SKEMA TEST 1 BFC23013_SEM 1 SESSI 20152016 QUESTION 1(a): (6 (6 Marks) Marks)
(a) Gradation Degradation: Erosion results from wearing of rocks by water, air and ice. Aggradation: Deposition results in accumulation of sediment sediment and ultimate building up of rock strata. (b) Tectonism Plate tectonics is a dynamic process of the lithospheric plate which moves over a weak plastic layer in the upper mantle known as asthenosphere. These plates interact with one another along their boundaries. Indicative of crustal instability, produce faulting (fracture and displacement), displacement), folding, subsidence and uplift of rock formation. Responsible for formation of mountain ranges. (c) Volcanism A volcano is a vent in the earth's crust through which molten rock materials within the earth, lavas, ashes, steam and gas are ejected and responsible for the formation of plutonic rocks, once solidified at great depth. Majority of volcanoes are located along the margins of tectonic plates.
QUESTION 1(b): (4 ( 4 Marks) Marks)
1. 2.
3. 4.
Divergent boundaries boundaries - where new crust i s generated as the plates pull away from each other. Convergent boundaries boundaries - where crust is destroyed as one plate dives under another. It can divide into three: (1) Oceanic - continental convergence (2) Oceanic - oceanic convergence (3) Continental - continental convergence Transform boundaries boundaries - where crust is neither produced nor destroyed as the plates slide horizontally past each other. Plate boundary zones zones - broad belts in which boundaries are not well defined and the effects of plate interaction interaction are unclear. unclear.
QUESTION 1(c): (4 ( 4 Marks) Marks)
They occur naturally as inorganic solids. They have a specific internal structure; that is, their atoms are precisely arranged into a crystalline solid. They have a chemical composition that varies within definite limits and can be expressed by chemical formula. They have definite set of physical properties (hardness, cleavage, crystal form etc) that result from their crystalline structure and composition.
QUESTION 1(d): (1 ( 1 Mark based on any two of properties below )
Crystal form and shape (external appearance) appearance) Color Streak Cleavage and fracture Luster Hardness Reaction with acid
TEST 1: BFC21303
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QUESTION 2(a): (3 Marks)
1)
Volcanic or extrusive igneous rocks: form when the magma cools and crystallizes on the surface
2)
of the Earth. Intrusive or plutonic igneous rocks: form when the magma crystallizes at depth in the Earth.
QUESTION 2(b): (5 Marks based on sketches, label & any related explaination of below facts )
Dykes
Tabular or wall like mass. Results from magma injected into cracks and joints in rocks. Vary in width from a few cm to a few meters but not more than 3 meters wide. Largest known dyke in Zimbabwe, Africa (600 km long and average width of 10 km).
Sills
Rising magma follows path of least resistance such as bedding plane, which separates layers of sedimentary rock. Magma injected between the layers form tabular intrusive body parallel to layering. Sills range from few centimeters to hundreds of meters thick and can extend to several kilometers.
Laccoliths
Viscous magma injected between layers of sedimentary rock, tend to uparched the overlying strata forming mushroom shaped. Usually thicker in center and thinner near margin and may give rise to dome shaped hill. Can be several kilometers in diameter and thousands of meters thick and typically porp hyritic.
Batholiths
Largest rock bodies in the Earth's crust, generally granitic composition. Cover several thousand square kilometers and may be 60 km thick. Typically form in the deeper zones of mountain belts and are exposed only after considerable uplift and erosion.
TEST 1: BFC21303
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SEMISESSION20152016
QUESTION 2(c): (5 Marks based on any related explaination of below facts )
The genesis of sedimentary rocks involves four major processes which are: a) b) c) d)
Weathering Transportation Deposition Lithification (Compaction and Cementation)
Weathering
Weathering is a number of chemical and mechanical processes that act to break up rocks such as an interaction between rocks exposed at the Earth's surface and elements in the atmosphere. The preexisting rocks can disintegrate and decompose either by physically or chemically and forms layer of loose, decayed rock debris or soil. The unconsolidated material can then be transported easily by various agents such as streams, wind, groundwater and glaciers. For example, once surface rocks have been broken up into fragments by weathering processes, erosion (by wind and moving water) can transport the detrital material away from its source region to a new location where these new sediments can be deposited.
