Welcome to my new series of posts called Rock Box. I will work on these as and when the mood takes me, but my aims are thus; 1) when work commitments prevent me from adventuring out and about, it gives me a chance to do some ‘arm-chair geology’; 2) over the years like many a geogeek I have gathered up a small collection of rock samples…and they could do with organising, so this gives me an excuse; 3) it allows me to explain some basic geological principles to the masses. So let’s dive in to my rock box. My first post will be a basic introduction to the rock cycle and the major rock groups. This is to provide a foundation for the rest of the Rock Box series.
The Rock Cycle
The above diagram is a simplified look at the rock cycle. The rock cycle starts with magma – molten rock. As it cools it solidifies into one of the many types of igneous rocks. From this it can either be eroded away into sediment, or re-heated to form a metamorphic rock. The sediment will eventually be deposited to form a sedimentary rock and this in turn can either be eroded again into sediment or put under intense heat & pressure to form a metamorphic rock. And as you’ll have probably guessed by now, the metamorphic rock will either be eroded into sediment or melted into magma.
Igneous rocks are usually put in to two categories; plutonic and volcanic. In the case of the first, the molten rock moves towards the surface, through pre-existing rock layers, but cools & solidifies before it actually reaches the surface. Under these circumstances the magma usually cools slowly allowing large crystals to form from minerals with a higher melting temperature. As the temperature reduces, minerals with a lower melting point start to solidify around the larger crystals. Examples include granite, gabbro and diorite.
Volcanic rocks occur where the magma has reached the surface (now referred to as lava) and then cooled. Contact with the air or water usually means that these rocks solidify very rapidly, this causes the crystals to be smaller as they don’t have time to grow. In the case of more explosive volcanic eruptions the rock can contain air bubbles (both from the dissolved gases in the magma, and from the air it enters whilst cooling). Basalt, rhyolite, pumice and andesite are examples of volcanic rocks. Igneous rocks where the crystals are large & easily visible are referred to as having a phaneritic texture. Rocks where the crystal are small have a aphanitic texture.
To complicate things even further igneous rocks are also divided by their mineral composition into felsic, intermediate, mafic and ultramafic. Some examples of good places in the UK to look at igneous rocks;
- The Uriconian Hills of Shropshire (yeah I’m a Shropshire lad so you can guess why this is top of the list)
- The Snowdon (Yr Wyddfa) range in Wales
- Cairngorm & Nevis Ranges in Scotland
- Central Lake District
- The Granite domes of Cornwall – Dartmoor, Bodmin Moor & Lands End
- The Isle of Skye (highly recommend)
- The Giants Causeway, N. Ireland
Hay Tor – Granite (Dartmoor, Devon, UK)
Common characteristics of Igneous Rocks are;
- Solid & hard rock
- Dense crystal structure
- Often associated with hills & mountains as they tend to be more resistant to erosion
- Often contain heavy metals & precious stones
Sedimentary rocks are formed by the layered deposition of material, often sourced from other rocks. Sedimentary rocks can be put into several broad categories;
- Organic Sedimentary
- Inorganic Chemical
- Volcaniclastic sediments
Detrital or Clastic rocks are made up of the eroded remains of other rocks cemented together. Those rocks made up of larger grains (in this case pebbles) cemented together are called conglomerates and breccias. As the gains get smaller you come to the sandstones, which sometimes contain small pebbles, but are dominated by sand-sized grains. Finally you reach the mudstones. siltstones and shales which are made up of very small grains (<0.063mm). These rocks are typically the result of a watery depositional environment such as a river, lake or shallow sea. This is however not always the case, and other environments such as hot deserts also leave clastic remains. An example would be the red sandstones from the Permian & Triassic Periods that are common across Europe. The orange/red colour coming from a layer of hematite that coats the grains.
Organic sedimentary rocks are a mixed group, but all have an organic origin. One type is limestone which is made up of calcite. There are a few varieties of limestone from the oolitic with its calcium covered spherical grains to the fossiliferous with its mass of broken fossils. There are also lime-rich muds and chalk which is made up of the calcium rich remains of microbes called coccolithophores. As well as the limestones there are the organic ‘rocks’ we commonly refer to as fossil fuels; coal, oil and mineral gas. OK oil and gas aren’t technically a solid rock, but they have the organic origin and are made from the remains of marine organisms. These are characterised by the high content of hydrocarbons…something we have found very useful in the 20th century.
Inorganic chemical sedimentary rocks are those that are the result of an inorganic process that deposits the rock layer. One of the most common of these are the evaporites, rocks that form when water containing dissolved salts evaporates, leaving the salts behind. Common evaporites are halite, gypsum and anhydrite. Last of all is a cross over of igneous and sedimentary rock.
The volcaniclastic sediments are mainly the result of layers of ash (called tuff), along with pyroclastic flows and occasionally volcanic mud flows called lahars.
