ABE101_Lesson_5-1 (2).pdf

ABE101 Introduction to Geo-physical Environment Lecture notes LESSON 5: Rocks and rock forming processes 4.1 Introduction In this lesson, we will continue expounding on the idea of minerals and rocks specifically by looking...

ABE101 Introduction to Geo-physical Environment Lecture notes LESSON 5: Rocks and rock forming processes 4.1 Introduction In this lesson, we will continue expounding on the idea of minerals and rocks specifically by looking into the rock cycle process. This will enable the students to understand the three major classification of rocks and their formation processes. This lesson will help you understand further in terms of the types of rocks found in different parts of Kenya as a result of ancient geologic processes. 5.2 Lesson Learning outcomes By the end of this lesson, you will be able to: 5.2.1 Describe the rock cycle 5.2.2 Explain the formation of igneous rocks 5.2.3 Explain the formation of sedimentary and metamorphic rocks 5.2.1 Rock cycle Rocks can form in a variety of ways. Igneous rocks form from magma (molten rock) that has either cooled slowly underground (e.g., to produce granite) or cooled quickly at the surface after a volcanic eruption (e.g., basalt). Sedimentary rocks, such as sandstone, form when the weathered products of other rocks accumulate at the surface and are then buried by other sediments. Metamorphic rocks form when either igneous or sedimentary rocks are heated and squeezed to the point where some of their minerals are unstable and new minerals form to create a different type of rock. An example is schist. Page 1 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes Source: https://opentextbc.ca/geology/wp-content/uploads/sites/110/2015/07/image003.png Examples of rocks are granite, basalt, sandstone, limestone, and schist. 5.2.2 Formation of Igneous rocks Igneous rocks are formed from molten rock (magma) which cools and crystallizes either above (extrusive igneous rocks) or below (intrusive igneous rocks) the earth’s surface. Molten rocks happen between 800° to 1300°C depending on the material and existing pressure underground. As the minerals in rocks melt, the atoms and molecules they contain are released to form a ‘bath’ of liquid called magma. This liquid mixture of atoms and molecules is usually less dense than the surrounding rock and so tends to rise. As magma rises, it cools down, so that minerals begin crystallizing again. As cooling continues, the first crystals to form become larger, as more molecular building blocks come together and the atomic structures grow. The longer the liquid has to cool, the larger the crystals become. Eventually all the liquid crystallizes and the rock has become a solid mass with a texture of randomly-orientated interlocking crystals. Rocks formed by crystallizing from magma are called igneous rocks. Most magmas crystallize underground, but if magma flows to the surface it is called lava. So solidified lavas are also igneous rocks. There are of two main types of igneous rocks: Plutonic (or Intrusive) rocks Plutonic rocks are formed when the magma does not reach the surface. It solidifies deep in the crust producing a variety of geologic structures such as batholiths and dykes which are exposed on the surface by prolonged action of erosion. Plutonic rocks are also called intrusive igneous rocks. Volcanic (or Extrusive) rocks Volcanic rocks which are poured on the Earth’s surface are called lava. The lava that reaches the Earth's surface through volcanoes or through great fissures hardens and become igneous rock. These are also called extrusive igneous rocks. Some of the more common types of extrusive igneous rocks are lava rocks, cinders, pumice, and volcanic ash and dust. The most common minerals found in igneous rocks are quartz, a compound of silicon and oxygen (SiO2) with a simple but well-bonded atomic structure, and feldspar, another silicon/oxygen (silicate) compound but with extra aluminum, sodium, potassium and calcium, and with an atomic Page 2 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes structure that is nearly as well bonded as quartz. A third common mineral is mica, another silicon/oxygen compound with extra elements added; this is poorly bonded, particularly in one direction, and so has a strong cleavage, is soft and