Transportation
Running water is the most effective form of sediment transport. Large quantities of sediment are carried towards the sea and deltas are formed from sediment transported by rivers. Wind and glaciers also transport sediment although restricted to certain climatic zones. Sorting that occurs during transportation is an important factor in the genesis of sedimentary rock. Water and air are fluids, thus the size of detrital material that can be transported depends on the velocity (i.e. energy) of the fluid. In other words, rapidly moving water or air can transport larger grain size detrital material than more slowly moving water or air. Ice, on the other hand, is a solid. Thus, ice can transport all sizes of sediment independent of the velocity at which the ice is moving. In the case of transport by water or air, sediments are deposited at l ocations where the velocity of the fluid decreases. For example, consider a river flowing out of the mountains into a lake.
Deposition
Deposition process takes place due to settlement of sediments and loose aggregates. The most significant factor in the origin of sedimentary rocks is the environment that exists where the sediment is deposited. The depositional environment determines the characteristics of sedimentary rock formed (physical, chemical and biological condition) e.g. type of transporting agent, geochemical parameters such as pressure, oxygen, temperature, and flow characteristics of depositing fluid (velocity). Distinctive types of texture, composition, internal structure, and fossil assemblages are thus developed in each deposition.
TEST 1: BFC21303
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SEMISESSION20152016
L ith if ication (Compaction and Cementation)
The process of converting unconsolidated sediments into sedimentary rocks is called lithification (Compaction and Cementation). Compaction is the process whereby loose sediments are compacted to a denser state by additional stress from accumulated material deposited from time to time or even tectonic forces. The process of expulsion of water from the void spaces between particles takes place as they are forced closer together. Clayey-rich sediments can be compacted much better than sands.
QUESTION 2(d): (6 Marks based on any related facts below) Contact Metamorphism Occurs when country rocks are surrounded by igneous intrusion and altered by intruding magma.
Physical changes such as recrystallisation occur due to contact metamorphism when original minerals in country rock are permeated by magmatic fluid. For example, limestone (country rock/preexisting rock) intruded by hot magma may be altered from distance of few inch to several miles from line of contact between the two rocks. Heat is the most significant influence in contact metamorphism. Metamorphic rock formed in a baked-zone of the altered country rock.
Igneous Intrusion
Figure 3.56 Contact metamorphism occurs at X region
TEST 1: BFC21303
Regional Metamorphism The zones of rock alteration are much more extensive (large scale) than rocks altered under contact metamorphism.
Regional metamorphism occurs due to the effects of both pressure and temperature. The rock layers undergo structural deformation (folded, crushed or fractured) due to great pressures exerted on it and therefore results in the obliteration of any indication of fossils or stratification and realignment of mineral grains.
Dynamic Metamorphism Dynamic metamorphism is produced by variable strain, variable pressure, variable temperature and high fluid pressure and normally occurs in active fault zones.
Dynamic metamorphism is metamorphism of rock masses caused primarily by stresses that yield relatively high strain (deformation) rates. More simply, it is metamorphism resulting from deformation. Temperatures during dynamic metamorphism are typically elevated and may be caused by the deformation process.
Regional metamorphism must occur deep within the crust, at least at depths Fluids commonly of 10 km or more and is contribute to the known to be responsible metamorphic process, for the forming of both by altering chemistry mountain ranges. and by aiding recrystallization. Specific group of minerals present in rock can be Dynamic metamorphism used to infer a certain occurs in fault zones when metamorphic grade. country rock is ground up and partially Rocks subjected to high recrystallized. temperatures and pressures are of high Rocks formed by dynamic
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grade. Under various metamorphic grades, different minerals can be produced from the same original rock. For example, kyanite, sillimanite and andalusite have the same chemical composition but different internal structures.
Q3
metamorphism display a sugary texture, moderate foliated texture, and small round rock fragments within the foliation. Rocks types of Dynamic Metamorphism such as breccias and mylonite.
With reference to the geological map attached in Appendix 1, answer the following questions: (a)
Construct the lines of strike for each rock boundary. (4 marks)
(b)
Examine the dip direction and dip angle for each rock boundary. (4 marks)
(c)
Construct the topographical cross section along the line X-Y with a suitable scale. (3 marks)
(d)
Construct the geological information (distribution of rocks, rocks thickness and their dip angles) into the topographical cross section between X-Y. (4 marks)
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SEMISESSION20152016
Q3(a)(b)
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Q3(c)(d)
TEST 1: BFC21303
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