Geologists once believed that sudden catastrophes shaped the Earth, but by the mid-1800s the view called uniformitarianism had taken over, summed up as the present is the key to the past. Under that view, the slow processes we can observe today, such as wind and rain wearing big rocks into smaller pieces, help explain the rocks of the distant past. The rock cycle, the never-ending change of rocks from one type into another over geologic time, is the grand result, and because the Earth is so old, rocks that formed in one place may have been altered or carried far from where they started.

Even the vocabulary of the subject is less settled than a textbook suggests. Geologists define a rock as an aggregate of one or more minerals, or a body of undifferentiated mineral matter, and a mineral must pass five tests: it forms naturally, so steel is not a mineral, it is inorganic, it is usually solid at room temperature, it has an ordered internal structure that generally forms crystals, and it has a definite chemical composition that may vary only within limits. Yet reference guides still list some natural substances that lack a crystal structure, such as opal and liquid mercury, as minerals even though they do not meet every part of the definition, and as of November 2026 even the question of the most common mineral has two answers, because quartz is the most common individual mineral while the feldspar group as a whole is more prevalent.

The tools that settle such questions are less glamorous than the questions. Thin sections, slices of rock or mineral only 0.03 millimeters thick mounted on glass slides, are how most optical mineralogy is done today, because transmitted light lets geologists determine and measure properties of minerals that are otherwise not discernible, and in thin section it is often easier to tell igneous, sedimentary and metamorphic rocks apart than in a hand specimen. Powder X-ray diffraction is the common method for routine mineral identification: the sample is ground to a fine powder and different X-ray peaks correspond to different crystals in the powder. It is not normally used to work out atomic arrangements, however, because powder data are more ambiguous than single-crystal data, although the Rietveld method sometimes overcomes this.

Carbon shows how much a mineral's structure matters. Diamond is made entirely of carbon, and each atom is joined to four others by strong covalent bonds in a very tight, even pattern, while graphite, the other mineral form of carbon, is extremely soft. Diamond is rated 10 on the Mohs scale and is about four times harder than corundum, which is 9, and its hardness is directional, since it is hardest parallel to its octahedral planes and softest parallel to its cubic planes. It forms at the high temperatures and pressures of Earth's mantle, about 100 miles down, and most diamonds found have been carried up by deep-source volcanic eruptions. Many people think diamonds form from coal, because both contain carbon, but geologists say that is not true; graphite, by contrast, can form when coal seams are metamorphosed and heat destroys the coal's organic molecules, leaving nearly pure carbon.

Igneous rocks record the speed at which melt cooled. Magma rises because it is hotter and lighter than the surrounding rock; if it reaches the surface it becomes lava and cools quickly into fine-grained rock, but if it stays below, slow cooling over thousands or millions of years lets the crystals grow large. Granite is the classic example, a coarse-grained, light-colored igneous rock that forms by the slow crystallization of magma below Earth's surface and is mostly quartz and feldspar with some mica and amphiboles. Half Dome in Yosemite stands nearly 8,800 feet above sea level and is the remains of a magma chamber that cooled slowly thousands of feet underground, and introductory geology textbooks say granite is the most abundant rock in the continental crust.

Basalt is the rock of the ocean floor and of great volcanoes. Basalt is a dark, fine-grained rock that usually forms from lava flows, and it underlies more of Earth's surface than any other rock type, including most of the ocean basins; most of it formed at oceanic divergent boundaries, at oceanic hotspots, and above mantle plumes and hotspots beneath continents. Shield volcanoes are built from thousands of fluid basalt flows that cool as thin sheets, and as of November 2026 Mauna Loa, the largest shield volcano, rises 13,677 feet above sea level, with its top over 28,000 feet above the deep ocean floor. Olympus Mons on Mars is a basalt shield volcano about 375 miles across and 15 miles high, the largest known volcano in the solar system as of November 2026.

Fast cooling gives different rocks, and obsidian shows the extreme case. It forms when lava cools so fast that its atoms cannot arrange into crystals, which makes it a volcanic glass called a mineraloid. Obsidian breaks with a conchoidal fracture into curved surfaces and very sharp edges, which may be why people first used it as a cutting tool. Archaeologists matched the chemistry of obsidian tools to the outcrops they came from, and a laboratory study used X-ray fluorescence to map how obsidian moved across the western United States.

