Mineralogy and Petrology Lecture Slides.pptx
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UNIVERSITY OF MINES AND TECHNOLOGY (UMaT), TARKWA Faculty of Geosciences and Environmental Studies Department of Geological Engineering MINERALOGY AND PETROLOGY MR 160 Lecturer: Dr Mrs Etornam Bani FIADONU Assistant: Mr Albert Ka...
UNIVERSITY OF MINES AND TECHNOLOGY (UMaT), TARKWA Faculty of Geosciences and Environmental Studies Department of Geological Engineering MINERALOGY AND PETROLOGY MR 160 Lecturer: Dr Mrs Etornam Bani FIADONU Assistant: Mr Albert Kafui KLU Semester Two 2022/23 1 Organisational Aspects Geological Engineering Department Email: [email protected] Office hours: Tuesdays and Thursdays 12- 16hrs Assessment o Class attendance (10 marks) o Continuous Assessment: Assignments+ Exercise + Mini Projects (30 marks) o End of semester exams (60 marks) Others NB: Marks will be allocated for class participation What are your expectations for this course? 2 Course Outline 1. Internal Structure of the Earth Petrology 2. Plate Tectonics 1. Introduction to Petrology 2. Geological Classification of Rocks Mineralogy 3. Rock Cycle 3. Introduction to Mineralogy 4. Igneous Petrology 4. Minerals 5. Igneous Textures 5. Classification and Naming of 6. Common Structures of Igneous Minerals Rocks 6. Properties of Minerals 7. Metamorphic Petrology 7. Mineral Groups 8. Common Structures of Metamorphic Rocks 9. Sedimentary Petrology 10. Common Structures of Sedimentary Rocks 3 Earth’s Interior Compositional Layers – Crust (~3-70 km thick) Very thin outer rocky shell of Earth – Continental crust - thicker and less dense – Oceanic crust - thinner and more dense – Mantle (~2900 km thick) Hot solid that flows slowly over time; Fe-, Mg-, Si-rich minerals – Core (~3400 km radius) Outer core - metallic liquid; mostly iron Inner core - metallic solid; mostly iron 4 Earth’s Interior Mechanical Layers – Lithosphere (~100 km thick) Rigid/brittle outer shell of Earth Composed of both crust and uppermost mantle Makes up Earth’s tectonic “plates” – Asthenosphere Plastic (capable of flow) zone on which the lithosphere “floats” 5 Properties of Earth‘s Interior Density P-wave Laye (g/cm³ velocity r ) (km/sec) Continental 2.6 - 6 2.8 crust Oceanic 7 3.5 crust Mohorovicic discontinuity (Moho) Mantl 8- e 4.5 - 10 12 Gutenberg discontinuity Core - (average) 12 - 8- Outer 10 core Lehman Bullen discontinuity (liquid) 13. 11 - 5 12 Inner core (solid) Determined from seismic activity through the Earth 6 Plate Tectonics: Earth's Plates and Continental Drift 7 Some questions we will answer today: How is the earth always changing? What forces inside the earth create and change landforms on the surface? What is the theory of plate tectonics and how does it work? What two theories help make up the theory of plate tectonics? What is continental drift and sea floor spreading? 8 DID YOU KNOW? 9 Land and Water Photographs of the earth taken from space show clearly that it is a truly a ”watery planet.” More than 70 percent of the earth’s surface is covered by water, mainly the salt water of oceans and seas. 10 Land The large landmasses in the oceans are called continents. Landforms are commonly classified according to differences in relief. The relief is the difference in elevation between the highest and lowest points. Another important characteristic is whether they rise gradually or steeply. The major types of landforms are mountains, hills, plateaus, and plains. 11 Most people know that Earth is moving around the Sun and that it is constantly spinning. But did YOU know that the continents and oceans are moving across the surface of the planet? Volcanoes and earthquakes as well as mountain ranges and islands all are results of this movement. Why is this? 12 Plate Tectonics 13 Most of these changes in the earth’s surface take place so slowly that they are not immediately noticeable to the human eye. The idea that the earth’s landmasses have broken apart, rejoined, and moved to other parts of the globe forms part of the Plate Tectonic Theory. 14 Plate Tectonic Theory About forty years ago, scientists exploring the seafloor found that it is full of tall mountains and deep trenches, a single seafloor mountain chain circles Earth and contains some of Earth’s tallest mountains. Along this mountain chain is a deep crack in the top layers of earth. Here the seafloor is pulling apart and the two parts are moving in opposite directions, carrying along the continents and oceans that rest on top of them. These pieces of Earth’s top layer (crust) are called tectonic plates. They are moving very slowly, but constantly (Most plates are moving about as fast as your fingernails are growing -- not very fast!). Currently Earth’s surface layers are divided into nine very large plates and several smaller ones. 15 According to the theory of plate tectonics, the earth’s outer shell is not one solid piece of rock. Instead the earth’s crust is broken into a number of moving plates. The plates vary in size and thickness. 