(a) Describe the characteristics of any two of the following types of landforms (i) delta (ii) atoll (iii) caldera (b) Explain with the aid of diagrams the ...

Choosing (i) delta and (iii) caldera.

(a) Characteristics

Delta

• A roughly triangular or fan-shaped mass of alluvium built up where a river enters a sea or lake and drops its load.
• The main channel splits into many branching distributaries separated by low, flat, marshy islands and mud-flats.
• Composed of fine, fertile silt, sand and clay; very low-lying and liable to flooding (for example the Nile and Niger deltas).

Caldera

• A very large, roughly circular, basin-shaped hollow at the summit of a volcano, far wider than an ordinary crater (often several kilometres across).
• Bounded by steep inward-facing walls; the floor may hold a crater lake or later small cones (for example Ngorongoro).

(b) Mode of formation (with diagrams)

Delta: As a heavily-laden river reaches the still water of a sea or lake, its speed and carrying power suddenly fall. It deposits its sediment, coarsest first, building up beds of alluvium. Continued deposition blocks the mouth so the river divides into distributaries that push the land seaward. It forms best where the load is large, the current gentle and the tide/waves weak. A diagram shows the river dividing into distributaries across a triangular deposit.

Caldera: A caldera forms when a violent eruption empties the magma chamber beneath a volcano, or when the summit is blown off by an explosion. Losing this support, the unsupported top of the cone collapses inward (subsides) into the emptied chamber, leaving a huge circular depression far wider than the old vent. A diagram shows the cone before eruption, the emptied chamber, and the collapsed wide basin (caldera) afterwards.