Earth is made of a wide range of solid, liquid and gaseous materials organised into interacting layers and systems. By mass, Earth consists predominantly of iron, oxygen, silicon and magnesium, with smaller amounts of nickel, sulphur, calcium, aluminium, sodium, potassium and other elements. These elements occur in different chemical combinations and physical states depending on depth, temperature and pressure. The solid Earth contains minerals and rocks, while the surface environment also includes oceans, an atmosphere, ice and living organisms. Earth's composition is therefore not uniform. Instead, materials became differentiated during planetary formation as gravitational settling and melting allowed denser substances to move toward the centre while less dense silicate materials became concentrated nearer the surface.
Minerals are naturally occurring solid substances with relatively well defined chemical compositions and ordered internal atomic structures. Most rocks are aggregates of one or more minerals, although some rocks contain volcanic glass or other non crystalline materials. The three major rock groups are igneous, sedimentary and metamorphic. Igneous rocks form from molten material that cools and solidifies. Sedimentary rocks form from accumulated sediments or from chemical or biological processes. Metamorphic rocks form when existing rocks are transformed by heat, pressure or chemically active fluids without completely melting. These rocks participate in the rock cycle, in which geological processes continuously create, destroy and transform Earth materials.
Silicate minerals dominate the crust and mantle because silicon and oxygen combine abundantly with metals such as magnesium, iron, aluminium, calcium, sodium and potassium. Quartz, feldspars, pyroxenes, amphiboles, micas and olivine are important examples of silicate minerals. Their structures contain silicon oxygen units arranged in different ways, producing large differences in physical and chemical properties. Carbonate minerals such as calcite are also important in many sedimentary environments. Metallic elements can occur in minerals that are economically valuable because of their concentrations of iron, copper, aluminium, lead, zinc, gold and other elements. Geological processes can concentrate elements sufficiently for them to become exploitable mineral resources.
Earth's composition varies strongly with depth. The crust is dominated by relatively low density silicate rocks. Beneath it lies the mantle, composed mainly of magnesium and iron rich silicate minerals under enormous pressures and temperatures. The outer core is composed predominantly of iron and nickel rich metallic material and is liquid, while the inner core is predominantly iron rich and solid because pressure raises its melting temperature. This chemical and physical differentiation is the basis of Earth's major internal structure. The differences in composition and physical state affect density, seismic wave speeds, electrical conductivity, heat transport and the behaviour of Earth's magnetic field.
Earth also contains enormous quantities of water. Most of the planet's water is in the oceans, with smaller amounts stored in glaciers, ice sheets, groundwater, lakes, rivers, soil moisture, the atmosphere and living organisms. Water continuously cycles among these reservoirs through evaporation, condensation, precipitation, runoff, infiltration, freezing, melting and biological processes. The oceans contain dissolved salts and gases rather than pure water. Earth's surface environment is therefore a complex mixture of minerals, rocks, water, atmospheric gases, organic matter and living systems. These components interact continuously through the water cycle, carbon cycle, rock cycle and other biogeochemical cycles.
The atmosphere consists mainly of nitrogen and oxygen, with smaller concentrations of argon, carbon dioxide, water vapour and trace gases. Its composition varies with altitude, location and time, particularly for water vapour and aerosols. The atmosphere is not chemically isolated from the solid Earth or oceans. Gases are exchanged between air and seawater, volcanic eruptions add gases and particles to the atmosphere, plants and microorganisms alter gas concentrations, and weathering can consume or release carbon dioxide. The composition of the atmosphere has changed substantially through geological history, and these changes have influenced climate, ocean chemistry and the evolution of life.
Living organisms are also an important component of the Earth system. The biosphere contains organisms whose bodies are made primarily from elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. Organisms interact with rocks, soils, water and atmosphere through processes including photosynthesis, respiration, decomposition, nutrient cycling and mineral formation. Some organisms contribute directly to geological processes, such as reef building, soil formation and sediment production. Microorganisms can influence mineral weathering and chemical transformations. Consequently, Earth's composition cannot be fully understood by studying rocks alone; biological activity has modified Earth's surface and atmosphere for billions of years.
Earth's materials are continuously recycled and redistributed. Weathering breaks rocks down at the surface, erosion transports sediments, burial and lithification create sedimentary rocks, metamorphism transforms existing rocks, and melting can produce magma from which new igneous rocks form. Plate tectonics transports crustal material, while volcanic activity returns material from the interior to the surface. Rivers move dissolved and particulate material to oceans, glaciers reshape landscapes and organisms redistribute elements through biological cycles. Earth is therefore not a static collection of materials but a dynamic system in which matter is conserved while its chemical forms, locations and physical states continually change.
