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Aug 31 2018

Igneous rock composition and texture

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Ask GeoMan.

How do composition and texture relate to igneous rocks?

Igneous rocks are crystalline solids which cool from magma: the liquid phase of solid rock. Magmas occur at depth in the crust, and are said to exist in magma chambers, a rather loose term indicating an area where the temperature is great enough to melt the rock, and the pressure is low enough to allow the material to expand and exist in the liquid state. Many different types of igneous rocks can be produced. The key factors to use in determining which rock you have are the rock’s texture and composition.

Texture relates to how large the individual mineral grains are in the final, solid rock. In most cases, the resulting grain size depends on how quickly the magma cooled. In general, the slower the cooling, the larger the crystals in the final rock. Because of this, we assume that coarse grained igneous rocks are intrusive, in that they cooled at depth in the crust where they were insulated by layers of rock and sediment. Fine grained rocks are called extrusive and are generally produced through volcanic eruptions.

Grain size can vary greatly, from extremely coarse grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineral grains at all. Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic. Granite and gabbro are examples of phaneritic igneous rocks. Fine grained rocks, where the individual grains are too small to see, are called aphanitic. Basalt is an example. The most common glassy rock is obsidian. Obviously, there are innumerable intermediate stages to confuse the issue.

The other factor is composition: the elements in the magma directly affect which minerals are formed when the magma cools. Again, we will describe the extremes, but there are countless intermediate compositions. (Composition relates to the mafic and felsic terms discussed in another question. If these terms are confusing, please refer to that discussion before continuing.)

The composition of igneous magmas is directly related to where the magma is formed. Magmas associated with crustal spreading are generally mafic, and produce basalt if the magma erupts at the surface, or gabbro if the magma never makes it out of the magma chamber. It is important to remember that basalt and gabbro are two different rocks based purely on textural differences – they are compositionally the same.

Intermediate and felsic magmas are associated with crustal compression and subduction. In these areas, mafic seafloor basalt and continental sediments are subducted back into the crust, where they re-melt. This allows the differentiation process to continue, and the resulting magma is enriched in the lighter elements. Intermediate magmas produce diorite (intrusive) and andesite (extrusive). Felsic magmas, the final purified result of the differentiation process, lead to the formation of granite (intrusive) or rhyolite (extrusive).

Click here for a chart summarizing the classification of igneous rocks.

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