Hull Geological Society

Common urban rocks -

(a work in progress)

A glossary of some geological terms

Frost damage - a porous rock can absorb water which freezes in cold weather; when water freezes it expands and can cause the rock to shatter. Porous rocks used in buldings or memorials are prone to frost shattering at their base if the absorb goundwater unless they have a damp course.

Grain size - a measure of the size of the crystals or particles in the rock. This can be measured exactly, but here is a rough unscientific guide -

• very fine grained - cannot be distinguished with the naked eye

• fine grained - can just be distinguished without the aid of a magnifying glass

• medium grained - average

• coarse grained - the crystals look big - for example most granites

• very coarse grained - the crystals look very big - for example Larvikite

Matrix - the background finer-grained grains in the rock, or the rock around the fossils.

Phenocryst (in igneous rocks) - a crystal that is is significantly bigger than the average for the rock

Weathering - the  breakdown of a rock by water, frost, acid rain, plants &c. This can change to colour of the rock; for example iron in the rock can turn a rusty orange colour.

Xenolith (in igneous rocks) - literarily a foreign stone - this is a piece of existing rock that has fallen into molten magma and ends up as part of the igneous rock; these are often a different colour and have been rounded by partial melting.

Copyright - Mike Horne and Hull Geological Society 2020

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