Classification of Igneous rocks.

Josey Bellamy

Classification of Igneous rocks

Igneous Rocks are classified is several ways, and methods of classification have evolved a lot over the past 100 years. Each classification is useful for a certain purpose and reflects a particular way of looking at igneous rocks. Early in the days of geology there were few rocks described and classified. In those days each new rock described by a geologist could have shown characteristics different than the rocks that had already been described, so there was a tendency to give the new and different rock a new name. Because such factors as cooling conditions, chemical composition of the original magma, and weathering effects, there is a potential to see an endless variety of igneous rocks, and thus a classification scheme based solely on the description of the rock would eventually lead to a lot of rock names.

There are various ways that could be used to classify igneous rocks…

Crystal size: - Igneous rocks are formed by the crystallisation of a rock melt or magma. The crystallisation occurs during cooling, as the atoms become organised into crystals. Eventually all the crystals will grow until they meet each other, forming an interlocking three dimensional structure when crystallisation is complete.

Magmas that reach the surface of the Earth in volcanoes cool quickly, forming fine-grained extrusive volcanic rocks. If the rock is cooled extremely rapidly a volcanic glass results, where no crystals have had time to form.

Rocks forming mid- point within the Earth form medium- grained hypabyssal rocks. These are intrusive, that is they are forced into existing rocks.

Crystallisation also takes place very slowly deep within the earth’s crusts, to form coarse-grained plutonic rocks. These are also intrusive.

Coarse=

> 5mm, and easily visible with the naked eye.
They form at plutonic depth- with slow cooling rate.
Batholiths
Limited nuclei-, which grow to bigger crystals- ionic diffusion.
Example- Granite

Medium=

1-5mm- crystals are visible with the naked eye, but you may need ...

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Rocks forming mid- point within the Earth form medium- grained hypabyssal rocks. These are intrusive, that is they are forced into existing rocks.

Crystallisation also takes place very slowly deep within the earth’s crusts, to form coarse-grained plutonic rocks. These are also intrusive.

Coarse=

> 5mm, and easily visible with the naked eye.
They form at plutonic depth- with slow cooling rate.
Batholiths
Limited nuclei-, which grow to bigger crystals- ionic diffusion.
Example- Granite

Medium=

1-5mm- crystals are visible with the naked eye, but you may need a hand lense to identify them.
Medium rate of cooling.
Hypabyssal- dykes and sills are formed.
More nuclei- competing for same ions therefore crystals will be smaller.
Examples- Microgranite, Dolerite.

Fine=

< 1mm- not visible a hand lense is needed/ a microscope.
Volcanic/ hypabyssal.
Rapid cooling.
Also found in chilled margins.
Large amount of nuclei- smaller crystals.
Examples- Basalt, Rhyolite, Andesite.

Glass (Hyaline)=

Have no crystal grain size- cooling is too quick (super cooling) to form.
Volcanic.
Example- Obsidian.

Such rock descriptions are made purely by observation with a hand lens or microscope of the texture and minerals present, and lead into discussions of how the rock was formed. A more precise way of defining a rock is to analyse its chemical composition.

Colour: - Colour of a rock depends on the minerals present and on their grain size. Generally, rocks that contain lots of feldspar and quartz are light coloured, and rocks that contain lots of pyroxenes, olivines, and amphiboles (ferromagnesian minerals) are dark coloured. But colour can be misleading when applied to rocks of the same composition but different grain size. For example granite consists of lots of quartz and feldspar and is generally light coloured. But a rapidly cooled volcanic rock with the same composition as the granite could be entirely glassy and black coloured (i.e. an obsidian). Still we can divide rocks in general into felsic rocks (those with lots of feldspar and quartz) and mafic rocks (those with lots of ferromagnesian minerals). But, this does not allow for a very detailed classification scheme.

Controlled by chemical composition
Looking at % of dark minerals.
Wide variety in colour
Dark grey/ black> light grey/ white

There are 3 measurements of % of dark minerals they are…

Leucocratic- light coloured, 0-30% dark minerals- Example- Granite

Mesocratic- medium coloured, 30-60% dark minerals- Example- Andesite

Melanocratic- dark coloured, > 60% dark minerals- Example- Basalt

We can only use colour if the rock is crystalline, weathering also affects rock colour.

We can divide rock colour into 3 general measurements.

1= Felsic= light coloured- leucocratic, feldspar and Quartz, silica. Major constituents- Granite.

2= Mafic= dark coloured- 60%+- Basalt- melanocratic, Iron and Magnesium rich, with feldspar plus ferromagnesians.

3= Ultra Mafic= Rocks composed almost entirely of ferromagnesians, melanocratic- 90%- Peridotite.

Chemistry composition= Chemical composition of igneous rocks is the most distinguishing feature.

The composition usually reflects the composition of the magma, and thus provides information on the source of the rock.
The chemical composition of the magma determines the minerals that will crystallize and their proportions.
Chemical composition cannot be easily determined in the field, making classification based on chemistry impractical.
The percentage of SiO2 present is vital.

General Chemical Classifications

SiO2 (Silica) Content

> 66 % - Acid, example= Granite

52-66 % - Intermediate, example= Syenite

45-52 % - Basic, example= Gabbro

< 45 % - Ultra basic, example= Peridotite

The table on the next page shows the relationship between the percentage of SiO2 in the rock compared to other elements.

Looking at the results shown you can see that the percentage of silica decreases from acid- ultra basic
Also Iron and Magnesium increase from acid- ultra basic

Mineral content- related to chemical composition.

Rocks are made up of essential minerals and accessory minerals. Essential minerals are those that have to be there to make the rock what it is. Accessory minerals are minerals that are present but don’t make the rock type. Example- Basalt- essential= plagioclase feldspar and pyroxene. Accessory= Olivine.

The chemistry of igneous rocks is reflected in the minerals that they contain.

If magma is over saturated with Silica, such as quartz, should form from the magma, and be present in the rock. On the other hand, if the magma were under saturated with silica, then a silica mineral wouldn’t be present in the magma, and so shouldn’t be present in the rock. The silica saturation concept can be used to divide rocks in silica under saturated, silica saturated, and silica over saturated rocks.

Silica under saturated Rocks - In these rocks we should find minerals that, in general, do not occur with quartz. We can relate this to a mineral contents table of well-known igneous rocks, to prove this theory.

Concluding from this table is that Quartz isn’t present in ultra basic and basic rocks because there is a low content of SiO2 present. Pyroxene and plagioclase feldspar form first- what’s left over forms Quartz. Ultra basic rocks and basic rocks form at very high temps and form quickly. To the lack of timing involved, the amount of SiO2 formed is small- Quartz only formed when there is an excess amount of SiO2.

I conclude that using these 4 main ways of classifying igneous rocks, you are able to identify and understand the make up and composition of Igneous rocks. I have also found that each classification relates to another- in a circle so using these together proof can be made of age of rock, type, and the relationship between the rate of cooling and the crystal grain size.