Physical Geography: Earth Structure, Rocks, Weathering and Landform Processes
Earth structure and plate tectonics
The Earth has a crust (continental sial, oceanic sima), a mantle, a liquid outer core and a solid inner core. The lithosphere is broken into plates that diverge, converge or slide past each other. Malawi lies at the southern end of the East African Rift, a divergent zone where tension has produced normal faults, grabens and a rift lake.
Rocks and the rock cycle
Igneous rocks form from magma (intrusive: granite, gabbro; extrusive: basalt). Sedimentary rocks form by deposition and compaction (sandstone, limestone, shale). Metamorphic rocks form under heat and pressure (marble from limestone, gneiss from granite). The rock cycle links them through melting, weathering, erosion, deposition and metamorphism. The Mulanje Massif is an intrusive syenite body; Karoo sandstones and coal occur in the north.
Weathering and landforms
Weathering is in-situ breakdown; erosion involves transport. Mechanical weathering includes freeze-thaw, exfoliation and salt crystallisation. Chemical weathering includes carbonation, oxidation and hydrolysis of feldspar to clay. In Malawi's hot wet lowlands hydrolysis and carbonation dominate; freeze-thaw operates above about 2,000 m on Mulanje and Nyika. Inselbergs and dambos are typical residual landforms.
Worked example
Why does the Livingstone–Nyasa rift have steep escarpments? Reasoning: tensional stress at a divergent margin causes normal faulting; one block drops to form a graben; adjacent blocks are uplifted as horsts and escarpments; rivers then steepen and erode the fault lines. The chain plate force → fault type → landform must be stated.
Traps
Mixing weathering with erosion; calling every igneous rock volcanic; attributing carbonation to acid rain; ignoring altitude control on freeze-thaw; confusing earthquake magnitude with intensity.