Plate tectonics (Part 3): The rock cycle
Helen Gould (UK)
What is the rock cycle?
Usually, the first thing that budding geologists learn about rocks is that there are three kinds: igneous, sedimentary and metamorphic. These three major kinds are divided up into many different types of rock. For example, marble, slate and metaquartzite are all metamorphic rocks; basalt, granite, obsidian and andesite are all igneous rocks; and limestone, sandstone, clay and siltstone are all sedimentary rocks.
What is a rock made of?
Rocks are made of minerals. Therefore, particular combinations of minerals help us to identify rocks. Minerals are chemical compounds, consisting of chemical elements, which in turn are made of atomic particles.
Who first thought of the rock cycle?
James Hutton was the first geologist to propose a cycle of rock creation and change. In 1785, he gave a talk to the Scottish Geological Society in Edinburgh. In it, he suggested that rocks undergo processes that change them from one type of rock into another. He later developed the idea in his book, ‘Theory of the Earth with Proof and Illustrations’. He thought there was a relationship between the three basic rock types: igneous, sedimentary and metamorphic. We now know this is correct.
However, it was not until this idea was set in a plate tectonics context that it really made sense to geologists. At about the same time, another Scotsman, James Hall, invented experimental geology. He demonstrated the crystallisation of basalt under slow-cooling conditions, and produced marble by heating calcium carbonate under pressure – the first artificial metamorphic rock.
What is the rock cycle’s relationship with plate tectonics?
The theory of plate tectonics has a very special relationship with the rock cycle. When the Earth was very young indeed, it probably had a molten “ocean” of magma instead of a solid crust. This allowed the separation of the various layers of the Earth: the crust, the inner and outer mantle, and the inner and outer core.
The Earth’s current plate tectonic regime allows the eruption of igneous rocks. These interact with the atmosphere through weathering and erosion, forming some types of sedimentary rocks. Others sedimentary rocks are formed by living organisms using inorganic materials around them to make their shells.
Sedimentary and igneous rocks may be subjected to heat and/or pressure, changing these rocks into metamorphic rocks. But metamorphic rocks can also be broken down by weathering, in turn producing sedimentary rocks, or melted to produce more igneous rocks. And so the cycle goes on.
What evidence is there for the existence of a rock cycle?
As the processes are very slow, it took a long time for Earth scientists to realise what was going on. However, the evidence is, in fact, all around us:
- The chemical elements, present in rocks as minerals, determine what rocks are made. For example, to make limestone, calcium and carbon must be present and be able to combine to form calcium carbonate.
- Distorted fossils are sometimes found in metamorphic rocks.
- Certain metamorphic minerals can be used as indicators for temperature and pressure conditions. This is because some can only exist, or exist together, under particular conditions.
- And on the famous journey of the Beagle, Charles Darwin found fossils high up in the Andes. This showed that the rocks had originally formed under the sea and had then been uplifted during mountain building.
These lines of evidence would be used together with other field evidence.
Igneous rocks tend to be hard and hardwearing. The temperatures at which they are erupted, as well as their interlocking crystalline structure, mean that they are less likely to be affected by heat and pressure. Fossils are not usually preserved in them.
Similarly, metamorphic rocks are hard enough to resist heat and pressure. However, even they suffer weathering and erosion, subduction and melting at subduction zones, and the heat and pressure of earth movements. Eventually they are changed into new rocks.
Rocks break down during weathering and erosion to form new sedimentary rocks. These are rocks that are made of fragments of other rocks, called “clastic rocks”. They contain particles of metamorphic, igneous or other sedimentary rocks. Their variability provides powerful evidence for the origin of sedimentary rocks.
Water is the major agent of rock particle transport, both in the sea and on land in rivers. Wind, gravity and glaciers are less important as rock transporters. Therefore, the rock cycle depends on, and interacts with, the water cycle. In effect, the Earth is a giant recycling system.
How can we define these kinds of rocks?
Rocks are defined on the basis of grain size. Fine grain size is invisible to the naked eye and requires a microscope to view it; medium grain size requires a hand lens; and coarse grain size is easily visible to the naked eye. There are other ways of classifying rocks, but this is the easiest to use.
Igneous rocks are the results of molten rock cooling. In the case of obsidian, which is a volcanic glass, it cools too quickly for crystals to form. Basalt has small crystals, so we say it is “fine-grained”. Andesite has larger crystals (“intermediate grain size”), and granite contains large crystals so it is described as “coarse-grained”.
Grain size is an important indicator of cooling history. A granite such as Shap Granite contains large pink feldspars that took a long time to cool and so grew large. However, it also contains small black crystals of biotite mica that did not have long to grow. The crucial thing about igneous rocks is that their crystals form an interlocking structure.
Images by Stuart Handley.
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