Geological Agents: Shaping Earth’s Surface
1. Introduction to Geological Agents
Geological agents, including the atmosphere, water, ice, sea, and wind, shape Earth’s surface through various external processes. These processes include weathering, erosion, transport, and sedimentation. Latitude, altitude, climate, and human actions also play a role.
2. Weathering: Breaking Down Rocks
Weathering is the breakdown of rocks. There are three types of weathering:
- Physical Weathering: Rocks break into fragments without chemical alteration. Examples include:
- Freeze-thaw (gelivacion): Water enters cracks, freezes, expands, and widens the cracks, eventually breaking the rock.
- Dilation and Contraction: Rocks expand and contract with temperature changes, leading to cracks and breakage.
- Chemical Weathering: Chemical reactions alter the rock’s composition. The main agent is water. Examples include:
- Oxidation: Oxygen combines with minerals, giving rocks a yellowish or rusty appearance.
- Carbonation: CO2 reacts with rocks, especially limestone, dissolving them.
- Biological Weathering: Living organisms contribute to rock breakdown. Examples include plant roots widening cracks, burrowing animals, and the action of bacteria and lichens.
3. Surface Water Action
Running water carves channels in soft materials, forming rills and gullies. In heterogeneous terrain, valleys and canyons are formed. Ramblas and wadis are dry riverbeds in arid regions that carry water only after heavy rainfall. Rivers, fed by rain, snowmelt, and groundwater, carve valleys. In mountainous areas, they create V-shaped valleys. In flatter areas, they deposit sediment, forming alluvial plains and deltas.
4. Groundwater Action
Rainwater infiltrates the soil, forming groundwater. When it encounters an impermeable layer, it forms an aquifer. The water level in the aquifer is called the water table. Wells extract groundwater, but over-extraction can deplete the resource. Karst landscapes form in limestone areas where groundwater dissolves the rock, creating features like sinkholes, caves, stalactites, and stalagmites.
5. Glacial Action
Glaciers are large accumulations of ice. Polar ice caps, like those in Greenland and Antarctica, contain most of the planet’s glacial ice. When they reach the coast, they break off, forming icebergs. Valley glaciers flow down mountainsides, carving U-shaped valleys.
6. Wind Action
Wind is a significant agent in deserts and semi-deserts. It carries loose material, creating deflation hollows and depositing sand dunes. Coastal dunes can also form, posing a threat to agriculture and settlements.
7. Coastal Processes
Waves, currents, and tides shape coastal landforms through erosion and sedimentation. Erosion creates cliffs, caves, and arches. Sedimentation forms beaches and barrier islands.
8. Sedimentary Rock Formation
Sedimentary rocks form from the accumulation of sediments. They are classified into three types:
- Detrital: Formed from fragments of other rocks (e.g., conglomerate, sandstone, shale).
- Chemical: Formed from precipitation of dissolved minerals (e.g., limestone, salt).
- Organic: Formed from the remains of living organisms (e.g., most limestone, coal).
Rocks of organic origin include coal (formed from plant remains in swamps) and petroleum (formed from the remains of organisms).