layers are easily broken off. In igneous rocks, quartz is grey, feldspar is white or sometimes pink, and mica is black or colorless. You can see all these minerals in the photograph of granite (Figure 1), which also shows how the minerals have crystallized to fill all the space, in a tough interlocking texture. Figure 1. left is A close up view of a piece of the igneous rock, granite. Right is A close up view of a piece of the metamorphic rock, gneiss. Most of Earth's continental crust is made up of igneous rocks such as granite, diorite, and granodiorite. Feldspars are also important constituents of gabbro and basalt, which are the primary types of rock in Earth's oceanic crust. Page 3 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes Beneath the Earth’s crust is the mantle and all the material that now forms the crust originally came from the mantle. Mantle rock is relatively rich in iron and poor in silicon; it is called an ultramafic rock (‘mafic’ meaning magnesium, ‘ma’ and iron, ‘fic’). When this becomes heated, it partially melts and, since the silicon-rich minerals melt first, the melt is richer in silicon than the original mantle rock. The new magma rises and penetrates the rock above. This is a mafic melt, found mainly at mid-oceanic ridges, above the mantle source rock. Mafic melts cool to form mafic rocks. Where these mafic rocks become heated again, as can happen if they are carried to oceanic trenches, they partially melt, producing a magma which is even richer in silicon, called an intermediate magma. If, in turn, the rocks formed by intermediate magmas are partially melted, as can happen if they are carried beneath continents, a melt even richer in silicon is formed, called a silicic melt. This shows that the four main different compositions of magma are produced by the partial melting of the others in sequence: ultramafic rocks partially melting to mafic melts; mafic rocks partially melting to intermediate melts; and intermediate rocks partially melting to form silicic melts and rocks. IGNEOUS ROCKS MINERAL COMPOSITION On the basis of composition, the igneous rocks are classified in four groups according to the amount of silica (SiO2) content: - Sialic (silica >65%) - Intermediate (silica 65-52%) - Mafic (silica 52-45%) - Ultramafic (silica < 45%) Common silicate minerals present on the igneous rocks can be divided in two big groups: sialic and mafic Sialic Silicates Rich in silicon (Si) and aluminum (Al) Generally light in colour Quartz, Potassium feldspar, Plagioclase, Muscovite Mafic Silicates Rich in iron (Fe) and magnesium (Mg) Dark green or black in colour Pyroxene, Amphibole, Olivine, Biotite Obviously a sialic and intermediate igneous rock is rich in sialic minerals and a mafic and ultramafic igneous rock is rich in mafic minerals. Page 4 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes 1) Sialic igneous rocks: Contain abundant silicon aluminum silicates (Si, Al) Usually light in color Rock types include: Granite (intrusive) Rhyolite (extrusive) Principal Minerals White: Quartz and Potassium feldspar Dark: Biotite 2) Mafic igneous rocks: Contain abundant ferromagnesian silicates (Mg, Fe) Usually dark in colour Rock types include: Gabbro (intrusive) Basalt (extrusive) Principal Minerals White: Plagioclase Dark: Pyroxene 3) Intermediate igneous rocks: Intermediate in composition between sialic and mafic Usually light in color Rock types include: Diorite (intrusive) Andesite (extrusive) Principal Minerals White: Plagioclase Dark: Amphibole Page 5 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes 3) Ultramafic igneous rocks: Almost entirely magnesium and iron silicates Usually dark in color Rock types include: Peridotite (intrusive) Minerals Dark: Olivine (green) and Pyroxene E-tivity 5.2.1 – Igneous rocks Numbering, pacing 5.2.1 and sequencing Title Formation of Igneous rocks Purpose To introduce you to igneous rocks, their characteristics and origin. Page 6 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes Brief summary of Watch videos: overall task Rock cycle: review of basic types of rocks Igneous Rocks Plutons: differences between intrusive and extrusive igneous rocks Read chapter 1 book by Chris King Read open Physical Geology - 2nd Edition E-book by Steven Earle: Chapter 3: Intrusive igneous rocks Online forum