Assignment Code: BGYCT-135 / TMA / 2021 - 2022

Course Code: BGYCT-135

Assignment Name: Petrology

Year: 2021 – 2022

Verification Status: Verified by Professor

Note: Attempt all questions. The marks for each question are indicated against it. Write all answers in your own words; do not copy from the course material.

Part A

Q1. Write short notes on the following:

Q1. a) Classification of igneous rocks (5)

Ans) By bulk, igneous and metamorphic rocks account for 90–95 percent of the upper 16 kilometres of the Earth's crust. About 15% of the Earth's present land surface is made up of igneous rocks. Igneous rock makes up the majority of the Earth's marine crust.

Classification of Igneous rocks

The classification of the many forms of igneous rocks can reveal a lot about the circumstances under which they arose. Particle size, which is mostly dependent on the cooling history, and the mineral content of the rock are two major factors utilised for igneous rock categorization. Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are all essential minerals in the creation of igneous rocks and are used to classify them. In practically all igneous rocks, all other minerals are considered non-essential and are referred to as accessory minerals. Carbonatites, which contain important carbonates, are examples of igneous rocks that contain other vital minerals.

Phaneritic igneous rocks have crystals large enough to be seen with the naked eye, while aphanitic igneous rocks have crystals too small to be seen. In general, phaneritic denotes an intrusive origin, whereas aphanitic denotes an extrusive origin. Porphyry is an igneous rock having bigger, easily identifiable crystals contained in a finer-grained matrix. When some of the crystals grow to a significant size before the bulk mass of the magma crystallises as finer-grained, homogeneous material, porphyritic texture occurs.

Texture and composition are used to classify igneous rocks. The size, shape, and arrangement of the mineral grains or crystals that make up the rock are referred to as texture.

Q1. b) Colour Index (5)

Ans) The colour index is a geological phrase that indicates the minerals present as well as the type of rock. The ratio of dark coloured, or mafic, minerals to light coloured, or felsic, minerals is measured by the colour index of an igneous rock. Mineral colour, in general, indicates the specific gravity of the mineral, as lighter-colored minerals tend to weigh less.

The proportion volume of ferro-magnesium or dark-colored minerals contained in the rock determines this classification.

The following are the groups:

1. Leucocratic: ‘Leuco’ means light ‘cratic’ means coloured. When the rock is dominantly composed of light-coloured minerals and poor (<0.33%) in dark coloured minerals it is known as leucocratic.

2. Mesocratic: ‘Meso’ means medium when the dark coloured minerals vary between 33-67%. It represents intermediate colour, i.e., neither dark nor light in appearance.

3. Melanocratic: ‘Melano’ means dark when the dark coloured minerals are more than 67% in the rock.

Q2. Discuss the different types of textures found in igneous rocks with the help of neat well labelled diagrams. (10)

Ans) Igneous rock is made up of both crystallised and non-crystallized elements. Fabric is the study of crystal form and the mutual connection between mineral grains and glassy materials. The arrangement, orientation, and mutual interaction of mineral grains or crystals and/or glass are therefore termed as fabric. Fabric is a non-compositional rock characteristic that includes both textures and structures.

Crystallinity

The degree or number of crystals created during the solidification of magma is referred to as crystallinity or degree of crystallisation. Crystals, partly crystals and partly glassy materials, or completely glassy matter can all be found in igneous rocks. The ratio of crystallised materials to glass in an igneous rock is used to determine the degree of crystallisation. It is the modal percentage of mineral grains in relation to glass, which ranges from 0 to 100%.

Glass:

Rapid cooling of silicate melts during crystallisation produces a very viscous liquid with a disordered atomic structure. This phrase is used to describe a rock or a piece of a rock that lacks crystalline structure. When a highly viscous magma is rapidly supercooled, the atomic groups and molecules existing in it are not correctly and consistently ordered in a defined sequence, resulting in a crystalline solid.

Textures of igneous rocks are classified according to their crystallinity or degree of crystallisation:

1. Holocrystalline: The prefix 'holo' denotes a rock that is totally or almost entirely made up of well-defined crystal faces of constituent minerals, such as orthoclase in granite and augite in gabbro. In plutonic rocks, holocrystalline texture can be visible.

