Rock (geology)

Rocks, also referred to as stones, are naturally occurring solid masses made up of minerals or mineraloid materials. Geologists classify them based on their mineral content, chemical makeup, and the processes that formed them. These materials make up Earth’s outer solid layer—the crust—and the vast majority of its interior, with the exceptions being the liquid outer core and pockets of magma in the asthenosphere. The scientific study of rocks draws on several branches of geology, including petrology and mineralogy, and can focus on Earth’s rocks or extend to planetary geology, which examines rocks on other celestial bodies.

The study of rocks, known as geology, investigates Earth’s components and rock formations. Petrology specifically looks at the character and origins of rocks, while mineralogy examines the mineral components that form them. This research has deepened our understanding of Earth’s history, informed archaeology about human history, and driven advances in engineering and technology. Although ideas about rocks and their origins have existed throughout human history, the formal science of rock study took shape in the 19th century. During that period, the theory of plutonism emerged, and the 1896 discovery of radioactive decay enabled radiometric dating. The understanding of plate tectonics developed later, in the second half of the 20th century.

Rocks are mainly composed of mineral grains—crystalline solids with atoms chemically bonded into orderly structures. Some rocks also contain mineraloids, rigid but non-crystalline substances like volcanic glass. The types and amounts of minerals in a rock depend on how it formed. Most rocks contain silicate minerals, which feature silica tetrahedra in their crystal lattice; these make up about one-third of all known mineral species and roughly 95% of Earth’s crust. The silica content in rocks and minerals is a key factor in naming and classifying them.

Rocks are grouped by characteristics such as mineral and chemical composition, permeability, texture, and particle size—properties that stem from their formation processes. Over time, rocks can change from one type to another through a geological model called the rock cycle. This cycle produces three main classes: igneous, sedimentary, and metamorphic. Each class is further divided into many groups, but boundaries between related rocks are not rigid. As mineral proportions shift, rocks gradually transition from one type to another, so the definitions used in rock names simply mark selected points along a continuous series.

Igneous rock comes from the cooling and solidification of magma or lava. Magma can originate from partial melting of pre-existing rocks in a planet’s mantle or crust, typically triggered by increased temperature, decreased pressure, or changes in composition. Igneous rocks fall into two main categories: plutonic (or intrusive) rocks, which form when magma cools slowly within the crust—granite is a common example—and volcanic (or extrusive) rocks, which result when magma reaches the surface as lava or fragmental ejecta, producing minerals like pumice or basalt. As magma rises, it tends to become richer in silica through a process called magma differentiation. This happens because low-silica minerals crystallize out first (as described by Bowen’s reaction series) and because magma assimilates high-silica crustal rock as it ascends. Silica content is therefore the most important chemical criterion for classifying igneous rocks, followed by alkali metal oxide content. About 65% of Earth’s crust by volume is igneous rock, with 66% of that being basalt and gabbro, 16% granite, 17% granodiorite and diorite, 0.6% syenite, and 0.3% ultramafic rock. The oceanic crust is 99% basalt, a mafic igneous rock, while granite and similar granitoids dominate the continental crust.

Sedimentary rocks form at Earth’s surface through the accumulation and cementation of fragments from earlier rocks, minerals, and organic material. These fragments, called sediments, are created by weathering, transport, and deposition of existing rocks. Over time, diagenesis and lithification turn these sediments into solid rock. Metamorphic rocks arise when existing rocks are subjected to high pressures and temperatures, transforming them without significant melting.

Humans have used rocks since the earliest times. The Stone Age saw the development of many stone tools, and stone later became a major building material for structures and early infrastructure. Mining emerged to extract rocks from the Earth and obtain the minerals within them, including metals. Modern technology has led to the creation of new human-made rocks and rock-like substances, such as concrete.