16 The North American Plate stretches from the mid-Atlantic Ocean to the northern top of Japan. The Cocos Plate covers a small area in the Pacific Ocean just west of Central America. These plates are not anchored in place but slide over a hot and bendable layer of the mantle. 17 To really understand how the earth became to look as it does today, and the theory of plate tectonics, you also need to become familiar with two other ideas: Continental Drift and Seafloor Spreading 18 Less than 100 years ago, many scientists thought the continents always had been the same shape and in the same place. A few scientists noted that the eastern coastline of South America and the western coastline of Africa looked as if they could fit together. Some also noted that, with a little imagination, all the continents could be joined together like giant puzzle pieces to create one large continent surrounded by one huge ocean. 19 So, if our continents fit together, why does the earth look like it does today? 20 Continental Drift Theory When the tectonic plates under the continents and oceans move, they carry the continents and oceans with them. In the early 1900s a German explorer and scientist proposed the continental drift theory. He proposed that there was once a single “supercontinent” called Pangaea. 21 22 Wegner’s theory was that about 180 million years ago, Pangaea began to break up into separate continents. To back this theory up, he perserved remains and evidence from ancient animals and plants (fossils) from South America, Africa, India, and Australia that were almost identical. 23 Seafloor Spreading The other theory supporting plate tectonics emerged from the study of the ocean floor. Scientists were suprised to find that rocks taken from the ocean floor were much younger than those found on the continents. The youngest rocks were those nearest the underwater ridge system which is a series of mountains that extend around the world, stretching more than 64 thousand 24 The theory of seafloor spreading suggests that molten rock (think of a melted chocolate bar that has been left in your pocket for too long) rises under the underwater ridge and breaks through a split at the top of the ridge (the crust... Remember, the plate). The split is called a rift valley. The rock then spreads out in both directions from the ridge as if it were on two huge conveyor belts. As the seafloor moves away from the ridge, it carries older rocks away. Seafloor spreading, along with the 25 continental drift theory, became part of The blue and red arrows represent the magnetic pull of the earth when the rock was created. Scientists use these marks to determine how old the ocean is. 26 Plate motions also can be looked at into the future, and we can have a stab at what the geography of the planet will be like. Perhaps in 250 million years time there will be a new supercontinent. 27 What two theories help make up the theory of plate tectonics? What is continental drift and sea floor spreading? 28 So.... When a geologist or a geographer looks at a piece of land they often ask, “What forces shaped the mountains, plains, and other landforms that are here?” 29 What is their answer? 30 Plate Tectonics But this doesn’t actually tell me how the mountains or volcanoes were formed or how earthquakes happen, does it? 31 YES! As mentioned earlier, those tectonic plates are always moving. They are always moving: pulling away from each other crashing head-on or sliding past each other Depending on which way these plates are moving will decide what is happening on the earth you and I are standing on. 32 They’re Pulling Apart! When plates pull away from one another they form a diverging plate boundary, or spreading zone. Thingvellir, the spreading zone in Iceland between the North American (left side) and Eurasian (right side) tectonic plates. January 2003. 33 34 East African Rift Valley 35 The Crash! What happens when plates crash into each other depends on the types of plates involved. Because continental crust is lighter than oceanic crust, continental plates ”float” higher. Therefore, when an oceanic plate meets a continetnal plate, it slides under the lighter plate and down into the mantle. The slab of oceanic rock melts when the endges get to a depth which is hot enough. A temperature hot enough to melt about a thousand degrees!. This process is called subduction. Molten material produced in a subduction zone can rise to the 36 earth’s surface and cause volcanic 37 38 39 When they Crash When two plates of the same type meet, the result is a process called converging. What type of plates these are, depends on what occurs. 40 Converging... They crash! And they’re both ocean plates! When both are oceanic plates, one slides under the other. Often an Oceanic island forms at this boundary. 41 42 Wadat i- Benio ff Zone 43 Converging...They Crash! And they’re both Continental Plates When both are continental plates, the plates push against each other, creating mountain ranges. 44 45 They Crash and are both continental plates! Earth’s highest mountain range, the Himalayas, was formed millions of years ago when the Indo-Australian Plate crashed into the Eurasian Plate. Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about 5 cm a year! 46 They meet and slide past each other! Sometimes, instead of pulling away from each other or colliding with each other, plates slip or grind past each other along faults. This process is known as faulting. 47 48 These areas are likely to have a rift valley, earthquake, and volcanic action. For example: Here, the San Andreas Fault lies on the boundary between two tectonic plates, the north American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of 5 to 6 centimeters each year. This fault frequently plagues California with earthquakes. 49 What forces inside the earth create and change landforms on the surface? What happens when the plates crash together, pull apart, and slide against each other? 50 Assignment 51 Introduction to Mineralogy 52 Mineralogy/Mineral Science The study of the chemistry, atomic structure, physical properties, and genesis of minerals. 53 Subfields of Mineralogy Descriptive Mineralogy – documenting physical and optical properties Crystal Chemistry – relationship of chemical composition to atomic structure Crystallography – relationship of crystal symmetry and form to atomic structure Mineral Genesis – interpreting the geologic setting in which a mineral forms from its physical, chemical, and structural attributes and its associated minerals 54 Fundamental Position of Mineralogy to all other Earth Science Disciplines Petrology – the study of the origin of rocks is largely determined by evaluating the structure, texture, and chemistry of the minerals they contain. Geochemistry – study of the chemistry of earth materials which reflects the collective chemistry of the minerals they contain Structural Geology and Tectonics – Deformation of rocks is controlled by the orientation and crystal structure of its constituent minerals Environmental Geology/Hydrogeology – the study of how the biosphere, hydrosphere, and atmosphere interacts with rock and minerals (the lithosphere). Economic Geology and Metallurgy – study of the origin and beneficiation of mineral deposits 55 Definition of a Mineral A mineral is a naturally occurring homogeneous solid, inorganic form with a definite chemical composition and ordered atomic arrangement. There are over 2000 minerals identified. 56 History of Mineralogy Mineral “arts” dates back to early human civilization. Mineral science begun with Renaissance/ Age of Reason (Agricola, 1556; Steno 1669). 1700’s measurements of crystal geometry and symmetry. Early 1800’s precise measurements of crystal symmetry heralds the field of crystallography; analytical chemistry leads to chemical classification of minerals. Late 1800’s – creation of polarizing microscope opens field of petrography and the study of 57 History of Mineralogy (cont.) Early 1900’s - X-ray diffraction measurements allows for precise measurement of internal symmetry and structure of minerals. 1960 – development of the electron microprobe, which allows for accurate in situ analysis of mineral chemistry. 1970 – development of transmission electron microscope, which allows for visualization of atomic structure and symmetry. 1980 – ion microprobe allow for study of isotopic composition of minerals. 58 Economic Importance of Minerals 59 Classification & Naming of Minerals Classified by major anionic components (oxides, silicates, sulfides, halides,...). New minerals – must be accepted by the CNMNMNIMA after careful description of atomic structure and chemical composition. Names – few rules (appearance, major chemical attribute, location of discovery, discoverer...). 60 Properties of Minerals Physical properties of minerals – Color – Streak – Luster – Transparency – Form – Hardness – Fracture – Cleavage – Density 61 Color – Depends on absorption of some and reflection of other colored rays or vibrations composed ordinary white light. Streak – It is a color of powder of mineral produced on rubbing. – It is determined by rubbing a mineral on unglazed porcelain plate. 62 Luster – It is appearance given to a mineral by light reflected from its surface. – Metallic- Metallic minerals e.g. pyrite, galena – Vitreous- Luster of broken glass reflection. E.g. quartz – Resinous Light reflection like of resins. Eg. Opal, amber – Pearly Sheen of a pearl, eg. Talc. – Silky Luster of silk. Fibrous minerals like asbestoses and gypsum – Adamantine Brilliant reflection like diamond 63 64 Transparency – A mineral is transparent when the outlines of the objects seen through it appear sharp and distinct. – When an object looks indistinct then it is called as semitransparent. – Minerals which are capable of transmitting light but can't see through it are called translucent. – Minerals which do not transmits the light are called as opaque minerals. 65 Forms – Crystalline Minerals which show well developed crystals are termed as crystalline. – Cryptocrystalline Mineral which shows mere traces of crystalline structure. 66 – Acicular – fine needle-like structure; natrolite – Bladed – shape of knife blade; kyanite – Botryoidal- spheroidal forms resembling bunch of grapes. Botryoidal hematite – Columnar- resembles columns; hornblende – Fibrous- fine thread like strands; asbestos – Granular- grain shape like lump of sugar. – Radiated- needle like crystal radiating from a centre; pyrite concentration 67 68 Hardness 5. Apatite – It is measured by scratching 6. Orthoclase 7. Quartz a mineral surface. 