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

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 (interpreting "heterolithic unconformity" as nonconformity, a common type where different rock types meet) 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.