The youngest rock layers are at the top and the oldest are at the bottom, which is described by the law of superposition.

The oldest layers are on the bottom, and the youngest layers are on the top. Because sediments sometimes include once-living organisms, sedimentary rock often contains a lot of fossils. Fossils are once-living organisms that have been turned into rock, in which the shape or form of the organism can still be seen.

Some cratons in Canada and Greenland shelter the oldest continental rocks in the world, about 4 billion years old. Other cratons in South Africa, Australia and Asia have continental rocks that are 3 billion years old. The oldest seafloor is comparatively very young, approximately 280 million years old.

4 billion years old

Microscopic grains of dead stars are the oldest known material on the planet — older than the moon, Earth and the solar system itself. By examining chemical clues in a meteorite’s mineral dust, researchers have determined the most ancient grains are 7 billion years old — about half as old as the universe.

The location of fossils in rock layers provides evidence of Earth’s past landscapes. It is hard to guess the age of rock. Scientists have to act like detectives, piecing together a mystery to determine how long ago rocks formed. Fossils found in a particular rock layer help scientists determine the age of the rock.

For radiocarbon dating to be possible, the material must once have been part of a living organism. This means that things like stone, metal and pottery cannot usually be directly dated by this means unless there is some organic material embedded or left as a residue.

Radiocarbon Dating Because wood can be dated directly and by radiocarbon, scientists used bristlecone pines to calculate a new calibration curve, and convert radiocarbon results into accurate calendar dates.