Sedimentary Rocks

Earth Science Laboratory SEDIMENTARY ROCKS

Sedimentary rocks originate at the earth's surface. They are created by the geological processes of weathering and erosion, which break, wear down, and change the mineral content of pre-existing rocks (collectively called parent rock Erosion represents processes that transport and deposit sediment through the action of water, wind, and/or glacial ice.

Physical weathering affects mainly the size (reduction) and shape (rounding) of sediments. Parent rock is fragmented into smaller pieces, and eventually the particles are rounded through the action of water, wind, and ice (which transport sediment particles). The sun and extremes of weather break apart rocks by alternate freezing and thawing (rocks expand upon heating and contract upon cooling). Once cracks form, water and plant roots enter the cracks and further break them apart.

Chemical weathering causes changes in the chemical composition of minerals, and is enhanced by warm, moist surface conditions. Depending upon their individual stability, minerals may change their chemical composition to a more stable state at the earth's surface. The silicate minerals in the Bowen's Reaction Series of rock-forming minerals may be used as examples. Recall that olivine crystallized at the highest temperature from a magma, while quartz crystallized at the lowest temperature. At the earth's surface, low temperatures predominate - therefore, quartz is a much more stable mineral than olivine. Generally, the mafic (ferromagnesian) minerals chemically weather to form clay minerals (fine-grained sediments which make up shale), and so do feldspars. Quartz may become finer-grained silt, but still remains quartz.

Sedimentary rocks may be classified into two major groups:

1. Clastic (or detrital): Made up of fragments of pre-existing igneous, sedimentary, or metamorphic rocks, collectively called source or parent rock. Clastics make up about 75% of all sedimentary rocks.

2. Chemical (or non-clastic): Made up of chemical precipitates (minerals which form from a dissolved state, or seashells made by living aquatic creatures).

Sedimentary rocks form when layers of deposited sediments become lithified (are held together by a cement or matrix). Cement is usually silica, calcite, or iron oxide. Matrix material is typically clay or silt. Shales generally require no cement or matrix since clay is "sticky." The characteristic texture of sedimentary rocks is the presence of any of these features: rounded particles, layering (stratification), or fossils - these all indicate surface processes of the earth.

How clastic rocks form: Sediments are transported by water (streams) and wind from source areas, usually in hilly or mountainous terrain, to a sedimentary basin, usually an ocean, lake, or floodplain. How far the sediment is transported depends upon the size and weight of the particles, and the turbulence of the water or wind. When water or wind are no longer able to hold sediment particles in suspension, they are dumped on the ground. Typical examples of clastic rocks: sandstone, shale, conglomerate, breccia.

How chemical sedimentary rocks form: The most common chemical sedimentary rock is limestone, which is made up of calcium carbonate (usually calcite or aragonite). Calcium carbonate sediments generally form in warm, clear, ocean waters. Inorganic carbonates may precipitate as needles of aragonite or as small, spherical grains called oolites. More commonly, limestones are made up of the remains of the shells of sea creatures such as snails, coral, clams, etc., and are called fossiliferous limestones. Dolomite (dolostone) is a limestone which has been altered by groundwater containing magnesium ions; it is composed of both the minerals dolomite and calcite. Other chemical sedimentary rocks may form from oceans or saline lakes which have a high concentration of evaporite minerals such as halite, gypsum, or anhydrite.Coal and peat are chemical rocks made up of organic material, which are not minerals.