Geomorphology

Continental drift and plate tectonics

Continental drift is the idea that continents have moved across the surface of the Earth over geological time, rather than remaining fixed in their present positions. The evidence for it is of several independent kinds, and a strong answer names more than one. The coastlines of continents on opposite sides of an ocean, most famously South America and Africa, fit together like pieces of a puzzle. The same fossil plants and animals, and the same distinctive rock sequences, occur on continents now separated by wide oceans, which is hard to explain unless those continents were once joined. Evidence of ancient glaciation is found today in warm tropical lands, while coal formed in warm swamps is found in cold polar lands, showing that continents have changed latitude. Finally, the ocean floor itself carries the record: magnetic stripes of alternating polarity run symmetrically on either side of mid-ocean ridges, showing that new crust is created at ridges and moves outward. No single line of evidence is decisive alone; together they make drift difficult to deny.

Plate tectonics gives drift a mechanism and a global framework. The outer shell of the Earth, the lithosphere, is broken into a small number of large plates and several smaller ones that move slowly over the weaker asthenosphere beneath. Plates are not the same as continents: a single plate may carry both continental and oceanic crust, and it is the plate that moves, carrying its continents with it. At divergent boundaries, plates move apart and new crust is created, as at mid-ocean ridges where magma rises and solidifies. At convergent boundaries, plates move together; where oceanic crust meets continental crust, the denser oceanic plate is forced down into the mantle in subduction, producing deep ocean trenches, volcanic arcs, and powerful earthquakes. Where two continental plates collide, neither sinks readily, and the crust is crumpled upward into great fold mountain ranges. At transform boundaries, plates slide past each other sideways, producing earthquakes along faults but little new crust or mountain building. The distribution of the world's volcanoes, earthquakes, and young fold mountains follows plate boundaries closely, which is why a map of plate boundaries is also, to a first approximation, a map of the world's active tectonic hazards. Mountain building, or orogeny, is thus not a mysterious uplift of a single range but the local expression of a global system of moving plates, and the same system explains why some continental margins are quiet and others are among the most earthquake-prone places on Earth.

Denudation, weathering and geomorphic cycle

Denudation is the general term for the wearing away of the land surface, covering weathering, mass movement, and erosion together. It is important to keep its parts distinct. Weathering is the breakdown and decay of rock in place, without transport: the rock is weakened where it stands. Erosion is the removal and transport of weathered material by a moving agent such as a river, glacier, wind, or sea. Mass movement is the transfer of material downslope under gravity alone, without a transporting agent carrying it in a stream. A boulder cracked by frost and still sitting on a hillside has been weathered but not eroded; the same boulder carried downstream by a river has been eroded; a hillside that slumps in one wet mass has undergone mass movement. Examination questions often hinge on exactly this distinction, so definitions should be learned precisely rather than loosely.

Weathering itself takes three main forms. Mechanical or physical weathering breaks rock into smaller fragments without changing its chemical composition: frost action, where water in cracks freezes, expands, and wedges the rock apart; thermal expansion and contraction in deserts, where repeated heating and cooling stresses the outer layers; and unloading, where removal of overlying rock allows buried rock to expand and sheet off in layers. Chemical weathering changes the composition of minerals: solution dissolves soluble minerals, especially in limestone regions; hydrolysis alters silicate minerals in the presence of water; oxidation rusts iron-bearing minerals; and carbonation, where carbon dioxide dissolved in water forms a weak acid, attacks carbonate rocks. Biological weathering is the work of living things: roots prying cracks wider, burrowing animals disturbing soil, and lichens and bacteria producing acids that attack rock surfaces. Climate controls which type dominates: chemical weathering is strongest in warm, humid climates where water and heat are abundant, mechanical weathering dominates in cold climates with frequent freeze and thaw and in hot deserts with large temperature ranges, and weathering is slowest in very dry or very cold places where water is scarce or frozen.

The geomorphic cycle, or cycle of erosion, is an attempt to describe how a landscape evolves through time after uplift. In the classic Davis model, a landmass is rapidly uplifted and then left relatively stable while rivers and other agents wear it down through youth, maturity, and old age. Youth is marked by steep gradients, narrow V-shaped valleys, waterfalls, and limited floodplains, because downcutting dominates. Maturity shows graded rivers, wider valleys, meanders, and floodplains, as lateral erosion grows in importance and relief declines. Old age produces a low, gently undulating surface, the peneplain, close to base level, with broad valleys and isolated resistant hills. Penck offered a different picture, emphasising that uplift and denudation often proceed together rather than in separate phases, so that slopes evolve by parallel retreat and the landscape passes through stages of waxing, uniform, and waning development. Later work has criticised both models for assuming a simple starting surface, ignoring climatic change, and underplaying the role of tectonic activity that never truly stops. Their value today is as frameworks for comparison and for interpreting the stage of a landscape, not as literal histories that every landscape must follow.

Earth movements

Earth movements are the displacements of the crust produced by internal forces, and they are classified first by speed. Slow movements, or diastrophism, act over long periods and build the large structures of the crust. Sudden movements, such as earthquakes and volcanic eruptions, release energy in moments and are treated as hazards as well as land-forming events. Slow movements are further divided into vertical and horizontal movements. Vertical movements, or epeirogeny, raise or lower large areas of crust without much folding: a broad region may be uplifted into a plateau or depressed to admit the sea, while the rock layers remain nearly horizontal. Horizontal movements, or orogeny, compress or stretch the crust and produce folding and faulting; they are responsible for mountain ranges and rift valleys, and they deform rock layers strongly.

