Weathering, Erosion and Sedimentary Rock Formation

1. Weathering

Weathering is the alteration of surface rocks by atmospheric agents, such as moisture or air.

1.1 Physical weathering

Physical weathering consists of fragmentation or disintegration of rocks by physical processes, without chemical modification of their composition. Thermal variations cause rock rupture. In cold latitudes or at high altitude, water freezes in cracks and holes in rocks, forming ice wedges that increase fracturing. In warm climates, evaporation can concentrate dissolved salts and form salt crystals, which function much like ice wedges and promote disintegration.

1.2 Chemical weathering

Chemical weathering involves changes in the chemical composition of rocks because of chemical reactions. Examples include:

• Oxidation: oxygen oxidizes minerals in the rocks.
• Hydrolysis: water breaks down certain mineral constituents.

2. Erosion, transport and sedimentation

The rock surface is acted on by external geological agents: running water, rivers, groundwater, the sea, ice and wind. All these agents ultimately derive their energy from the sun, and their effect is enhanced by gravity. The forces of these geological processes alter surface rocks: erosion mobilizes weathering products, transport moves those products, and finally sedimentation deposits them in low-lying areas of the Earth's surface.

2.1 Erosion

Erosion is the process that involves the detachment and movement of material.

2.2 Transport

The ability of different geological agents to transport material depends on how much energy they possess:

• Wind: carries finer particles at high speed and long distances; intermediate particles move more slowly; larger fragments usually remain in place.
• Rivers: erode channel material and transport sediments to floodplains and downstream basins.
• Ice (glaciers): transports materials within the mass of huge ice bodies and deposits them on the bed or margins.
• Coastal processes: rock fragments eroded by waves may be transported by longshore currents across the continental shelf, toward the abyssal plain, or along the shore.

2.3 Sedimentation

Sedimentation occurs when the energy of the transporting medium decreases and its capacity to carry material is reduced, so particles are deposited.

3. Sedimentary rocks

Sediments transported by different agents accumulate in sedimentary basins and, after undergoing a number of processes, are transformed into sedimentary rocks. The physical, biological and chemical processes that transform sediments into sedimentary rocks are called diagenesis. Sedimentary rock types include:

• Conglomerates
• Sandstones
• Clays (mudstones, shales)
• Evaporites
• Carbonates
• Organogenic (organic) rocks