Rocks

Types of rock

There are 3 types of rock:

Sedimentary
Metamorphic
Igneous

Sedimentary rocks

Sedimentary rocks are made from layers of sediment. They are grainy and crumbly and may contain fossils.
Examples of sedimentary rocks are sandstone and limestone.

Metamorphic rocks

Metamorphic rocks are formed by the effect of extreme pressures and temperatures deep within the Earth.
This alters the structure of the existing rock.
Metamorphic rocks show small crystals in layers (bands).
Metamorphic rocks are hard and smooth.
Examples of metamorphic rocks are slate and marble.

Schists

Schists are formed from basalt (an igneous rock) or shale (a sedimentary rock) that has been pressurised.

Marble

Marble is made from limestone that has been subjected to moderate heat and pressure.

Igneous rocks

Igneous rocks are formed by the solidification of molten material from below the earth's surface.
Extrusive igneous rocks are cooled quickly so small crystals form (e.g. basalt).
Intrusive igneous rocks are cooled more slowly so large crystals form (e.g. granite).

Granite

It was made from molten rock from inside the Earth (actually the mantle). If the magma doesn’t make it out of the crust, but cools down inside the crust it makes granite.
Where the magma intrudes the crust it makes granite intrusions.

Granite Intrusions

A vertical intrusion is called a dyke. If it’s softer than the rest of the rock, it makes a dip called a trench.
If the dyke is harder than the surrounding rock it sticks up leaving a ridge.
If it makes a big, dome-shaped intrusion it is called a batholith.
A sideways intrusion that goes along a bedding plane is called a sill.

Basalt pillars

These pillars were formed when lava cooled on the Earth’s surface.

What is a batholith ?

The top of the granite might be exposed if the rock on top gets eroded away.
Granite is quite hard to erode, so it gets left sticking up.
The top of the batholith isn’t smooth, there are bits that stick up more. These bits are called tors.

What is a tor ?

Granite is a jointed rock (it has small cracks in it). These joints are lines of weakness.
Granite doesn’t get weathered or eroded very easily, but these cracks can be weathered by frost-shattering.
Frost-shattering is when water gets into cracks and then freezes (when it freezes it expands). This forces the joint to get bigger.
A TOR is a rocky peak with expanded joints. Eventually bits break off the tor, fall to the bottom and make a blockfield.

Limestone

Limestone is a sedimentary rock, made by lots of skeletons and shells of sea creatures falling to the bottom of clear tropical seas and getting compacted there.
Made of calcium carbonate (can be dissolved slowly by rain-water or sea water.
Usually jointed – the joints are lines of weakness so they get weathered more.
Quite strong – so it can make steep hills and cliffs without collapsing.

Limestone Features

Swallow Holes
•These happen when a stream dissolves a joint and then flows down it rather than over the ground.
•The hole swallows the stream.

Dry Valleys
•Old river valleys. When there was a lot of water around and the ground was saturated after the last ice age, the rivers flowed over the surface and made these valleys.
•Now the ground isn’t saturated, the water can permeate into the limestone (down swallow holes) and so the valleys are left dry.

Caves
•These happen when the underground stream dissolves an underground hole called a cave.
•Inside caves, the dripping water deposits pure limestone in hanging stalactites and standing stalagmites.

Gorges
•If a cave system (a group of caves) collapses, it leaves a stream in the bottom of a steep-sided narrow gorge.
•Some people think there’s another reason for gorges : powerful rivers of meltwater from the glaciers that melted at the end of the last ice age.

Limestone Pavements
•These are areas where the limestone is exposed. The joints are weathered by solution and this leaves a pattern of dips (enlarged joints called grykes) and flat surfaces (called clints). This looks like a pavement.

Dolines
•These are hollows in the limestone that don’t have a stream going into them. They are formed either by water dissolving the limestone near the surface or by a cave collapsing.

Chalk

It’s a sedimentary rock, a type of soft limestone, it’s made of calcium carbonate.
It was made by lots of shells of sea creatures falling to the bottom of the sea and then getting compacted over the centuries.
Chalk is used in making plaster, putty, cement, mortar and rubber.
Strong types of chalk can be used for building and for blackboard chalk.
Chalk is very porous so it lets water through. This means there aren’t many rivers on chalk but there are dry valleys found on chalk.
Chalk gets eroded mostly by solution (slowly) this is why it tends to stick up above other rocks that have been eroded more quickly.
Chalk doesn’t have joints so it doesn’t make pavements and caves. It tends to make steep cliffs.

Clay

Clay is a fine-grained sedimentary rock. Clay is made from the chemical weathering of other rocks and minerals. It builds up in layers on top of these rocks, or it can be transported by rivers and glaciers and then deposited in lakes and seas.
Clay isn’t strong enough to make steep slopes. It collapses under its own weight. If clay gets wet the water acts like a lubricant and the clay slides downhill, this is called slumping.

Clay is impermeable, this means it doesn’t let water in.
Clay is used for making pots.
It’s also used in paper making, for the tips of spark plugs, in chemical filters and in musical instruments like the ocarina.