(a) What are lithospheric plates? Discuss the theory of plate tectonics. Which plates were involved when a major earthquake…

(a) Lithospheric plates, plate tectonics and the 2004 tsunami

Lithospheric plates are rigid slabs of crust and upper mantle, about 100 km thick, that move over the weaker asthenosphere. There are about seven major plates (Pacific, Eurasian, African, North American, South American, Indo-Australian and Antarctic) and many minor plates.

Plate tectonics states that the lithosphere is broken into plates that move a few centimetres a year, and that most earthquakes, volcanoes and mountain building occur at their margins. Margins are of three kinds: divergent (new crust at mid-ocean ridges, as the Mid-Atlantic Ridge), convergent (subduction at ocean–ocean and ocean–continent margins, and collision at continent–continent margins, as in the Himalaya), and transform (sliding, as the San Andreas Fault). Driving forces are mantle convection, ridge push and slab pull.

Evidence: the fit of continents, matching fossils and rocks, palaeomagnetic polar wander, symmetrical magnetic stripes and increasing age of ocean floor away from ridges (sea-floor spreading), the Wadati–Benioff zones of subduction earthquakes, and hotspot tracks. Oceans open and close in a Wilson cycle.

The 26 December 2004 Sumatra–Andaman earthquake (about Mw 9.1) occurred where the Indian plate (part of the Indo-Australian plate) subducts beneath the Burma microplate, a part of the Sunda plate. A great thrust rupture, over 1,000 km long, lifted the sea floor and generated the Indian Ocean tsunami.

(b) Geomorphic processes; fluvial aggradational and degradational landforms

Geomorphic processes are the physical and chemical actions that modify the Earth's surface. They are endogenic (diastrophism, folding, faulting, volcanism, earthquakes), which build relief, and exogenic (weathering, mass wasting, erosion, transport and deposition by water, wind, ice and waves), which wear it down. Exogenic work levels relief by degradation and builds low relief by aggradation, together called gradation.

Fluvial processes are the work of running water: erosion by hydraulic action, abrasion and solution, transport in suspension, saltation and bed load, and deposition where velocity falls.

Degradational landforms: V-shaped valleys, gorges and canyons, waterfalls and rapids, potholes, river terraces, and entrenched meanders, ending in a peneplain.

Aggradational landforms: alluvial fans, flood plains, natural levees, point bars and meander scars (oxbow lakes), braided bars, and deltas.

(c) Attitude of a bed and its representation on a map

The attitude of a bed is its orientation in space, given by strike and dip.

Strike is the direction of the horizontal line formed by the intersection of the bedding plane with a horizontal plane. Dip direction is the compass direction in which the bed slopes down, at right angles to the strike. Dip amount is the angle of the steepest slope of the bed below the horizontal, measured in a vertical plane at right angles to the strike.

On a map, a strike line is drawn with a short tick at its middle pointing in the dip direction and the dip angle written beside it. A horizontal bed is shown by a circle with a cross inside it. A vertical bed is shown by a strike line with a short bar crossing it, extended on both sides.

Outcrop patterns: horizontal beds follow the contours and give a dendritic pattern; vertical beds outcrop in straight lines regardless of relief; inclined beds give V-shaped outcrops across valleys, according to the rule of V's, with the V pointing in the direction of dip where the dip is steeper than the valley slope.