Discussion: where are igneous rocks found in Kenya? Spark Igneous rock. Source: https://commons.wikimedia.org/wiki/File:Igneous_Rock_show ing_Magma_Mixing_Structures_Kosterhavet_Sweden_2.jpg Can you identify an intrusive/extrusive igneous rock by colour/texture? Individual task Watch videos provided state where igneous rocks are found in Kenya give examples of uses of various types of rocks in the built environment. Next Sedimentary and metamorphic rocks 5.2.2 Sedimentary and Metamorphic rocks Metamorphic rocks When rocks are heated or come under great pressure, the minerals in them can be recrystallized without melting and this recrystallisation process is called metamorphism. Page 7 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes When rocks are heated or come under great pressure, the minerals in them can be recrystallized without melting and this recrystallisation process is called metamorphism. During metamorphism the sizes and shapes of the original minerals can be changed or original minerals can be changed into new minerals. The result is a tough rock with an interlocking texture. If the metamorphism is caused by high temperature alone, then the new metamorphic minerals have random orientations. However, if the metamorphism is caused by high temperatures and pressures in the roots of mountains during mountain building episodes, the metamorphic minerals recrystallize at 90° to the direction of the pressure. In high pressure/temperature metamorphic rocks, many of the minerals are therefore aligned into a rock texture of sheets or bands, as seen in the photograph in Figure 1.7. The minerals quartz, feldspar and mica found in igneous rocks are also common in metamorphic rocks, but a mineral that is not frequently found in other rocks, but is common in higher grades of metamorphic rock, is garnet. Garnet (Figure 1.8) is another silicate mineral, where the silicon/oxygen has combined with aluminium and calcium, iron or magnesium. It has a strongly- bonded structure making it a hard mineral. Due to this hardness and its pleasing red or pink colours, it is sometimes used as a semi-precious gem mineral. If the metamorphosed rock was originally made of calcium carbonate (i.e. limestone), then the rock produced by metamorphism is also composed of the calcium carbonate mineral, calcite. The new rock, formed of interlocking calcite crystals, is called marble (Figure 1.9). Page 8 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes The main types of metamorphism are: 1) Contact metamorphism 2) Cataclastic metamorphism 3) Regional metamorphism Contact metamorphism Contact metamorphism occurs locally, at and near the igneous intrusions. The metamorphic agent is the high temperature near the igneous intrusion. Most common rocks are: HORNFELS Cataclastic metamorphism Metamorphism is due to motion on faults. The metamorphic agent is the direct pressure (a major pressure in one single direction). Page 9 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes Most important rocks are MYLONITES. They are located near the fault plane. Regional metamorphism Regional metamorphism is usually caused during mountain building (ore genesis). The metamorphic agents are high pressure and high temperatures. The intensity of metamorphism (metamorphic grade) is the amount of difference between the original parent rock and the metamorphic rock. It is indicated by the temperature: ▪ Low (200°-300°) ▪ Intermediate (300°-450°) ▪ High (450°-800°) An original rock can be changed into different metamorphic rocks on the basis of the metamorphic grade. Page 10 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes Classification of metamorphic rocks based on structures: Foliated/Non foliated ▪ Foliation refers to flat or wavy planar features (looking like layers) caused by the alignment of platy minerals such as argillaceous minerals and mica. Page 11 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes Evaporites When salt water evaporates as lakes or arms of the sea dry out, salts crystallise out as deposits that cover the old lake or sea bed (Figure 1.10). These are called evaporite deposits and the most common of these is the compound of sodium and chlorine, halite. The sodium chloride (NaCl) molecules are weakly bonded, so halite is a mineral with a very low hardness. The