Hull Geological Society

Common urban rocks -

There are three main types of rocks - Igneous, Sedimentary and Metamorphic.

  Igneous rocks are formed from molten rock (Magma) that cools slowly within the earth (Plutonic rocks) or cools rapidly when erupted from a volcano (Volcanic Rocks). Typically they are made up of crystals of minerals such as quartz, feldspar, mica and olivine. The crystals tend to have distinctive angular shapes. If the rock cooled slowly big crystals form, if it cooled rapidly the crystals are small and you may need to us a magnifying glass to see them. Normally there is no layering in igneous rocks - the crystals are randomly orientated. Granite and basalt are types of igneous rock.

  Sedimentary rocks form in the sea, on the land or in lakes and are made up of rounded mineral grains. They are often layered. They may contain fossils (remains of animals and plants). Sandstone and limestone are types of sedimentary rocks.

  In Metamorphic rocks the minerals of an existing rock have been altered by intense pressure and/or heat. The crystals may have been reorientated into layers and the layers may have  bent and contorted. Some minerals may have grown bigger or new minerals may have grown through chemical reactions. Marble, slate and gneiss (pronounced 'nice') are metamorphic rocks.

   To read more about the Rock Cycle click here 

Copyright - Mike Horne and Hull Geological Society 2020

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