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Module 19 - Rocks & Weathering

Module 19 - Rocks & Weathering - Revision

Here the key ideas from Module 19. Make sure that you also look at your written notes and the sites on the links page.

Rock and minerals

All rocks are the same aren't they?

No! There are hundreds of different types of rock that have been formed from mixtures of different minerals, and formed in different ways. The appearance, hardness and chemicals in a rock tell us a story about how and where that rock was made.

So, what is a mineral?

A mineral is a single substance. Each mineral has its own unique appearance. Here are some examples:

Quartz (silicon dioxide)

Malachite (copper carbonate)

Iron pyrites (iron sulphide) - often known as "fool's gold"

Olivine (magnesium iron silicate)

Rocks are not just one colour - they are formed from lots of minerals mixed together.

What types of rocks are there?

We divide rocks into three main types. They are:

  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks

Each of these types is formed in a different way.

The Three Rock Types

Igneous rocks are formed from molten material (liquid rock) from inside the Earth. When this material comes out of a volcano as lava, or seeps up into cracks in the Earth's crust, it cools down and becomes solid rock. Examples of igneous rocks are granite and basalt.

Granite

Sedimentary rocks are formed from pieces of other rocks or shells dropping in layers to the bottom of lakes or seas. The pieces build up in layers. Eventually the pressure of the layers building above turns the lower layers into solid sedimentary rock. Examples of sedimentary rocks are sandstone and limestone.

Limestone

Metamorphic rocks are formed when heat (from the Earth's mantle) and pressure (from other rocks above) changes either igneous or sedimentary rocks, baking them into a new type of rock. An example is marble, which is made when limestone is changed.

Marble

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Weathering and erosion

Sedimentary rocks are formed from pieces of other rock on the bottom of lakes or seas. How do these pieces of rock get there? The answer lies in two words: weathering and erosion.

Lots of people (even some teachers!) get confused about the meaning of the words weathering and erosion. Let's sort this out:

Weathering is the breaking up of rock into small pieces. There are two types of weathering: mechanical weathering and chemical weathering.

Erosion is the movement of these small pieces of rock. They may be moved by wind, rainwater, streams and rivers, or even accidentally by humans or vehicles.

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Mechanical Weathering

Freezing water

Rainwater will collect in cracks in rock. If this water freezes, it expands as it forms ice and pushes on the inside of the crack, making the crack wider and helping to break the rock apart.

Hot / cold cycles

When a rock is heated up it expands. When it cools down it contracts (gets smaller). Repeated heating and cooling weakens the rock and eventually breaks it apart. (This is a slower version of what happens if you put a very hot glass object into very cold water - it cracks). This type of waethering is very important in hot deserts, where it is very hot in the day time but cold at night.

Plants

If seeds are blown into cracks in rocks, then plants may start to grow in the crack. The roots of these plants push on the inside of the crack, widening it and helping to break the rock apart.

Wind

Wind blows pieces of sand over rocks, wearing away softer rock. This can lead to strange scenes like this:

Wind has weathered the softer pieces of rock leaving only this strange arch shape.

Sea

The constant beating of the waves eventually breaks off pieces of rock from cliffs. The waves move these pieces of rock backwards and forwards, making them smaller and smaller. Eventually the pieces are so small they are sand.

Glaciers

Heavy sheets of ice break up rock and carry the pieces away.

This is the Fox Glacier in New Zealand. The sheet of ice is constantly moving down the mountain side, breaking off rock as it goes and carrying those pieces down the valley.

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Chemical Weathering

Chemical weathering is caused by chemical reactions wearing away rock.

Rainwater is an acid?

All rainwater is a little acidic because when carbon dioxide from the atmosphere dissolves in rainwater it makes a weak acid called carbonic acid. This weak acid will slowly dissolve some types of rock (e.g. limestone of marble). This effect can lead to fantastic cave formations like this:

This cave was formed as rainwater trickled through cracks in the limestone and dissolved away the rock to form caves. The stalactites (spikes) are formed as the dissolved rock decomposes to reform the substance calcium carbonate.

Acid rain

Polluting gases, like sulphur dioxide and nitrogen oxide dissolve in rainwater to make stronger acids. When this rainwater falls, we get acid rain. This acid attacks many rock types, seriously damaging buildings and monuments.

The Taj Mahal in Agra, India, is one of the world's most beautiful buildings. Acid rain is wearing away at the superb marble carvings on the outside of the building.

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