Igneous Rocks and Rates of Cooling

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Crystal Grain Size

Examining crystals in igneous rocks is one of the quickest ways to identify rocks in the field or from samples.

The rate of cooling is the main factor that controls crystal size. The general rule is the slower the rate of cooling, the larger the crystal size.

However, there are other factor that can be important. Where an intrusion is large, the inside of the intrusion will cool more slowly than the outer part. This is because it is better insulated and will lose heat more slowly.

If a large intrusion cooled at a depth of 1km, it could have a range of crystal grain sizes from fine at the edges to coarse in the centre.

Crystal Grain Size and Depth of Cooling

Glassy

No crystals

Forms very quickly - in hours, usually in the sea as water is the quickest way of cooling magma

At the surface - they are always volcanic extrusive

Fine

Crystals are <1mm so individual crystals cannot be seen by eye

Forms due to rapid cooling over weeks or months

At the surface in extrusive volcanic rocks as lava flows. In chilled margins at the edges of minor intrusions

Medium

1-5mm in size, so individual crystals can be seen by eye but are difficult to identify

fairly slow cooling - over a few thousand years

Below the surface in minor intrusions - 1km down (hypabyssal)

Coarse

Crystals are >5mm, so can be seen and identified in hand specimens

Cools very slowly over millions of years

Found in major intrusions at great depth >10km down as batholiths or plutonic rocks

Glassy Texture

If lava flows into the sea, cooling can be very rapid - in minutes on the outside of the flow. The inside could still be molten. This is why some pillow lavas show a layered structure with glass on the outside and fine crystals inside. Where cooling is rapid, there may be no time for crystals to form. Obsidian forms in this way. It looks black because light will not pass through it, but in thin section is clear glass.

Pegmatites

A pegmatite is an igneous rock that has exceptionally large crystals. The crystals are normally larger than a few centimetres and can be often tens of centimetres or even metres long. Pegmatites form in the last stages of crystallisation from the water-rich residual magma. The pegmatite crystals of quartz, K feldspar and muscovite grow in a supersaturated solution. Pegmatites can produce large crystals in a short period of time.

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