8. Topaz – Moh’s scale of hardness 9. Corundum 1. Talc 10. Diamond 2. Gypsum 3. Calcite 4. Fluorite 69 Fracture – Mineral when breaks from cleavage plane it forms a fracture. – These are not linear nor parallel 1. Even fracture 2. Uneven fracture 3. Splintery- observed in fibrous minerals look like woodstick 4. Concoidal-minerals break in curved shaped. Shown by natural glass 5. Hackly- surface elevations of minerals. 70 Cleavage- – The property of mineral to split under the influence of force, more or less parallel to crystal face. – It may cleave in one, two, three or more directions. Density- the weight of the mineral compared to equal volume of water. 71 Mineral Groups Silicate group Silicates include large number of minerals. They constitute 95% of silicate minerals. Which are considered common minerals of earth’s crust. 72 Quartz group (Silica group) Quartz appears in hexagonal dipyramid form. color- colourless Streak- color less Cleavage- absent Fracture- uneven Sp. gravity - 2.8 Luster- vitreous O. prop.- piezoelectric Identified from- hardness, cleavage and crystal form Other types- 73 Rock crystal- colorless Amethyst- violet Rose quartz- rose color Milky quartz- milkyness is due to presence of air cavity. Smoky quartz- black color Agate Jasper- red to brown color, due to presence of iron oxide particles. Flint Occurrence- it is common mineral in crustal layer of earth. It is essential mineral in sandstone. Uses- in electrical appliances. 74 Feldspar group These are available in igneous rocks. There are two groups, – Albite; sodium – Anorthosite; potassium Orthoclase (potassium aluminum silicate) – Color- white – Luster- vitreous – Streak- white – Cleavage- 2 set – Fracture- uneven – Hardness- 6 – Occurrence- in acid igneous rocks 75 Plagioclase ( group of Ca and Na) – Color- colorless – Streak- white – Luster- subvitreous – Cleavage- perfect – Fracture- uneven – Hardness- 6 – Occurrence- in all igneous rocks. These are also common in low grade metamorphic rocks. 76 Pyroxene group Pyroxenes are silicates of iron, magnesium and calcium. They crystallize in orthorhombic and monoclinic system. Two groups – Orthorhombic system- Astatine Hypersthenes – Monoclinic system- Augite Diopside 77 Hypersthenes- – Color- brownish yellow – Streak- colorless – Hardness- 5-6 – Luster- sub metallic – Fracture- uneven – Occurrence- found in igneous rocks like gabbros – Composition- iron magnesium silicate 78 Diopside – Color- white – Luster- vitreous – Hardness- 6 – Cleavage- parallel – Fracture- uneven – Occurrence- metamorphosed dolomite limestone. 79 Amphibole group It is double chain structure. They crystallize in monoclinic system Hornblende- – Color- light green – Luster- resinous – Cleavage- 2 set – Hardness- 5 – Fracture- uneven 80 Mica group They are characterized by cleavage in one direction. Muscovite- – Color- colorless – Luster- vitreous – Streak- colorless – Hardness- 3 – Cleavage- present – Fracture- uneven – Occurrence- in acid igneous rocks. Also, in sandstone and schist. 81 82 Assignment 83 PETROLOGY AND ITS DEFINITION Petrology (from Greek: Petra, rock; and logos, knowledge) is the branch of GEOLOGY that studies rocks, and the conditions in which rocks form. The subject matter of PETROLOGY consists the origin, association, occurrence, mineral composition, chemical composition, texture, structure, physical properties, etc., of rocks. Whereas PETROGRAPHY deals with the descriptive part of rocks and PETROGENY deals with the mode of formation of rocks. These two together makeup 84 Petrology This is the science dealing with rocks. The earth crust is built up of different mineral aggregates called rocks. The rock may be mono-minerallic or poly-minerallic. The chemical composition of any rock depends naturally on the kind of constituent minerals. The rock is formed under certain geological conditions that exert an important control upon the mode of its occurrence, nature and relation to its constituent minerals – texture. Every rock type is also distinguished by its own physical properties like color, density, mechanical strength, etc. A rock is therefore an aggregate of more or less quantitatively and qualitatively constituent mineral grains different from each other in certain textural features, physical properties and in geological conditions in which they were formed. 85 Rocks Monomineralic Polymineralic 1 Mineral More than 1 Mineral Rocks are classified by how they are formed!!! 