A new idea about the Earth met doubt in 1912. Alfred Wegener introduced continental drift in 1912, arguing that about 200 million years ago the supercontinent Pangaea began to split apart, and his idea was not well received at first. In 1947 scientists on the research ship Atlantis found that the sediment layer on the Atlantic floor was much thinner than expected, since they had believed the oceans were at least 4 billion years old. Basalt keeps a magnetic record of its own: basalt on the ocean floor contains the magnetic mineral magnetite, and magma that cools into volcanic rock locks in the direction of Earth's magnetic field at that moment. The global mid-ocean ridge, a submarine mountain chain more than 50,000 kilometers long, winds around the globe like the seam on a baseball.

Sedimentary rocks are made in steps: weathering breaks rock into fragments, wind, water, ice or living things carry them away, and once enough sediment settles, the lowest layers are compacted so tightly that they form solid rock. Limestone usually forms in clear, calm, warm, shallow seas, so if limestone is found on land, the area was probably once under water, and Tibetan sandstones hold fossils from when the Tethys Sea covered the region, which show their geologic age and the environment and climate in which they formed. Coal forms from plant debris that piles up in oxygen-poor swamp water, where the lack of oxygen keeps the debris from decaying, and about ten feet of that debris becomes only one foot of coal, and the debris builds up very slowly.

Metamorphic rocks are rocks that were squeezed and heated deep in the Earth or where tectonic plates meet, without melting, and metamorphism creates new minerals, either by rearranging a rock's mineral components or by reactions with fluids that enter the rock. Regional metamorphism can change shale into slate, then phyllite, then schist, and finally gneiss, and foliation, the striped look, forms when pressure squeezes flat or elongate minerals so they line up, although rocks made of minerals that are not flat, such as limestone, do not align no matter how much pressure is applied. With enough heat and pressure, limestone, sandstone and bituminous coal can become marble, quartzite and anthracite coal, and even glacier ice qualifies, because it is a rock made of one mineral, ice, which forms mostly by metamorphism as thousands of snowflakes recrystallize into larger, denser crystals.

To put dates on all this, geologists read atomic clocks. Radiometric dating applies where a rock was last melted or disturbed enough to reset its radioactive elements, so the date tells when the rock formed or last changed, and a half-life is the time for half of a sample's parent atoms to decay. The pairs used for old rocks include uranium-238 to lead-206, with a half-life of 4.5 billion years, uranium-235 to lead-207, with 704 million years, potassium-40 to argon-40, with 1.25 billion years, and rubidium-87 to strontium-87, with 48.8 billion years. The method is simple in theory but the lab work is complex, because the main difficulty is measuring very small amounts of isotopes precisely, so two or more methods are used on the same rock where possible, and ancient rocks dated by several methods give consistent results that give scientists confidence that the ages are correct to within a few percent.

The age of the Earth itself cannot be read from Earth rocks, because the oldest rocks have been recycled and destroyed by plate tectonics. The best age, 4.54 billion years with an uncertainty of less than 1 percent, comes not from dating single rocks but from lead isotopes in old lead ores and in iron meteorites, treating Earth and meteorites as one system, and more than 70 meteorites of different types have been dated radiometrically, giving a formation time for the Solar System of between 4.53 and 4.58 billion years ago. The oldest dated Moon rocks are between 4.4 and 4.5 billion years old, though only a small number of rocks were returned by the six Apollo and three Luna missions. Earth's oldest rocks named by the U.S. Geological Survey are the Acasta Gneisses in northwestern Canada, at 4.03 billion years, as of 2007, so as of November 2026 newer finds may differ.

Even the oldest rocks are not a primordial crust: they are lava flows and sediments laid down in shallow water, which shows that Earth history began well before they formed. So the oldest rocks we can hold are not the beginning of the story. The Acasta Gneisses, at 4.03 billion years as of 2007, are only the oldest pages that survive, and the Earth they came from was already old.

Words to know

• uniformitarianism: slow-change idea
• magnetite: magnetic mineral
• foliation: mineral striping
• isotopes: atom versions
• obsidian: volcanic glass