All formed when molten rock (magma) from the mantle cools down and hardens. There are two types depending on where the magma has cooled down:
Intrusive eg. Granite
- Form when magma cools down below the Earth's surface.
- It cools very slowly forming large crystals that give the rocka a coarse texture.
- Large domes of cooled magma form domes of igneous rock called batholiths.
- Where the magma has flowed into gaps in the surrounding rock it forms dykes (in vertical gaps) and sills (in horizontal gaps).
Extrusive eg. Basalt
- Form when lava cools down after it's erupted from a volcano onto the earths surface.
- It cools down very quickly formiing small crystals that give the rocks a fine texture.
Formed when layers of sediment are compacted together until they become solid rock. The process of compaction is called lithification.
- Carboniferous limestone and chalk are formed from calcium carbonate. Layers of tiny shells and skeletons of dead sea creatures are deposited on the sea bed and compacted together over time.
- Clays and shales are made from mud and clay minerals. The particles have been eroded from older rocks, deposited in layers on lake or sea beds then compacted together.
* Sedimentary rocks often contain fossils.
Formed when other rocks (igneous, sedimentary or older metamorphic rocks) are changed by heat and pressure.
- Rocks deep in the Earth are changed by the pressure from the weight of the material above them.
- When tectonic plates collide, rocks are changed by the massive heat and pressure that builds up.
- Magma from the mantle heats the rocks in the crust causing them to change.
The new rocks are harder and more compact eg. Limestone becomes Marble. Clay becomes Slate.
- When it rains water gets into the cracks in the rocks
- At night the water freezes and expands, which puts pressure on the rock
- The water thaws the next day, releasing the pressure, then re freezes the next night
- Repeated freezing and thawing widens the cracks and causes the rock to break up
- Each day the surface layers of rock heat up and expand faster than the inner layers
- At night the surface layers cool down and contract faster than the inner layers
- This creates pressure within the rock and causes thin surfaces to peel off
- Some minerals that make up rock dissolve in water
- The minerals dissolve in rainwater, breaking the rock down
- Rainwater has carbon dioxide dissolved in it, which makes it a weak carbonic acid
- Carbonic acid reacts with rocks that contain calcium carbonate, eg Carboniferous limestone, so the rocks are dissolved by the rainwater
- Plant roots break down rocks by growing into cracks on their surfaces and pushing them apart
- Burrowing animals may loosen small amounts of rock material
- Granite has lots of joints which aren't evenly spread ( they're closer together in some parts )
- Freeze-thaw and chemical weathering wear down the parts of the rock with lots of joints faster because there are more cracks for the water to get into.
- Sections of granite hat have fewer joints are weathered more slowly than the surrounding rock and stick out at the surface forming tors
Granite is also impermeable - it doesn't let water through
This creates moorlands - large areas of waterlogged and acidic soil, with low growing vegetation
Chalk and clay landscapes
- Horizontal layers of chalk and clay are sometimes tilted diagonally by earth movements
- The clay is less resistant than the chalk so is eroded faster
- The chalk is left sticking out forming escarpments/cuestas ( hills ). Where they clay has been eroded it forms vales - wide areas of flat land
- Escarpments have steep slopes ( scarp slope ) on one end and gentle slopes (dip slope ) on the other
Chalk is an aquifer - a permeable rock that stores water
Water flows throughout the chalk and emerges where the chalk meets impermeable rock
Where the water emerges is called a spring line
Areas of chalk can also have dry valleys - valleys that don't have a river or stream flowing in the, because the water is flowing underground
Carboniferous limestone features
- Limestone pavements are flat areas of limestone with blocks separated by weathered-down joints
- Swallow holes are weathered holes in the surface
- Caverns form beneath swallow holes where the limestone has been deeply weathered
- Limestone gorges are steep sided gorges formed when caverns collapse
- Limestone is permeable so limestone areas also have dry valleys and resurgent rivers ( rivers that pop out at the surface when limestone is on top of impermeable rock )
- Water seeping through limestone contains dissolved minerals. When the water drips into a cavern the minerals solidify and build up over time to produce stalactites ( on the ceiling ) and stalagmites ( on the ground )
- When stalagmites and stalactites meet in the middle they form a pillar. When water flows in as a sheet a curtain builds up