Shaping the earth's surface

Why is the earth's surface uneven? The crust is dynamic: it has moved, and still moves, vertically and horizontally. Two sets of forces act on it:

• Endogenic forces come from within the earth. They are mainly land-building: they raise and build up parts of the surface.
• Exogenic forces come from the atmosphere, driven ultimately by the sun's energy. They are mainly land-wearing.
• Exogenic forces cause degradation (wearing down high ground) and aggradation (filling up basins and depressions). Together this levelling is called gradation.
• Because endogenic forces keep building relief up, exogenic processes never manage to make the surface flat. Unevenness lasts as long as the two opposing sets keep acting.

Geomorphic processes are the endogenic and exogenic forces that cause physical stress and chemical action on earth materials and change the shape of the surface.

• Endogenic processes: diastrophism and volcanism.
• Exogenic processes: weathering, mass wasting, erosion and deposition.

Processes and agents:

• A process is a force applied to earth materials.
• An agent is a mobile medium that removes, transports and deposits materials: running water, groundwater, glaciers, wind, waves and currents. These become mobile because of gradients (slopes).
• Gravity drives all downslope movement and also stresses earth materials. Without gravity and gradients, there would be no erosion, transport or deposition. All movement goes from higher to lower levels, or high to low pressure.

Why it matters: people depend heavily on the land surface, and overuse is reducing its potential quickly. Understanding how it was shaped helps us use it without destroying it for the future.

Example (new): in a sand pit, a child keeps heaping sand into a mound while rain keeps washing it down. As long as the child keeps building, the pit never becomes flat: the child is like the endogenic forces, the rain like the exogenic ones.