Sedimentary Rock

These rocks are formed at the Earth's surface by the hydrological system. These rocks are the most interesting to 'read' because they contain the history of life and ancient environments. While only about 5% of the Earth's outer crust are sedimentary, they make up about 75% of continental rocks.

Types of Sedimentary Rock:

1. Clastic rocks - consists of rock and mineral fragments.
   1. Sand --> sandstone
   2. Silt and clay --> shale or mudstone
   3. Gravel --> conglomerates
2. Chemical rocks and Biochemical - consists of chemical precipitates.
   1. Carbonate sand and mud --> limestone
   2. Iron oxide sediments --> iron formations
   3. Siliceous sediments --> chert
   4. Peat --> coal, oil, gas

The origin of a sedimentary rock involves:

1. Weathering (the breaking down) of pre-existing rocks at the Earth's surface.

   Chemical Weathering water dissolving minerals oxidation dissolving by acids (carbonic and sulfuric)

   Mechanical Weathering changes in temperature frost action plant growth gravity abrasion by moving particles

2. Erosion (removal of weathered materials) and transportation of the weathered material. The causes of erosion are:
   1. wind
   2. running water
   3. waves
   4. glaciers
   5. gravity
3. Deposition (when sediments are laid down in a new location) of the material in the sea or some other sedimentary environment. This usually happens when the transporting agent (wind, water, etc) slows down and loses energy. It takes energy to carry sediment. If there isn't enough energy the sediment settles out and is deposited. How well the sediments are separated by particle size is called sorting.
4. Burying of the sediments and their lithification. Lithification is when sediments turn to hard rock by being:
   1. compacted by the weight of more sediment.
   2. cemented by minerals like calcite, silica and iron oxides. This is the most important lithification process.