Plate tectonics (Part 4): More on the rock cycle

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Helen Gould (UK)

In Plate tectonics (Part 3): The rock cycle, I presented an overview of the relationship between the rock cycle and plate tectonics, and then went on to look more closely at igneous rocks. This time, I want to discuss sedimentary and metamorphic rocks, and review the occurrence of the rock cycle in the Solar System.

Sedimentary rocks

Sedimentary rocks, by contrast with igneous and metamorphic rocks, have no crystalline structure, being made up of little lumps of non-crystalline material derived from weathering other rocks. However, where they have been built up in horizontal layers, they may contain large-scale structures such as bedding.

What are bedding planes?

When sedimentary rocks deposited underwater (for example, limestones), they may be periodically exposed to the atmosphere due to tectonic uplift or a fall in sea level, perhaps because water is locked up on land as ice. The fact that it may take a thousand years to deposit a centimetre’s-worth of limestone places bedding planes into a context of millions of years.

Fig. 1. Garnet micashist.
Fig. 2. Garnet.

Limestones are deposited in shallow seas, forming from the rain of billions of shells of sea animals onto the seafloor. Deposition stops if the area is exposed to the air and restarts when the sea covers it again, so a gap (bedding plane) forms. Many sedimentary rocks are laid down underwater and may contain bedding planes. In addition, the grain size, the fossils that are present and other lithological features all may vary from bed to bed, as conditions change during deposition. The type of deposit (sand, mud or lime) can be related to water depth, helping us work out where ancient oceans were.

What happens to the rocks during a pause in deposition?

The rocks are exposed to the atmosphere and become weathered – the mechanical breakdown of rocks. When covered by water again, deposition restarts, producing the next layer of bedding. Sometimes, much of the rock is worn away before deposition recommences, forming an unconformity. James Hutton was the first geologist to recognise this. A good example is when one layer of rocks have been tilted and eroded (weathered material carried away), and more rocks (perhaps of a different kind) have been laid down horizontally on top (an angular unconformity).

Are there other kinds of sedimentary rocks?

Some sedimentary rocks (for example, sandstones) are sometimes deposited on land. These represent ancient deserts and, perhaps, preserve the pattern of winds blowing across the landscape (cross-bedding), so telling us the direction of the prevailing wind. Some are formed by evaporating seawater, in which case the salts dissolved in the oceans are left behind. These chemically-formed sedimentary rocks (which are also minerals) are known as evaporites. The sea contains various chemical salts, and the first to be deposited following evaporation is the least soluble. Layers of different salts form as evaporation proceeds.

Fig. 3. Gneiss.
Fig. 4. Marble.

Some sedimentary rocks consist almost entirely of sea-animals’ shells and lime mud, while some rocks, such as breccias and conglomerates, are mixtures of large and small particles, such as pebbles in a sandy or muddy matrix. If the larger particles are angular, the rock is a breccia (and may be the result of a flash flood), but if they are rounded, the rock is a conglomerate.

Metamorphic rocks

Like igneous rocks, metamorphic rocks are made of crystals that have grown into an interlocking structure, but in the solid state rather than during cooling after melting. Metamorphic rocks started out as either igneous or sedimentary rocks and were changed by either heat or pressure, or by both. There are two kinds of metamorphic rocks.

Fig. 5. Metaquartzite.
Fig. 6. Mica schist.

(1) Which metamorphic rocks are formed by heat?

Rocks metamorphosed by heat are formed by contact (thermal) metamorphism, where igneous rocks have pushed into the pre-existing rocks in an area. Around this intrusion, the rocks have changed to a high-temperature form, such as hornfels, marble and metaquartzite. The area around an igneous intrusion is called an aureole and may contain bands of different rocks as the heat decreases away from the intrusion.

(2) Which metamorphic rocks are formed by pressure?

As rocks are shifted over millions of years by tectonic activity, pressure builds up in them, and so does heat, leading to recrystallisation. This is regional metamorphism, which occurs during mountain-building (orogeny). Some examples of regionally metamorphosed rocks are phyllite, schist and gneiss, though marble and metaquartzite can also be formed in this way.

What is foliation?

When tectonic activity squeezes rocks, their minerals align in the same direction instead of randomly. As the pressure increases, the minerals separate into light and dark bands. The dark minerals are mafic (iron-related) ones, while the light ones are felsic (for example, feldspar and quartz). As regional metamorphism proceeds, the bands may become “wavy”. These processes are known as foliation – the sign of a regionally metamorphosed rock. Contact metamorphosed rocks are said to be unfoliated.

Could the rock cycle exist without plate tectonics?

If it ever had plate tectonics, Mars’ climate is now too dry and its internal heat has gone. However, it still has rocks and an atmosphere with winds, albeit thinner than the Earth’s and of a different composition. Wind is an agent of erosion, so sedimentary rocks should be able to form there. Some planetary scientists think Mars is still volcanically active. However, without plate tectonics, regionally metamorphosed rocks cannot be formed, although igneous activity below ground causes contact metamorphism.

Fig. 7. Phylilite.
Fig. 8. Slate.

So, although not all stages of the rock cycle can be present, it is theoretically possible some could. The short answer is, until we go there, we won’t know for sure. Igneous rocks occur on the Moon and Venus. Moondust is a sediment, but sediments count as sedimentary rocks (think of gravel). Climatic conditions on Venus keep lavas hot for longer, making long single flows more likely; and our twin planet is known to be still volcanically active. Meanwhile, on the moons of the outer planets, a strange form of volcano-tectonism occurs, based on water and ice. Our Solar System holds many variations on the rock cycle.

Further reading

Introducing Metamorphism, by Ian Sanders, Dunedin Academic Press Ltd, Edinburgh (2018), 148 pages (Paperback), ISBN: 9781780460642.

Introducing Mineralogy, by John Mason, Dunedin Academic Press, Edinburgh (2015), 118 pages (Paperback), ISBN: 978-17-80460-28-4.

Introducing Volcanology: A Guide to hot rocks, by Dougal Jerram, Dunedin Academic Press Ltd, Edinburgh and London (2011), 118 pages (Paperback), ISBN: 978-19-03544-26-6.

Introducing Tectonics, Rock Structures and Mountain Belts, by Graham Park, Dunedin Academic Press, Edinburgh (2012), 132 pages (Paperback), ISBN: 978-19-06716-26-4.

Planetary Geology: An Introduction (2nd edition), by Claudio Vita-Finzi and Dominic Fortes, Dunedin, Edinburgh (2015), 206 pages (Paperback), ISBN: 978-17-80460-15-4.

Rocks and minerals: The definitive visual guide, by Ronald Louis Bonewitz, Dorling Kindersley (2008), 356 pages (hardback), ISBN: 978-14-05328-31-9.

Other articles in this series comprise:
Plate tectonics (Part 1): What are they?
Plate tectonics (Part 2): A closer look
Plate tectonics (Part 3): The rock cycle
Plate tectonics (Part 4): More on the rock cycle
Plate tectonics (Part 5): A simple key to identifying rocks in the field

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