Weathering, Erosion, and Resource Sustainability Practice Flashcards

Fundamental Concepts of Weathering, Erosion, and Deposition

Weathering constitutes a group of essential processes that break down soils, rocks, and minerals into smaller pieces through direct contact with the Earth's atmosphere, plant and animal life, and the various waters of the world. While it is a notably slow process, weathering is fundamentally responsible for the creation of soil and the formation of many unique geological features on the surface of the planet. There is a critical distinction to be made between weathering, erosion, and deposition. Weathering refers specifically to the breaking down of materials in situ without the movement of the eroded material. Erosion, by contrast, is the process by which soil and rock, having been weakened by weathering, are moved from one area of the Earth's surface by natural processes like wind or water flow and transported to other locations. Deposition marks the conclusion of this cycle, occurring when the eroded material reaches a new location and settles, forming features such as sand dunes and beaches.

Mechanisms and Types of Physical Weathering

Physical weathering, also known as mechanical weathering, involve the physical breakdown of rocks into smaller components without altering their chemical composition. This process is driven by physical forces such as temperature fluctuations and the action of frost. One primary mechanism is granular disintegration, which occurs when rocks break up into small grains or pieces of gravel, eventually forming sand. Another significant process is exfoliation, where rocks weather by peeling off in sheets or thin layers, specifically on large rock surfaces, rather than crumbling into gravel.

Freeze-thaw action is a particularly powerful form of physical weathering that occurs when water collects in the cracks of rocks. As temperatures drop and the water turns into ice, its volume increases, putting immense pressure on the sides of the rock. Because the top layer of water in a crack often freezes first, the expanding ice below cannot move upward and is instead forced to expand sideways against the rock walls. Over many cycles of freezing and thawing, the rock is weakened until large fragments fall off. This is closely related to frost shattering and block separation, where ice forms in the joints of rocks, causing large fragments to separate based on the rock's natural joints. Furthermore, extreme temperature changes cause different minerals within a rock to expand and contract at different rates, leading to internal stress and the eventual breaking up of the rock.

Chemical and Biological Weathering Processes

Chemical weathering involves the breakdown of rocks into particles with a different mineral composition from the original rock. The primary agents facilitating these changes are water, oxygen, and carbon dioxide. This occurs through several specific processes. Solution happens when minerals or soluble chemicals in the rock dissolve directly into water. Oxidation occurs when minerals react with dissolved oxygen to produce red or yellow oxides or hydroxides. Hydration is a process where minerals absorb water and expand, causing the rock to break. Hydrolysis involves a chemical reaction between the minerals in the rock and hydrogen present in rainwater. Finally, carbonation occurs when carbon dioxide dissolves in water to form a weak carbonic acid, which is particularly effective at weathering carbonate rocks such as chalk and limestone.

Biological weathering is the result of activities by living organisms and involves both physical and chemical actions. Physical biological weathering includes the growth of plant roots into rock cracks or the burrowing of animals, both of which apply pressure that breaks rocks. Chemical biological weathering includes the excretion of acids by organisms or the decomposition of organic matter, which releases corrosive substances. Additionally, the release of carbon dioxide by organisms can react with water to form carbonic acid, further contributing to the degradation of specific rock types.

The Role of Human Activity in Weathering and Erosion

Human activities significantly accelerate the rate of weathering and erosion through various industrial and developmental practices. The burning of fossil fuels releases chemicals into the atmosphere, creating acidic conditions. This leads to acid rain, which chemically eats into materials like limestone on buildings. Construction projects contribute to erosion by removing vegetation that holds soil together, compacting the soil, and altering natural drainage patterns. Covering land with concrete increases surface runoff, which further accelerates bank erosion. During road construction, the use of dynamite physically breaks rocks into smaller pieces. Mining operations, both open-cast and underground, have profound impacts. Open-cast mining involves removing large amounts of earth to reach minerals, leaving permanent scars and enhancing the rate of erosion. Underground mining can lead to surface collapses or depressions that fill with water, and the waste rocks brought to the surface can contaminate topsoil and flow into streams during rainfall, causing further erosion and environmental degradation.

