ROCKS
Overview
- Rocks are naturally occurring solid aggregates of minerals or mineraloids (a mineral-like substance that does not exhibit crystallinity)
- The Earth’s outer solid layer, the lithosphere, is made of rocks
- Rocks are generally classified into three types
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
- The structure and composition of rocks change over time, causing one type of rock to be reclassified as another
- The study of rocks is called petrology
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IGNEOUS ROCKS
Overview
- Igneous rocks are rocks which form from the cooling and solidification of magma
- They are the results of volcanic processes
- The magma can be derived from melts of pre-existing rocks in either the crust or mantle. Typically, rocks melt under conditions of extremely high temperatures, low pressures or changes in composition
- Igneous rocks can be of two types:
- Intrusive (plutonic) rocks
- Extrusive (volcanic) rocks
- Igneous rocks make up about 90% of the Earth’s crust. However, they are hidden from the surface by a thin layer of sedimentary and metamorphic rocks
- Igneous rocks can be seen at mid ocean ridges, areas of volcanism and intra-plate hotspots
- They are crystalline and impervious
- They are resistant to erosion and weathering
Geological significance of igneous rocks
Crystallisation of magma leading to igneous rocks
- Since igneous rocks come from the mantle, the minerals and chemistry of igneous rocks give information about the composition of the mantle
- Their features are characteristic of a particular tectonic environment, allowing reconstitution of tectonic conditions
- They host important mineral deposits such as uranium, tungsten, tin, chromium, platinum
Mineralogical composition of igneous rocks
- Felsic rock: highest content of silicon with predominance of quartz and feldspar. These rocks are usually light coloured and have low density
- Mafic rock: lesser content of silicon, predominance of mafic minerals (manganese and iron). These rocks are usually dark coloured and have higher density than felsic rocks
- Ultramafic rocks: lowest silicon content, with more than 90% of mafic minerals
Felsic Mafic Ultramafic Intrusive Granite Gabbro Peridotite Extrusive Rhyolite Basalt Komatite
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Intrusive igneous rocks (plutonic rocks)
- Intrusive igneous rocks are formed from magma that cools and solidifies within the crust
- These rocks are coarse-grained. Mineral grains in these rocks can be identified by the naked eye
- The central cores of most mountain ranges are made of intrusive rocks (usually granite). These large formations of intrusive rocks are called batholiths
- Examples of intrusive igneous rocks include granite and diorite
Extrusive igneous rocks (volcanic rocks)
- Extrusive igneous rocks are formed at the surface, from magma released into the surface from volcanic eruptions
- Extrusive rocks cool and solidify quicker than intrusive
- Extrusive rocks are fine grained in nature
- Examples of extrusive rocks include basalt and rhyolite
Large Igneous Province (LIP)
The Deccan Traps in the Western Ghats
- Large Igneous Provinces are extremely large accumulations of igneous rocks (both intrusive and extrusive)
- They refer to igneous rocks extending over 100,000 sq km, that formed in a short geological time scale of a few million years or less
- LIPs usually consist of basalt and rhyolite rocks
- When created, LIPs often have an area of few million sq km and volume on the order of a million cubic km. Majority of the LIP’s volume is emplaced in less than a million years.
- LIP’s are postulated to arise from hotspots of linear chains of volcanoes
- LIPs are often linked to mass extinction events. This is said to arise from the enormous quantities of sulphuric acid released into the atmosphere, the subsequent global cooling and absorption of oceanic oxygen.
- The Deccan Traps, one of the largest volcanic features on Earth, is an example of a Large Igneous Province. The Traps consist of multiple layers of basalt, more than 2 km thick and cover an area over 500,000 sq km, and were formed as a result of volcanic eruptions in the Western Ghats about 66 million years ago. It is believed that the enormous volcanic eruptions led to global cooling of around 2C, and were instrumental in the mass extinction of non-avian dinosaurs.
