SEDIMENTARY
ROCKS
These are the
secondary rocks that are formed from the deposition and compression of rocks
and mineral fragments. The word “sedimentary” is derived from the Roman word
“Sedimentum” that means “Settling”, as these rocks are formed from grains
broken away from existing rocks by the action of wind, water, ice and other
agents. After this their compaction occurs and rock is formed. Most of the
sedimentary rocks started their journey as loose deposits of sand or gravel at
the bottom of sea or desert, later on this sediment is lithified and compressed
into a rock.
FORMATION
OF SEDIMENTARY ROCKS
As the
sedimentary rocks are formed at the bottom of sea or desert the gravel is
deposited on the bed in form of thin layers called sediments. Due to the
addition of these sediments the water and air between the lower layers is
removed due to the weight of upper added layers. This process is called
compaction. At this stage another process called cementation starts as most of
sediment particles have thin layer of water that act as cement and glues them
in form of a layer. This coordinated action of these two processes transforms
the bed of loose sand and gravel into layer of cohesive Sedimentary Rock.
CLASSIFICATION
OF SEDIMENTARY ROCKS
ON THE BASIS OF
FORMATION
Marine
Sedimentary Rocks
These are the
rocks that are formed in the Oceans near coastal lines by the sedimentation of
sand, gravel and other sediments brought by river water to the Ocean and by
wind action.
Riverine
Sedimentary Rocks
The sediments
deposited in the flood plains or river basins are known as Riverine Sedimentary
Rocks.
Lacustrine
Sedimentary Rocks
Accumulation of
sediments at the basin of a lake and then their convergence to the Sedimentary
rocks
Loess
When wind blows
the desert particles (Sand, Clay or Silt) to far off places and these are
accumulated there and transformed into rocks. Such are called Loess. This
phenomenon occur in North China, North America and West Europe.
Glacial
Sedimentary Rocks
The moving ice
erodes off the earth particles which are deposited at the places where the ice
melts to form Sedimentary Rocks.
ON BASIS OF COMPOSITION
ORGANICALLY
FORMED
These are formed
as a result of decay and decomposition of dead plants and animals and contain
fossils of Flora and Fauna
Calcareous
These are formed
from the remains of the animals such as Corals, Shell fish, etc whose fleshy
part decomposes and remaining hard shell material is deposited at sea bottom
and latter transformed into calcareous sedimentary rocks e.g. Chalk, Limestone,
Dolomite etc.
Carbonaceous
These rocks are
formed after decomposition of the vegetative matter and forests after high
temperature and pressure e.g. Coal, Lignite, Graphite and Peat.
Silicacious
These rocks are
formed from the remains of the animals such as Sponges and Radiolaria and
plants such as Diatoms.
Ferruginous
These are formed
when the minute bacterial organisms produced by decomposing vegetative matter
assist in the precipitation of hydrated iron oxide from water of lakes and
marshes e.g. Iron Stone
INORGANICALLY
FORMED
Chemically
formed Sedimentary Rocks
These rocks are
formed after evaporation of solutions which dissolve different type of minerals
and salts by chemical action and after evaporation these minerals accumulate as
rocks e.g. NaCl, Gypsum, Nitrates and Sulphates.
Mechanically
formed Sedimentary Rocks
These are also
called “Clastic” that is Latin word that means “broken material”. These rocks
are formed by eroded and weathered material.
Arenaceous
These are rich
in sand that makes more than 70% part of these rocks. Sand stones are its most
familiar form, made from sand grains and Quarts fragments derived from Granite.
These vary in color, texture and composition. Many types of sand stones are
used for building purposes e.g. Grit.
Rudaceous
These are
coarser grained elastic rocks made up of mixture of pebbles, gravels, stones,
boulders etc and their examples are Conglomerate, Braccia.
PROPERTIES OF
SEDIMENTARY ROCKS
·
These
rocks are porous
·
Stratified
·
Non-crystalline
·
Rich
in fossils of Flora and Fauna
·
High
erosion rate
·
Softer
than Igneous Rocks
ECONOMIC
IMPORTANCE
·
Coal,
Peat, Anthracite are all used as fuel
·
Petroleum
is also found in the bed of these rocks
·
Provide
raw material for Industry and Construction Purposes
·
Lime
Stone, Gypsum are used in Agriculture for treatment of Saline Sodic soils
·
NaCl
is used in foods