2. Hemicrystalline/ Microcrystalline: Hemicrystalline texture is defined as a rock that has crystalline and glassy material in equal amounts, such as dolerite or basalt. This is most common in rocks that crystallise near the surface or at a depth between the surface and the surface. The phrases merocrystalline and hypocrystalline are also used as synonyms.

3. Holohyaline: Glassy or non-crystalline materials makes up the entirety of the rocks with this texture (like crystallites and microlites). When the rate of cooling is extremely fast, this happens. Obsidian, pitchstone, and nephelinite are examples of volcanic rocks with this texture.

Q3. Explain physical properties of magma in detail. (10)

Ans) Magma's physical characteristics. The following factors influence magma crystallisation: Magma's physical characteristics.

The following factors influence magma crystallisation:

1. Temperature: Directly measuring the temperature is difficult since it is dangerous. Remote control allowed for direct temperature measurements of 12000C in streaming lava. The temperature of magma crystallises between the temperatures of 12000C and 6500C, according to laboratory experiments and field observations. Temperatures of magmas show that the temperature of various magmas during eruption is as follows:

• Basaltic magma - 1000 to 1200oC

• Andesitic magma - 800 to 1000oC

• Rhyolitic magma - 800 to 650oC.

2. Viscosity: It is measured as the resistance to flow (the polar opposite of fluidity) that controls magma motion. It is mostly determined by the magma's composition and temperature. The volatiles give the magma a low viscosity. When compared to mafic magma, felsic magma (with greater SiO2) has a higher viscosity. This is due to the intricacy of felsic magma's crystal structure (growing silica content). As a result, when compared to rhyolitic magmas, basaltic magmas have a lower viscosity. The viscosity of magma is also controlled by temperature. Magmas with a lower viscosity are more viscous than magmas with a higher viscosity. As a result, viscosity is an essential factor in determining how magmas erupt.

3. Density: The density of magma is 2.65 gm/cm3, while the density of the mantle is 3.3 gm/cm3. Magma with a higher iron content has a higher density. Basalts are often higher in Fe, Ca, and Ti than rhyolites, while rhyolites are higher in Na, Al, and Si. Basaltic magma has a density of 2.65 to 2.80 gm/cm3; andesitic magma has a density of 2.45 to 2.50 gm/cm3; and rhyolitic magma has a density of 2.18 to 2.25 gm/cm3. Temperature and pressure, in addition to composition, are major factors in determining density. When the temperature of the magma rises, it expands, lowering the density. Higher pressure, on the other hand, increases the density of the magma.

4. Volatiles/gases: Almost all magmas have dissolved gases and volatiles. According to petrologists, water makes up the majority of the volatiles in magma (about 90%). The magma rises towards the surface as the pressure drops, and the gas separates into a distinct vapour phase. Let's use an analogy to help us understand this. Carbonated beverages bottled at high pressure are analogous to gas developing as a distinct phase. The high pressure keeps the CO2 dissolved in the beverage, but when the pressure is reduced, such as when the bottle is opened, the gas escapes and forms a distinct gas phase, which is visible as bubbles. Because the volume of gas expands as the pressure is released, it gives magma an explosive nature. The explosiveness of magma increases as the number of dissolved gases or volatiles in the magma increases. Carbon dioxide is the most prevalent volatile after water. Basaltic magmas are often dry, with H2O concentrations of less than 0.5 percent. While felsic rocks, such as granite and rhyolite, have water content ranging from 0.5 to 0.7 percent.

Q4. Discuss the megascopic and microscopic characters of gabbro with the help of neat well labelled diagrams. (10)

Ans)

Megascopic Characters of Gabbro

Gabbro is made up of a lot of ferromagnesian minerals. As a result, it's a mesocratic mafic rock with a dark colour. It happens in a plutonic state. Plagioclase, pyroxenes, hornblende, and opaque are the primary constituents. It has a densely uniform appearance and generally has the same texture and composition throughout the entire gabbroic bulk.