Folding occurs when rocks under compression bend rather than break. The upfold is an anticline and the downfold is a syncline, and in simple, symmetrical folds the oldest rocks are exposed in the core of an anticline after erosion. Folds vary from gentle open folds to tight, overturned, and recumbent folds where the fold is pushed so far that it lies almost flat, and the limbs may be stretched and faulted. Faulting occurs when rocks under stress break and the blocks on either side move relative to each other. In a normal fault, produced mainly by tension, the hanging wall moves down relative to the footwall, and repeated normal faults can drop a central block to form a rift valley with uplifted horst blocks on either side. In a reverse or thrust fault, produced by compression, the hanging wall is pushed up over the footwall. In a strike-slip or transform fault, blocks slide horizontally past each other. The line of the fault at the surface is the fault trace, and the vertical displacement is the throw. Faults matter for landforms in two ways: directly, by creating fault scarps, rift valleys, and fault-block mountains, and indirectly, because fractured rock along a fault weathers and erodes faster than intact rock, so valleys often follow fault lines even where the original scarp has been worn away.

Isostatic adjustment is a slower vertical movement of a different kind. The crust floats, in a broad sense, on the denser mantle, so adding a load such as a thick ice sheet or a heavy sediment pile depresses the crust, and removing the load allows it to rebound. Regions once covered by thick ice are still rising today as the crust recovers, while regions loaded with sediment in deltas slowly subside. Together, folding, faulting, uplift, subsidence, and isostatic rebound explain why the crust is not a fixed platform but a surface constantly adjusting, and why landscapes preserve a record of movements that are still, in many places, under way.

Landforms and geomorphic hazards

Rivers are the most widespread land-forming agent on land. In their upper course, where gradient is steep, they cut narrow V-shaped valleys, gorges, and waterfalls where a resistant rock layer holds up the channel. In the middle course, lateral erosion widens the valley, meanders develop as the channel swings from side to side, and a floodplain of deposited alluvium forms. In the lower course, the river deposits more than it erodes, building levees along its banks, deltas where it meets the sea or a lake, and oxbow lakes where a meander neck is cut through and the old loop is abandoned. River terraces, flat benches above the present floodplain, record earlier floodplain levels and tell a story of renewed downcutting or changing base level. A graded river, in equilibrium between erosion and deposition along its length, is an ideal that real rivers approach but rarely maintain perfectly, because climate, tectonics, and sea level keep changing.

Glaciers produce a very different assemblage. A valley glacier erodes by plucking rock from its bed and by abrasion, as rock fragments frozen into the ice scour the bed like sandpaper, producing U-shaped valleys with flat floors and steep sides, hanging valleys where tributary glaciers could not cut as deeply as the main glacier, cirques or corries — armchair-shaped hollows at the glacier head — and sharp ridges and pyramidal peaks where cirques meet back to back. Deposition by ice, which unlike water can carry debris of every size unsorted, produces till, moraines along the sides and end of a glacier, drumlins of streamlined till, and outwash plains of sorted material deposited by meltwater beyond the ice margin. Wind, working where vegetation is sparse and sediment is dry, erodes by deflation, lifting fine particles, and by abrasion, sandblasting rock near the ground, producing mushroom rocks, yardangs, and desert pavements; it deposits loess, fine wind-blown dust, and sand dunes whose form — barchan, transverse, longitudinal — depends on sand supply and wind direction. Waves erode headlands into cliffs, caves, arches, and stacks, and deposit beaches, spits, and bars. Groundwater dissolving limestone produces karst: sinkholes, underground caverns, stalactites and stalagmites, and disappearing streams. Each assemblage can be recognised by its characteristic forms, and a good answer groups forms by agent and by whether they are erosional or depositional.

Geomorphic hazards arise when these processes operate at a scale or speed that threatens people. Earthquakes shake the ground, collapse buildings, and trigger landslides and tsunamis; their damage depends less on magnitude alone than on depth, distance, local ground conditions, and the quality of construction. Volcanic eruptions threaten through lava flows, ash fall, pyroclastic flows, and poisonous gases, and their hazard zone depends on eruption style: fluid basaltic eruptions are generally less explosive than viscous, gas-rich eruptions. Landslides, rockfalls, mudflows, and debris flows move material downslope rapidly, commonly triggered by heavy rain, earthquakes, undercutting by rivers or waves, or human activity such as road cutting and deforestation. Flooding, although hydrological, is geomorphically destructive, scouring channels and depositing sediment across floodplains where people have settled. Coastal erosion and sea-level rise threaten settlements built on soft cliffs and low-lying shores. Mitigation is never simply about controlling nature: it combines hazard mapping, land-use zoning that keeps vulnerable building out of the most dangerous places, early warning, engineered defences where they are justified, and community preparedness. The most dangerous landscapes are often those where a hazardous process is well understood but settlement has nevertheless expanded into its path.

Plate boundaries compared

Weathering types compared