86 Geological Classification of Rocks There are three basic kinds of rocks, each type is determined by the process by which the rock forms. Igneous Rocks - form by solidification and crystallization from liquid rock, called magma. Sedimentary Rocks - form by sedimentation of mineral and other rock fragments from water, wind, or ice and can also form by chemical precipitation from water. Metamorphic Rocks - form as a result of increasing the pressure and/or temperature on a previously existing rock to form a new rock. 87 Igneous Petrology 88 Igneous Rocks are formed by crystallization from a liquid, or magma. They include two types: Volcanic or extrusive igneous rocks form when the magma cools and crystallizes on the surface of the Earth Intrusive or plutonic igneous rocks wherein the magma crystallizes at depth in the Earth. Magma is a mixture of liquid rock, crystals, and gas, characterized by a wide range of chemical compositions, with high temperature, and properties of a liquid. Magmas are less dense than surrounding rocks, and will therefore move upward. If magma makes it to the surface it will erupt and later crystallize to form an extrusive or volcanic rock. If it crystallizes before it reaches the surface it will form an igneous rock at depth called a plutonic or intrusive igneous rock. 89 These are primary rocks, most abundant rocks in the earth’s crust. These are formed at a very high temperature and pressure conditions directly as a result of solidification of magma or lava. MAGMA: The term magma is applied when the melt is underground. LAVA: The melt when it reaches the earth’s surface and flows over it, is called lava. 90 Igneous: - Form when liquid rock cools and solidifies Intrusive Extrusive Cools below the earths Cools at the Earths surface (slowwwwly!) surface (quickly!) Magma Lava “Plutonic” “Volcanic” 91 The longer the rock takes to cool, the larger the crystals! Cools slow …..Large crystals Cools fast …….small crystals Cools immediately……NO Crystals (glass) 92 93 Vesicular- gas pockets 94 Magmatic Differentiation Processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma. These processes are referred to as magmatic differentiation and include assimilation, mixing, and fractional crystallization. Assimilation - As magma passes through cooler rock on its way to the surface it may partially melt the surrounding rock and incorporate this melt into the magma. Because small amounts of partial melting result in siliceous liquid compositions, addition of this melt to the magma will make it more siliceous. 95 Mixing - If two magmas with different compositions happen to come in contact with one another, they could mix together. The mixed magma will have a composition somewhere between that of the original two magma compositions. Evidence for mixing is often preserved in the resulting rocks. Fractional Crystallization - When magma crystallizes it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. The processes is called magmatic differentiation by Fractional Crystallization. Because mafic minerals like olivine and pyroxene crystallize first, the process results in removing Mg, Fe, and Ca, and enriching the liquid in silica. Thus crystal fractionation can change a mafic magma into a felsic magma. Crystals can be removed by a variety of processes. If the crystals are denser than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind. Removal of crystals can thus change the composition of the liquid portion of the magma. 96 Bowen's Reaction Series Bowen found by experiment that the order in which minerals crystallize from a basaltic magma depends on temperature. As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize first. Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase. But, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, then the remaining liquid will be more SiO2 rich. If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature. 97 98 Igneous Environments and Igneous Rocks The environment in which magma completely solidifies to form a rock determines: 1. The type of rock 2. The appearance of the rock as seen in its texture 3. The type of rock body. In general, there are two environments to consider: The intrusive or plutonic environment is below the surface of the earth. This environment is characterized by higher temperatures which result in slow cooling of the magma. Intrusive or plutonic igneous rocks form here. Where magma erupts on the surface of the earth, temperatures are lower, and cooling of the magma takes place much more rapidly. This is the extrusive or volcanic environment and results in extrusive or volcanic igneous rocks. 99 In relatively shallow environments intrusions are usually tabular bodies like dikes and sills or domed roof bodies called laccoliths. Dikes are small (< 20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude. Discordant means that they cut across preexisting structures. They may occur as isolated bodies or may occur as swarms of dikes emanating from a large intrusive body at depth. Sills are also small (< 50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude. Sills usually are fed by dikes, but these may not be exposed in the field. 