Surface Forces That Shape Earth: Weathering, Erosion, Deposition, and Human Impact

Comprehensive vocabulary flashcards covering surface forces that shape the Earth, including physical, chemical, and biological weathering; fluvial, coastal, glacial, and arid erosion and deposition landforms; and human impacts accelerating soil erosion.

Weathering

The wearing away and breaking down of rocks and soil in place by the action of the weather, with very little movement other than loose material falling under gravity.

Agents of Weathering

Natural forces that drive the breakdown of rocks and soil, the two most important being temperature and water.

Physical Weathering

The breakdown of rocks into smaller fragments through mechanical forces such as frost and temperature changes, without altering the rocks' chemical composition.

Freeze-Thaw Action (Frost Shattering)

A physical weathering process where water enters cracks in rocks and freezes from the top downward; because ice is about $9\%$ larger in volume than liquid water, it exerts lateral pressure on crack walls and shatters the rock over repeated cycles.

Exfoliation

A physical weathering process (the 'onion-peeling effect') occurring in dry desert environments with daily temperature fluctuations exceeding $30\,^\circ\text{C}$, where repeated daytime expansion and nighttime contraction weaken rock surfaces, causing outer layers to peel away.

Chemical Weathering

The decomposition of rocks caused by chemical reactions that change their mineral composition, weakening their structure and causing them to break down.

Carbonation

A chemical weathering process where rainwater containing dissolved carbon dioxide forms weak carbonic acid ($H_2CO_3$), reacting with insoluble calcium carbonate ($CaCO_3$) in rocks like limestone to create soluble calcium bicarbonate ($Ca(HCO_3)_2$), which dissolves and washes away.

Oxidation

A chemical reaction in which oxygen dissolved in water mixes with iron present in rocks to form iron oxide (rust), weakening rock structures and producing distinct reddish stains (such as at Thabazimbi).

Hydrolysis

A sequence of chemical reactions between water and silicate minerals (such as feldspar in granite) that converts silicates into soluble clay minerals, causing the remaining rock to crumble grain by grain.

Biological Weathering

The disintegration of rocks and soil caused by living organisms, including burrowing animals, prying plant roots, and chemical excretions from organisms such as lichens.

Lichens

Symbiotic organisms consisting of algae and fungi that weather rocks physically by penetrating root-like structures between mineral grains, and chemically by secreting acids, eroding approximately $1\,\text{cm}$ of sandstone every $100\,\text{years}$.

Erosion

The detachment and movement of rock, sand, and soil from one place to another by natural mobile agents, specifically water, wind, and ice.

Deposition

The geological process by which weathered and eroded material is laid down or dropped by water, wind, or ice, creating landforms such as beaches, sand dunes, and river deltas.

River Load

The weathered material transported by a river—chiefly sand, stones, and silt—which acts as an abrasive tool wearing away the riverbed and banks while gradually reducing in size from source to mouth.

Vertical Erosion (Downward Erosion)

The downward cutting of a riverbed caused by the abrasive grinding of large boulders and rocks in steep, high-energy upper courses, producing deep V-shaped valleys.

Waterfall

A feature formed along a river's course when water plunges over a sudden vertical drop in gradient, occurring where river water flows over a resistant layer of hard rock that undercuts softer underlying rock, creating an overhang that collapses into a plunge pool.

Rapids

Stretches along a river course where water accelerates over uneven rock bands without an abrupt vertical drop in river height.

Gorge

A deep, steep-sided, relatively straight valley formed by backward erosion as a waterfall retreats upstream over thousands of years (such as Oribi Gorge in KwaZulu-Natal).

Canyon

A wide, deep-sided valley formed by intensified downward river erosion triggered by increased discharge, regional land uplift, or a fall in sea level (such as the Fish River Canyon).

Meander

A prominent loop or bend in a river's middle or lower course formed by simultaneous lateral erosion on the fast-flowing outer bank and sediment deposition along the slower-flowing inner bank.

Meander Neck

The narrow strip of land separating the two outer bends of an exaggerated river loop.

Ox-Bow Lake

A crescent-shaped lake formed when a river cuts directly through a meander neck during high water flow, leaving the abandoned channel loop separated from the main river by sediment deposition.

Levee

A raised natural embankment of sand and silt built along riverbanks through repeated overbank flooding events.

River Delta

A low-lying, fan-shaped or D-shaped depositional area at a river's mouth formed when the river enters a standing body of water, loses velocity, and dumps its sediment load into multiple distributary channels.

Headland

A prominent, high, rocky landform composed of resistant rock that projects outward into the sea beyond adjacent bays.

Bay

A broad, curved coastal indentation situated between headlands, formed where waves erode bands of softer rock more rapidly than surrounding hard rock.

