Geography Reference
In-Depth Information
The thickened crust sticks up like an iceberg floating on top of the
mantle with a deep root. All ten of the Earth's highest peaks are
found in the Himalayas. As the high mountains are eroded and
mass removed then the floating root at the bottom moves up and
exposes more rocks and minerals which have been altered by high
temperatures and pressures.
WEATHERING AND EROSION
Weathering is the physical breakdown of rock whereas erosion is
the transportation of the weathered material. Mountains that are
built by tectonic processes are eventually worn down by weather-
ing and erosion. The sediments produced are moved around often
over vast distances by water, ice or wind and can be reincorporated
into rock formation again over long timescales. Thus the Earth's
surface is in a constant state of change. The rates of change vary
with rock type, climate, slope conditions, ice and vegetation cover.
Types of rock
There are three main types of rock found around the Earth's
surface:
igneous
,
sedimentary
and
metamorphic rocks
. Tem-
perature at the time of formation, the mix of minerals present and
pressure all interact to create varieties of these main rock types.
Igneous rocks are formed when molten lava cools and hardens. If
the molten rock is from a volcano, then the subsequent cooled and
hardened basalt rock has small crystals. If the rock is able to cool
slowly then larger crystals may grow producing a coarse grained
rock such as granite.
Sedimentary rocks are produced by weathering of rocks fol-
lowed by subsequent erosion and deposition of material. The
deposited sediment can accumulate and eventually build up before
being compacted and hardened over long time periods by the
weight and pressure of sediments above and internal chemical
changes. Rocks such as sandstone, siltstone or shales are good
examples. These rocks often contain a record of the physical con-
ditions present when the rocks were deposited, including fossils. In
fact some rocks such as chalk or coal are almost entirely made from
the remains of animals and plants.
