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].
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.
