The Geo RoomGeomorphology

Definition of the Day
River Competence
The largest particle that can be carried by a river

Sedimentary Rocks; Formation, Characteristics & Types

Sedimentary rocks are rocks formed from the remains of organic matter or the cementation of rock particles by rock juices or from the precipitation of rock minerals.

The former two formations are most common among sedimentary rocks.

Common Characteristics

Sedimentary rocks usually consists of several minerals from other rocks; thus can inherit some igneous or metamorphic rock characteristics. Clustered sedimentary rocks are good examples. For example, the abundant quartz (from igneous) in sandstones make it a hard sedimentary rock.

In addition, sedimentary rocks can consist of wide range materials such as plants, animal shells, bones, rock particles etc embedded within the rock.

Sedimentary rocks have a poorly sorted arrangement of minerals and particles e.g conglomerates, breccias and arkose.

Sedimentary rocks are soft compared to igneous and metamorphic rocks and weather rather easily. They often form deep eroded landforms such as gorges

Dead Organic Matter

When plants or animals die the dead remains can merge and compress with rock particles lithifying into solid rock. The dead animal matter can be marine organisms such as fish, shells, worms e.t.c. Most sedimentary rocks contain dead fish and shells embedded within the rock. These rock types are usually found along coasts. Examples of organic sedimentary include limestone (coquina) and chalk. The White Cliffs of Dover in UK which are chalk cliffs formed from marine organisms.

Clustered Sedimentary Rocks

These sedimentary rocks are formed when rock debris is glued and cemented by water juices. These juices are actually rock juices dripped out as water percolates within the rock spaces. These rock juices can glue rock particles together upon drying forming a hard and solid sedimentary rock e.g sandstone, conglomerate, arkose, mudstones etc. The juices can also be excreted by plant matter.

Common Examples

Breccia
A sedimentary rock composed of sharp rock shades cemented together.
Conglomerate
A sedimentary rock composed of large pebbles and cobbles cemented together with sand particles or gravel
Sandstone
A hard sedimentary rock formed from cemented quartz grains.
Arkose
A sedimentary rock of cemented feldspar.
Shale
A weak, fine grained sedimentary rock made of cemented clay.
Other sedimentary rocks of clay and silt particles include mudstone anf siltstones.
Breccia and ConglomerateBreccia and Conglomerate
sandstone sample; source Sand atlas

Precipitated Sedimentary Rocks

Some sedimentary rocks doesn't have an organic origin, but rather are formed when minerals or salts laid in water are deposited (left) as the water evaporates. This is usually described as mineral precipitation. The minerals or salts left behind can compact and cement together forming rocks such as rock salt, gypsum or precipitated limestone.

Some sedimentary rock types called jaspers either have organic or inorganic origin or both.



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