GeologyRocks

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Jon
University of Edinburgh

Introduction

Igneous rocks are formed form the cooling of molten rock, magma. They are crystalline, which means 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 tens of thousand of years to cool. They have large crystals, tens of millimetres in size. Extrusive rocks are those which have erupted from volcanoes. They have very small crystals, not visible to the naked eye, as they cooled quickly. Of course there is every grain size possible in between these two extremes.

Chemistry

The chemistry of igneous rocks is quite complicated. It depends on two things; evolution and silica saturation. In this tutorial 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 as it crystallises at a higher temperature than B and C. Liquid is relatively enriched in minerals B and C.
3. Remove mineral B as it crystallises at a higher temperature than C. Liquid is now completely mineral C.

The minerals are removed in order of Bowen's Reaction Series (Figure 1).

Figure 1: Bowen's reaction series

As you can see, if you remove olivine, the magma will become more enriched in pyroxenes etc. This process continues until only quartz is left. This leads us to the following, simple, identification (Table 1).

Identification 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	low high
Amount of Na2O and K2O	low high
Amount of CaO, FeO and MgO	high low

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 a 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:

• Porphyritic texture - large grains (phenocrysts) surrounded by much finer grains (groundamss). 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.

The name given to an igneous rock depends on it's mineralogy (basic, intermediate or acidic) and the grain size. This is sumemrised in Table 2.

Names 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
Fine Grained Name	Basalt	Andesite	Rhyolite
Coarse Grained Name	Gabbro	Diorite	Granite

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).

Figure 2: Active, ropy pahoehoe on the south-southwest flank of Alae in Hawaii Volcanoes National Park. Courtesy of USGS Hawaiian Volcano Observatory

Intrusive bodies form when magma is injected into existing rock layers. A dyke is a body which cuts across the country (host) rock (Figure 2). 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.

Figure 3: Intrusive igneous bodies. Redrawn from Plummer & McGeary (1997)

Some Pictures Of Igneous Rocks

A xenolith (dark patch) in a basaltic sill. Photo by Tim Ivanic.

A close up of some granite. The grains are roughly 2cm in size. The dark grains are biotite. The white grains are quartz and the pink grains are orthoclase feldspar. Photo by Tim Ivanic.

A basaltic rock. This rock is part of a lava flow (pahoehoe). The red colour is due to oxidising of the iron minerals. Photo by Tim Ivanic.

A photo of some pillow lavas. These formed underwater and cooled quite quickly. The shape is due to the surface cooling very quickly, forming a "skin". The next pillow then broke the "skin" forming the next pillow. Photo by Tim Ivanic.

This rock shows a porphyritic texture. The large grains (phenocrysts) are feldspar, which are surrounded by a matrix of quartz, feldspar and mica. Photo by Tim Ivanic.

Conclusion

Igneous rocks form from the cooling of molten rock: magma. They are classified using crystal size and chemistry. The simplest classification type uses three chemical descriptors; basic, intermediate and acidic; and two grain sizes; fine and coarse. The chemistry of magma alters as it cools according to Bowen's Reaction Series. Igneous rocks form geological bodies such as dykes and sills as intrusive structures and volcanoes and lava flows as extrusive features.

References

C. Plummer, D. McGeary, 1997. Physical Geology. Buy It