Geomorphology: the processes that shape landforms

The Greater Caucasus is still rising, and every year mudflows and landslides carry millions of tonnes of rock down its slopes. So every mountain is the result of a “contest” between two forces: one builds relief and the other destroys it. Geomorphology is the science of landforms, their origin and their development.

Endogenous and exogenous processes

The change in height at any point can be written as a simple balance: uplift (tectonic and isostatic) minus denudation, the lowering of the surface by weathering and erosion. When uplift is faster than denudation, mountains grow; when it is slower, they are worn down; when the two are equal, the relief is in dynamic equilibrium.

Weathering

Weathering is the breakdown and alteration of rock in place, without transport. In physical weathering rock breaks apart without changing its chemistry: water freezing in cracks expands by about 9% and splits the rock like a wedge; day–night temperature swings peel off the surface layer; growing salt crystals push grains apart. In chemical weathering minerals react with water, CO₂ and oxygen: limestone dissolves (karst caves and sinkholes), feldspar turns into clay, and iron-bearing minerals oxidise into red soils. Biological weathering involves roots and micro-organisms.

The type of weathering depends on climate. In hot, humid climates chemical weathering is very strong: reactions speed up in the heat and water is plentiful, so thick weathered mantles form in the tropics. In cold, wet mountains frost weathering dominates, and in hot deserts temperature and salt weathering do. In cold, dry lands both types are weak.

Erosion and transport: stream power

On land, running water changes relief more than anything else. A river's work is measured by the rate at which it loses potential energy — its stream power. When a volume Q of water (mass ρQ) flows each second down a slope S, it releases ρgQS of energy per metre of channel length. This energy goes into friction, scouring the bed and carrying sediment. According to the Hjulström curve, medium sand is the easiest to pick up: coarse gravel is heavy, and fine clay particles stick together.

Besides rivers, gravity (landslides, rockfalls, mudflows), glaciers (U-shaped valleys, moraines), wind (barchan dunes, deflation) and sea waves (cliff erosion, beaches) also shape relief. Mudflows on the southern slope of the Greater Caucasus and the mud volcanoes of Absheron and Gobustan are characteristic features of Azerbaijan's relief.

Isostasy and landform evolution

The crust “floats” on the denser, slowly flowing mantle. In the Airy model, a mountain has a root sinking into the mantle. At the depth of compensation the pressure must be the same everywhere: under the mountain it is ρc·(h + T + r)·g, and under the plain ρc·T·g + ρm·r·g (T is the normal crustal thickness). Setting them equal gives ρc·h = (ρm − ρc)·r.

Isostasy also slows denudation: when a thickness e of rock is eroded, the crust becomes lighter and rises by about e·ρc/ρm. So the surface is lowered by only Δz = e·(1 − ρc/ρm) ≈ 0.15·e — to reduce mean elevation by 1 km, about 6.6 km of rock must be removed. That is why old mountains such as the Urals still exist after hundreds of millions of years.

The first general theory of landform evolution was put forward by W. M. Davis in 1899: after uplift, relief passes through “youth”, “maturity” and “old age” and ends as an almost flat surface, a peneplain. Later researchers proposed parallel slope retreat and the idea of dynamic equilibrium (J. Hack, 1960): when uplift and erosion are equal, the shape of the landscape stays the same for a long time. Modern geomorphology studies these processes with erosion rates measured by cosmogenic isotopes and with computer models.