Agents of Erosion and the Force of Gravity

There are five primary agents of erosion: gravity, water (rivers), glaciers (moving ice), sea waves, and wind. Gravity is considered the underlying agent for the other four, as it is the fundamental force that pulls materials downward. Gravity causes erosion through a process known as mass wasting, which is the down-slope movement of rock and sediment. This force moves material from higher elevations to lower elevations, where other transporting agents like streams or glaciers can then pick up the sediment and move it even further. These natural phenomena work together to form many of the features seen in the natural world, such as the wearing away of stream banks, known as bank erosion.

Stages of a River and Fluvial Landforms

A river progresses through three main stages: the youth stage, the mature stage, and the old age stage. The youth stage is characterized by fast-flowing water with high energy, often found on steep slopes and mountains. This stage is dominated by vertical erosion, which deepens the river channel and creates V-shaped valleys, waterfalls, and rapids. The mature stage features a wider valley floor with a shift toward lateral erosion, resulting in U-shaped valleys, meanders, and narrow floodplains. The old age stage occurs where the surrounding relief is very flat. The river flow is very slow, and meanders become the dominant feature. In this stage, deposition is the primary process because the flow lacks the energy to carry its load, often leading to the formation of oxbow lakes, which are U-shaped bodies of water created when a meander is cut off from the main river stem.

Features of River Erosion and Deposition

River erosion occurs as water flows over land, carrying a load of sand, stones, and silt that wears away the river bed and sides. The size of this load typically decreases from the source to the mouth as boulders are broken down. Faster-flowing water in the upper course can transport larger, heavier loads, causing significant downward erosion. Most rivers eventually flow into others as tributaries rather than directly into the sea. Waterfalls and rapids can occur at various points, often where the river flows over alternating bands of hard and soft rock. Waterfalls form as the softer rock erodes faster, creating an undercut and an overhang of hard rock. When this overhang collapses into the plunge pool below, the waterfall gradually retreats upstream, often leaving behind a gorge. Gorges are deep-sided valleys that are typically straighter than canyons, whereas canyons usually form when increased water volume or land uplift accelerates downward erosion.

In the middle and lower courses, lateral erosion becomes more prominent, widening the valleys. As the gradient gentles and energy levels drop, deposition increases. Meanders, large winding bends in the river, are formed through simultaneous erosion and deposition. Water flows faster on the outside of a bend, creating a river cliff through lateral erosion, while it flows slower on the inside, depositing material to form a slip-off slope or point bar. Over time, erosion on the outer bend and deposition on the inner bend causes meanders to migrate across the floodplain. When the neck of a meander becomes sufficiently narrow, the river may erode through it during a flood, creating a new, straighter channel. Deposition then seals off the old meander, forming an oxbow lake that eventually dries up into a meander scar. Near the mouth, rivers may form levees, which are natural mounds on the banks created by repeated flooding and silt deposition. Deltas form where a river enters a sea or lake, losing its energy and dropping its remaining silt. This silt can block the main channel, forcing the river into multiple smaller channels called distributaries, often resulting in a D-shaped landform.

Coastal Landforms and Wave Action

Wave action possesses immense power to cause erosion and deposition along coastlines. Coastal erosion processes include hydraulic action, where the sheer force of water breaks rocks apart, and corrasion, where rocks and pebbles flung by waves wear down coastal rocks. Attrition occurs as broken rock particles rub against each other and become smaller and rounded, while corrosion involves the dissolving of certain rocks like limestone in seawater. These processes create various features, starting with cliffs, which are steep rock faces formed by erosion and abrasion at the foot of a slope. Headlands project from the coastline into the sea, while bays are areas of water enclosed by land with a wide mouth. Sea caves are hollow openings at the base of cliffs formed at weak points in the rock. If erosion continues through a cliff, it may form a natural bridge known as an arch. When an arch collapses, it leaves an isolated pillar of rock called a stack, which further erodes into a low outcrop known as a stump. Wave-cut platforms are flat surfaces at the base of sea cliffs formed by continuous wave erosion.

Deposition by waves creates features such as beaches, spits, and barrier islands. Beaches form as sand and small rocks accumulate along the shore. Spits are sand bars or ridges that extend from the shore across a bay. Barrier islands form where the shoreline is flat or gently sloping and serve as the first line of defense against storms. These features are the result of waves carrying materials eroded from cliffs or brought to the coast by rivers and depositing them in areas where water energy is lower.