100 101 102 Types of Dykes and Sills Dykes Sills Simple Simple sills dykes Multiple sills Multiple Composite sills dykes Differentiated Composi sills te dykes Interformational Ring sills dykes 103 Laccoliths are somewhat large intrusions that result in uplift and folding of the preexisting rocks above the intrusion. They are also concordant types of intrusions. Deeper in the earth intrusion of magma can form bulbous bodies called plutons and the coalescence of many plutons can form much larger bodies called batholiths. Plutons are large intrusive bodies, of any shape, that intrude and replace rocks in an irregular fashion. Stocks are smaller bodies that are likely fed from deeper level batholiths. Stocks may have been feeders for volcanic eruptions, but because large amounts of erosion are required to expose a stock or batholith, the associated volcanic rocks are rarely exposed. If multiple intrusive events occur in the same part of the 104 crust, the body that forms is called a batholith. 105 Rates of Cooling and Igneous Textures Fast cooling on the surface results in many small crystals or quenching to a glass. Gives rise to aphanitic texture (crystals cannot be distinguished with the naked eye), or obsidian (volcanic glass). Slow cooling at depth in the earth results in fewer much larger crystals, gives rise to phaneritic texture. Porphyritic texture develops when slow cooling is followed by rapid cooling. Phenocrysts = larger crystals Matrix or groundmass = smaller crystals. 106 107 Aphanitic Texture Porphyritic Texture 108 Classification of Igneous rocks Two main types of classification are: Based on silica percentage and Based on depth of formation Based on silica percentage The amount of silica present that is related to the mineral composition gives the FOUR main groupings of igneous rock: Acid (Felsic), Intermediate, Basic (Mafic) and Ultrabasic (Ultramafic) igneous rocks. 109 Acid Igneous Rocks Acid igneous rock is relatively rich in silica with more than 66% of it making up the rock. These are also called felsic rocks. Minerals generally present are quartz, orthoclase, sodium – plagioclase, muscovite, and biotite (plus or minus hornblende). They are made up of minerals with low melting temperatures. These tend to be light coloured rocks because of the low amount of ferromagnesian minerals. Intermediate igneous rocks have silica ranging between 52 – 66% and have a mineral composition of quartz, orthoclase, plagioclase, biotite, hornblende, (plus or minus Augite). Andesite is the most common intermediate volcanic rock. 110 Basic Igneous Rocks The basic igneous rocks have silica content between 45 and 52% and also have mineral composition of calcium – plagioclase, augite (plus or minus olivine and hornblende). They are also termed mafic rocks. Ultra basic rock has silica content less than 45%. It is almost entirely (or entirely) composed of ferromagnesian minerals. The mineral compositions making up the ultrabasic igneous rock are, calcium – plagioclase, olivine (plus or minus augite). It is entirely free from feldspars and quartz. 111 Classification Based on depth of formation Plutonic - This is formed by slow cooling at considerable depth within the crust of enormous masses of magma; the depth of formation may extend down many kilometers. As the heat dissipates very slowly from such vast bodies of material, the crystallization process is slow, and the rocks cool into a coarse- grained texture. Hypabyssal – It is formed as small bodies of rock called dykes and sills. The cooling process is more rapid than with plutonic rocks with their crystals still reasonably large that they assume a medium grained texture. Volcanic - This is formed at the planetary surface and originate as lavas emitted through volcanic activity and, as these cool relatively rapidly, attain a fine grain texture. Most occurrence of extrusive rock depends on the types of magma eruption and its viscosity. 112 Common structures of igneous rocks Physical appearance of rocks including size, shape and forms. Types Vesicular structure Amygdaloidal structure Columnar structure Sheet structure Flow structure & Pillow structure 113 Vesicular structure: Vesicular structure is a volcanic rock structure characterized by a rock being pitted with many cavities (known as vesicles) at its surface and inside. Basalt 114 Amygdaloidal Thestructure: drop in pressure that magma experiences as it flows from underground to the Earth's surface allows water and gases in the lava to form bubbles. If the bubbles do not get large enough to pop, they are frozen in the lava as vesicules. Amygdaloids are simply vesicles that have been filled in with a secondary mineral long after the flow cooled. Such secondary minerals are commonly white: quartz, calcite, or zeolite. (A secondary mineral is one that formed after the rock originally formed). Olivine basalt 115 Columnar structure The structure of a mineral aggregate that is made up of nearly parallel slender columns and that is intermediate between an equant and acicular structure (as in some amphiboles) C 116 Sheet structure The development of one set of well-defined joints sometimes. Brings about a slicing effect on the massive igneous rock body. If all such slices are horizontal, the structure is said to be sheet structure. R 117 Flow structure These structure is planar or linear features that result from flowage of magma with or without contained crystals. Various forms of faintly to sharply defined layering and lining typically reflect compositional or textural in homogeneities, and they often are accentuated by concentrations or preferred orientation of crystals, inclusions, vesicles and other features. 