1. Colour: Gabbro is dark grey, greenish, greenish black, brownish in colour. It is mesoocratic.

2. Appearance: It's a coarse-grained intrusive igneous rock with a dark colour. Because of the high concentration of ferromagnesian minerals in gabbro, it is generally dark in colour. They look as a tightly homogeneous rock with same texture and composition across the entire rock mass.

Mineralogical Composition: The minerals that are found in tiny amounts or as accessory, essential, or secondary minerals in these rocks are detailed below.

Minerals that are essential: Plagioclase, clinopyroxene (diopside and augite) and/or orthopyroxenes (hypersthene), olivine, and amphibole are examples of calcium-rich minerals.

Accessory minerals include apatite, magnetite, and ilmenite, with less quartz, alkali feldspar, and feldspathoids. Chlorite, titanite, serpentine group minerals, and epidote are secondary minerals.

Texture: The following are the look, size, and arrangement of different mineralogical ingredients inside the rock:

Gabbro has a hypidiomorphic structure and is holocrystalline. It is coarse-grained with equigranular grains. It is phaneritic in grain size. Gabbro is made up of equigranular dark-colored pyroxene grains and light-colored plagioclase laths arranged in a mutual pattern. Gabbro has a classic salt-and-pepper texture. Plagioclase laths are sometimes positioned parallel to layers.

Structure: Gabbro is commonly layered and may show alternating light and dark layers. Layered structure is called as cumulate structure.

Macro-and microscopic photographs

Different igneous rocks sampled in the Ceret gabbro-dioritic stock: (a) diorite with large crystals of biotite (b) plagioclase-rich gabbro with distinct large crystals of clinopyroxene; (c) ultramafic rock; (d) leucogranite dyke crosscutting the gabbro-diorite; (e) diorite with amphibole, idiomorphic plagioclase and interstitial quartz; (f) clinopyroxene almost completely surrounded by hornblende and biotite. Idiomorphic plagioclase and pyroxene form an ophitic texture; (g) abundant crystals of olivine included in brown amphibole and altered to serpentinite; and (h) large garnet crystals surrounded by a leucocratic halo in a diorite close to the contact with the host rock.

Microscopic Characters

Gabbro is a hypidiomorphic holocrystalline rock. As a result, various minerals found in the rock can be detected with the naked eye or with a petrological microscope at low magnification. Let's look at gabbro under a microscope now.

1) Mineralogy: The rock is mostly made up of plagioclase laths and a variety of equigranular ferromagnesian minerals. Pyroxene dominates in ferromagnesian minerals. There are primarily two types of clinopyroxene minerals: diopside and augite.

2) Texture: The crystals' form and size are as follows:

• Crystallinity: Gabbro is hypidiomorphic and holocrystalline. The majority of the crystals are granular in texture and are anhedral to euhedral in shape.

Gabbro is a medium to coarse-grained rock with a high degree of granularity. It's normally granular, with most of the grains being nearly similar in size (phaneritic), but it can also have a poikilitic texture.

3) Rock types: The following are the several types of gabbro rocks.

• Norite is a gabbro that contains hypersthene and plagioclase.

• Alkali gabbro: This type of gabbro is dominated by orthoclase and microcline, as well as alkali amphibole.

• Troctolite is a gabbro containing olivine and plagioclase with a little quantity of pyroxene.

• Anorthosite is gabbro that contains >90% Ca-rich plagioclase (anorthite-rich).

4) Classification: Gabbro is classified as mafic plutonic rock.

5) Occurrence: Batholiths and laccoliths are common forms of gabbro. It's common in mid-ocean ridges and in ancient mountains made primarily of crushed and elevated oceanic crust. Gabbro is an important component of the oceanic crust that can be found in ophiolite sequences all over the world. Pyroxeneplagioclase orthocumulate is a better word for cumulate gabbros.

