sedimentary rocks

sediments

sediments

small pieces of rock that are moved and deposited by water, wind, glaciers, and gravity

chemical weathering

The process in which rock is broken down by changes in its chemical makeup

physical weathering

during physical weathering, minerals remain chemically unchanged. rock fragments break off the solid rock along fractures or grain boundaries.

Erosion

the removal and transport of sediment is call erosion

the four main agents of erosion are wind, moving water, gravity, and glaciers.

after rock fragments and sediments have been weathered out of the rock, they often are transported to new locations through erosion. eroded material is almost always carried downhill.

Deposition

occurs when transported sediments are deposited on the ground or sink to the bottom of a body of water, forming layers with the largest grains of sediment at the bottom.

energy of transporting agents

as moving water slows down, some sediment deposits are sorted into layers of different-sized particles. the largest particles settle out first, then the next largest, and so on.

wind can move only small grains, so sediment deposits made by wind are usually fine and well sorted.

some sediment deposits contain particles of all sizes because they are dumped in unsorted piles when, for example, a glacier melts or there is a landslide.

Lithification

as more sediment is deposited in an area, the bottom layers are subjected to increasing pressure and temperature.

these conditions cause lithification, the physical ad chemical processes that transform sediments into sedimentary rocks

lithification begins with compaction. the weight of overlying sediments forces the sediment grains closer together, causing physical changes.

Cementation

it occurs when dissolved minerals precipitate out of groundwater and their growth glues sediment grains together into a solid rock.

Sedimentary features

Bedding: The primary feature of sedimentary rock is horizontal layering called bedding, which results from the way sediment settles out of water or wind.

Bedding in which the particle sizes become progressively heavier and coarser toward the bottom layers is called graded bedding.

Cross-bedding is formed as inclined layers of sediment are deposited across a horizontal surface.

ripple marks

When sediment is moved into small ridges by wind or wave action or by a river current, ripple marks form.

Angular vs. Rounded: As sediment is transported, pieces that began with an angular shape knock into each other and become rounded as their edges are broken off.

Evidence of past life

Fossils that are preserved, remains, impressions, or any other evidence of once-living organisms.

During lithification, parts of an organism can be replaced by minerals and turned into rock, such as shells or petrified wood that has been mineralized.

clastic sedimentary rock

The most common sedimentary rocks, clastic sedimentary rocks, are formed from the abundant deposits of loose sediments that accumulate on Earth's surface.

Clastic refers to rock and mineral fragments produced by weathering and erosion. These rocks are further classified according to the sizes of their particles.

Clastic sedimentary rocks consisting of gravel-sized rock and mineral fragments are classified as coarse-grained rocks.

Conglomerates have rounded, gravel-sized particles. Breccias are composed of angular, gravel-sized particles.

Sedimentary rocks consisting of silt- and clay-sized particles are called fine-grained clastic rocks. Fine-grained sedimentary rock has low porosity and often forms barriers that hinder the movement of groundwater and oil.

chemical sedimantary

When the concentration of dissolved minerals in a body of water reaches saturation, crystals can precipitate out of solution and settle to the bottom. The resulting layers of chemical sedimentary rocks are called evaporites.

The constant evaporation from a body of salt water results in precipitation of large amounts of salts.

Biochemical sedimentary rocks

Biochemical sedimentary rocks are formed from the remains of once-living organisms. The most abundant of this type of rock is limestone, which is composed primarily of calcite.

After the death of organisms that used calcium carbonate to make their shells, the shells settle to the bottom of the ocean and can form thick layers of carbonate sediment.

During burial and lithification of the carbonate sediment formed from the shells of once-living organisms, calcium carbonate precipitates out of the water, crystallizes between the grains of carbonate sediment, and forms limestone.