5.1.1 Rocks and minerals
Understanding a planet's geologic history requires development of techniques to read the record left by geologic processes. Solid bodies like Mars are composed of rocks, which are made up of minerals. A mineral is a naturally formed substance with a specific chemical composition. It can be composed entirely of one element or it can be a compound consisting of two or more elements. Minerals usually have a specific crystalline structure and changes in crystal structure, even when chemical composition remains constant, result in a different mineral.
Rocks are composed of a mass of minerals. A rock can be composed of a single mineral type or be a mixture of different minerals. Igneous rocks are rocks that solidify from molten material. The molten rock is called magma when it occurs underground and lava once it is extruded onto the planet' s surface. Intrusive igneous (plutonic) rocks cool slowly underground and are characterized by large crystals. Extrusive igneous rocks form on the surface and typically cool faster than intrusive rocks, resulting in small crystals or amorphous (non-crystalline) structures.
Sedimentary rocks are composed of small fragments of other rocks, produced from weathering processes. Metamorphic rocks form when heat and/or pressure change the characteristics of an original rock without melting it. Rocks can be transformed from one type of rock into another, as demonstrated in the rock cycle (Figure 5.1).
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