Endogenic and exogenic processes

Endogenic processes get their energy from inside the earth: mostly radioactivity, rotational and tidal friction, and primordial heat left from the earth's origin. Uneven heat flow, crustal thickness and strength make their effects uneven too.

Diastrophism includes all processes that move, raise or build up parts of the crust:

• Orogeny: mountain building through severe folding of long, narrow belts.
• Epeirogeny: uplift or warping of large parts of the crust: continent building, with simpler deformation.
• Earthquakes: local, relatively minor movements.
• Plate tectonics: horizontal movement of plates.
• These cause faulting and fracturing, and changes in pressure, volume and temperature (PVT) that lead to metamorphism of rocks.

Volcanism: the movement of magma towards or onto the surface, and the intrusive and extrusive forms it builds.

Exogenic processes take their energy from the atmosphere, ultimately from the sun, and from gradients made by tectonic factors.

• Gravity pulls material down any slope. Force per unit area is stress; stress from pushing or pulling causes deformation. Shear stresses act along surfaces and break rocks. Molecular stresses come from temperature changes, crystal growth and melting; chemical processes loosen the bonds between grains.
• So the basic cause of weathering, mass movement and erosion is stress in earth materials.
• All exogenic processes together are called denudation ('denude' means to strip off or uncover): weathering, mass wasting, erosion and transportation.
• They vary by climate: temperature and precipitation, vegetation, and even slope direction (north- or south-facing) change their rates.
• With climate equal, the rate depends on rock type and structure: folds, faults, joints, bedding, hardness, permeability and chemical make-up. A rock may resist one process and give way to another.

Example (new): a cut stone wall built of granite on one side and sandstone on the other, standing in the same rain and sun, will wear unevenly: the softer, more porous sandstone crumbles first. Rock type, not climate, makes the difference.