118 Pillow structure Consists aggregates of ovoid masses, resembling pillows or grain-filled sacks in size and shape, that occur in many basic volcanic rocks. The masses are separated or interconnected, and each has a thick vesicular crust or a thinner and more dense glassy rind. The interiors ordinarily are coarser- grained and less vesicular. Pillow structure is formed by rapid chilling of highly fluid lava. 119 Assignment 120 Metamorphic Petrology 121 Metamorphism The term metamorphism comes from the Greek (meta morph) meaning change of form. In petrology, metamorphism refers to changes in a rock's mineralogy, texture, and/or composition that occur pre- dominantly in the solid state. Metamorphic Agents Temperature Pressure Fluid State of Stress 122 Temperature Increasing temperature has several effects on sedimentary or volcanic rocks. It promotes recrystallization, which results in increased grain size of minerals. Rocks being heated may eventually reach a temperature at which a particular mineral is no longer stable, or a group of minerals is no longer stable together. When this happens a reaction will take place that consumes the unstable mineral(s) and produces new minerals that are stable under the newly achieved conditions. Among the most common are devolatilization reactions (usually dehydration or de-carbonation reactions). 123 Pressure In/on the Earth surface/interior where the temperatures are high happens, of course, together with pressure increases also. Remember that pressure increases with depth, due to the weight of overlying rocks, and is called litho-static pressure (also called confining pressure). Metamorphic Fluids Most metamorphic rocks at depth contain an intergranular fluid phase. We use the term fluid to avoid specifying the exact physical nature of the phase. At low pressures, the fluid is either a liquid or a gas, but at pressures and temperatures above the critical point of water there is no difference between liquid and gas Deviatoric Stress Only when the pressure is unequal in various directions will a rock be deformed. Unequal pressure is usually called deviatoric stress (whereas lithostatic pressure is uniform stress). 124 125 Types of Metamorphism The processes that change existing rocks into metamorphic rock(s) is term as metamorphism. These processes can be grouped into; Contact (thermal) metamorphism. Dynamic (pressure) metamorphism and Thermodynamic (both temperature & pressure) or Regional metamorphism 126 Contact metamorphism This occurs adjacent to igneous intrusions as a result of the thermal effects of hot magma intruding cooler shallow rocks. Contact metamorphism can occur wherever igneous activity occurs. This is not restricted to any particular setting, such as plate boundaries. 127 128 Dynamic metamorphism Fault-zone metamorphism occurs in areas of high shear stress. The term "fault" is to be interpreted broadly and includes zones of distributed shear that can be up to several kilometers across. In this type of metamorphism, the pressure is high than temperature. 129 Thermodynamic Metamorphism This results when temperature and stresses are combined, as in orogenic belts. 130 Classification of Metamorphic Rocks Metamorphic rocks have been variously classified based on: Texture of rocks e.g fine, medium or coarse grained. Structure of the rocks e.g foliated or non – foliated. Degree of the metamorphism e.g low, medium or high grade. Mode of origin e.g ultramafic, mafic, shales, carbonate, quartz and quartzo – feldspathic rocks. Mineralogical composition e.g metamorphic zone where an index mineral characterizes the rocks like chlorite, biotite, garnet, staurolite, kyanite or sillimanite zones. 131 But the most common classification of metamorphic rocks is based on the presence of foliation, the property to split up into thin sheets, into the following two groups: Foliated rocks This group includes the rocks that can be split up into thin sheets (quartzites) and those with recrystallized minerals as in gneiss. Non-foliated rocks This group includes the rocks that cannot split up into thin sheets. E.g marble, granulite, eclogite, serpentinite, migmatite, etc. 132 133 134 135 Metamorphic: Rocks that are changed due to extreme heat and/or pressure. DO NOT MELT!!! (they recrystalize) Metamorphic rocks become… 1. Harder 2. More dense 3. Banded or foliated 4. Distorted 136 Banding 137 Foliated 138 Non-foliated In general, many non-foliated rocks have not undergone a great amount of stress, and therefore, do not show foliation. Also, the minerals that compose non-foliated rocks are equidimensional crystals. As a result, no foliation would appear because all the mineral grains look similar. 139 Importance of Metamorphic Rocks The metamorphic rocks are extensively used as building stones. The foliated rocks like slate, gneiss and schist are used as a roofing material, tabletops, stair-case etc. The non-foliated rocks are used as building stones. The most important non-foliated rock is marble. They are an excellent building material for important monumental, historical and architectural buildings. Marble is extensively used, in modern buildings also, for decorative purposes in columns, stair-case, floor Quartzite is also used in outdoor decorations, especially in Accra (i.e in Ghana). 