Rock Types

Within the three major categories of igneous, sedimentary and metamorphic, geologists recognise many more specific rock types.These are classified according to their appearance and composition, and according to the conditions in which they formed.

Igneous rocks are divided between those which reached the surface and so were extrusive, and those which cooled near but below the earth's surface and so can be considered intrusive.



Basalt's black volcanic rock texture and small crystals can be seen in this photograph.

Copyright Geological Survey of Ireland 2006.

  is a good example of an extrusive igneous rock that formed as lavas surfaced and spread. It is black and has very small crystals (reflecting fast cooling).
Granite is an example of an intrusive igneous rock. It is white or pink-coloured and, reflecting a much slower cooling, may have large crystals. In the early nineteenth century, granite taken from Dalkey hill in south Dublin was used to build the great 'asylum harbour' at Dun Laoghaire

Dun Laoghaire Harbour

This is built with granite rock quarried from Dalkey Hill, Co. Dublin.

Photo courtesy of Arnold Horner 2006.

Also, sedimentary rocks can be classified in more detail. Examples are shales, limestones, and sandstones. Limestones, as already noted, formed in shallow, quiet, warm seas, whereas sandstones formed in desert conditions. Ripple marks

Ripple Marks

Ripple marks are sometimes evident on sedimentary rocks formed in shallow water.

Courtesy of Arnold Horner 2006.

  are sometimes evident on sedimentary rocks formed in shallow water.

Metamorphic rocks can likewise be subdivided, some of the major types being quartzites, slates, schists, and gneisses. The Valentia Island slate quarry flouished during the nineteenth century. Valentia slate was exported to Britain and used to roof the House of Commons in London.


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