Wave-Cut Platform

A flat or gently sloping rocky expanse exposed at low tide, formed at the base of a retreating coastal cliff by continuous wave undercutting and cliff collapse.

Sea Arch

A coastal opening through a headland formed when wave erosion carves two back-to-back sea caves that eventually join together.

Stack

An isolated vertical column of rock standing offshore in the sea, left behind after the roof of a sea arch weakens and collapses.

Beach

A depositional coastal feature composed of loose sand, gravel, or stones covering the shore between the low-tide and high-tide boundaries.

Longshore Drift

The zig-zag transport of loose sediment along a shoreline, caused by incoming waves driving material up the beach face at an angle while gravity pulls backwash directly perpendicular to the coast.

Spit

A narrow coastal landform consisting of a ridge of sand or shingle extending out from the coastline across a bay or river mouth in the direction of longshore drift.

Bar (Coastal)

A ridge of deposited sand that grows completely across a bay, connecting two headlands and isolating a coastal lagoon behind it.

Lagoon

A shallow body of brackish or salt water isolated from the open ocean behind a coastal sand bar, which often fills with sediment to become a salt marsh.

Glacier

A massive body of dense ice that forms from compacted snow and moves slowly downhill across the land under the pull of gravity.

Glacial Abrasion

A mechanical erosion process where rock fragments frozen into the base of a glacier grind, scrape, and polish underlying bedrock as the ice mass moves downhill.

Plucking

A glacial erosion mechanism occurring when meltwater freezes around fractured bedrock blocks, tearing them away from the valley floor and embedding them into the moving glacier.

Cirque

A steep-sided, bowl-shaped amphitheater hollow carved into high mountain slopes where glacial ice originates and accumulates.

Arête

A sharp, narrow, knife-edged knife-like ridge of rock formed when two opposing cirques erode back-to-back into a mountain crest.

Glacial Horn

A sharply pointed, pyramid-shaped mountain peak carved by three or more cirques eroding headward toward a central summit.

U-Shaped Valley

A glaciated valley characterized by steep, vertical walls and a broad, flat floor, scoured out and widened by an advancing valley glacier that planed off pre-existing spurs.

Truncated Spur

A steep, triangular cliff wall along a glaciated valley where a valley glacier has sheared off the tips of interlocking spurs.

Moraine

An accumulation of unsorted glacial till (rock debris, boulders, and soil) deposited along the edges or terminus of a glacier as the ice melts.

Lateral Moraine

A ridge of rock fragments and debris deposited along the lateral margins or sides of a valley glacier.

Terminal Moraine

A prominent ridge of glacial debris dumped at the furthest point reached by the snout of an advancing glacier before it melted.

Esker

A long, winding ridge of layered sand and gravel deposited by subglacial streams flowing within tunnels beneath melting glaciers.

Drumlin

A smooth, rounded, teardrop-shaped hill composed of glacial deposits, typically about $50\,m$ high and $500\,m$ long, streamlined in the direction of ice flow.

Wind Abrasion

The scouring and wearing away of rock faces by loose windblown sand grains, reaching its maximum intensity close to the ground where saltating sand concentrations are highest.

Mushroom Rock

An isolated, pedestal-shaped rock formed in arid regions by wind abrasion, where the base is heavily eroded and undercut because sand-sized particles travel close to the ground surface.

Yardang

An elongated, wind-carved ridge of resistant rock separated by parallel troughs, aligned parallel to prevailing desert winds.

Sand Dune

A wind-built accumulation of sand formed when wind velocity drops, displaying a gentle windward slope and a steeper leeward slip face.

Barchan

A crescent-shaped sand dune with pointed horns facing downwind, developing in arid zones with a constant wind direction and moderate sand availability.

Soil Erosion

The process by which topsoil is stripped and carried away by natural forces like wind and water, becoming problematic when erosion outpaces natural soil formation rates.

Open-Cast Mining

A form of surface mining for coal or iron ore that removes all vegetation and topsoil to expose rock strata, piling soil into loose spoil heaps vulnerable to wind and water erosion.

Agricultural Furrows

Linear grooves dug across farm fields by ploughs that break up soil cohesiveness, drying out soil in arid climates and channeling runoff down slopes to accelerate erosion in wet climates.

Monoculture

The repeated commercial farming practice of planting the exact same crop (such as maize) in a field year after year, stripping the soil of vital nutrients like nitrogen and leaving it degraded and prone to erosion.

Crop Rotation

The planned practice of growing a sequence of different crops across successive seasons to preserve nutrient balance, restore nitrogen levels, and protect soil against erosion.

Overstocking

The management practice of maintaining a greater number of farm animals on a pasture than the land can sustainably feed.

Over-Grazing

The severe degradation of land caused by excessive livestock feeding on leaves and roots (notably goats and ostriches), eliminating protective root systems and exposing bare soil to erosion.