Basic Geography/Geology

What is geology? Defined broadly, geology is the study of rocks. Geology has a number of topics: palaeontology (study of fossils), Plate Tectonics (Movement of the plates), Erosion (breaking down of rocks), and a whole number of different topics. The aim is to give you basic knowledge of geology. To know more, try looking round your local library and have a look at the References and External Links.

NOTE: There is a reasonable amount of confusion of how to classify geology: some classify it as a subject on its' own, others classify it as a form of science. I personally class it as a topic of geography.

The basics

It is vital in geology to understand the three ways rocks form:

Sedimentary

Sediments (such as teeth, rocks and sand) settle on the bottom of a riverbed. Over millions of years, these sediments compress together to form sedimentary rock. The sediments may have got to the riverbed in a number of ways: they could be from eroded cliffs, and gradually layers got deposited ('dumped' in one place by the forces of the currents) and compressed over time. Sedimentary rocks are layered into strata, with the newest rocks above the older. If a really strong force (such as movement of the plates) occurs, the layers, or strata, could be ordered differently: for example, the rocks may be older the further along the cliff you walk opposed to the height of the cliff. Different sediments form different sedimentary rocks: for example, sandstone is formed from sand sediments and mudstone is formed from mud sediments. Sedimentary rocks tend to be the 'weakest' of the three, as Igneous and Metamorphic rocks both undergo extreme pressures to form.

Igneous

These rocks are characteristic of their resistance to weathering and their crystals. Igneous rocks are formed in two ways: from molten rock (magma) underground or from lava (from an erupted volcano). Both forms involve lots of heat and pressure. You can tell which way Igneous rock formed by analysing the crystals. The Igneous rocks that have formed underground in the mantle tend to have quite large crystals, because rock takes a long time to cool underground, opposing being spewed out to sea- which cools the molten rock very quickly. Granite is a typical example of Igneous rock that has formed underground: it is very tough, has quite large crystals, and is very resistant to weathering. Dykes and Sills are formed when lava runs through other rocks, giving it characteristic layering.

Metamorphic

These rocks have undergone extreme pressures to form and, like Igneous rocks, are very tough and resistant to weathering. They are formed in a number of ways: a Sedimentary, Igneous or Metamorphic rock could come into contact with molten rock in the mantle, therefore altering its' composition. Metamorphic rocks tends to be common in mountainous regions: mountains form when plates push together, forming a subduction zone where one plate sinks under the other. The forces involved are unimaginable. Marble is a quite valuable example of Metamorphic rock, other rocks and minerals can be seen inside the Marble, as all kinds of rocks are caught in the process of metamorphism.

The Plates

Plate Tectonics is the movement of the Plates. The Continents of the Earth may seem immovable, but they have been moving throughout history and gradually still are. Perhaps the most famous past position of the continents is during the Triassic era, when all the continents were together, forming the Supercontinent Pangea. The past positions of the Plates can help Geologists solve a number of mysteries: like why the White Cliffs of Dover are exactly the same as those off a shore in France. When Plates meet they behave in a number of different ways. They could move apart, collide or move along the fault line that is caused by the meeting. Fault lines are the plate boundaries that rise when the two plates collide, forming jugged, rocky formations, a bit like when cars crash. They also cause Metamorphism, explaining why much of the world's mountains are made mainly of Metamorphic rock. India is currently further colliding with the Asian plate. The immense force of this collision is what formed the Himalayas, which Everest is in. However, the colliding of Plates can also lower land level, because one Plate always sinks in the process of Subduction (explained in Metamorphic section).

Erosion

I like to think of Erosion as the 'Digesting of rocks', because Erosion 'breaks down' rocks, making it an important part of forming Sedimentary rocks and therefore the Rock Cycle. The Sea is a big part of erosion: when the waves lash at the cliffs, the rock may dissolve, or it may break off the cliff, sometimes forming interesting patterns such as caves and arches, where the 'weaker' material is eroded first. Glaciers can erode rocks in the same way as the sea can, cracking rock in narrow gauges and valleys. Much of the valleys we see today were 'calved' by Glaciers. Ice is chemically less dense than liquid water, which is why it is always the top of rivers and lakes which freezes over first. If there is a chip in a rock, drops of rain can find its' way into the rock. If a really cold day follows, the water freezes, therefore expanding, which cracks open the rock. Examples include cracked rock near drainpipes. There was a significant amount of this form of Erosion during the Ice Age. Corrosion, or chemical weathering, is common where there is lots of pollution. Rain is slightly acidic, due to the dissolved carbon that it contains, which over time can roughen the sides of rock and monuments. Industrial chemicals (such as Sulphur) can also dissolve in the rain, further increasing the acidity. The power of river currents is a big part of erosion. The current in the river erodes away the bedrock, and loose rock travels with the river. It may be deposited which over time, may form sedimentary rocks. The erosive power of a river is revealed when it floods. The wind is a major force behind erosion, particularly in deserts where sand is forcefully flung onto rocks. The wind can also speed up other erosive forces (such as the sea). Biological weathering is common on about any old building. The process of biological weathering is similar to the erosion caused by freezing: as the plant grows, it may find itself trapped by rocks (such as in a patio). As the plant grows more and more the crack it is growing in expands further, damaging the rock. Last but not least is onion skin weathering. When the temperature cools, a rock will contract. When the temperature rises, rock expands. If these temperature changes are frequent and severe, the constant expanding and contracting makes the rock weaken and 'peel off' like layers of an onion. Onion Skin is common in Desert climates.

Minerals

Minerals are the elements or compounds we find in rock. Some minerals are worth lots as jewellery, others in industry. Oxygen and Silicon are the most abundant elements in minerals, making up over half the minerals on Earth. The atomic formation in minerals make them have different characteristics. Pollutants and impurities can change some of the characteristics of minerals: hence the wide variety of Quartz colours and the bluish tinge of copper on iron. One characteristic that is useful in telling minerals apart is the hardness scale, devised by Friedrich Mohs. The scale shows resistance to scratching by other minerals.Diamond is the hardest mineral and will scratch all other minerals, without getting scratched itself, Talc is at the other end of the scale, being the weakest mineral.

Fossils

Fossils are the preserved remains of an organism that lived in the past. As sediment settles in the seabed or riverbed, organisms may be trapped in the process, the sediment protecting the body from decay. It is only usually the teeth, shell, or bones of an organism that get preserved, and rarely do any organic remains preserve: they usually get replaced by minerals such as Pyrite. Fossils tell us many clues about Earth's history: such as the past position of the Plates or mass extinctions or disasters. Fossils can tell us how old a rock is. But to be a 'zone' fossil, the organism must be common throughout the globe, but at the same time, have a relatively short period of time on the planet e.g. an organism that lived for over 200 million years would not tell us the age of the rock accurately, unless the organism had many short-lived species.

Quiz

1. Which rock type can be produced when other rocks erode?
2. Which rock type is produced when other rocks melt;
3. and how would the rocks melt;
4. and why is the new rock not metamorphic?

Answers

1. Sedimentary rock
2. Igneous rock
3. In the mantle
4. Because not all Igneous rock is formed from other rock; metamorphic rocks are 'second-hand'.

References

Investigations: Rocks and Minerals, Jack Challoner and Rodney Walshaw

Net-based Rock and Mineral, DK, in association with Google