sodium chloride bonds are broken in water so that halite is soluble and easily dissolves again. Because of this solubility and its salty taste, a refined form of halite is used as table salt in many of the foods you eat. Page 12 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes Sedimentary rocks When small rock fragments and minerals are eroded and transported and deposited by water or glacier to new locations as sediments, sedimentary rocks are formed. The forming action is through continuous deposition of sediments which pile up and consequently piling pressure which eventually lead to cementation of the sedimentary rocks at depths of hundreds of meters and above. Often the grains of sediment are single crystals which have been broken and worn down. These have often become rounded during transportation and sorted into grains of similar sizes. After they have been deposited, water flows in the pore spaces between the grains. This slow-flowing water dissolves minerals from some parts of the deposit and they recrystallize between the grains in other parts of the deposit, ‘gluing’ the grains together. This natural ‘glue’ is called cement and changes soft sediments into harder sedimentary rocks. When these rocks are heated up in deeper depths, in the earth’s crust, due to piling pressure, they are transformed into a new structure resulting into metamorphic rocks. Three types of sedimentary rocks are recognized: Mechanically-Formed Sedimentary Rocks: They are deposited by water, ice, or wind. They include clays, gravels, and alluviums (deposited by water), moraines, boulder clay, boulder clay and gravels (deposited by ice), and loess (deposited by wind). Organically Formed: They are formed from plants (peat, coal and lignite) and animals (chalk and coral). Chemically Formed: These include potash, gypsum, nitrates, certain types of limestone and rock salt. Sedimentary rocks can be distinguished in three different types: Clastic rocks Chemical rocks Organic rocks i. Clastic rocks: Clastic rocks derive by the weathering of pre-existing rocks, which have been transported to the depositional basin. These rocks are classified according to their texture (grain size) Page 13 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes ii. Chemical rocks Chemical sedimentary rocks are formed by chemical precipitation. Page 14 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes iii. Organic rocks Organic sedimentary rocks are formed by accumulation of shells of organisms or plants Page 15 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes The Sedimentary Stages of the Rock Cycle SUMMARY OF MAIN ROCKS Page 16 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes E-tivity 5.2.2 – Sedimentary and Metamorphic rocks Numbering, pacing 5.2.2 and sequencing Title Sedimentary and metamorphic rocks Purpose To introduce you to Sedimentary and metamorphic rocks, their characteristics and origin. Brief summary of Watch videos: overall task Sedimentary Rocks: o Weathering and sedimentation, o Identifying sedimentary rocks. Metamorphic Rocks: o Metamorphism o Identifying metamorphic Rocks Read chapter 1 book by Chris King Read open Physical Geology - 2nd Edition E-book by Steven Earle: Chapter 6 & 7: Sedimentary and metamorphic rocks Spark Layers of sedimentary rocks. Metamorphic rock. Source: Source: https://commons.wikimedia.org https://commons.wikimedia.org /wiki/File:Metamorphic_rock.j /wiki/File:Layers_of_sedimenta pg ry_rock_in_Makhtesh_Ramon_ (50755).jpg Can you identify Sedimentary and metamorphic rocks by colour/texture? Individual task Watch video provided in the links state where Sedimentary and metamorphic rocks are found in Kenya and their possible use in built environment. Next Surface Weathering and Erosion Rocks and Stones used in Construction1 COAL: 1 Source: Minerals, Rocks and Stones used in Building Construction in Civil Engineering (aboutcivil.org) Page 17 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes A sedimentary rock, formed from decayed plants, is mainly used in power plants to make electricity. LIMESTONE: A sedimentary rock, it is used mainly in the manufacture of Portland cement, the production of lime, manufacture of paper, petrochemicals, insecticides, linoleum, fiberglass, glass, carpet backing and as the coating on many types of chewing