Stand at the rim of the Grand Canyon and you are looking down through more than a billion years. Each band of color on the canyon walls was once a beach, a muddy sea floor, a field of sand dunes or a coral flat. Earth science begins with a simple but unsettling discovery: the planet is about 4.5 billion years old, and almost all of that history happened before any human was around to see it. Geologists call this immense span deep time. Learning to read it took about three centuries of patient work.

Layers in order

In 1669 Nicolas Steno, a Danish anatomist working in Florence, set down rules for reading rock layers that geologists still use. The first is superposition: in undisturbed layers, the lowest layer is the oldest, because each one was laid down on top of the one before. The second is original horizontality: sediment settles in flat layers, so layers that are tilted or folded were moved after they formed. A third rule, developed later, deals with things that slice through rock. A feature that cuts across rock layers, such as a fault or a vein of volcanic rock, is younger than the layers it cuts, since the layers had to exist before anything could break through them.

Gaps in the record

In 1788 the Scottish farmer and naturalist James Hutton sailed along the coast to Siccar Point and found grey layers standing almost on end, with gently sloping red sandstone resting on their worn tops. The scene told a long story: the grey layers were laid down flat, buried, tilted upright, raised and worn away, then sunk again and buried under new sediment. A surface like this is called an unconformity. An unconformity is a gap in the rock record, a surface where erosion removed rock or deposition stopped for a long time before new layers formed. Hutton concluded that the world's history had no visible starting point. In his words, "we find no vestige of a beginning, no prospect of an end."

Charles Lyell built on Hutton in Principles of Geology (1830 to 1833). His approach came to be called uniformitarianism: the slow processes we can watch today, such as erosion and deposition, working over vast spans of time, can explain most of the rock record. Modern geologists keep this idea but also accept that rare catastrophes, such as the asteroid that struck about 66 million years ago, leave their own marks.

Fossils as clocks

William Smith, a self-taught English surveyor who dug canals, noticed that each rock layer held its own set of fossils, always in the same order from bottom to top. This became the principle of faunal succession. Because fossils appear in a consistent order, layers holding the same fossils can be matched as the same age even when they lie far apart. In 1815 Smith published the first geological map of a whole country, England and Wales. Its cost ruined him, and he spent time in a debtors' prison before the Geological Society honored him in 1831.

From order to numbers

Steno's rules and Smith's fossils give relative dates. Relative dating puts events in order without giving ages in years; absolute dating assigns an age in years. Absolute dating became possible after Henri Becquerel discovered radioactivity in 1896. Some atoms are unstable: over time a parent atom, such as uranium, decays into a daughter atom, such as lead, at a steady rate that heat and pressure do not change. A half-life is the time it takes for half of the parent atoms in a sample to decay. Measure how much parent and daughter a mineral holds, and you can work out how long it has been ticking.

Different atoms suit different jobs. Uranium-238 has a half-life of about 4.5 billion years, ideal for old rocks. Carbon-14 has a half-life of only about 5,730 years. Carbon-14 decays too fast to date ancient rocks; it works only on things that were once alive and are younger than about 50,000 years. A favorite target for geologists is the tiny mineral zircon. Zircon crystals take in uranium when they form but shut lead out, so any lead found inside them later must have come from decay. That makes zircon an excellent natural clock.

The British geologist Arthur Holmes pioneered radiometric dating in his 1913 book The Age of the Earth. The final answer came from the American chemist Clair Patterson. In 1956 Patterson measured lead isotopes in meteorites and found that Earth is about 4.55 billion years old, a figure later refined only slightly. While fighting stray lead in his laboratory, Patterson discovered lead pollution almost everywhere, and he spent years campaigning against leaded petrol, a reminder that careful measurement can protect human health. The oldest known pieces of Earth itself are zircon grains from the Jack Hills of Western Australia, about 4.4 billion years old.

If all of Earth's history were squeezed into one day, our species would appear only in the last few seconds before midnight.

Worked example: reading the Grand Canyon

At the bottom of the inner gorge lies dark Vishnu Schist, metamorphic rock about 1.7 billion years old, cut by pink veins of granite. By the cross-cutting rule, the granite is younger than the schist. Above the schist runs the Great Unconformity, where hundreds of millions of years of record are missing. Then come flat layers of sandstone, shale and limestone, stacked by superposition from older at the bottom to younger at the top, each with its own fossils: trilobites low down, later reptile tracks higher up. To put numbers on the story, geologists date zircons in volcanic ash beds; a fossil layer sandwiched between two dated ash beds must have an age between them. The river that carved all this is among the youngest features in the scene, cutting downward over the last few million years.