Most people have heard the terms Igneous, Metamorphic and Sedimentary but what do they mean and how can you tell them apart?
The earth is made up of rocks that comprise three categories and are all interlinked. These categories are: Igneous, Sedimentary and Metamorphic.
Igneous rocks form from several means but the most well known are those of the volcanic range. Magma builds up and is expelled, cooling and becoming rock. While this is the most well known rock-type within this group, there are other means by which igneous rocks form. Igneous rocks are formed from solidified magma.
Granite is a plutonic igneous rock. This means that it cooled in a magma chamber within the earth. Granite is not a volcanic rock. Most igneous rocks are not volcanic, they form underground (and are known as intrusions).
How do you recognise an igneous rock? Firstly, rocks are made up of minerals and these minerals interact with one another in different ways. If these minerals appear to be crystalline, and can be identified in specific groupings (such as quartz with mica and feldspar, the basic granitic composition), then they are igneous.
Form from processes on the earth’s surface, such as beach formation, river beds, dunes, the sea floor (not including rocks forming near Mid Ocean Ridges etc), glacial.
Examples of sedimentary rocks include sandstone. As the name suggests, it forms from the gathering and compaction of sand over millions of years. Desert sandstone forms when the quartz in the sandstone become oxidised (rusts). As the name implies, this tends to occur in desert conditions.
Limestone is another common sedimentary rock that forms most often in marine conditions. It is made up primarily of calcium carbonate, and often includes fossil remains. An interesting note is that Mount Everest is made up of marine limestone, meaning that at one point, it was under the sea.
This final category of rock forms when igneous or sedimentary rocks are buried deeply, exposed to high temperatures and pressures, causing many of the minerals that make up the rock to become altered.
A prime example of a metamorphic rock is marble. Marble is metamorphosed limestone.
Another example is gneiss – the result of granite (or several other rock types) being subjected to the conditions above. Gneiss in many of its occurrences looks like a courser granite.
How are they connected?
The geological system is a circular one. Igneous rocks form, are weathered and then transported by rivers to create river beds or go out to sea and are deposited as sands and muds. These are then buried and heated to form metamorphic rocks such as pelites and psammites.
Igneous rocks can directly become metamorphic rocks as well, such as granite being altered into Gneiss, as mentioned above.
Metamorphic rocks also weather and become sedimentary rocks. And igneous, metamorphic and sedimentary rocks can be returned to the mantle via subduction zones (areas of the crust/mantle which are destroyed along certain plate boundaries, such as the pacific ring of fire).
All rock types are subject to earth processes and all can be altered several times over in a variety of ways, some of which are mentioned above. It is interesting to pick up a pebble from the beach and wonder at how many of these processes it may have seen.
Source: Holmes’ Principles of Physical Geology by D. Duff