The geological record

Different rocks for different climates

Deposits of some types of rocks and minerals are evidence of particular types of climate. Museum scientist Richard Herrington analyses deposits in different parts of the world and works out what the climate was like when they formed.

Rocks and rainfall

Richard can tell how wet or dry a region once was by looking at its geology.

In the hot, dry conditions of a desert, all the water evaporates from the ground, leaving deposits of minerals behind. For example, layers of salt are commonly found in desert regions.

If a region has been very wet at some point in its history then coal may have formed there.  Coal cannot form in dry environments. 

Evidence of glaciation

Rocks can also be used as a guide to temperature.  Glaciers in particular leave geological clues behind them, which help scientists to work out which areas of land they covered during past ice ages.

As glaciers move across a surface they often pick up rocks and transport them to the end of the glacier, where they are deposited.

If the glacier flows into the sea then the rock could be carried out into the ocean in an iceberg and then dropped to the ocean floor when it melts. These rocks are known as drop stones.

On land, drop stones can be deposited on layers of rock far from where they originally formed. When an isolated coarse-grained rock is found in an area of fine-grained sediment, it is evidence that the rock was brought there by a glacier.

Laterite research in tropical regions

Richard is particularly interested in laterites, which are commonly found In the tropics.  They are rocks that have been heavily weathered.

They can be identified by a high concentration of minerals and metals that don’t dissolve easily, such as gold. Minerals that dissolve more quickly will have been removed by the weathering and will form a layer of soil over the rock. This soil sometimes becomes fossilised, allowing geologists like Herrington to find out where weathering took place in the past.

Rocks only become heavily weathered in warm, wet regions, as warm rain is needed to dissolve most minerals. So, laterites are evidence that a region was once tropical. 

For example, Richard has found laterites in the Ural mountains in Russia. The climate here is now very cold but it must once have been tropical.

Importance of the geological record

The geological record gives scientists important clues about changes in the Earth's climate over very long periods of time.

Other methods of studying temperature, such as examining ice cores, only reveal the climate of the last few hundred thousand years.  The geological record goes much further back in time.