Rock types and the Rock Cyle

© Craven & Pendle Geological Society

Geologists' work with three types of rock - Igneous, Sedimentary and Metamorphic.  Lets take them one at a time beginning with igneous rocks.

Igneous Rocks:
As hot magma rises to the surface and cools, the chemical elements within it combine to make-up the minerals which in turn begin to crystallise.  As cooling continues crystals grow.  The slower the rate of cooling the larger the crystals e.g. granite and gabbro. Conversley the faster the cooling the smaller the crystals (so small that frequently a powerful microscope is needed) e.g. basalt and rhyolite.

Field geologists differentiate between igneous rocks formed above and below the surface of the earth.  The rocks formed below ground are commonly referred to as intrusive or plutonic rocks (coarse grained).  The rocks formed at the earth's surface are known as volcanic or extrusive rocks (fine grained lavas).

Sedimentary Rocks:
Sedimentary rocks are formed at the earth's surface by the action of weathering and erosion on pre-existing rocks.  A variety of methods from physical to chemical are used to break down rocks.

Particles vary in size from boulders through gravel, sand, silt and mud.  These particles are frequently subject to transportation and deposition into new environments where they may be compacted to form sedimentary rocks.  Because sedimentary rocks are formed by the accumulation of layers of deposited material, they frequently have a layered appearance.  They are excellent rocks for preserving structures such as ripples and animal / plant remains (fossils).

Metamorphic Rocks:
These kinds of rocks form when pre-existing rocks are subjected to increases in pressure and/or temperature.  The original rock could be igneous or sedimentary.  The kind of changes that take place are as follows:

a)  Recrystallisation to form a coarser grained rock e.g. gneiss

b)  New minerals form overgrowths on the original minerals e.g. slate

c) Foliation or strong banding develops along which the rock splits into sheets, bands or flakes.  Foliation is caused by the orientation of flattish or elongate minerals on a planar surface e.g. schist.

Types of metamorphism
a) Regional metamorphism covers large areas and is related to major tectonic adjustment of the crust characteristic of many mountain chains.

b) Contact or thermal metamorphism is the metamorphism that results from the rise in temperature in the surrounding country rocks near to igneous intrusions.

c) Dynamic metamorphism is local in extent and concentrated along major dislocations of the crust, faults and shear zones.

d) Low pressure / temperature burial metamorphism occurs in the depths of sedimentary basins.

e) Hydrothermal metamorphism involves chemical change as an integral part of the process and is the result of the circulation of hot water through a body of rock along fissures and cracks.