gum. SHALE: A sedimentary rock, well stratified in thin beds. It splits unevenly more or less parallel to bedding plane and may contain fossils. It can be a component of bricks and cement. CONGLOMERATE: A sedimentary rock with a variable hardness, consisted of rounded or angular rock or mineral fragments cemented by silica, lime, iron oxide, etc. Usually found in mostly thick, crudely stratified layers. Used in the construction industry. SANDSTONE: A sedimentary rock more or less rounded. Generally thick-bedded, varicolored, rough feel due to uneven surface produced by breaking around the grains. Used principally for construction, it is easy to work, the red-brown sandstone of Triassic age, better known as "brownstone," has been used in many eastern cities. GRANITE: An igneous-plutonic rock, medium to coarse-grained that is high in silica, potassium, sodium and quartz but low in calcium, iron and magnesium. It is widely used for architectural construction, ornamental stone and monuments. PUMICE: An igneous-volcanic rock, it is a porous, brittle variety of rhyolite and is light enough to float. It is formed when magma of granite composition erupts at the earth’s surface or intrudes the crust at shallow depths. It is used as an abrasive material in hand soaps, emery boards, etc. GABBRO: An igneous-plutonic rock, generally massive, but may exhibit a layered structure produced by successive layers of different mineral composition. It is widely used as crushed stone for concrete aggregate, road metal, railroad ballast, etc. Smaller quantities are cut and polished for dimension stone (called black granite). BASALT: An igneous volcanic rock, dark gray to black, it is the volcanic equivalent of plutonic gabbro and is rich in ferromagnesian minerals. Basalt can be used in aggregate. Page 18 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes SCHIST: A metamorphic uneven-granular, medium to coarse grained, crystalline with prominent parallel mineral orientation. Goes from silvery white to all shades of gray with yellow to brown tones depending on the mineral concentration. Some schists have graphite and some are used as building stones. GNEISS: A metamorphic uneven granular medium to coarse grained crystalline with more or less parallel mineral orientation. Colors are too variable to be of diagnostic value. Due to physical and chemical similarity between many gneisses and plutonic igneous rocks some are used as building stones and other structural purposes. QUARTZITE: A metamorphic or sedimentary rock with crystalline texture, consists of rounded quartz grains cemented by crystalline quartz, generally white, light gray or yellow to brown. Same uses as sandstone. MARBLE: A metamorphic even-granular grain to medium grained and may be uneven granular and coarse grained in calc-silicate rock. The normal color is white but accessory minerals act as coloring agents and may produce a variety of colors. Depending upon its purity, texture, color and marbled pattern it is quarried for use as dimension stone for statuary, architectural and ornamental purposes. Dolomite rich marble may be a source for magnesium and is used as an ingredient in the manufacture of refracting materials. AMPHIBOLITE: Amphibolite is a coarse-grained metamorphic rock that is composed mainly of green, brown, or black amphibole minerals and plagioclase feldspar. Amphibolite has a variety of uses in the construction industry. It is harder than limestone and heavier than granite. These properties make it desirable for certain uses. Amphibolite is quarried and crushed for use as an aggregate in highway construction and as a ballast stone in railroad construction. It is also quarried and cut for use as a dimension stone. Uses of rocks and minerals in the built environment The table below shows a list the materials used to build a house and explanations on how they are formed/sourced. Material Use Source Sand Pad level Fossil dunes or deposits from fast flowing rivers Pad Cement Fossil dunes or deposited from fast flowing rivers Mortar between bricks As above plus limestone Concrete Pad & floors Treated limestone Page 19 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes Clay Brick Fossil river beds Gyprock Wall ceilings Gypsum from old salt lakes and paper from trees Tiles & porcelain Fossil river beds