andesites are most common. Dacites and liparites are less common. Alkaline effusive rocks, such as phonolites and leucites, and ultrabasic effusive rocks, such as comateite, are even rarer. Effusive rocks were found on the moon. Effusive rocks are contrasted to intrusive rocks.