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.
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).
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