Introduction

Igneous rocks are formed form the cooling of molten rocks. They are crystalline, that is they are made up of crystals joined together. There are many different types of igneous rocks but they fall into two (very) broad categories; intrusive and extrusive. Intrusive rocks are igneous rocks which form at depth. They cool slowly, taking ten's of thousand of years to cool. They have large crystals, ten's of millimetres in size. Extrusive rocks are those which have erupted from volcanoes. They have very small crystals, not visible to the naked eye.

Chemistry

The chemistry of igneous rocks is quite complicated. It depends on two things; evolution and silica saturation. In this chapter we will concern ourselves with the effect of evolution only, the silica saturation will be assumed to be constant.
Igneous rocks evolve as they cool. This process is called differentiation. The mechanism for this process is as follows:

1. Liquid rich in minerals A,B and C
2. Remove mineral A. Rock is relatively enriched in minerals B and C
3. Remove mineral B. Rock is now completely mineral C

The minerals are removed in order of Bowen's Reaction Series. This is shown below (don't worry about the mineral names yet)



As you can see, if you remove olivine, the rock becomes more enriched in pyroxenes etc. This process continues until only quartz is left. This leads us to the following (simple) classification.

Classification of Igneous Rocks

Rock Type	Basic (unevolved)	Intermediate	Acidic (evolved)
Minerals	olivine, pyroxene, amphibole, calcium plagioclase	calcium and sodium plagioclase, with some olivine, pyroxenes and amphiboles	sodium plagioclase, potassium plagioclase and quartz
Amount of silica	lowhigh
Amount of Na2O and K2O	lowhigh
Amount of CaO, FeO and MgO	highlow

Textures & Names

Igneous rocks have many textures which tell us about their cooling histories and/or chemistry.
In general rocks which have cooled rapidly are fine grained, that is with grains which are not visible to the naked eye. Rocks which have cooled slowly have large grains, sometimes as large as several centimetres across. This size variation arises as grains grow around a nucleus of some sort, i.e an minute grain. The slower the cooling the more time grains have to grow and amalgamate. Grains which show their true shape are said to be euhedral. Grains which show no shape are anhedral. Using this information, the order of grain growth can be worked out. For example, a rock may have large euhedral quartz grains, which are surrounded by anhedral feldspar. The quartz grew first as it had space, the feldspar then grew around the quartz.
Other features seen are:

• Porphoritic texture - large grains surrounded by much finer grains. This implies that the large grains grew slowly at depth, the magma with the grains in it, then rose up in the crust, cooling much more quickly forming the fine grains (the matrix).
• Exsolution - occurs within grains on certain minerals (pyroxenes and feldspars). This can give an indication of pressure and hence depth.
• Xenoliths - bits of the rock into which the magma intruded
• Cumulate layer - when a mineral grows which is denser than the magma, it will sink to the base of the chamber causing a cumulate layer. Minerals may form from liquid trapped between the grains - interstitial minerals.

Igneous Rock Formations

Igneous rock bodies are either intrusive or extrusive. Extrusive bodies are lava flows. If these occur under water they form pillow lavas. On land they can form lava tubes, aa (pronounced ah-ah and looks blocky) or pahoehoe (which looks ropey).
Intrusive bodies are shown on the diagram below.
A dyke is a body which cuts across the country (host) rock. A sill is parallel to the bedding layers. The baked margin is an area in the country rock, in contact with the igneous body, which has been thermally metamorphosed. The chilled margin is the area in an igneous body, in contact with the country rock, which cooled quicker than the rest of the rock due to the temperature difference between the magma and the country rock. These features are not always visible. The scale of these bodies is from millimetres to tens or even hundreds of metres.
The largest of igneous bodies is a pluton or batholith. These are massive, hundreds of kilometres in size. The moors of Cornwall and Devon are outcrops of a massive batholith.

Information provided by: http://www.hill.anorak.org.uk