Igneous rocks are those that solidify from a molten or partially molten state. These rocks are characterized as either extrusive or intrusive.
Extrusive igneous rocks solidify from molten material that flows over the earth’s surface (lava). Extrusive igneous rocks typically have a fine-grained texture (individual minerals are not visible unless magnified) because the lava cools rapidly when exposed to the atmosphere, preventing crystal growth. Common extrusive rocks are basalt, andesite, and rhyolite.
Basalt: Basalt is characteristically a dense, black, massive rock, high in calcium and iron-magnesium- bearing minerals and low in quartz content. Great examples of basaltic lava flows can be found in the Black Rock Desert, Millard County.
Andesite: Andesite has a higher quartz content than basalt and is usually lighter in color. Crystals of the minerals amphibole, biotite, and feldspar are sometimes visible without magnification. In Utah andesite can be seen at Signal Peak in the Tushar Mountains, Piute County.
Rhyolite: Rhyolite is typically a fine-grained, white, pink, or gray rock, high in quartz and feldspar content with some amphibole and biotite. A well-known example is the Topaz Mountain rhyolite in the Thomas Range, Juab County.
Intrusive rocks form from molten material (magma) that flows and solidifies underground. These rocks usually have a coarse texture (individual minerals are visible without magnification), because the magma cools slowly underground, allowing crystal growth. Common rock types within the intrusive category are granite and diorite.
Granite: Granite is the intrusive equivalent of rhyolite but has a coarser texture. A 12-square-mile outcrop of granite is visible on the southwestern flank of the Sheeprock Mountains, Tooele and Juab Counties.
Diorite: Diorite has the same texture as granite but has the mineral composition of an andesite, which is diorite’s extrusive equivalent. Diorite forms the summits of Haystack Mountain, Mt. Tomasaki, Mt. Mellenthin, and Mt. Tuckuhnikivatz in the La Sal Mountains, Grand and San Juan Counties.