ORIGIN OF EARTH
Friday, June 18, 2010
Sunday, June 13, 2010
ROCKS
ROCKS
Rocks are the materials that form the essential part of the Earth’s solid crust. “Rocks are hard mass of mineral matter comprising one or more rock forming minerals”.
ROCK FORMATION
• Rocks are formed from the molten material known as magma.
• The study of rocks is called Petrology (in Greek, petra
• Petrology deals with the description of rocks; petrogenesis is the study of the origin of rocks
SOIL FORMING ROCKS
Rocks are solid measure, an aggregation of minerals on the basis of their formation rocks are generally grouped into three classes
a) Igneous
b) Sedimentary
c) Metamorphic
1. Igneous rocks (primary or massive rocks)
Igneous rocks (Latin ignis - fire)
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 rocks (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 solidification of 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, Gabbro 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
1. Acid rocks : >66% SiO2 (Granite, Rhyolite)
2. Intermediate : 56 to 65% SiO2
(Sub acid rocks 60 to 66% SiO2 (Syenite and Trachyte))
(Sub acid rocks 60 to 66% SiO2 (Syenite and Trachyte))
(Sub basic rocks 56 to 60 % SiO2 (Diorite and Andesite))
3. Basic rocks : 40 to 55% (Gabbro, basalt)
Igneous rocks
S.No | Rocks | Origin | Essential minerals | Common minerals | Average specific gravity | Remarks |
i. | Granite | Plutonic holocrystalline | Quartz (20 to 30%) | Hornblende, magnetite, mica | 2.64 | Light coloured white or reddish |
ii. | Syenite | Plutonic Holocrystalline | Quartz, orthoclase | Hornblende, magnetite, biotite | 2.80 | Light coloured white or reddish |
iii. | Diorite | Plutonic Holocrystalline | Quartz | Hornblende, magnetite, biotite | 2.85 | Darker |
iv | Gabbro | Plutonic Holocrystalline | Labradorite, augite, olivine | Hornblende, ilmenite | 3.0 | Blakish |
v. | Dolerite | Hypabasal | Labradorite, augite, olivine | Hornblende, ilmenite | 3.0 | Blakish |
vi. | Basalt | Volcanic crystalline with glassy mass | Labradorite, augite, olivine | Hornblende, ilmenite | 3.0 |
Brief description of important igneous rocks
Basalt
This is the most abundantly formed rock from molten material. It is a fine grained and dark coloured rock which contains 50 per cent feldspars and 50 per cent ferromagnesian minerals, including pyroxene and olivine. The coarse-grained rock with the comparable composition is gabbro.
Pumice
It is a very light weight rock which has a lower specific gravity than water. As a result, it floats on water. In composition, it is comparable to granite / rhyolite and in texture, it is like a sponge.
Granite
A coarse-textured and light coloured rock that contains 60 to 70 per cent feldspars, of which orthoclase constitutes 40 to 45 per cent, plagioclase 20 to 25 per cent, ferromagnesian minerals 3 to 10 per cent and quartz 20 to 30 per cent. The fine grained rock with the above composition is rhyolite.
2. Sedimentary rocks
Sedimentary rocks (Latin sedimentum - settling)
These rocks are formed from the consolidation of sediments accumulated 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 may 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
1. Residual : Laterite
2. Transported
a. Deposited as solids in suspension : Sandstone, shale
b. Deposited by chemical precipitation : Limestone, ironstone
c. Deposited through agency of organic matter : Peat, Phosphatic deposits
Based on the grain size, sedimentary rocks are classified as
1. Rocks with boulder pebbles sized minerals (Rudaceous) : Conglomerate
2. Rocks with sand size particles (Arenaceous) : Sandstone
3. Rocks with silt size particles (silt rocks) : Siltstone
4. Rocks with clay size particles (Argillaceous) : Shale
Sedimentary rocks
S.No | Rock | Mineral composition | Colour and structure |
1. | Sandstone | Mainly quartz with some CaCO2, iron oxides 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 oxides, clay, phosphate and organic matter | Light grey to yellow, fine grained and compact |
The sedimentary rocks are formed from sediments, derived from the breaking down of pre-existing rocks. The sediments are transported to new places and deposited in new arrangements and cemented to form secondary rocks. Stratification is the most common feature of these rocks and as such these are also termed as stratified rocks.
Formation
In the formation of sedimentary rocks, four stages are recognized. They are
i. Weathering
The igneous and other rocks disintegrate owing to physical, chemical and biological weathering. The percolating water having carbonic and humic acids decompose the rocks and minerals giving rise to quartz and other secondary minerals and soluble substances like calcium, magnesium, iron and salts.
2KAlSi3 O8 + H2CO3 + H2O 4l2Si2O5 (OH)4 + K2CO3 + 4SiO2
The products of these weathering agencies provide the basic materials (gravels, sand, silt, mud) formation of sedimentary rocks.
ii. Transportation
The disintegrated material is transported by the agencies such as water, wind, glaciers, run off and gravity.
Water : Alluvium
Wind : Loess or Aeolian
Ice : Moraine
Gravity : Colluvial
Sea : Marine
iii. Deposition or sedimentation
The detrital materials, comprising minerals and rock fragments are deposited when the carrying agent has no longer energy enough to move it further. For instance, a stream flowing at a certain velocity possess enough energy to move particles of a certain size. With the decreasing velocity, the carrying capacity also decreases. This is how the coarser particles settle first and the finer particles later. This kind of deposition is called graded bedding.
iv. Diagnosis
It refers to the transformation of unconsolidated sediments to hard rock. It involves compaction and cementation.
