The surface of the earth is not static; it is constantly changing. The Himalaya mountains are still rising, rivers are carving out valleys, glaciers are grinding away rocks, and the wind is shifting sand dunes. Some of these changes happen so slowly that we cannot see them in a lifetime, while others, like earthquakes and volcanoes, can transform the landscape in seconds.
This chapter explores the two great sets of forces that shape the earth's surface: the endogenic forces that work from inside the earth, such as earthquakes and volcanoes, and the exogenic forces that work on the surface, such as rivers, wind, glaciers, and the sea. We will also learn how the movements of the earth's plates create major landforms like mountains, and how weathering, erosion, and deposition create minor landforms.
According to the theory of plate tectonics, the lithosphere (the crust and the upper part of the mantle) is broken up into a number of large slabs called lithospheric plates. These plates are in constant slow motion, moving a few centimetres every year. They float over the hot, semi-molten part of the mantle below.
The movement of the plates causes many of the earth's major features and events. Where plates collide, mountains are formed. Where they move apart, new crust is created. The boundaries of the plates are the most geologically active areas of the earth, where earthquakes and volcanoes are most common.
The forces that originate from inside the earth are called endogenic forces. These include the movements of the plates and the heat of the earth's interior. Endogenic forces cause sudden movements like earthquakes and volcanic eruptions, as well as slow movements that build up mountains and continents. They are responsible for creating major landforms.
The forces that work on the surface of the earth are called exogenic forces. These include the action of running water, wind, glaciers, and the sea. Exogenic forces wear down the surface of the earth through the processes of weathering, erosion, and deposition. They are responsible for creating minor landforms like valleys, plains, and sand dunes.
An earthquake is a sudden shaking or trembling of the earth's surface. It is caused by the sudden movement of rocks along a fault line in the earth's crust. When the plates of the earth move and get stuck at the edges, stress builds up, and when the stress becomes too great, the rocks break and release energy in the form of waves, which cause the shaking.
The point where the earthquake originates inside the earth is called the focus, and the point on the earth's surface directly above the focus is called the epicentre. Earthquakes are measured on the Richter scale. The instrument used to measure earthquakes is called a seismograph. Earthquakes can cause immense damage, destroying buildings and causing tsunamis in the oceans.
A volcano is an opening in the earth's crust through which molten rock (magma), gases, and ash escape. When magma comes to the surface, it is called lava. Volcanoes can be active (erupting regularly), dormant (sleeping for long periods), or extinct (no longer likely to erupt).
The most active volcano in the world is Mount Kilauea in Hawaii, while Mount Etna in Italy is Europe's most active volcano. Volcanoes can be destructive, destroying cities and farmland, but the lava and ash they bring can also make the surrounding soil very fertile.
Rivers are powerful agents of change. A river erodes (wears away) the land through which it flows, transports the eroded material, and deposits it where its speed decreases. The processes of erosion and deposition by running water create many landforms.
In desert areas, the wind is a major agent of erosion and deposition. Wind erodes rocks by blowing away loose sand and small particles, creating landforms like mushroom rocks, where the base of a rock is worn away more than the top. The deposits of wind-blown sand form sand dunes, and the largest dunes are called barchans. Wind erosion can also create deep, narrow canyons called gorges.
The waves of the sea erode the coast and deposit material. Wave erosion creates sea caves, arches, and stacks along the coasts. The deposition of sand by waves and currents forms beaches. The work of the sea thus continuously reshapes the coastline.
Glaciers, the slow-moving rivers of ice in cold mountain regions, are powerful agents of erosion. A glacier picks up rocks and debris and grinds the valley floor as it moves, carving out U-shaped valleys. When glaciers retreat, they leave behind moraines (deposits of rock debris) and other features. Glacial erosion creates some of the most dramatic mountain landscapes in the world.
| Force | Origin | Examples | Landforms |
|---|---|---|---|
| Endogenic | Inside the earth | Earthquakes, volcanoes, plate movement | Mountains, continents |
| Exogenic | On the earth's surface | Rivers, wind, glaciers, sea | Valleys, deltas, sand dunes |
| Agent | Erosional Landform | Depositional Landform |
|---|---|---|
| River | V-shaped valley, waterfall | Delta, floodplain, levee |
| Wind | Mushroom rock, gorge | Sand dunes, barchans |
| Sea | Sea cave, arch, stack | Beach |
| Glacier | U-shaped valley | Moraines |
The earth's surface is alive with change, driven by forces both within and upon it. The movement of lithospheric plates raises mountains and triggers earthquakes and volcanoes, while rivers, wind, glaciers, and the sea continuously wear down the land and build new landforms. Every valley, delta, sand dune, and beach is the result of these patient and powerful processes. By understanding how the earth changes, we learn to read the landscape around us and to appreciate the immense forces of nature that have shaped - and continue to shape - our planet.