Relative dating

While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.

[2] A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past."

For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.

These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.

[4] The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it.

As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found.

As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.

This is because inclusions can act like "fossils" – trapping and preserving these early melts before they are modified by later igneous processes.

In addition, because they are trapped at high pressures many melt inclusions also provide important information about the contents of volatile elements (such as H2O, CO2, S and Cl) that drive explosive volcanic eruptions.

The study of melt inclusions has been driven more recently by the development of sophisticated chemical analysis techniques.

They occur in most of the crystals found in igneous rocks and are common in the minerals quartz, feldspar, olivine and pyroxene.

The formation of melt inclusions appears to be a normal part of the crystallization of minerals within magmas, and they can be found in both volcanic and plutonic rocks.

For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.

These foreign bodies are picked up as magma or lava flows and are incorporated later to cool in the matrix.

Relative dating is used to determine the order of events on Solar System objects other than Earth; for decades, planetary scientists have used it to decipher the development of bodies in the Solar System, particularly in the vast majority of cases for which we have no surface samples.