Geomorphology

Exfoliation is a specific effect of physical or mechanical weathering, characterized by the flaking off of curved sheets or shells from the surface of rocks or bedrock. It is important to note that exfoliation is considered a *result* or an outcome, rather than a geomorphic *process* itself. This phenomenon leads to the formation of smooth and rounded rock surfaces. The primary causes of exfoliation include expansion and contraction induced by temperature changes, as well as pressure release, also known as unloading. Exfoliation can lead to the formation of distinct landforms such as exfoliation domes, which are typically a result of unloading, and tors, which are formed due to thermal expansion. This process is also known as 'onion peeling' due to the successive splitting off of outer layers. While temperature changes in deserts are a known cause, exfoliation is not confined to desert areas and can also be significantly induced by repeated wetting and drying, particularly in tropical regions.

Soil creep represents a form of mass movement characterized by the slow, gradual, and generally continuous downhill movement of soil on slopes. This movement is often subtle and not readily noticeable, particularly when occurring on gentle gradients or in areas well-covered by grass or other vegetation. The phenomenon is most prevalent in damp soils, where the presence of water acts as a lubricant, facilitating the movement of individual soil particles over each other and across the underlying rock. Another contributing factor to soil creep is the continuous trampling by grazing animals on slopes, which generates vibrations that loosen the soil and encourage its downward shift. Despite its slow pace, which makes direct observation challenging, the effects of soil creep become evident over time through various indicators.

Physical or mechanical weathering refers to the physical disintegration of a rock, where separate particles are prised apart. This process can occur even in completely fresh rock, but it is significantly facilitated when the rock's surface has already been weakened by chemical weathering. Mechanical weathering acts through several distinct mechanisms, including repeated temperature changes, cycles of wetting and drying, frost action, and biotic factors. Each of these processes leads to the breakdown of rocks into smaller fragments without altering their chemical composition. Dry climates, for instance, offer favorable conditions for physical or mechanical weathering, contrasting with warm, wet climates which promote rapid chemical weathering.

Festoons, also known as island arcs, are a type of continental island formation. They consist of an archipelago, or group of islands, arranged in the shape of a loop or arc around the edge of a mainland. These islands are not isolated oceanic formations but are geologically linked to the continent, representing a continuation of mountain ranges that can be traced back to the mainland. As a category of continental islands, festoons were formerly part of the mainland and became detached. This separation could have been caused by the subsidence of a part of the land or a rise in sea level that submerged the lowland links. The historical connection to the continent is often evident in the similar physical structure, flora, and fauna found on both the islands and the adjacent mainland.

Mid-Oceanic Ridges are extensive, interconnected mountain systems that are submerged within the ocean basins, forming the longest mountain chain on the surface of the Earth. These ridges represent one of the three major divisions of the ocean floor, typically situated between continental margins and abyssal plains. A mid-oceanic ridge is characterized by a central rift system at its crest, accompanied by a fractionated plateau and flank zones extending along its length. The rift system at the crest is a region of intense volcanic activity, where frequent eruptions bring significant amounts of lava to the surface. This volcanic activity, combined with shallow-depth earthquake occurrences, indicates that mid-oceanic ridges are geologically very active areas. These geological features are fundamental to the concept of "sea floor spreading." Scientists like Hess (1961) proposed that constant eruptions at the crest of oceanic ridges cause the oceanic crust to rupture. New lava then wedges into this rupture, pushing the oceanic crust outwards on either side, thereby causing the ocean floor to spread. This process means that mid-oceanic ridges are "spreading sites," also known as divergent boundaries, where new crust is continuously generated as tectonic plates move apart. The Mid-Atlantic Ridge is a prime example of such a divergent boundary. Studies of rocks from mid-oceanic ridges reveal that rocks located equidistant on either side of the crest share remarkable similarities in their constituents and age. Rocks found closer to the ridge crest are the youngest and exhibit normal magnetic polarity, with their age progressively increasing as the distance from the crest grows. The oceanic crust itself is significantly younger than continental areas, with rocks in the oceanic crust rarely exceeding 200 million years in age. The magnetic patterns observed parallel to these ridges are crucial for determining the rates of plate movement.