Compaction: The weight of the upper incumbent layers – thousands of metre thick – causes compression of the lower layers or deposits. The interstitial water and air are removed by the pressure of overlying sediments. The fine grained deposits under such environments are transformed to clays, shales etc.
Cementation: The most common materials that serve as cementing agents are lime, silica and iron oxide.
The sedimentary rocks are divided into six groups. They are
1. Arenaceous rocks are formed by the deposits of coarse grained particles. (eg.) sandstone, grit, Conglomerate and Breccia.
2. Argillaceous rocks consist of small size particles usually known as clay (eg.) clay, mudstone, shale.
3. Calcareous rocks consist of carbonate of lime or carbonate of lime and magnesia (eg.) timestone, chalk, ferruginous limestone.
4.Carbonaceous rocks are formed from the decomposing vegetation under anaerobic conditions (eg.) peat, lignite, coal, anthracite etc.
5. Siliceous rocks are also of organic origin having been formed from parts of minute plants and animals like diatoms and deposited either from salt or fresh water (eg.) Flint
6.Precipitated salts consists mainly of deposits formed as rock masses either by cooling, evaporation or by direct chemical precipitation
(Eg) Oxides : Haematite, limonite, bauxite, quartz
Carbonates : Limestone, Magnesite
Sulphates : Gypsum
Chlorides : Rock salt
Brief description of important sedimentary rocks
Conglomerate
It is a detrital rock made up of more or less rounded fragments. If the fragments are more angular than rounded, the rock is called breccia.
Sandstone
It is an intermediate between fine grained shale or mudstone and coarse grained conglomerate. The predominant mineral present is quartz; it is called arkose if, quartz and feldspars are predominantly observed.
Shale: Fine grained detrital rock composed of clay and silt sized particles.
Limestone
A rock, chiefly composed of calcite mineral, that has been deposited by an inorganic process. Some may be formed by an organic chemical process.
3. Metamorphic rocks
These are formed from igneous and sedimentary rocks under the influence of heat, pressure, chemically active liquids and gases. Change may occur in mineral composition or texture or both. The changes due to water is called hydrometamorphosis and due to pressure is called dynamometamorphosis
Sand stone : Quartizite
Shale : Slate/mica, schist
Lime stone : Marble
Granite : granite gneiss
Dolerite : Hornblende gneiss
Metamorphic rocks
S.No. | Rock | Mineral composition | Colour and structure |
1. | Gneiss | Formed from granite | Alternating light and dark colours, banded and foliated |
2. | Schist | Formed from basalt or shale | As original rock, foliated |
3. | Quartzite | Formed from sandstone | Light ot brown, compact and uniform texture, foliated structure |
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 |
Pre-existing and their equivalent metamorphosed rocks
Pre-exiting /original rock | Metamorphosed rock |
Conglomerate | Gneiss |
Sandstone | Quartzite – quartz – schist |
Shale | Slate – phyllite – mica schist |
Limestone | Marble |
Dolomite | Dolomite marble – soapstone or serpentine |
Iron ores | Haematite – schist |
Basalt | Schists |
Gabbro | Gneiss – chlorite schist |
Granite and Syenite | Gneiss – mica schist |
Coal | Graphite |
Brief description of important metamorphic rocks
Gneiss: A crystalline rock, with a banded appearance. The light coloured minerals, such as feldspars, quartz and mica roughly alternate with bands of dark coloured ferro - magnesian minerals.
Schist: A finely foliated or laminated rock, composed dominantly of micas and chlorite, together with some quartz and ferro-magnesian minerals. It can be derived from igneous and sedimentary rocks.
Marble: A non-foliated crystalline rock, composed of calcite and dolomite.
Slate: A very finely foliated rock with mineral particles too small to be seen. It splits into thin, smooth sheets. Mica, quartz and chlorite are the chief minerals in it.
Classification of metamorphic rocks (based on texture and structure)
Texture | Rock name | Mineral composition | Parent rock |
Foliated (Parallel structure): Latin foliatus, means leaved or leafy: The rocks that contain micas and ferro- magnesian minerals and show foliation as the minerals are flattened and arranged in parallel layers. | |||
Coarse –grained | Gneiss | Feldspars, quartz, micas, amphiboles | Granite |
Coarse-grained | Schist | Micas and other elongated silicates with minor amounts of quartz (and feldspars) | Gabbro, basalt |
Medium-grained | Phyllite | Micaceous (mica and chlorite) with larger grains representing a transition from schist to slate | Shale |
Very fine grained | Slate | Micaceous minerals (mica, chloirte) with quartz | Shale |
Unfoliated (massive structure) : The rocks containing quartz and feldspars do not show foliation even under pressure because of large-sized crystals. | |||
Fine-grained | Anthracite | Carbon (92-93%) | Peat, lignite, coal |
Talc – schist | - | Talc | |
Amphibolite | - | Hornblende | |
Granular: The rocks consisting of mostly equidimensional grains. | |||
Medium to coarse – grained | Quartzite | Quartz and quartz cement | Sandstone |
Fine – grained | Marble | Calcite, dolomite | Dolomite, Calcite |
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