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THE STRUCTURE OF THE EARTH
CRUST
MANTLE
OUTER CORE
INNER
CORE
6000 OC
4000 OC
The diameter of
the Earth is
approximately
12,800km.
3000 OC
2000 OC
ATMOSPHERE
Crust
The crust, a thin layer of solid rock only 50km thick, has a mean density of about
2.5g/cm3.
Continental (or granitic) crust, which is rich in silicon and aluminium, is pale in colour.
Oceanic (or basaltic) crust, which is rich in iron and magnesium, is dark in colour. It is
here that we obtain all of the resources that we depend on.
Mantle
The mantle, which is approximately 5700km thick, has a mean density of about
4.5g/cm3. The mantle consists of SOLID rock which is rich in magnesium, iron and
silicon. Although the mantle is solid, its high temperature means that the rocks are able
to move slowly (creep). Convection currents in the mantle are responsible for the
movement of the continents.
Outer Core
The outer core, which is a very hot, dense liquid, consists mainly of iron and nickel. It
extends to a diameter of about 6900km (just over half the Earth’s diameter) and has a
mean density of about 11g/cm3. The Earth’s magnetic field arises here.
Inner Core
The inner core, like the outer core, consists mainly of iron and nickel, but because of the
immense pressure, it is solid. It extends to a diameter of about 2500km and has a mean
density of about 15g/cm3.
TOPIC 10.1.7: OUR CHANGING PLANET 1
PLATE TECTONICS
The Earth originated as a cloud of hot gas which condensed as it cooled to form a
sphere of hot liquid. The densest substances sank to the middle of this sphere, forming
the core, while the least dense materials stayed on the surface, where they cooled and
solidified to form the crust.
Until as late as the 1960’s, some scientists believed that the Earth continued cooling to
the present day. The interior cooled and contracted as it did so, the solid crust buckled
and puckered, forming mountain ranges. This effect is similar to the effect seen on the
surface of hot toffee as it cools.
Theory of Plate Tectonics
Alfred Wegener
Today it is believed that the Earth’s crust is cracked into a
number of large pieces (tectonic plates). These plates are
constantly moving at relative speeds of a few centimetres per
year as a result of convection currents within the Earth’s
mantle, driven by heat released by natural radioactive
processes. This causes the continents, which sit on the plates,
to change positions relative to each other. This theory was first
put forward by Alfred Wegener in 1911; at that time people
dismissed it, believing him to be a crank.
Drifting Continents
Material is subducted in an ocean trench and added to at a mid-ocean ridge, with the
net result that material is transferred from one edge of a plate to the other.
As plates move, driven by convection currents in the mantle, the continents ride on
them. Although the process is very slow, in geological time (millions of years) the
continents can move hundreds or even thousands of kilometres.
The current arrangement of the continents is thought to have developed in the following
way:
Pangaea
TOPIC 10.1.7: OUR CHANGING PLANET 2
The original land mass on the Earth
was named Pangaea.
Laurasia
Gondwanaland
180 million years ago
180 million years ago, Pangaea
began to split into two major parts,
Laurasia and Gondwanaland, and
Gondwanaland itself was beginning
to break up.
135 million years ago
Laurasia and Gondwanaland drifted
northwards. The North Atlantic and
Indian Oceans widened. The South
Atlantic rift lengthened.
65 million years ago
South America had separated from
Africa. Australia and Antarctica were
still joined. India was moving towards
Asia. The Mediterranean Sea had
appeared.
Today
South America has joined with North
America. Australia has separated
from Antarctica. India has collided
with Asia.
TOPIC 10.1.7: OUR CHANGING PLANET 3
EVIDENCE TO SUPPORT THE
THEORY OF PLATE TECTONICS
1. Different Climates
Although the Earth’s climatic regions are not thought to have changed significantly over
time, it is possible to find in great Britain examples of rocks which were formed:
 under desert conditions
 under tropical swamp conditions
 from corals in warm seas
 under glacial conditions etc.
This could be explained, however, if Britain itself had moved through the different
climatic regions over time. About 300 million years ago, in the Carboniferous period,
Britain was near the equator and tropical forests grew there. These later decayed to
form coal deposits. By the Permian period, Britain had moved further north and had a
desert climate. Most of the rocks deposited during this period are desert sandstones.
