Module 6: Rocks

There are three types of rock that are different due to the way in which they form. All three rock types have unique textures and characteristics that distinguish them from each other. The three rock types are igneous, sedimentary, and metamorphic. The rock cycle diagram on this page shows all of the processes by which a rock forms and changes.

Igneous rocks form when melted rock cools and solidifies into solid rock. Melted rock exists in two forms: magma, which is located beneath the Earth's surface, and lava, which erupts and flows at the surface. As a result, igneous rocks form both at the surface of the Earth (extrusive) and beneath the surface (intrusive) wherever magma exists. Due to the way these rocks form, igneous rocks exhibit interlocking mineral crystals. Basically, the minerals form throughout so that there are no gaps between the individual minerals that make up the rock. The rate at which the melted rock cools will determine the size of the mineral crystals. Intrusive rocks cool slowly, and therefore have large mineral crystals. Extrusive rocks cool so quickly that little or no mineral crystals form.

Unlike the other two types, the methods of forming sedimentary rock can easily occur most anywhere on Earth. The simple processes of weathering, erosion, deposition, compaction, and cementation are responsible for forming sedimentary rock. All sedimentary rocks form at or very near the Earth's surface. This enables sedimentary rocks to record conditions and clues to the environments that exist at the surface over time. From sedimentary rocks, scientists discover fossils that can provide insight to the Earth's past.

Metamorphic rocks form deep beneath the surface of the Earth where temperature and pressure are the most intense. At depth, rocks can change chemically or physically in response to intense heat and pressure. Metamorphism, or the process of forming metamorphic rock, can result in foliated texture in the resulting rock. Foliated texture is the result of recrystallized minerals flattening and aligning into parallel bands. This parallel alignment can cause the rock to appear striped or layered.

 

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