140 141 142 Common Structures of Metamorphic Rocks The most common structures found in metamorphic rocks are; 1) Gneissose structure: bands of flaky minerals 2) Schistose structure: parallel layers 3) Granulose structure: having granular minerals 143 BONUS: CLASSIFY this rock as igneous, sedimentary or metamorphic and EXPLAIN why you classified it that way. 144 BONUS: Name the mineral that has the following properties: Non-metallic Can scratch fluorite but cannot scratch quartz Exhibits cleavage Contains the elements sodium & hydrogen 145 Assignment 146 Sedimentary Petrology 147 Why Study Sedimentary Petrology? The application of the knowledge of sediments and sedimentary processes can help to; Trace source of sediments (provenance) Deduce depositional environments of sediments Locate economic resources Infer paleoclimatic conditions Decipher tectonic settings under which sediments were formed Indicate the stratigraphic succession Deduce the paleo life/living organisms – Evolution 148 Weathering and the Sedimentary Cycle It is appropriate to study the genesis of sediment particles, before proceeding to study sedimentary rocks, their petrography, transportation and deposition. A sedimentary rock is the product of provenance and process. This is concerned primarily with the provenance of sediments. That is to say the preexisting rocks from which it forms and the effect of weathering on sediment composition. 149 The Rock Cycle 150 Formation of Sedimentary Rocks 151 Sedimentary Cycle The sedimentary cycle consists of the following phases: weathering erosion transportation deposition lithification uplift and weathering again 152 Classification of Sedimentary Rocks For the identification of sedimentary rocks in the field, two principal properties are considered – composition (mineralogy) and grain size (texture). On the basis of origin, sedimentary rocks can be classified broadly into four categories; 153 The most common lithologies are the sandstones, mud rocks and Carbonates/carbonate-bearing rocks. Other types – evaporites, ironstones, cherts and phosphates – are rare or only locally well developed, and volcaniclastics are important in some places. NB: In some cases, you may have to think twice as to whether the rock is even sedimentary in origin or not. Greywacke, for example, can look very much like dolerite or basalt, especially in hand-specimens away from the outcrop. 154 Greywacke 155 Parameters generally indicating a sediments origin include the presence of; stratification specific minerals of sedimentary origin (e.g., glauconite, chamosite) sedimentary structures on bedding surfaces and within beds fossils grains or pebbles which have been transported (i.e. clasts). 156 Different Categories of Clastic Rocks RUDACEOUS ROCKS: made up of rounded or sub- rounded Pebbles and cobbles e.g. Conglomerate. ARENACEOUS ROCKS: made up of mainly sand e.g. Sandstone. These rocks are either accumulated by wind action or deposited under water action or marine or lake environment. ARGILLACEOUS ROCKS: made up of clay size sediments e.g. Shale, mudstones, siltstones. 157 Field exposures of sandstones (a–d) in the Kwahu/Bombouaka and Oti/Pendjari groups 158 Limestones and dolomites Limestones are composed of more than 50% CaCO3 and so the standard test is to apply dilute hydrochloric acid (HCl). The rock will fizz. Many limestones are a shade of grey, but white, black, red, buff, cream and yellow are also common colours. Fossils are commonly present, in some cases in large numbers. Dolomites (also dolostones) are composed of more than 50% CaMg(CO3)2. They react little with dilute acid (although a better fizz will be obtained if the dolomite is powdered first). Most dolomites have formed by replacement of limestone and as a result in many cases the original structures are poorly preserved. Poor preservation of fossils and the presence of vugs (irregular holes) are typical of dolomites. 159 Dolomite Limestone Limestone Dolomite 160 Discontinuous limestone layers from central Turkey Massive limestone layers from central Turkey Brecciated dolomite from central Turkey 161 162 Identifying Characteristics of Rocks Igneous Sedimentary Intergrown crystals Cemented fragments Glassy texture (sediments) Fossils Organic material Metamorphic Banding Foliated 163 Common Structures of Sedimentary Rocks Sedimentary structures are those structures formed during sediment deposition. Stratification: A layered arrangement in sedimentary rock. Different layers also called beds or strata may be similar or dissimilar. Lamination: Layered structure similar to stratification but layers are quite thin. 164 Cross bedding: layers lying above one another are not parallel having inclined relation. Graded bedding: sediments are arranged according to their grain size. Mud cracks: having many fine sized grains with irregular cracks. Ripple marks: symmetrical wave-like undulations in a layer. 165 General Quiz 166
