Sedimentary Rock

Classification Sedimentary rocks are classified by how they form. There are three basic types of sedimentary rocks: –1. Clastic rocks: formed by fragments of other rocks being glued together. –2. Organic rocks: formed by plant and animal remains. –3. Chemical rocks: formed when a body of water dries up. These are also called evaporates.

Clastic rocks Clastic rocks: the fragments which form clastic rocks occur when large rocks are worn down by weathering or erosion. –Usually, running water carries the particles and wears them down. As the running water slows, the particles are deposited.

Particle size Particles range in size from large (gravel) to small (sand) to microscopic (clay). They are cemented by silica (SiO2), calcite (CaCO3), and rust (FeO). Clastic rocks may be described by the size and shape of their particles.

Descriptions Breccia: large (1 cm or more) angular rocks Conglomerate: same as breccia, but rocks are rounded Sandstone: small (1-2 mm) rocks. Sandstone has no more than 30% air space Shale: microscopic rocks, usually made of clay. Shale is usually soft, layered, and shiny.

Organic rocks Organic rocks: these rocks (not true minerals) contain carbon because they are the remains of ancient plants and animals.

Descriptions Coal: pure carbon, the remains of ancient trees. Bacteria digest the living parts of the tree. If oxygen becomes absent, and the carbon is compressed, it goes through many changes. Peat: compressed dry material- leaves and twigs can still be seen

Lignite: soft brown coal. Plant parts can’t be distinguished. Only 40% carbon Bituminous: soft black coal, 85% carbon Anthracite: hard black coal, 95% or more carbon. Burns hot and clean Calcite: the shells of many mollusks (clams, oysters, snails, etc) and animal bones contain CaCO3. These shells and bones don’t digest or dissolve, but remain. Usually they are crushed by weathering forces.

Fossils Fossils: after an animal or plant dies, the hard parts (shell, bone, or wood) can absorb CaCO3 from the environment. This creates a “mold” of the organism. This mold can be preserved for millions of years. If the organism has no hard parts, it may leave an impression that is preserved.

Chemical Rocks Chemical rocks: when a lake or a river bed or an estuary evaporates, chemicals dissolved in the water remain. These form large crystals if left undisturbed. Stratification: one feature that makes sedimentary rocks almost immediately recognizable is that they form layers or strata. The size of the layers and their arrangement give clues to the way the rocks formed.

Metamorphic Rock Metamorphic rock is a unique rock form that forms from either igneous rock or sedimentary rock. As a rock is subjected to intense heat and pressure, its crystal structure changes. This changes its appearance, its hardness, and other properties.

Types of metamorphism Regional metamorphism: as mountains are built, they cover large areas of crust underneath. These areas of crust contain metamorphic rock. Contact metamorphism: sometimes, magma flows under an existing mountain. This magma will harden, then be crushed under the mountain.

Process When metamorphosis of rock occurs: 1.Pressure squeezes the crystals together. This can cause the previous crystals to form parallel sheets which flake easily. This process of forming sheets of rock is called foliation. Foliation only occurs with regional metamorphism.

2. intense heat can cause a rearrangement of the crystals and can drive out all air space. This creates a more dense crystal that resists breaking. It also increases the hardness of the crystal.

Examples of metamorphic rock Original rockMetamorphic rock SandstoneQuartzite Chalk, limestoneMarble GraniteGneiss ShaleSlate Rhyolite, basaltSchist Coaldiamond

Summary: the Rock Cycle James Hutton (1795) proposed the theory of Uniformitarianism. This theory suggested that the physical geography of the world (mountains, oceans, deserts, and canyons) have always existed somewhere in the world.

Uniformitarianism Uniformitarianism suggests that rock forming takes millions of years and that rocks are constantly forming and re- forming. Before Hutton, science believed that our physical geography came into being suddenly and catastrophically.

The Rock Cycle