Q5 Explain the crystallization behavior of SiO2 system. (10)

Ans) Crystallisation SiO2 System Behaviour In a single component system, such as silica polymorphs, each polymorph has a distinct crystal structure and is stable throughout a defined T and P range. The stability field depicts the P and T circumstances under which each mineral is stable, whereas the phase boundaries describe the stability field's limits as well as the conditions under which phases from neighbouring fields coexist. In the phase diagram, there are several phases. We can arbitrarily modify P and T for any of the phases without affecting the character of the phases. It's worth noting that the phase rule doesn't tell you how many phases are present in the system or how many are possible to develop. The SiO2 system, which has numerous phases as illustrated in the diagram below, is a good example of a unicomponent system.

Part B

Q6. Describe any five primary structures found in sedimentary rocks. (10)

Ans) The formation of primary structures occurs during the deposition of constituent sediments. These structures, such as bedding, cross stratification, and ripple patterns, are created by physical or mechanical processes that occur throughout the deposition process and are thus referred to as mechanical structures. They're made of inorganic materials.

Primary sedimentary structures include the following:

1. Bedding and Stratification

Stratification is defined by bedding and lamination. The thickness of bedding is greater than 1 cm, whereas the thickness of lamination is less than 1 cm. Lamination is made up of laminae, while bedding is made up of beds. A frequent internal structure of beds is parallel (also known as planar or horizontal) lamination. It refers to the technique of laying down sediments one after the other such that they appear to be one set on top of the other. Bedding, stratification, and layering are all phrases that are frequently used interchangeably. Stratification is the most prevalent sedimentary structure.

2. Cross- Bedding

It's made up of layers of bedded material deposited by wind or water and angled up to 35 degrees from horizontal. Assuming some horizontality, crossed beds usually have truncated tops and asymptotic bottoms. It is generated when the velocity and/or direction of the flow of streams changes. A cross bedded layer's oblique lines usually meet the upper bedding at a greater angle and the lower section tangentially. The laminations in wind-formed current beddings are bent and of greater magnitude. Current-bedding or fake bedding are other terms for cross stratification.

3. Graded Bedding

They demonstrate a gradational progression of grain size variation from coarser at the bottom to finer at the top of a bed (Fig. 10.7). Each layer of sediment in a graded bed has a mixture of coarse and fine grains, but the average particle size decreases as you move upward. A graded bed is made up of a series of coarse to fine beds that are typically a few centimetres to several metres thick. The total thickness can reach hundreds of metres. Turbidity currents running down the ocean floor may deposit them in deep-ocean waters. Larger, heavier clasts settle first, followed by smaller, lighter clasts, resulting in vertically sorted “graded” beds with larger clasts on the bottom and finer clasts on top. In standing water suspension deposits when seasonal fluctuations are effective, gradational, fining-upward sequences are typical.

4. Ripple Marks

A wavy structure with a crest and dip is known as a ripple mark. In other words, they are very small sand or silt dune-like landforms whose long dimension is perpendicular to current movement. The crests, which might be sharp, rounded, or flattened, normally run parallel to each other. Low, thin ridges, usually 1-2 cm broad, are divided by wider troughs.

5. Mud Cracks

They're also known as shrinkage cracks or sun cracks, and they're prevalent structural features that form in clayey sediments after prolonged exposure to the atmosphere. As the sediment shrinks, cracks appear on the surface. The shrinkage aperture is larger at the top and narrows as it gets closer to the bottom, when it is filled with sand. They are polygonal in plan view and measure around 0.5 m across. Because drying of the sediments would not occur beneath a body of water, the presence of mud cracks shows that the sediment was exposed at the surface immediately after deposition.

Q7. Differentiate the following :

Q7. a) Importance of studying sedimentary rocks (5)

Ans) Sedimentary rock research is both vital and fascinating, and it has a lot of practical applications. Sedimentologists use the structure, texture, and composition of sediments and sedimentary rocks to derive the environment of deposition, such as ancient shorelines, mountains, riverine flood plains, deserts, and swamps in other locations.

