Earth Systems

The water cycle describes how water moves through Earth's systems. Water evaporates from oceans, condenses into clouds, falls as precipitation, and flows back to the ocean.

Weathering breaks rocks into smaller pieces through physical processes (like freezing water expanding in cracks) and chemical processes (like acid rain dissolving limestone).

The rock cycle shows how rocks change from one type to another. Igneous rocks form from cooled magma, sedimentary rocks form from compressed sediment, and metamorphic rocks form under heat and pressure.

Sedimentary rocks form in layers, with the oldest layers at the bottom. Geologists use these layers to determine the relative age of rocks and the fossils they contain.

Erosion moves weathered rock and soil from one place to another by water, wind, ice, or gravity. Rivers carve valleys, wind shapes sand dunes, and glaciers grind mountains.

Fossils are preserved remains or traces of ancient organisms. They form when organisms are buried in sediment that hardens into rock over millions of years.

Clouds form when water vapor rises, cools, and condenses on tiny particles in the atmosphere. Different cloud types—cumulus, stratus, cirrus—indicate different weather conditions.

Precipitation is water that falls from clouds to the ground. It comes in several forms—rain, snow, sleet, and hail—depending on the temperature of the air the water passes through as it falls.

Air forced up over mountains cools, and cool air holds less water vapour, so it rains on the windward slope. By the time the air comes down the other side it is dry, which is why deserts often sit just behind mountain ranges.

Soil begins as rock broken down by frost, water and roots, then mixes with the remains of dead plants and animals. It takes centuries to build a few centimetres, which is why soil lost to erosion is not quickly replaced.

A fast river picks up and carries sand, silt and even boulders. When it slows at a bend or a river mouth it drops the heaviest load first, which is how sandbanks, beaches and deltas are built.

Fossils preserve hard parts far more often than soft ones, and only in the right kind of sediment. The record is therefore patchy: an animal missing from the rocks may simply never have been fossilised.

Sediment settles in layers, so in undisturbed rock the deepest layer is the oldest. Reading the order of layers and the fossils inside them is how geologists work out the sequence of events without knowing exact dates.