Rocks: Types, Formation, and Human Use

Definition and role of rocks in geology

In geology, a rock (also called a stone) is any naturally occurring solid mass or aggregate of minerals (or mineraloid matter). Rocks are defined and distinguished by the minerals they contain, their chemical composition, and the processes by which they formed. They make up Earth’s outer solid layer (the crust) and most of its interior, excluding the liquid outer core and pockets of magma in the asthenosphere. Because rocks record the physical and chemical history of Earth, their study is central to geology and is handled through subdisciplines such as petrology and mineralogy. Rocks also play a major role in understanding Earth’s evolution and processes. They are commonly grouped into three main classes—igneous, sedimentary, and metamorphic—each produced by different formation conditions: igneous rocks form from cooling magma or lava; sedimentary rocks form through deposition and subsequent compaction/cementation of sediments; and metamorphic rocks form when existing rocks are transformed by high pressure and temperature without significant melting. Over time, rocks can change from one type to another through the rock cycle, and their properties (like mineral content, texture, and permeability) reflect the processes that formed them. Beyond Earth science, rocks have been crucial to human technology and culture (e.g., stone tools, building materials, mining), and humans have even created rock-like materials such as concrete.

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Rock composition: minerals and mineraloids

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