These rocks are the source of the most lucrative energy sources, such as oil, gas, and coal. Oil and gas are produced by the maturation of organic materials in sediments, which then migrate to a suitable reservoir, which is primarily permeable sedimentary rocks. Much of the world's iron, aluminium, potash, salt, construction materials, and other critical raw minerals come from sedimentary rocks. Sedimentary processes pique the interest of geologists because they provide crucial clues to the palaeoclimate. Fossils preserved in sedimentary rocks provide a wealth of information on the evolution of life on Earth. The study of sedimentary rocks is crucial for determining the environment and deposition processes, as well as palaeogeography and palaeoclimatology, all of which contribute to our knowledge and understanding of the earth's geological past.

Q7. b) Physical weathering (5)

Ans) Physical weathering (also known as mechanical weathering) is the disintegration of rocks and minerals into smaller pieces as a result of direct interaction with atmospheric conditions such as heat, water, ice, and pressure. Physical weathering occurs as a result of a number of processes, including abrasion, frost action, unloading, thermal cycling, and plant and animal activity, as listed below:

Abrasion: The size of the sediments and other clasts is reduced during weathering. Water and wind carrying sediments have immense strength to cut through the rocks they pass through. Huge glaciers churn up rocks in their path and transport massive amounts of material.

Frost action: It happens primarily in locations with a lot of moisture and frequent temperature fluctuations above and below the freezing point. Water percolated in the crevices and joints of the rock and froze during the night in high-altitude places. The repeated freeze-thaw cycle puts strain on the surrounding rock, forcing it to fracture. Cracks become larger and split up along the joints of the rocks into angular pieces as the process repeats.

Pressure release or unloading: Granite, for example, is an intrusive igneous rock that forms at vast depths. They are under a lot of strain. Pressure on the rock is released as material is removed from above owing to erosion or any other reason, and the outer rock surface expands. The rock expands, creating forces that cause the outer surface to break into sheets parallel to the rock surface.

Thermal cycling: There are significant temperature differences between day and night in dry environments. Because the days are hot and the nights are cold, some minerals in the rocks expand and contract differently. Sharp temperature variations cause differential strains in the rocks, producing fracture and expansion of joints, and therefore block breaking.

Plant and animal activities: Plant roots that grow in the rocks can exert pressure on the fissures, causing them to spread. Animal burrowing also breaks down rock, resulting in fragmented particles.

Q8. Explain the chemical reactions leading to metamorphism. (10)

Ans) Chemical reactions that cause alteration or modification in the mineral composition of metamorphic rocks include the following:

1. Solid-Solid Reactions

The solid phases of the reactants are involved in these reactions. All solid-solid reactions have the ability to measure pressure and temperature, making them excellent geobarometers and geothermometers.

2. Exchange Reactions

An exchange solid-solid metamorphic process occurs when components are exchanged only between particular anhydrous or volatile conserving mineral phases.

3. Polymorphic Reactions

In such solid–solid processes, a phase transition occurs, in which the mineral chemistry remains unchanged but the mineralogical phase shifts to a different polymorph.

4. Solvus Reactions

This sort of reaction occurs either in limited solid solution (at a temperature below the critical temperature where a solvus is specified) or in phases with complete solid solution (at a temperature above the critical temperature where a solvus is defined) (at high temperature). This sort of reaction involves the exsolution of a new phase along a slip plane from the previous phase. Geospeedometry (determining the rate of metamorphism) and geothermometry both benefit from these types of reactions.

5. Decarbonation Reactions

Carbon dioxide is released during the metamorphism of calcareous rocks and other CO2-bearing rocks. The breakdown of carbonate minerals releases CO2 (dolomite, calcite etc.). However, the occurrence of pure CO2 is highly speculative; as a result, CO2 is found in combination with H2O or other volatiles.

6. Mixed Volatile Reactions

The reaction involves two fluid phases, CO2 and H2O, and it occurs in one of two ways: either both fluid phases are on one side (either reactant or product side) or both fluid phases are on opposite sides in the same reaction.

