The Earth is almost a sphere. These are its main layers, starting with the outermost:
crust - relatively thin and rocky
mantle - has the properties of a solid, but can flow very slowly
outer core - made from liquid nickel and iron
Inner core - made from solid nickel and iron
The Earth's lithosphere is the rigid outer layer that is made up of the crust and the part of the mantle just below it. The lithosphere is made up of a mixture of minerals.
Caption: : Cross section showing structure of the Earth
Continental Drift & Wegener’s Theory
The theory of continental drift was proposed at the beginning of the last century by German scientist Alfred Wegener.
Before Wegener developed his theory,
it was thought that mountains formed because the Earth was cooling down,
and, as it cooled down, it contracted This process formed wrinkles, or mountains, on the Earth’s crust.
If this was the case, then mountains would be spread evenly over the Earth's surface. We know that this is not the case.
Wegener suggested that mountains formed
when the edge of a drifting continent collided with another, causing it
to crumple and fold. For example, the Himalayas formed when India came
into contact with Asia.
It took more than 50 years for
Wegener’s theory to be accepted. One of the reasons was that it was
difficult to work out how whole continents could move. It was not until
the 1960s that enough evidence was discovered to support the theory
The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates.
These are constantly moving at a few centimetres each year. Although
this doesn't sound like very much, over millions of years the movement
allows whole continents to shift thousands of kilometres apart. This
process is called continental drift.The plates move because of convection currents in the Earth’s mantle. These are driven by the heat produced by the natural decay of radioactive elements in the Earth.
Where tectonic plates meet, the
Earth's crust becomes unstable as the plates push against each other, or
ride under or over each other. Earthquakes and volcanic eruptions
happen at the boundaries between plates, and the crust may ‘crumple’ to
form mountain ranges.
Caption: : The earth's continents are constantly moving due to the motions of the tectonic plates. Closely examine the map below, which shows the 15 major tectonic plates.
Types of Rock
Igneous RockIgneous Rocks are formed by magma from the molten interior of the Earth. When magma erupts it cools to form volcanic landforms.
When it cools inside the Earth it forms intrusive rock, which may later be exposed by erosion
and weathering. Intrusive rock will have large crystals as it has
cooled slowly. Magma that has cooled on the surface is known as extrusive rock. This will have small crystals as it has cooled quickly
A river carries, or transports,
pieces of broken rock as it flows along. When the river reaches a lake
or the sea, its load of transported rocks settles to the bottom. We say
that the rocks are deposited. The deposited rocks build up in layers,
called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.
These are the different processes in order:sedimentation → compaction → cementation
Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock include: chalk, limestone, sandstone, shale
Caption: : Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed. Sedimentary rocks are often quite soft and are susceptible to erosion.
Metamorphic rock has been subjected to tremendous heat, pressure
or both. This has caused it to change into another type of rock. It is
usually resistant to weathering and erosion and is therefore very
Examples of metamorphic rock include:
marble - which originates from chalk or limestone
slate - which originates from clay
schists – which have formed from sandstone or shale (sedimentary rocks)
Limestone - which is a sedimentary rock- is a valuable resource from the Earth’s crust. It has many uses. For example:
it can be used as a building material
it is a major ingredient in toothpaste
it can be used as a food additive to provide calcium ions for strong teeth and bones
it can be processed as a useful raw material in the chemical industry
Most limestone is obtained by quarrying, where the rock is blasted out of the ground in huge pits.
Reactions with AcidsCarbonates react with acids to produce carbon dioxide, a salt and water. For example:
calcium carbonate + hydrochloric acid → carbon dioxide + calcium chloride + water
CaCO3(aq) + 2HCl(aq) → CO2(g) + CaCl2(aq) + H2O(l)
Since Limestone is mostly calcium carbonate, it is damaged by acid rain.
Sodium carbonate, magnesium
carbonate, zinc carbonate and copper carbonate also react with acids -
they fizz when in contact with acids, and the carbon dioxide released
can be detected using limewater.
Uses of Limestone
When limestone is heated strongly, the calcium carbonate it contains decomposes to form calcium oxide.
calcium carbonate → calcium oxide + carbon dioxide
CaCO3(s) → CaO(s) + CO2(g)
This reacts with water to form calcium hydroxide, which is an alkali
calcium oxide + water → calcium hydroxide
CaO(s) + H2O(l) → Ca(OH)2(s)
Calcium hydroxide is used to neutralise
excess acidity, for example, in lakes and soils affected by acid rain.
Calcium hydroxide dissolves in water to produce calcium hydroxide
(limewater), which is used to test for carbon dioxide.
Cement, mortar and concrete
Cement is made by heating powdered limestone with clay.
Cement is an ingredient in mortar and concrete.
Mortar - used to join bricks together - is made by mixing cement with
sand and water. Concrete is made by mixing cement with sand, water and
aggregate (crushed rock).
Advantages and disadvantages of various building materialsConcrete is easily formed into
different shapes before it sets hard. It is strong when squashed, but
weak when bent or stretched. However, concrete can be made much stronger
by reinforcing it with steel.
Limestone, cement and mortar slowly
react with carbon dioxide dissolved in rainwater and wear away. This
damages walls made from limestone, and leaves gaps between bricks in
buildings. These gaps must be filled in or ‘pointed’.
Pollution from burning fossil fuels makes the rain more acidic than it should be, and this acid rain makes these problems worse.