Glacial and Wind-Driven Landforms

Glaciers are large masses of moving ice that erode landscapes through abrasion (scraping the bedrock with carried rocks) and plucking (pulling blocks of bedrock from the surface). Glacial erosion creates horns, which are pyramidal peaks; arêtes, which are narrow knife-like ridges; and cirques, which are bowl-shaped depressions at the head of glacial valleys. During the formation of a cirque, snow compresses into ice and moves down by gravity, deepening the hollow. This process involves a steep back wall and a bergschrund, which is a deep crack in the ice. Glacial deposition results in till or moraine. Moraines are categorized as terminal (at the furthest advance), recessional (during retreat), lateral (along the sides), or medial (where two glaciers merge). Other features include drumlins, which are smooth, oval hills, and eskers, which are meandering ridges of sand and stone deposited by sub-glacial rivers.

Wind erosion is driven by abrasion, where wind-carried sand wears down rocks, and deflation, where wind removes loose surface particles to lower the ground level. Coastal or desert features include mushroom rocks (pedestals) where the bottom of a rock is eroded more than the resistant top, and hamadas, which are areas of bare bedrock. Yardangs are landforms about $10\,m$ high and $100\,m$ long that align with prevailing winds. Wind deposition primarily results in sand dunes, which take various forms: barchans (limited sand, constant wind), transverse (plentiful sand), longitudinal or linear (parallel to wind), and blowout or parabolic (where vegetation anchors the edges while the center is blown out).

Natural Resource Use and Sustainability

Natural resources are categorized as renewable or non-renewable. Renewable resources, such as water, soil, fish, solar energy, and forests, can replace themselves through natural processes if used wisely. Non-renewable resources, including coal, oil, gold, and minerals like copper or platinum, are finite and will eventually run out. Unwise resource use, such as overfishing, failing to replant trees, or wasting electricity, leads to resource damage, faster depletion, and future unavailability. Pollution from agriculture, mining, and industry further harms ecosystems. Sustainable resource management aims to balance environmental, social, and economic needs. This includes large-scale government planning and small-scale personal actions. At home, sustainability can be achieved by using low-energy light bulbs, improving roof insulation, making compost from kitchen waste, harvesting rainwater, and utilizing renewable energy sources like solar power.

Challenges of Overfishing and Overgrazing

Overfishing is a critical global issue; scientists predict that world fish stocks may collapse by the year 2050 if current rates continue. This is driven by modern technology, such as factory ships and electronic fish finders, and a lack of international regulation. Consequences include food shortages and economic poverty for over a billion people. Solutions involve establishing fishing quotas (maximum catch limits), creating marine protected areas, and regulating net mesh sizes to avoid catching young fish. In South Africa, the SASSI program uses a color-coded system—Green (safe), Orange (avoid), and Red (protected)—to inform consumers.

Overgrazing occurs when livestock exceed the land's carrying capacity, leading to bare soil, weed invasion, and erosion. In the Sahel region of West Africa, overgrazing combined with climate change and population growth has led to severe desertification. Sustainable grazing methods include rotational grazing, where animals are moved between fields to allow recovery, and strip grazing, which uses mobile electric fences to ensure all grass species are eaten. Another method is holistic land management, used in Zimbabwe, which mimics the behavior of wild herds and uses animal manure as fertilizer to reclaim grassland.

Food Security and Agricultural Technology

Food security is defined by three pillars: food availability, food access, and food use. It requires all people to have permanent access to sufficient, safe, and nutritious food. Local solutions include school gardens, while regional efforts often fall to women in rural communities. Global food security is often tied to economic development, with Less Economically Developed Countries (LEDCs) facing higher insecurity. Modern food production relies on science and technology, such as factory farming, which uses antibiotics and hormones to raise large numbers of animals in small spaces. Genetic modification (GM) of crops like soybean and maize aims to create insect resistance and pesticide tolerance. However, for smaller communities, appropriate technology—simple, inexpensive, and labor-intensive—is often more sustainable. Sustainable techniques like crop rotation maintain soil fertility, break pest cycles, and reduce erosion, while natural soil management uses compost and manure instead of artificial fertilizers.