List and explain two factors that control the viscosity of magma.

5. (a) Explain how the following features are formed: (i) Zeugen: Zeugen are formed in arid regions by differential wind erosion (abrasion and deflation). They occur where horizontal layers of alternating hard and soft rock are exposed. Wind erodes the softer rock more quickly, leaving behind mushroom-shaped or pedestal-like remnants of the more resistant hard rock. (ii) Ox-bow lake: An ox-bow lake forms from a meandering river. Erosion on the outer bend and deposition on the inner bend cause the meander to become more pronounced. During a flood, the river cuts across the narrow neck of the meander, taking a straighter path. The old meander bend is then cut off from the main river channel by deposition, forming a crescent-shaped lake. (iii) Sea stack: Sea stacks are formed by coastal erosion of headlands. Waves exploit weaknesses (joints, cracks) in the headland, forming sea caves. If two caves on opposite sides meet, or a cave extends through, a sea arch is formed. Continued erosion of the arch's base and roof eventually causes it to collapse, leaving an isolated pillar of rock called a sea stack. 5. (b) Describe and account for the origin of three sedimentary structures that can be used to deduce palaeo environments. • Ripple Marks: Description: Undulating ridges and troughs formed on the surface of sediment. Origin/Palaeo environment: Formed by the movement of water or wind over loose sediment. Symmetrical ripple marks indicate oscillatory flow (e.g., waves on a beach), suggesting a shallow marine or lacustrine environment. Asymmetrical ripple marks indicate unidirectional flow (e.g., river currents, wind in a desert), suggesting fluvial or aeolian environments. • Cross-bedding: Description: Layers within a larger bed that are inclined at an angle to the main bedding plane. Origin/Palaeo environment: Formed by the migration of ripples or dunes under the influence of a current (water or wind). The inclined layers represent the foreset beds of the migrating bedforms. The direction of inclination indicates the direction of the ancient current. Common in fluvial (river), deltaic, shallow marine, and aeolian (desert dune) environments. • Mud Cracks (Desiccation Cracks): Description: Polygonal patterns of cracks formed on the surface of fine-grained sediment (mud). Origin/Palaeo environment: Form when wet, fine-grained sediment dries out and shrinks, causing it to crack. This indicates an environment that was periodically submerged and then exposed to air, allowing for drying. Common in tidal flats, floodplains, and lake margins. 5. (c) (i) Outline the qualities of basalt that can make it useful for construction. Basalt is useful for construction due to its hardness and durability, making it resistant to abrasion and weathering. It possesses high compressive strength, allowing it to withstand significant pressure, which is ideal for foundations and road aggregate. Its resistance to chemical weathering also contributes to its longevity in various construction applications. 5. (c) (ii) Discuss two types of springs. • Gravity Spring (or Depression Spring): This type of spring forms where the ground surface intersects the water table in an unconfined aquifer. Water flows out under the influence of gravity because the water table naturally slopes towards a discharge area, and this slope meets the land surface. • Artesian Spring: An artesian spring occurs where water from a confined aquifer finds a path to the surface, often through a crack or fault, and flows out under hydrostatic pressure. The water in the confined aquifer is under pressure from the weight of overlying impermeable layers, causing it to rise to the surface naturally if the potentiometric surface is above ground level. 6. (a) Briefly discuss Graptolites under the following headings: (i) Mode of fossilisation: Graptolites are typically preserved as carbonaceous films on the bedding planes of fine-grained sedimentary rocks, such as shales and slates. Their chitinous exoskeletons undergo compression and carbonization, leaving a dark, two-dimensional impression of their colonial structure. (ii) Evolutionary changes: Graptolites evolved rapidly, particularly during the Ordovician and Silurian periods. Early forms were sessile and bushy (dendroid graptolites), attached to the seafloor. Later, they became planktonic (free-floating) with simpler, linear colonies called stipes (uniserial or biserial forms). This rapid evolution and wide distribution make them excellent index fossils for dating Paleozoic rocks. 6. (b) With the aid of diagrams, describe the following types of unconformities: (i) Parallel unconformity: A parallel unconformity (also known as a disconformity or paraconformity) is an unconformity where the sedimentary beds above and below the erosional surface are parallel to each other. It represents a period of erosion or non-deposition without significant tilting or folding of the older strata. c Younger parallel beds \\ -------------------- \\ ~~~~~~~~~~~~~~~~~~~~ (Erosional surface) \\ -------------------- \\ Older parallel beds (ii) Heterolithic unconformity (Nonconformity): A nonconformity (assuming "heterolithic unconformity" refers to this common type) is an unconformity where sedimentary strata overlie older igneous or metamorphic rocks. It indicates a period of uplift and erosion that exposed deep-seated igneous or metamorphic rocks at the surface, followed by subsidence and deposition of new sediments. c Sedimentary beds \\ -------------------- \\ ~~~~~~~~~~~~~~~~~~~~ (Erosional surface) \\ XXXXXXXXXXXXXX \\ Igneous/Metamorphic rock (iii) Angular unconformity: An angular unconformity is an unconformity where younger sedimentary layers are deposited horizontally on top of older, tilted or folded sedimentary layers that have been eroded. It signifies significant tectonic activity (tilting/folding), erosion, and then renewed deposition. c Younger horizontal beds \\ -------------------- \\ ~~~~~~~~~~~~~~~~~~~~ (Erosional surface) \\ / / / / / / / / / / \\ Older tilted beds 6. (c) List and explain two factors that control the viscosity of magma. • Temperature: Explanation: Higher temperatures decrease magma viscosity. As temperature increases, the bonds between silica tetrahedra break down, allowing the magma to flow more easily. Conversely, cooler magma is more viscous. • Silica Content (Composition): Explanation: Higher silica content increases magma viscosity. Silica tetrahedra (SiO_4) link together to form long chains and networks. Magmas with high silica content (felsic magmas like rhyolite) have more of these complex structures, making them more resistant to flow. Magmas with low silica content (mafic magmas like basalt) have fewer silica linkages and are therefore less viscous.