Geomorphic Processes and Earth Movements

The surface of the Earth is shaped by a constant tug-of-war between forces that build it up from within and forces that wear it down from outside. These geomorphic processes and earth movements fold and fracture rocks. Together they create mountains, valleys, plateaus and rift valleys. Understanding them ties together much of physical geography.

Geomorphic processes are driven by two opposing sets of forces:

• Endogenic (internal) forces: powered by the Earth's internal heat. They cause uplift, subsidence, folding, faulting and volcanic eruptions, and so build up relief. They may act suddenly (earthquakes, volcanoes) or very slowly.
• Exogenic (external) forces: powered ultimately by the Sun (and gravity). Through weathering, mass movement, erosion and deposition, they wear down the land and smooth out relief.

Exogenic forces wear down high areas and fill in low ones. The overall result of this is called gradation (degradation + aggradation). The slow internal diastrophic movements are further classed as orogenic (mountain-building, horizontal) and epeirogenic (broad vertical uplift or subsidence of continents). Every landform records this contest. Where building outpaces wearing, the land rises. Where wearing wins, the land is lowered.

Six factors together bring dynamic changes to the Earth's surface:

• Electromagnetic radiation: the Sun's energy. It heats the surface unevenly and drives the exogenic agents.
• Geothermal energy: the Earth's internal heat. It powers the endogenic forces.
• Gravitational force: it pulls water, ice and loose rock downslope.
• Plate movements: they cause folding, faulting, earthquakes and volcanoes.
• Rotation of the Earth: the daily spin. It helps drive winds, ocean currents and tides, which power exogenic work.
• Revolution of the Earth: the yearly orbit. It produces the seasons, which set the rhythm of heat, rain and frost.

So surface change is not the work of heat and gravity alone. The Earth's own rotation and revolution also keep its surface dynamic.

The water, oxygen and acids that attack rocks come largely from rain. Rainwater is never pure. As it falls, it dissolves gases from the air:

• Carbon dioxide: in solution it forms weak carbonic acid. This acid slowly dissolves rocks such as limestone.
• Oxygen: dissolved atmospheric oxygen combines with iron-bearing minerals and rusts them (oxidation).

Both gases carried in rainwater therefore cause chemical weathering of rocks.

How easily water can enter and pass through a rock depends on two distinct properties:

• Porosity: the rock has pores (tiny spaces) that can hold and absorb water.
• Permeability: the rock lets water flow through it.

The two do not always go together. Chalk is porous, so it absorbs water, and it is also very permeable. Clay also has pores, but its particles are so fine that water cannot pass between them. Clay is therefore porous yet quite impermeable.

The Earth's outer shell is broken into huge slabs called plates, which move very slowly. Their movements unleash the sudden endogenic forces:

• Earthquake: a sudden shaking of the ground. It happens when plates slip past or push against each other and release stored energy. The underground point of origin is the focus. The point on the surface directly above it is the epicentre.
• Volcano: an opening in the Earth's crust through which molten rock (lava), ash and gases erupt from below.

These sudden forces build mountains, islands and valleys, and can cause great destruction.

When rocks are squeezed by compressional (horizontal) forces acting towards each other, the strata bend into folds. This is folding. It builds the great fold mountains.

The parts of a fold are:

• the anticline: the upfold (arch), and
• the syncline: the downfold (trough).

Folds vary in form: symmetrical (limbs equally inclined), asymmetrical (one limb steeper), overturned, isoclinal (both limbs parallel after intense pressure), and recumbent (lying on its side). The Himalayas are the classic example of fold mountains. They formed as the Indian plate pushed into Eurasia, compressing the rocks between.

When rocks are subjected to tensional (pulling-apart) forces, they cannot bend. Instead, they fracture, and the blocks shift along the break. This is faulting. The plane of fracture is called the fault plane. The main types:

• Normal fault: the crust is stretched and one block slips down relative to the other.
• Reverse (thrust) fault: the crust is compressed and one block rides up over the other.
• Step faults: a series of faults all slipping in the same direction.

Faulting produces dramatic landforms. An uplifted block between two faults is a horst (block mountain). A sunken block is a graben or rift valley. The Rhine valley in Europe is a classic example.

Weathering is the breaking down of rocks, soil and minerals where they lie, by sun, rain, frost, wind and living things. The key point is that nothing is carried away. The material simply crumbles in place. It comes in three forms:

• Physical weathering: breaks rock into smaller pieces without changing its minerals. Repeated heating and cooling makes rock expand, contract and crack. Water freezing in cracks widens them, a process called frost action.
• Chemical weathering: changes the very minerals of the rock through reactions with water, oxygen and acids. Rusting of iron-rich rock and the slow dissolving of limestone by rainwater are examples.
• Biological weathering: the work of living things. Tree roots pry open cracks. Burrowing animals and bacteria loosen the rock.

Weathering is vital for two reasons. It is the first step in the making of soil. It also prepares loose material that other agents can then carry away.

Not all material is carried by water, ice or wind. Mass movement is the downslope movement of rock and soil under the direct pull of gravity, without a carrying agent. It ranges from the very slow to the sudden:

• Creep: the imperceptibly slow downhill shift of soil over years.
• Landslides and rockfalls: rapid, often destructive movements that send rock crashing down a steep slope in moments.
• Slump: the sliding of a mass of earth along a curved surface.

Water often acts as a trigger. It adds weight and makes the material slippery. That is why landslides are common during heavy rain in hilly areas.

Once weathering loosens rock, the moving agents take over. Erosion is the wearing away and removal of material by a moving agent. Deposition is the laying down of that material somewhere else when the agent loses its energy. Erosion lowers the land. Deposition builds new, usually flat, land. Each agent carves its own typical landforms:

• Running water (rivers): the most powerful agent of all, especially in humid regions. Rivers cut deep valleys and gorges in their upper course. In the plains they build wide loops called meanders and flat floodplains. At the mouth they deposit sediment to form a delta.
• Glaciers: slow-moving rivers of ice in cold mountain and polar regions. They carve broad U-shaped valleys and leave behind deposits called moraines.
• Wind: the chief agent in dry deserts. It shapes mushroom-like rocks and builds sand dunes.
• Sea waves: erode coastlines into cliffs and sea caves, and build beaches.

A trained eye can often tell which agent shaped a given landscape from these signature landforms.

The interplay of all these forces produces three broad relief features that make up the Earth's surface:

• Mountains: the highest landforms, rising steeply well above their surroundings. Young fold mountains such as the Himalayas are still rising. Mountains influence climate, are the source of many rivers, and hold rich stores of minerals and forests.
• Plateaus: raised areas of fairly level land with steep sides, standing higher than the plains around them. The Deccan Plateau in India is one of the oldest. Plateaus are often rich in minerals, which is why many mining regions are found on them.
• Plains: large stretches of flat, low land, usually formed by the deposition of sediment by rivers. They are the most fertile and most densely settled parts of the world. The northern plains of India, built by the Ganga, the Indus and the Brahmaputra, are a classic example.