Summary
Highlights
Rocks are solid, naturally occurring formations made of minerals or mineral-like matter. They can be found in various sizes and formations, from large structures like Half Dome to small canyons. Rocks are categorized into three main types based on their formation: igneous, sedimentary, and metamorphic.
Igneous rocks form when magma or molten rock cools. They can be extrusive (volcanic) if they cool quickly on the Earth's surface, forming small crystals, or intrusive (plutonic) if they cool inside the Earth, resulting in larger crystals. Examples include obsidian (extrusive, made of quartz, alkali, feldspar), granite (intrusive, made of feldspar, quartz, mica), basalt (extrusive, made of plagioclase feldspar, pyroxene), scoria (volcanic, made of feldspar, augite, hornblende), and rhyolite (extrusive, made of feldspar, quartz, mica).
Sedimentary rocks are formed from pre-existing rocks or once-living organisms. They accumulate on the Earth's surface or under the ocean as sediment, which can be mineral or organic matter. They are divided into clastic (from rock fragments), chemical (from mineral precipitation), and organic (from plant and animal debris) types. Clastic rocks are subdivided by particle size (clay, silt, sand, gravel, pebble, boulder). Examples include shale (fine-grained, composed of mud), sandstone (clastic, composed of sand-sized grains), conglomerate (coarse-grained, composed of pebbles and boulders), and limestone (organic, composed of marine organism fragments).
Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to extreme heat and pressure, changing their original structure. The original rock is called the protolith or parent rock. They can form deep in the Earth's crust or in areas of plate tectonic movement. There are two major types: foliated (crystals in stripes) and non-foliated (no crystals in stripes). Examples include slate (foliated, parent rock: shale), marble (non-foliated, parent rock: limestone), quartzite (non-foliated, parent rock: sandstone), gneiss (foliated, parent rock: schist), and schist (foliated, parent rock: slate).