Rocks
Rocks are made from minerals. Rocks form in a variety of differing environments and conditions.
There are three differing types of rocks:
- Igneous rocks: From the cooling and crystallisation of molten magma (eg. erupting as a volcano).
- Sedimentary rocks: Either recycled from pre-existing rocks (eg. sand and silt in a river), organic buildup (eg. a coral reef) or chemical precipitation (eg. a salt lake).
- Metamorphic rocks: From recrystallisation arising from high temperatures and pressures deep wihtin the earth.
Methods of classifying and identifying rocks
Geologists have numerous classification schemes for the differing rock types. However, the most basic of principles will enable you to easily get close to the names that seasoned geologists would use.
There are generally two features that define rocks, chemistry and/or texture.
Chemistry (~mineralogy, ~colour)
Chemistry usually relates to the types of minerals within the rock. For example, basalts are mostly olivine, pyroxene and feldspars (refer image). Granites are mostly composed of feldspar and quartz. Granites contain a lot of felsic (light coloured) minerals tend to be light coloured rocks. Basalts contain a lot of mafic minerals so are dark coloured. Therefore colour can often be used as the simplest means to get an idea of what the chemistry of the rock could be.
Mineralogical classification schemes are important for distinguishing many igneous, metamorphic and some sedimentary rocks.
Texture (~grain size)
The texture or physical features of the rock is often as simple as measuring the size of the particles/minerals. Particles/minerals over 1 mm in size are usually considered to be course grained. Conversely if they are smaller than 1 mm they are usually cosidered to be fine grained.
Textural classification schemes are important for distinguishing many igneous and sedimentary rocks. Igneous rocks with large, clearly visible minerals tend to cool below the ground and are likely therefore to be granites. Whereas igneous rocks with small minerals or no visible mienrals are morely likely to have cooled above the ground and are more likely to be basalts
Using rock classification
It is important to be able to identify and classify rocks to gain an understanding of the significance of that rock. This often equates to the environment of formation. For example, basalts come from lava flows and indicate there was a volcano nearby. Limestones indicate the region was once a warm shallow sea. It is often possible to gain a great insight into how the earth formed by looking at one or two rocks. Picking up a rock on a hillside may enable you to envisage ancient seas, giant rivers, volcanoes or violent faults some time in the past.
Some tricks
As mentioned in the previous pages, there are always numerous exceptions in geology and there are numerous tricks. Quartz is commonly white but can be a variety of colours, including black, giving it the appearance of a mafic mineral. Fine-grained basalts may have preserved gas bubbles that are later filled by minerals giving the rock an overall appearance of being coarse-grained.
When attempting to classify a rock, it is important to use basic principles to get one (or several) likely answers to give you a good idea of what the rock is likely to be and how it likely formed. But always keep in the back of your mind that there may be some trick and when you learn more information, for example from the next rock beside it, that you may get an entirely differing understanding.