Cross-section through Earth Rocks are constantly being formed and deformed, worn down and formed again. This is known as the Shell-bearing limestone Sedimentary Rocks Inner core: rock cycle. For rocks to change takes millions of years. Rocks are divided into 3 main types: Solid - As mountains are worn away by · Igneous nickel & iron Conglomerate erosion over millions of years, Outer core: · Sedimentary most of the rocky debris ends up Liquid - in streams and rivers flowing · Metamorphic nickel & iron towards the sea. Eventually, as Mantel: the water slows down - this load Molten rock of sediments settles on the bot- tom of a lake or ocean, or along Crust: Young beach terrace Solid rock along the Skeleton Coast the river course. After further millions of years, pressure from the growing sediment pile com- pacts the lowermost layers and turns them into clastic sedimen- · The Crust makes up less than 1% of the Earth's tary rocks. Chemical sedimen- mass, and consists mainly of oxygen, magnesium, Ripple-marked sandstone tary rocks form by precipitation aluminum, silicon, calcium, sodium, potassium, and of non-organic or organic chemi- iron, which form rocks and minerals. Continental cals, e.g. the skeletons of tiny crust is about 35 km, oceanic crust some 7 km thick. Sedimentary organisms like diatoms. · The Mantle is the solid casing of the Earth´s core Examples of sedimentary rocks Rocks Burial, compacting, Shale containing plant fossil and is about 2900 km thick. It makes up about 70% cementing are sandstone, shale, and lime- of Earth's mass, and consists of silicon, oxygen, stone. Some sedimentary rocks Deep burial, contain fossils of plants or ani- aluminum and iron. heating, mals, which were preserved as · The Core is mainly made of iron and nickel and deformation they were buried under fresh makes up about 30% of Earth's mass. The Outer Sediments layers of sediment before they Core is ca. 2200 km thick and liquid, while the Inner had time to decay. Where there Core is 1300 km thick and solid. are large amounts of organic THE matter contained within the sedi- Garnet-bearing paragneiss Metamorphic mentary rocks, coal, oil, or natu- ROCK ral gas deposits may form from Rocks Calcrete CYCLE Weathering, their carbon content. transportation Pumice breccia Melting Igneous

Migmatite Rocks Granite Magma Metamorphic Rocks and Lava Crystallizing, Metamorphic rocks were cooling originally igneous or sedi- Igneous Rocks mentary, but due to move- Igneous means related to fire or heat. ment of the earth's crust, or When molten rock comes up to the Earth's Granite continued sedimentation and surface, new igneous rock is created. deep burial, have undergone While it is still liquid and inside the Earth´s conditions of extreme heat Trachyandesite crust, it is called magma. Below the sur- and pressure, which have face the magma cools and solidifies slowly Obsidian altered both their appearance to become granite, diorite or gabbro, Basalt and physical properties (from depending on its chemical composition Greek meta = change and Fresh lava flow (intrusive igneous rocks). Basalt morphos = form). However, when the magma reaches the Examples of metamorphic Earth’s surface and flows out during a vol- rocks are marble (originally canic eruption, the viscous liquid is called limestone), schist (originally lava. When lava cools and hardens, it turns shale) and gneiss (high- into obsidian, lava rock (e.g. basalt, rhyo- grade metamorphic rock lite, andesite) or pumice, depending on its derived from either igneous Cross-section through a volcano composition and the nature of the volcanic showing magma “channels” Gabbro or sedimentary precursors ). event, i.e. effusive or explosive (extrusive igneous rocks). Compiled by Geological Survey of Namibia