Summary
Highlights
Rocks are solid chunks of one or more minerals, found everywhere from huge formations to tiny pebbles. While most rocks are mixtures of multiple minerals, some, like chalk, are made of a single mineral (calcite). Geologists, or rock experts, classify rocks into three main categories.
Despite the vast number of rock types, they are simplified into three categories based on how they were made: sedimentary, igneous, and metamorphic. Understanding these categories helps explain the origin of each rock type.
Sedimentary rocks are formed from small particles called sediment, which can include sand or seashells. These sediments collect at the bottom of bodies of water like lakes and oceans. Over time, the pressure from the water causes the sediments to harden into rock, with water molecules acting like cement to bind them together.
Metamorphic rocks are created when existing rocks undergo significant heat or pressure. The term 'metamorphic' relates to 'morph,' meaning to change. These rocks are typically formed deep below the Earth's surface, within the Earth's crust, where moving tectonic plates generate intense heat and pressure, transforming the rock's appearance and mineral composition. Sedimentary rocks, for example, can become metamorphic if pushed deep enough.
Igneous rocks are formed when molten rock cools and hardens. Molten rock is known as magma when it's under the Earth's surface and lava when it erupts. This cooling process solidifies the molten rock into igneous rock. Igneous rocks can also form when metamorphic rocks are heated to the point of becoming molten and then cool.
Rocks are constantly changing through a process called the rock cycle. Igneous rocks can erode into sediment to form sedimentary rocks; sedimentary rocks can be subjected to heat and pressure to become metamorphic; and metamorphic rocks can melt into molten rock and then cool to form igneous rocks. This continuous cycle demonstrates the dynamic nature of Earth's geology.