Dating rocks and deep time

Relative dating: older or younger?

Relative dating puts rocks and events in order without giving an age in years. Geologists use a few simple principles, first written down by Nicolas Steno in 1669:

• Superposition: in undisturbed layers of sedimentary rock, the oldest layer is at the bottom and the youngest at the top, because each new layer settles on top of the ones before.
• Original horizontality: sediment is deposited in flat, horizontal layers. If layers are tilted or folded, something happened to them afterwards.
• Cross-cutting relationships: anything that cuts across rock layers, such as a fault or a dyke of igneous rock, must be younger than the layers it cuts.
• Unconformities: an uneven erosion surface between layers shows a gap in time, when rock was being worn away instead of laid down.

Index fossils

How do you match layers in different places, perhaps hundreds of kilometres apart? In the early 1800s the English surveyor William Smith, digging canals, noticed that each rock layer contained its own particular set of fossils, always in the same order. Index fossils are fossils of organisms that lived for only a short time but spread widely, so finding one tells you the rock's age range anywhere in the world. Good index fossils include certain trilobites, ammonites and microscopic plankton. Smith used fossils to make the first geological map of a whole country in 1815.

The fossils in a sequence of layers also tell a story. The oldest rocks contain only simple, single-celled life; younger rocks contain animals with shells, then fish, land plants, reptiles and, only in the youngest rocks, mammals and flowering plants. Life has changed through time.

Absolute dating: age in years

Since the early 1900s, scientists have been able to measure the actual age of many rocks in years, using radioactive elements as natural clocks. This is radiometric dating.

Some atoms, such as uranium-238 and potassium-40, are unstable. Over time they decay into other, stable atoms: uranium-238 eventually becomes lead-206, and potassium-40 becomes argon-40. Each radioactive isotope decays at its own fixed rate, unaffected by temperature, pressure or chemistry. The half-life is the time it takes for half of the radioactive atoms in a sample to decay. When a mineral crystallises, it traps radioactive atoms and starts its clock. By measuring how much of the original (parent) atom is left compared with the decay product (daughter), scientists can calculate how long ago the crystal formed.

Fossils themselves are rarely dated directly. Instead, geologists date volcanic ash or lava layers above and below a fossil-bearing layer. If an ash layer below is 70 million years old and one above is 66 million years old, the fossil is between 70 and 66 million years old. Combined with relative dating, this has given ages to every part of the rock record.

In 1956 the American geochemist Clair Patterson measured lead in meteorites, which formed at the same time as the Earth, and found the Earth is about 4.5 billion years old. Today's best estimate is 4.54 billion years. (Patterson's work also led him to discover that lead from petrol was polluting the whole planet, and he campaigned until leaded petrol was banned.)

The geological time scale

Geologists divide Earth's history into eons, eras and periods, mostly marked by big changes in the fossil record such as mass extinctions. The table shows the start of each period, in millions of years ago (Ma).

To picture deep time, squash the Earth's whole history into one calendar year. The Earth forms on 1 January. The first life (microbes) appears around March. Animals with shells don't appear until about 18 November. Dinosaurs die out on about 26 December. Modern humans appear at about 11.25 pm on 31 December, and all of recorded history fits into about the last 35 seconds.