Geologists once believed that sudden catastrophes shaped the Earth. By the mid-1800s a different view called uniformitarianism took over, and it was summed up as the present is the key to the past. Because the Earth is so old, rocks that formed in one place may have been altered or carried far from where they started.

To read that record, geologists first need to know what a rock is. They define it as an aggregate of one or more minerals, or a body of undifferentiated mineral matter. What a rock is made of, the shapes of its grains and how the grains fit together are clues to its history. Thin sections are one tool: slices of rock or mineral only 0.03 millimeters thick mounted on glass slides. They are how most optical mineralogy is done today, because transmitted light lets geologists determine and measure properties of minerals that are otherwise not discernible.

Under the microscope, a thin section can settle an argument. In thin section it is often easier to tell igneous, sedimentary and metamorphic rocks apart than in a hand specimen. Other tests are simpler. Hardness is a mineral's resistance to scratching, and luster is how light reflects from its surface, described as metallic, glassy, dull or earthy. Cleavage is a mineral's tendency to break along weak planes, and streak is the color of the powder left when a mineral is scratched on an unglazed porcelain tile.

For routine identification, the common method is powder X-ray diffraction. The sample is ground to a fine powder, and different X-ray peaks correspond to different crystals in the powder. Obsidian shows its story clearly. It forms when lava cools so fast that its atoms cannot arrange into crystals, which makes it a volcanic glass, called a mineraloid rather than a mineral. 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. Obsidian is also chemically unstable, because it is a glass. Over time some of it begins to crystallize, forming radial clusters of white or gray cristobalite crystals, and when cut and polished it is known as snowflake obsidian. So a stone that began as fast-cooled lava can be traced back to its source, and it can also slowly change within.

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 case: a coarse-grained, light-colored rock that forms by the slow crystallization of magma below Earth's surface, mostly from 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.

Basalt underlies more of Earth's surface than any other rock type, including most of the ocean basins. Basalt is a dark, fine-grained rock made mostly of plagioclase and pyroxene minerals, and it usually forms from lava flows. Most of it formed in three settings: oceanic divergent boundaries, oceanic hotspots, and mantle plumes and hotspots beneath continents. It also covers much of the Moon, where much of the surface is underlain by basaltic lava flows. Those areas are called lunar maria.

A new idea about the Earth met doubt at first. In 1912 Alfred Wegener introduced continental drift, arguing that about 200 million years ago the supercontinent Pangaea began to split apart. His idea was not well received at first. Then, 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. The thin layer did not fit what they had expected of an old ocean.

Basalt keeps a magnetic record. 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.

Sedimentary rocks show another kind of history. They form from fragments of older rocks or from the remains of once-living things, and they often show layers. Sediment forms rock in steps: weathering breaks rock into fragments, wind, water, ice or living things carry them away. 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. Most limestone forms on the ocean floor from calcium carbonate and the remains of marine animals with shells.

Metamorphic rocks form when rocks are squeezed and heated deep in the Earth or where tectonic plates meet. They do not melt. Regional metamorphism can change shale into slate, then phyllite, then schist, and finally gneiss. Foliation is the striped look that forms when pressure squeezes flat or elongate minerals so they line up. But limestone does not align no matter how much pressure is applied, because its minerals are not flat. Marble forms mainly at convergent plate boundaries, where large areas of crust get the heat and pressure of regional metamorphism, and it is easy to carve, so it is used for sculptures.

Dating a rock means reading an atomic clock. 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. A half-life is the time for half of a sample's parent atoms to decay. Pairs used for old rocks include uranium-238 to lead-206, with a half-life of 4.5 billion years, and potassium-40 to argon-40, with 1.25 billion years. 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.

Ancient rocks have been dated by several radiometric methods, and the consistent results give scientists confidence that the ages are correct to within a few percent. Even so, Earth's oldest rocks have been recycled and destroyed by plate tectonics, so scientists cannot find the exact age of the Earth directly from Earth rocks. 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. The stone beneath a hiker's boots is, in this sense, a clock that has been running for a very long time.

Words to know

• uniformitarianism: slow-change idea
• magnetite: magnetic mineral
• obsidian: volcanic glass
• isotopes: atom version