Rock Wall, slate floors, granite Depends on rock surfaces Wood Doors, windows, floors Trees feeding on soil Copper Wiring Igneous & metamorphic rock Iron & steel Structural support Igneous & metamorphic rock Paint Decoration/protection Petroleum products Rocks and the products of weathered rock are the source of these materials. What Earth processes are responsible for the formation of the following building materials? Material Product Granite for structure and surfaces Igneous activity and tectonic uplift. Sand for pad, cement, mortar and glass. Weathering and erosion of acid (silica rich) rocks. Deposition from medium flowing rivers or wind. Clay for pipes, roofing tiles, bathroom Deep weathering of Aluminium rich rocks furniture and fittings (both acid and basic igneous rocks). Deposition of fine clay particles by very slow- moving mature river system. Sand for glass has often been reworked by sea or wind to become almost pure SiO2. Iron for structural steel Iron ore - Deep weathering of volcanic iron rich rocks and rapid deposition in oxygen poor environment. (Banded Iron Formation of WA) Burial, compaction and cementation. Carbon – burning fossil fuels in oxygen poor environment Marble for surfaces and decoration Marine organisms in a clear sea lay down lime shells. Burial and groundwater cause compaction, cementation to form limestone. Moderate metamorphism converted the limestone into marble Slate for roofs and floors Metamorphosed clay (see above) Concrete for floors and structural concrete Heated limestone (see above) mixed with water and sand (see above) Gypsum CaSO4 2H2O for plaster and Salt rich and detritus poor water is allowed to plasterboard dry out to form salt lakes. Evaporite beds are created. Gypsum can also be formed from hydrothermal spring waters being evaporated. Since gypsum dissolves easily, groundwater can carry the original material over great distances to be deposited elsewhere. Page 20 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes 5.3 Assessment questions 1. Mafic rocks are rich in? a. Iron b. Potassium c. Magnesium d. sodium 2. Felsic rocks have high levels of what minerals than mafic rocks? a. Iron b. Potassium c. Magnesium d. sodium 3. Sedimentary rocks can be distinguished in three different types: a. Clastic rocks b. Organic rocks c. Sediment rocks d. Chemical rocks 4. Select the main types of metamorphism a. Contact metamorphism b. Global metamorphism c. Regional metamorphism d. Cataclastic metamorphism 5. What earth processes are responsible for formation of the following building materials? a. Granite ______________ b. Clay _________________ c. Iron __________________ d. Marble ________________ e. Slate ________________ 5.5 References 1 Earth Rocks. (2019, June 25). Rock cycle [Video]. YouTube. https://www.youtube.com/watch?v=N2BKjELzhfU&list=PLrhG2NtyHAZuPW5 HP3cyenGGTUqUhumeQ&index=8 2 Earth Rocks. (2015, June 26). Identifying Igneous rocks [Video]. YouTube. https://www.youtube.com/watch?v=cjyF- te5lQI&list=PLrhG2NtyHAZuPW5HP3cyenGGTUqUhumeQ&index=13 3 Earth Rocks. (2019, January 22). Plutons [Video]. YouTube. https://www.youtube.com/watch?v=uIQTfnJg_aA&list=PLrhG2NtyHAZuPW5H P3cyenGGTUqUhumeQ&index=15 4 Earth Rocks. (2019, February 5). Weathering and Sedimentation [Video]. YouTube. https://www.youtube.com/watch?v=ub9NCVIMBoY&list=PLrhG2NtyHAZuPW5 HP3cyenGGTUqUhumeQ&index=18&pbjreload=101 Page 21 of 22 ABE101 Introduction to Geo-physical Environment Lecture notes 5 Earth Rocks. (2015, June 26). Identifying sedimentary rocks [Video]. YouTube. https://www.youtube.com/watch?v=U3eR19ZfBqE&list=PLrhG2NtyHAZuPW5H P3cyenGGTUqUhumeQ&index=19 6 Earth Rocks. (2019, February 19). Metamorphism [Video]. YouTube. https://www.youtube.com/watch?v=N7SiZSa5csA&list=PLrhG2NtyHAZuPW5H P3cyenGGTUqUhumeQ&index=20 7 Earth Rocks. (2015, June 26). Identifying metamorphic rocks [Video]. YouTube. https://www.youtube.com/watch?v=HUydPhIaQQU&list=PLrhG2NtyHAZuPW5 HP3cyenGGTUqUhumeQ&index=21 8 Earle, S. (2015). Physical Geology. Victoria, B.C.: BCcampus. Retrieved from https://opentextbc.ca/geology/ 9 King, C. (2010). The planet we live on - the beginnings of the Earth Sciences. (R. McWeeny, Ed.) (Book 6). Manchester: http://www.learndev.org/. Retrieved from http://www.learndev.org/dl/Science/EarthScience/ThePlanetWeLiveOn.pdf Page 22 of 22