Minerals do not lose their identities when they make up a rock. Instead, they are merely associated together in varying proportions. Some rocks, as we will find later, instead of being composed of the minerals themselves, are made up of fragments of earlier-formed rocks. Ordinarily, we think of rocks as hard and solid substances, such as limestone and granite, but some geologists consider loose and uncemented materials, such as sand, gravel, or volcanic ash, to be rocks also. The words _sediments_ or _deposits_ are often used to describe this uncemented or loose material. Rocks are commonly grouped, according to how they formed, into three great classes known as _igneous_, _metamorphic_, and _sedimentary_. [Illustration: A rock is made up of minerals. The igneous rock _granite_, for example, consists chiefly of quartz and feldspar; other minerals such as mica and hornblende are commonly present.] IGNEOUS ROCKS Igneous rocks result from the cooling of hot, molten rock material or _magma_. Magma that reaches the surface through volcanoes is called _lava_. Magma comes from deep within the earth and is made up of a mixture of molten mineral materials. Igneous rocks have been forming throughout the geologic past and are still forming today. We can understand how they form when we look at pictures of hot, molten lava flowing from volcanoes, such as Mauna Loa in Hawaii. As lava cools, it hardens into rock. Extrusive or Volcanic Igneous Rocks The igneous rocks that form on the earth’s surface are called _extrusive_ or _volcanic_ igneous rocks. When magma flows to the surface, it cools and hardens quickly. The mineral grains that form during this fast cooling may be too small to be distinguished from each other. Some lava cools too quickly for minerals to crystallize—then the rock is volcanic glass. [Illustration: Extrusive igneous rocks form at the earth’s surface from lava that cools and hardens relatively quickly.] No volcanic igneous rocks are forming in Texas now. However, during Tertiary time, in the Big Bend area and in other parts of the Trans-Pecos country of west Texas, lava came to the surface and hardened. (The physiographic outline map, p. 42, shows where these areas are located.) Intrusive Igneous Rocks