SEDIMENTARY ROCKS
Overview
- Sedimentary rock is the type of rock formed sedimentation of material. This sedimentation can occur on the Earth’s surface or within bodies of water
- Sedimentary rocks form the thin outermost layer of the earth’s crust, making up about 5% of the total volume of the crust
- Sedimentary rocks are deposited in strata called bedding
- Coal is a sedimentary rock
- Examples of sedimentary rocks include shale, sandstone, limestone
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Geological significance of sedimentary rocks
Sedimentary rocks are the only rocks that contain fossils
- Study of sedimentary rocks provides information about subsurface, which is important in civil engineering for construction of roads, bridges etc
- Sedimentary rocks are also important sources of natural resources like fossil fuels, water, ores etc
- The study of sedimentary rock strata serves as the main source of scientific knowledge about the Earth’s geological history
- Sedimentary rocks are the only rocks that contain fossils.Sedimentary rocks contains fossils because, unlike igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossils
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Composition of sedimentary rocks
- Most sedimentary rocks contain either quartz or calcite
- Unlike igneous and metamorphic rocks, sedimentary rocks do not contain multiple major minerals
- Carbonate rocks contain carbonate minerals like calcite, aragonite or dolomite
- Siliclastic rocks contain silica-bearing minerals like quartz
Clastic sedimentary rocks
- Clastic rocks are composed of fragments, called clasts, of pre-existing rocks
- Clastic sedimentary rocks are those that are formed from rocks that have been broken down due to weathering, which are then transported and deposited elsewhere
- Clastic sedimentary rocks come in various grain sizes. They range from fine clay in shales, to sand in sandstone and gravel, cobbles and boulder size fragments in conglomerates and breccias
- Conglomerates are clastic sedimentary rocks with rounded fragments, while breccias consist of clasts with angular fragments. Both conglomerates and breccias contain clasts larger than sand (> 2 mm)
- Examples include shale, sandstone, siltstone
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Organic sedimentary rocks
- Organic sedimentary rocks contain materials generated by living organisms
- They usually contain carbonate minerals generated by these organisms
- Examples include corals, chalk, coal and oil shale
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Chemical sedimentary rocks
- Chemical sedimentary rocks are formed from minerals in solution that become oversaturated
- They usually occur as a result of evaporation
- Examples include limestone, barite, gypsum
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METAMORPHIC ROCKS
Overview
- Metamorphic rocks form as a result of transformation of an existing rock, in a process called metamorphism. The existing rock is called protolith
- Metamorphic rocks are formed when the protoliths are subject to extreme temperatures and pressures
- They form from tectonic process, intrusion of magma, or simply by being deep beneath the earth’s surface (being subject to high temperatures and pressures of rock layers above)
- Much of the lower continental crust is metamorphic
- Examples of metamorphic rocks include gneiss, slate, marble
Composition of metamorphic rocks
- Metamorphic rocks are composed of metamorphic minerals
- Metamorphic minerals are those that form only at high temperatures and pressures. These include sillimanite, kyanite, andalusite, staurolite and garnet (all of which are silicates)
- Metamorphic rocks also contain smaller amounts of micas, feldspars and quartz. However, these are not products of metamorphism, and are instead leftovers from the protoliths
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Contact metamorphic rocks
- Contact metamorphic rocks are those that form when magma is injected into surrounding rock
- The cooling magma leads to igneous rocks, and around this is a zone called contact metamorphism aureole where metamorphic rocks are formed
- The extreme temperatures cause sandstones to metamorphise into quartz, limestone into marble and shale into cordierite
- Igneous rocks are harder to transform than sedimentary rocks since they form at even greater temperatures
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Regional metamorphic rocks
- Regional metamorphic rocks are those that form due to metamorphism over a wide area
- Regional metamorphism tends to make rocks foliated
- Regional metamorphic rocks tend to form at great depths simply under the temperature and pressures of upper layers of rock
- Continental crusts are examples of regional metamorphic rocks
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IMPORTANT ROCK TYPES
| Rock | Classification | Composition | Notes |
| Basalt | Igneous – extrusive volcanic | Feldspar, pyroxene | Present on moon, Mars, Venus Basalt rocks sustain microbial life Fine texture |
| Granite | Igneous (intrusive, felsic) | Quartz, feldspar | Coarse texture Massive, hard and tough Exhibit radioactivity (uranium) |
| Shale | Sedimentary (clastic) | Clay | Contain organic matter Contains multiple thin layers |
| Limestone | Sedimentary | Calcite (calcium carbonate) | Used in quicklime, mortar, cement, concrete Soluble in water Host of most cave systems |
| Sandstone | Sedimentary | Quartz, feldspar | Common building material Porous, allows water percolation Host of water aquifers and petroleum reservoirs |
| Slate | Metamorphic | Clay, volcanic ash | Used to make roofing, flooring It is an electrical insulator, used for switchboards Can host even microscopic amounts of fossils |
| Gneiss | Metamorphic | Garnet, biotite | |
| Marble | Metamorphic | Calcite (calcium carbonate) | Comes from metamorphism of limestone Pure white marble comes from pure limestone Colours, swirls, veins come from mineral impurities Important source of calcium carbonate, used in toothpaste, paint |
| Quartzite | Metamorphic | Quartz | Comes from metamorphism of sandstone Used as a decorative stone Used for railway ballast |
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IMPORTANT ROCK FORMATIONS/STRUCTURES
| Formation/structure | Location | Classification | Notes |
| Deccan Traps | Deccan Plateau, India | Large Igneous Province (LIP) | One of the largest volcanic features on earth |
| Siberian Traps | Siberia, Russia | LIP | One of the largest known volcanic events (250 million years ago) |
| Acasta Gneiss | Quebec, Canada | Metamorphic | Oldest known rock in the world (4.28 billion years) |
| Devil’s Tower | Wyoming, USA | Igneous | Monolithic rock that rises 1200 feet above surrounding terrain |
| Blue Lias | England | Limestone and shale | Rich in dinosaur fossils |
| Red Fort | Delhi | Sandstone | |
| Hawa Mahal | Jaipur | Sandstone | |
| Mahabalipuram sculptures | Mahabalipuram | Granite | |
| Mount Augustus | Western Australia | Sandstone and conglomerate | Largest monolith in the world |
| Savandurga | Karnataka | Gneiss and granite | Largest monolith in India |
| Sphinx | Egypt | Limestone | Oldest known monumental sculpture Largest monolith statue in world |
| Phobos monolith | Mars | Igneous |
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