C2.3 Earth systems · KS3 Science

The Earth and its rocks

The Earth has a thin rocky crust, a hot mantle of rock that flows very slowly, a liquid outer core and a solid inner core, both made of iron and nickel.
Igneous rocks form when magma or lava cools and becomes solid. Slow cooling underground makes large crystals (granite); fast cooling at the surface makes small crystals (basalt). They are hard, with interlocking crystals.
Sedimentary rocks form from layers of sediment. They are often porous, made of grains, and may contain fossils: sandstone, limestone (from shells), chalk and mudstone.
Metamorphic rocks form when existing rocks are changed by heat and pressure without melting: limestone becomes marble, and mudstone becomes slate.

Weathering breaks rocks down where they are: physical (freeze-thaw, when water freezes in cracks, expands and splits the rock), chemical (acid rain) and biological (plant roots).
Erosion and transport carry the pieces away, for example in rivers. Deposition drops the sediment in layers, often at the bottom of the sea.
Compaction and cementation turn the layers into sedimentary rock over millions of years.
Heat and pressure make metamorphic rock; melting makes magma, which cools to form igneous rock. Uplift brings rocks back to the surface.

Water evaporates from the sea and transpires from plant leaves. The water vapour rises, cools and condenses into clouds. It falls as precipitation (rain, snow or hail), then flows back to the sea in rivers or through the ground.
Living things need water for transporting substances, for chemical reactions and for controlling temperature.

Dry air is about 78% nitrogen, 21% oxygen, 1% argon and 0.04% carbon dioxide.
Burning fossil fuels releases carbon dioxide (which causes global warming), sulfur dioxide and nitrogen oxides (which cause acid rain), soot particles (which cause breathing problems) and carbon monoxide (which is toxic).
Pollution can be reduced by using renewable energy and public transport, and by fitting catalytic converters to cars.

Crust: The Earth's thin, rocky outer layer.
Mantle: The hot layer of rock below the crust, which flows very slowly.
Core: The centre of the Earth, made of iron and nickel.
Igneous rock: Rock formed when magma or lava cools and becomes solid.
Sedimentary rock: Rock formed from layers of sediment squashed and cemented together.
Metamorphic rock: Rock formed when existing rock is changed by heat and pressure.
Weathering: The breaking down of rocks where they are.
Erosion: The wearing away and carrying off of rock pieces.
Deposition: When sediment is dropped in layers.
Rock cycle: The ways rocks change from one type to another over time.
Evaporation: Liquid water turning into water vapour.
Condensation: Water vapour cooling and turning into liquid water.
Precipitation: Water falling from clouds as rain, snow or hail.
Acid rain: Rain made acidic by gases such as sulfur dioxide.