Landform Creation: Geological Processes and Weathering

Geological Processes and Landform Development

The Geological Cycle

The geological cycle involves several key processes that shape the Earth's surface:

1. Orogenesis: Internal processes responsible for the formation of mountains.
2. Gliptogenesis: The wearing down of reliefs through the erosion and transport of materials to marine basins, driven by both internal and external processes.
3. Lithogenesis: The formation of rocks through internal and external processes.

External Geological Processes

Weathering

Weathering is the breakdown of rocks at the Earth's surface. It can be categorized as follows:

A) Mechanical (Physical) Weathering

This process is primarily driven by temperature changes and other physical conditions:

• Solar Radiation and Temperature: Temperature variations between poles and the equator, as well as daily fluctuations, cause stress in rocks.
• Pressure and Wind: Wind is generated by pressure differences and can contribute to abrasion.
• Precipitation: Creates stresses, particularly in high-altitude and equatorial regions.

Key mechanical weathering processes include:

• Frost Wedging (Gelidraction): Water seeps into cracks, freezes, and expands in volume by 9-10%, exerting pressures up to 2000 atm, which fractures the rock.
• Differential Expansion: Daily temperature changes cause minerals to expand and contract at different rates. For example, dark minerals heat up and expand more than light-colored minerals, leading to non-homogeneous expansion and contraction that fatigues and crumbles the rock.
• Pressure Release (Unloading): As erosion removes overlying material (overburden), the underlying rocks expand suddenly, causing them to fracture in sheets.

B) Chemical Weathering

This involves changes in the chemical structure of mineral grains within rocks.

• Solution: The process where composite particles are disaggregated by a solvent, like water, which acts as a dipole.
• Carbonation: A specific type of solution where carbon dioxide dissolved in water creates carbonic acid, which converts insoluble materials (like calcium carbonate) into soluble ones.
• Hydration: The incorporation of water molecules into a mineral's structure, causing it to increase in volume and become more susceptible to mechanical attack.
• Hydrolysis: The chemical breakdown of a compound due to reaction with water, often breaking down salts into their constituent acids and bases.
• Oxidation: The reaction of minerals (especially those containing iron) with oxygen, often transported by water, to form oxides and hydroxides.

C) Biological Weathering

This is the action of living organisms on rocks.

• Chemical Action: Algae can decompose calcium carbonate (CaCO3) deposits.
• Mechanical Action: Burrowing animals excavate the ground. Some organisms digest calcium from limestone, forming domes. Plant roots can grow into fractures (diaclases), acting as levers that widen the cracks, especially when moved by the wind.

Slope Processes

1. Gravity-Driven Processes

• Falls and Avalanches: The rapid descent of large masses of rock.
• Slides: The movement of material along an inclined surface, which can be slow or fast.
• Creep or Flow: The slow, gradual movement of soil and regolith downslope.

2. Fluvial Processes on Slopes

These processes are driven by water flow on steep hillsides.

• Gullies (Cárcavas): Large channels or cracks formed on steep slopes by torrential rain, especially on erodible materials like slippery clays.
• Fairy Chimneys (Hoodoos): Isolated rock pinnacles formed by erosion, where a more resistant caprock protects the softer rock underneath.
• Rills and Sinkholes: Surface water flowing over soluble rock (like limestone) can dissolve it, producing large grooves and depressions.

Karstic Landscapes

A karst landscape is a set of landforms developed over soluble bedrock, such as limestone, through dissolution. The process begins when carbon dioxide (CO2) and water (H2O) react to form carbonic acid. This weak acid attacks and dissolves limestone (calcium carbonate, CaCO3), which is not soluble in pure water but becomes soluble through the process of carbonation.

Exokarst (Surface) Features

• Dolines (Sinkholes): Closed, circular depressions that can originate from surface dissolution or the collapse of a cave roof. When two or more dolines merge, they form an uvala. The bottoms of these depressions may contain clay from the decalcification of insoluble impurities.
• Lapies (Karren): Micro-grooves or solution channels formed on the surface of limestone.
• Sinking Stream (Ponor): A point where a river disappears underground into the karstic system.
• Resurgence: A point where an underground river re-emerges on the surface.
• Canyons: Deep gorges carved by rivers through the limestone.

Endokarst (Subsurface) Features

These features form underground along fractures (diaclases) in the rock.

• Chasms and Caves: Vertical shafts (chasms) and horizontal passages (galleries) form a complex cave system.
• Speleothems (Stalactites and Stalagmites): When water rich in dissolved calcium carbonate enters a cave, the decrease in CO2 pressure causes the chemical reaction to reverse. The calcium carbonate precipitates out of the solution, forming stalactites (hanging from the ceiling) and stalagmites (growing from the floor).