11.13 MOUNTAIN BUILDING, WEATHERING AND EROSION
Earth’s natural landscapes are shaped by
the ongoing processes of mountain
building, weathering and erosion. Apart
from volcanos, mountain building occurs
mainly when tectonic plates collide and the
edges of one or both are crumpled and
thrust upwards over tens of millions of
years. The photo shows Kanchenjunga in
the Himalayas where the Indian plate is
colliding with the Eurasian plate. But at the
same time that mountains are being thrust
up, they are also being worn away by
weathering and erosion.
Weathering is the physical and chemical
break-down of rocks into smaller rocks and soil by natural processes, including the heat and light
of the sun, corrosion by gases in the atmosphere, and the action of wind, rain, snow, frost and
ice. Erosion refers to the carrying away of rocks and soil, mostly by water and gravity through the
action of rain, streams and rivers. Wind may also cause erosion, and in cold climates and high
mountains, glaciers (slow moving rivers of ice) grind down the rocks beneath them. Weathering
and erosion can wear away even the
highest mountains in time, leaving only
flat plains. The photo on the left shows
the Grand Canyon in the USA. A flat
plain, left behind after the erosion of
earlier mountains, was uplifted millions
of years ago. Now the plain is being cut
down again by weathering and erosion.
The canyon is 440 km long, 29 km wide
and almost 2 km deep. It has taken
about 17 million years for the Colorado
River to carve out the Grand Canyon.
In a natural landscape, erosion is slowed by vegetation. Roots may help to crack rocks, but they
also hold the soil together and prevent it being washed or blown away. Grass, trees and other
plants also shield the soil from the force of heavy rain and strong winds. Some animals contribute
to erosion, for example by digging burrows, but humans are the most destructive of all. When we
clear land to plant crops, or
allow our animals to
overgraze, the soil can be
washed or blown away.
Deforestation (cutting down
trees without replacing
them) can be very harmful.
On steep slopes, millions of
tonnes of good soil may be
washed away very quickly,
impoverishing the ground
and spoiling nearby rivers.
Villagers living near the
river in the first photo used
Deforestation and erosion
Terracing helps to prevent erosion
to eat fish, but now the
river is brown with eroded soil and fish can no longer live in it. The second photo shows a
traditional way of preventing erosion. Terraces slow
down run off, trap the soil, and help to retain water.
Soil is essential for the growth of most
plants. It is a mixture of sand and clay (from

1. What are (i) weathering, (ii) erosion, (iii) soil, (iv)
the weathering of rocks) and humus.
overgrazing, (v) deforestation, (vi) run off, and (vii) terracing?
Humus is organic matter from rotting leaves
and similar materials. Have look at soil with

2. What is the approximate time scale for building mountains,
a hand lens and see what you can find.
and for wearing them down again?
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