Geopix UK - Igneous Processes

Miscellany of Igneous Features

Igneous rocks are involved in a wide range of smaller topics.

Dating

Igneous events are slotted into the relative time scale by using the Law of Cross-Cutting Relations and the Law of Included Fragments. Igneous rocks are given an absolute date by radiometric methods. In this way the whole of the Geological Column can be assigned ages (in millions years).

Jointing

Many igneous masses are crossed by systems of joints which are caused by the stresses during their formation and cooling. Jointing is important in weathering and erosion, including the formation of tors (see below), and in quarrying.

Tors

Tors upstanding tower of unweathered blocks separated by widened joints most commonly found in granite, as for example in Devon and Cornwall. Theories for their formation suggest that differential weathering widens the joints and periglacial 'erosion' removes the weathered material from the joints.

China Clay

Minerals in granite may be altered to clay minerals by reacting with water containing acids and other chemicals. For the granites of Devon and Cornwall, the water probably came from depth, changing the feldspars into kaolin, or china clay.

Building stone

Igneous rocks are widely used for decorative building stone, as any visit to a city centre will show, and for other construction materials such as concrete blocks and roadstone.

Granite is widely because it