Q9. Describe the different types of textures found in metamorphic rocks with the help of neat well labelled diagrams. (10)

Ans) Because of the varied P-T (pressure and temperature) circumstances during metamorphism, metamorphic textures show a wide range of sizes, shapes, orientations, and spatial arrangements of crystals. Four types of metamorphic textures can be identified:

1. Palimpsest Texture

Palimpsest texture is the fundamental texture of the original rock that is occasionally discovered. Palimpsest texture is also known as relict texture since it has survived metamorphism and shows protolith rock textures even after metamorphism. Low-grade metamorphic rocks have strong relict texture preservation and are created in such a way that distortion is minimal, allowing for preservation.

2. Typomorphic Texture

These textures are common in metamorphic rocks, and they are formed by dynamic forces, heat action, or crystallisation.

3. Reaction textures

Reaction textures can be found on the corroded edges of crystals, as a consequence of corrosive rimming of crystals of one mineral by finer-grained aggregates of another, or as a result of other characteristics that suggest partial loss of crystalline material by reaction with magma or another fluid.

4. Corona Texture

Both the prograde and retrograde phases of metamorphism can produce this texture. One or more minerals form a complete rim around the older phase present in the texture's centre or core, forming a corona-like geometry. Depending on how many reactions have occurred, the number of rim layers created might range from one to five.

5. Intergrowth Texture

Symplectite texture is characterised by the worm-like appearance of minerals that originate along the reaction border of earlier minerals. Figure displays orthopyroxene– garnet–cordierite symplectite intergrowth. It's also known as a reaction texture, in which fine-grained minerals grow in an uneven pattern due to a reaction at the rim of a previously produced mineral.

Q10. Discuss the megascopic and microscopic characters of following rocks:

Q10. a) Limestone (5)

Ans) Marble is a non-foliated, high-temperature, high-pressure, thermally metamorphosed, and regionally metamorphosed rock created by the metamorphism of limestone or dolostone. Dolomarble is a type of marble that contains dolomite (18 mole percent Mg + limestone). Marble is white and primarily made up of the mineral calcite. The ancient shells, pore spaces, and differentiation between grains and cement vanish as the calcite in the protolith recrystallizes during marble formation. Calcite crystals are rebuilt in a denser and equigranular form during metamorphism, resulting in a compact rock, despite the fact that calcite has a hardness of just 3 on the Moh's scale. Marble commonly formed in convergent tectonic settings or when rising magma heats limestone or dolomite. Unfortunately, marble's calcium carbonate composition causes stone to deteriorate when exposed to acid rain. Marble made from limestone interbedded with shale has noticeable foliation and is banded.

Megascopic characters: Marble is crystalline, equigranular, hard, and compact with a medium to coarse grain. The texture of marble is granular and saccharoidal (sugary). It usually consists of a mass of interconnecting calcite crystals that is quite homogenous. Due to impurities inherited from parent rock or protolith, the marble can be pink, grey, green, or even black in colour. Marble without traces of iron is white, while marble with traces of iron is reddish or pink. Marble interacts with dilute acid because it is mostly made of calcium carbonate.

Q10. b) Gneiss (5)

Ans) Gneiss is a metamorphic rock that is formed of alternating dark and light-colored strata of varying thickness. Gneiss is a medium to coarse-grained rock that resembles schist in that it has light and dark coloured bands of felsic and mafic minerals, comparable to schist. The majority of gneisses are felsic in composition and are formed by high-grade granite metamorphism or shale. The light and dark components split during high-grade metamorphism, giving gneisses their distinctive banded or layered look. Dark minerals can also make up the majority of gneisses.

Megascopic characters: Gneisses are metamorphic rocks dominated by granular and elongated (rather than flaky/platy) minerals. It is distinguished by its compositional and/or structural variety, which is caused by alternate light and dark coloured bands made up of minerals of different colours. Unlike schist, the banding is not continuous. The foliation is prominent, but unlike slate and schist, the rock does not cleave along the foliation planes. The colour is changeable, with dark and light bands alternated. The gneissose structure is seen in gneiss. The gneissic bands in the rock are formed of quartz, feldspars, mica, and amphiboles with alternate light and dark coloured bands. Gneiss is characterised by light and dark bands of felsic and mafic minerals (biotite, pyroxene, amphibole, garnet, and so on).