Types of weathering
Physical (mechanical) weathering breaks rock apart without changing its chemical composition -- for example, water repeatedly freezing and expanding in cracks, or a rock's outer layers expanding and contracting from daily heating and cooling until they flake off. Chemical weathering changes a rock's chemical composition, weakening it -- for example, rainwater (slightly acidic) dissolving limestone, or oxygen reacting with iron-rich rock to form rust-like compounds.
Example 1: A rock in a hot desert repeatedly heats up strongly during the day and cools sharply at night, eventually cracking and flaking without any change to its mineral composition. Identify the type of weathering.
This is physical (mechanical) weathering -- specifically thermal expansion and contraction -- since the rock breaks apart from repeated stress, with no chemical change to the minerals themselves.
Agents of erosion
Water erosion (rivers, waves, rainfall runoff) is the most widespread agent, carving valleys and coastlines. Wind erosion is significant in dry, exposed areas with little vegetation to hold soil in place. Ice erosion (glaciers) carves distinctive U-shaped valleys, though this is not significant in most of West Africa's climate.
Soil formation and the soil profile
Soil forms gradually from weathered rock (the parent material) mixed with organic matter from decomposing plants and animals. A soil profile typically shows distinct horizons: topsoil (dark, nutrient- and organic-matter-rich, where most plant roots and biological activity are), subsoil (lighter, less organic matter, often where leached nutrients accumulate), and weathered parent rock below.