2. Continents Which Were Once Joined
The edges of land masses (continents)
which are separated by thousands of
kilometres of ocean (e.g. the east coast of
South America and the west coast of
Africa) have shapes which fit quite closely.
This suggests that they were once part of a
single land mass which has split and
moved apart.
Also, if South America and Africa are
rejoined, they can be seen to have similar
patterns of rocks and fossils. For example,
an old mountain belt and fossil remains of
the Mesosaurus can be seen to run
continuously between South America and
Africa.
3. Earthquake Zones
Most earthquakes and volcanoes occur along clearly defined lines in the Earth’s crust
which correspond to the margins of the plates, where they rub against each other. If a
large amount of stress builds up, the plates may deform. The built up tension can be
released by a sudden movement: this is an earthquake. Scientists are unable to predict
when earthquakes will happen.
TOPIC 10.1.7: OUR CHANGING PLANET 4
TOPIC 10.1.7: OUR CHANGING PLANET 5
The Earth’s atmosphere
The earth was formed about 4.6 billion years ago. During the first billion years of
the Earth.s existence there was intense volcanic activity. This activity released the
gases that formed the early atmosphere and water vapour that condensed to form
the oceans.
For the last 200 million years, the proportions of different gases in the
atmosphere have been much the same as they are today:
− about four-fifths (78%) nitrogen
− about one-fifth (21%) oxygen
− small proportions of various other gases, including carbon dioxide, water vapour
and noble gases.
Some theories suggest that during this period, the Earth’s atmosphere was mainly
carbon dioxide and there would have been little or no oxygen gas (like the
atmospheres of Mars and Venus today). There may also have been water vapour
and small proportions of methane and ammonia.
Bacteria and algae began to thrive and started to produce oxygen. As more
plants spread across the earth’s surface, oxygen became richer and richer and
eventually it was possible for animals to evolve
Some of the earliest bacteria could not tolerate oxygen and eventually died out..
The seas absorb huge amounts of carbon dioxide forming both soluble and
insoluble carbonates. Most of the carbon from the carbon dioxide in the air gradually
became locked up in sedimentary rocks as carbonates and fossil fuels.
The level of CO2 has remained constant as a result of a natural cycle. Nowadays
the release of carbon dioxide by burning fossil fuels increases the level of carbon
dioxide in the atmosphere. As CO2 levels in the atmosphere increase energy from
the sun is trapped within our atmosphere leading to global warming.
The Carbon cycle
TOPIC 10.1.7: OUR CHANGING PLANET 6
Life on Earth
Have you ever wondered where the molecules which make up the cells that all living
things are made of? One theory (Miller-Urey) suggests that methane (CH4), ammonia
(NH3), water (H2O) and hydrogen (H2), which were all present in the earth’s early
atmosphere, may have reacted to form the amino acids which we are all made of. The
energy required to make this happen could have been provided by lightning.
In the laboratory they mixed these gases and produced sparks to simulate
lightning over 7 days. When the mixture produced was analysed they found 22 amino
acids.
Other scientists suggested that the composition of the earth’s atmosphere was
different to those used by Miller and Urey, however, amino acids are still produced
when they replicated the original experiment.
Other theories suggest that meteors crashing into our planet brought a variety of
organic molecules with them. These are only theories and it has not been proven that
life on earth did start in this way.
TOPIC 10.1.7: OUR CHANGING PLANET 7
Gases in the atmosphere
The air we breathe is a mixture of several very useful gases. Oxygen is used in welding
as well as to help people with breathing difficulties in hospital. Nitrogen is unreactive
and is used to stop food going off (eg a packet of crisps). Liquid nitrogen is used to
freeze food rapidly (eg frozen fish fingers). Argon is used in light bulbs. In order to use
these gases we need to separate the mixture. To do this we use fractional distillation.
The liquefied air (-200oC) is passed into
the bottom of a fractionating column.
The column is warmer at the bottom
than it is at the top. The liquid nitrogen
boils (-196oC) at the bottom of the
column. Gaseous nitrogen rises to the
top, where it is piped off and stored.
Liquid oxygen (-183oC) collects at the
bottom of the column. The boiling point
of argon - the noble gas that forms 0.9%
of the air - is close to the boiling point of
oxygen, so a second fractionating
column is often used to separate the
argon from the oxygen.
TOPIC 10.1.7: OUR CHANGING PLANET 8