Denudation is a fundamental external geological process responsible for wearing away the Earth's surface, leading to its general lowering and levelling. It results from the vigorous action of external forces that counterbalance the internal forces which create new relief features. This comprehensive process involves four distinct but simultaneously occurring phases. The first phase is **weathering**, which refers to the gradual disintegration of rocks by atmospheric or weather forces. Following weathering, **erosion** takes over, actively wearing away the Earth's surface through the action of moving agents such as running water, wind, ice, and waves. The third phase, **transportation**, involves the removal of the eroded debris from its original location to new positions. Finally, **deposition** is the process where this transported debris is dumped in specific parts of the Earth, where it can accumulate and potentially form new rocks. These four phases of denudation occur concurrently across different regions globally, though their rates can vary significantly. The rate and effectiveness of denudation are influenced by several factors, including the nature of the local relief, the structural characteristics of the rocks, the prevailing climate, and human interference.

Chemical weathering is a fundamental process of denudation, involving the extremely slow and gradual decomposition of rocks. This occurs when rocks are exposed to air and water, which contain chemical elements that initiate reactions within the surface layers of the exposed rocks. These reactions can either weaken or entirely dissolve certain components of the rock, thereby loosening other crystals and diminishing the overall strength of the surface. For instance, in granite, felspar minerals are more rapidly weathered than quartz, leading to the loosening of quartz crystals and the formation of a coarse sandy residue. When a rock surface undergoes chemical weathering, some loosened material is removed by agents like wind or water, exposing a fresh surface. However, much of the weathered material, known as regolith, can remain in place and form the base for soil. The presence of a soil cover generally enhances chemical weathering of the underlying rocks, as the soil absorbs rainwater, keeping the rocks in continuous contact with moisture. This rainwater, having absorbed organic acids from the soil, becomes a more potent weathering agent than pure rainwater acting on bare rock. There are three primary chemical weathering processes: solution, oxidation, and decomposition by organic acids. Solution involves water dissolving minerals, a process significantly intensified when rainwater contains carbon dioxide, forming a weak acid. This is particularly effective in limestone regions, dissolving calcium carbonate and widening cracks to form caves. Oxidation is the reaction of oxygen in air or water with rock minerals, such as iron, forming easily erodible iron oxide (rust). Decomposition by organic acids is driven by bacteria in soil thriving on decaying organic matter, producing acids that accelerate weathering. Micro-organisms and plants like mosses or lichens on damp bare rock also contribute by absorbing chemical elements and producing organic acids. The rate of chemical weathering is significantly influenced by climate, with warm, wet conditions promoting rapid reactions, while dry climates inhibit it.

Soil impoverishment and soil erosion are described as "twin evils" that result from unsound farming practices. Soil impoverishment refers to the deterioration of the mineral and organic content of the soil, making it highly deficient in plant nutrients and leading to low yields. Soil erosion is the process where the fertile top layer of soil is completely removed, sometimes beyond replacement. (p. 241) Human interference and mismanagement in agriculture can greatly accelerate natural erosion processes. While natural conditions replenish soil fertility through processes like leaf-fall and organic decomposition, farming activities such as ploughing and harvesting crops upset this balance. Practices like overcropping, improper cultivation on slopes, and deforestation leave the soil exposed and vulnerable to erosion by agents like running water and wind. (p. 241-242)

Soil Flow, also known as Solifluction, is a distinct type of mass movement characterized by the downslope movement of soil that has become completely saturated with water. In this state, the individual soil particles are nearly suspended in water, allowing them to move easily over one another and over the underlying rock, causing the soil mass to behave much like a liquid. This phenomenon manifests under specific climatic and environmental conditions. In arid regions, a soil flow or mud-flow can be triggered when a layer of weathered debris becomes fully saturated with rainwater following a storm, resulting in its downslope movement as a semi-liquid mass. In temperate and tundra regions, solifluction commonly occurs during spring when the surface layers of previously frozen ground begin to thaw. The melt-water acts as a lubricant, enabling the soil and rock debris to flow readily over the still-frozen subsoil. Furthermore, in areas with extensive peat soils, which possess a high moisture absorption capacity, a soil flow can occur if the peat reaches its saturation point, leading to its downslope movement. In Ireland, such flows involving peaty soil are specifically referred to as 'bog-bursts'.