Types of rocks in soil science

1.    Igneous rocks (primary or massive rocks) :- these are first formed in the earth crust due to the solidification of molten magma, based on the mode of formation, they are further classified as extrusive and intrusive rocks.

Extrusive or volcanic rocks – these rocks are formed due to the consolidation of magma on the surface of the earth. The magma, when it flows on the earth surface is called lava. E.g.Basalt.

Intrusive rocks or plutonic rocks – these rocks are produced due to magma below the surface of the earth, plutonic – intrusive rocks solidifies at greater depth and hypabassal rocks solidifies at shallow depth from the surface. E.g. granite, syenite, diorite, gabbros etc. rocks formed in vertical cracks are called dykes and in horizontal cracks are called sills.

Vesicular rocks – molten magma cools on the surface, steam of water is entrapped into rocks and forms vesicles.

Based on the silica content,rocks are also classified as

a.     Acid rocks : 65% SIO₂(granite, rhyolite)

b.    Intermediate : 56 to 65% SIO₂

(sub acid rocks 60 to 65% SIO₂(Syenite and trachyte)

(sub basic rocks 56 to 60% SIO₂(dlorite and andesite)

c.     basic rocks : 40 to 55% (gabbro, basalt)

S.No	rocks	origin	Essential mineral	Common minerals	Average specific gravity	Remarks
1.	granite	Plutonic holocrystalline	Quartz (20 to 30%)	Hornvlende, magnetite, mica	2.64	Light coloured whicte or reddish
2.	syenite	Plutonic holocrystalline	Quartz, orthoclase	Hornblende, magnetite, biotite	2.80	Light colored white or reddish
3.	Diorite	Plutonic holocrystalline	quartz	Hornblende, magnetite, biotite	2.85	Darker
4.	gabbro	Plutonic holocrystalline	Labradorite, augite, olivine	Hornblende, ilmenite	3.0	blakish
5.	dolerite	hypabasal	Labradorite, augite, olivine	Hornblende, ilmenite	3.0	Blakish
6.	basalt	Volcanic crystalline with glassy mass	Labradorite, augite, olivine	Hornblende, ilmenite	3.0	-

2.    Sedimentary rocks:- These rocks are formed from the consolidation of sediments accumulates through wind or water action at the surface of the earth. Many are deposited in layer or formed through chemical reactions as precipitates from aqueous solutions. Sediments any contain various size particles cemented together by substances like SIO₂, Fe₂O₃, or lime. These rocks are also called as clastic rocks.

Based on the origin, the sedimentary rocks are classified as:

                                 i.            Residual                                                           :laterite

                              ii.            Transported

a)    Deposited as solids in suspension                  :sandstone, shale

b)   Deposited by chemical precipitation             :limestone, ironstone

c)    Deposited through agency of organic matter: peat, phosphate deposits

Based on the grain size, sedimentary rocks are classified as:

                            i.            Rocks with boulder pebbles sized minerals     :conglomerate

                         ii.            Rocks with sand size particles                              :sandstone

                       iii.            Rocks with silt and size particle                         :siltstone

                       iv.            Rocks with clay size particles                    :shale

Sedimentary rocks

sr. no	rock	Mineral composition	Color and structure
1.	sandstone	Mainly quartz with some CaCO3,IRON OXIDE AND CLAY	Light to red, granular
2.	shale	Clay minerals, quartz and some organic matter	Light to dark thinly laminated
3.	limestone	Mainly calcite with some dolomite, iron oxide, clay, phosphate and organic matter	Light gray to yellow, fine grained and compact

Metamorphic rocks:- these are formed from igneous and sedimentary rocks under the influence of heat, pressure, chemically active liquids and gases. Change any occur in mineral composition or texture or both. The change due to water is called hydro metamorphosis and due to pressure is called dynamo metamorphosis.

Sand stone                              :quartzite

Shale                                        :slate/mica, schist

Lime stone                               :marble

Granite                                     :granite gneiss

Dolerite                                    :hornblende gneiss

Sr. no	rocks	Mineral composition	Color and structure
1.	gneiss	Formed and granite	Alternating light and dark colors, banded and foliated
2.	schist	Formed from basalt or shale	As original rock, foliated
3.	quartzite	Formed from sandstone	Light or brown, compact and uniform texture,
4.	slate	Formed from shale	Grey to black, compact and uniform texture foliated structure
5.	marble	Formed from lime stone	Light red, green, black, compact fine to coarse texture, foliated structure