Some examples of good places in the UK to look at sedimentary rocks…well most places really, they are really common on the surface;
- For limestone the Wenlock Edge in Shropshire and the cliffs of Lyme Regis have a lot of fossil rich material, the Yorkshire Dales with their limestone pavements and the North & South Downs plus the White Cliffs of Dover with their chalk.
- Coal can be found in several layers in amongst the Carboniferous rocks of the Midlands, Pennines and up towards Leeds, along with the Glasgow area.
- Permo-Triassic rock, including evaporites forms the foundation of much of Cheshire.
- The Padarn Tuff Formation near Bangor in Wales will be the place to go for volcaniclastic rocks.
Bedding planes – Comley Sandstone (Ercall Quarry, Shropshire, UK)
Common characteristics of Sedimentary Rocks are;
- Wide variety of grain sizes depending upon the depositional environment.
- Often porous and form the source of aquifers and hydrocarbon stores.
- Contain fossils, and in some cases are almost entirely made up of fossils.
The last type of rock is metamorphic rock. Metamorphic rocks are formed when heat and/or pressure is applied to a rock. In the earth’s crust & upper mantle the temperature increases at an average rate of 20-30*C per kilometre of depth. A temperature of about 200*C is needed to start metamorphose of rock and is usually reached about 10km depth. Pressure is applied to rocks in two different formats. Lithostatic or confining pressure pushes on the rock from all sides, compressing it fairly evenly. This usually takes place as a result of deep burial. In this case the becomes compressed into a smaller, denser form, but maintains the same basic shape. Directed pressure results when force is applied from a principle plane of direction. This commonly occurs at tectonic plate boundaries, fold mountains and faults. This typically deforms on the plane on which the highest pressure is applied.
Rocks that have been metamorphisied are denser than their parent rock. This is because the pressure pushes the grains together, closing the pores between them. In directed pressure the grains usually line up perpendicular to the principle direction of pressure. This prefered alignment is known as foliation. This often results in metamorphic rocks having a coloured banded pattern. The increased pressure also has another side effect. The pressure releases fluids (such as water) from the rock. This helps facilitate chemical reactions, often resulting in the metamorphic rock having a different chemical composition to its parent rock.
There are several different types of metamorphism, with contact & regional being the most common types.
Contact Metamorphism: This is when a the metamorphism takes place as a result of contact with a heat source. This is common when you have a magma intrusion. The metamorphism decreases with increased distance form the heat source. This type is usually very local. A good place to see this is around the granite outcrops in Devon.
Regional Metamorphism: This is when the metamorphism is over a large region. This has two sub-types; burial and dynamothermal. Burial metamorphism takes place when rocks are overlain by 10km or more of rock. Dynamothermal metamorphism occurs during mountain building when the rocks are deformed and heated, with some of them being pushed upwards to form the mountains and some of it being pushed downwards to form the mountain base, where heat & pressure work on the rock. One of the best places in the world to see regional metamorphism this is in Canada where the Canadian Shield covers about half of the country and used to be at the base of a mountain range.
Hydrothermal Metamorphism: This is when a chemical alteration of the parent rock occurs when it comes into contact with hot water. This is common in volcanic areas with hydrothermal vents.
Fault-zone Metamorphism: Rocks on either side of the fault generate direct pressure on each other, along with frictional heat.
Shock Metamorphism: This occurs where a meteorite hits the ground and the intense heat & pressure causes the metamorphosis.
Pyrometamorphism: Probably the coolest sounding type (yeah I know, not a very scientific thing to say) and is the result of high temperatures but without the pressure. This can happen in such occurrences as lightning strikes and natural underground coal fires.
Good places to see metamorphic rocks in the UK are;
- Contact metamorphic zones around the granite outcrops in Devon & Cornwall
- The slate mine & hills of northern Wales
- By far the best place, and one that I would recommend if you ever get the chance is the north-west Highlands of Scotland.
Lewisian Gneiss Formation – gneiss with granite and dolerite (Laxford Bridge, North-west Highlands, Scotland, UK).
Common characteristics of metamorphic rocks;
- Dense, hard and heavy.
- Multiple coloured banding.
- Traces of parent rock.
I hope this has been of use/interest to people and not just me whittering on. I plan on using it as a foundation of other geologically related items, especially as I go through the laborious task of organising my rock collection. This is a very brief introduction to the rock cycle and the various rock groups. If you want to know more I suggest finding a good geology textbook.
References: Fundamentals of the Physical Environment (3rd Edition) by P. Smithson, K. Addison & K. Atkinson (2002). Geology (2nd Edition)by S. Chernicoff (1999). Geology of Shropshire (2nd Edition) by P. Toghill (2006). The Geology of Britain – An Introduction by P. Toghill (2006). Fossil Revolution – the finds that changed our view of the past by D. Palmer (2003). Sedimentary Petrology (3rd Edition) by Maurice E. Tucker (2001).