The STRUCTURE of the EARTH
Dating rocks & earthquake (seismic) wave analysis
Doc Brown's Chemistry - Earth Science & Geology Revision Notes
for KS4 Science, GCSE, IGCSE & O
INDEX OF EARTH SCIENCE PAGES
GCSE/IGCSE level chemistry revision
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The structure of the Earth is described in terms of
the three layers - core (inner and outer), mantle and the crust and comments on
the dating of rocks and how we find out the structure of the Earth from
The Structure of the Earth - A
sort of egg?
9. The structure of the Earth
The Earth is almost spherical and is composed of
three principal layers.
The three layered structure
of the Earth.
X is the crust: is the relatively thin and cool outer
solid layer of the Earth. The thickness of this upper part ranges from 6km to
100km surrounded by the atmosphere
of air W. It is much cooler,
harder, brittle and less dense than the other layers of the Earth. The crust is divided into
sections or 'tectonic plates' which 'float' and move on the very hot mantle
at the rate of a few cm per year relative to each other (for more details see
This plate movement means that most parts of the Earth's crust are in very
different locations from millions of years ago. It should
also be noted that 2/3rds of the surface of the Earth is water.
The lithosphere consists of the crust and the
almost solid upper part of the mantle next to the crust.
Y is the mantle: is very hot rock material, it is
almost solid, quite rigid, but behaves like 'thick, very viscous plastic' rock
and can be deformed. In the mantle there are huge slowly moving convection
currents driven by heat from radioactive decay in the metallic core. It is these
convection currents which move the 'plates' which float above the mantle. The mantle's 'thickness' is 3000 km
(about halfway to the centre of the Earth) and
its temperature is usually over 1000oC. The deeper you go, the hotter
the mantle gets and the rock gets less rigid i.e. flows more easily.
Chemically, the mantle consists mainly of
non-metallic silicates with some metal ions. Magma is heated molten rock, from
the more 'runny' mantle
material and comes up to the surface in volcanic activity or igneous
intrusions into and through the mantle. The mantle has a higher density and a different chemical composition
compared to the crust. It is relatively cold and rigid just below the crust, but
lower down it is much hotter and non-rigid and so is able to flow.
Technical note: Most of the heat (~90%) generated
in the Earth's interior is fuelled by the decaying of radioactive isotopes like
Potassium 40, Uranium 238, 235, and Thorium 232 present in the mantle. These
radioactive-isotopes generate heat as they lose excess energy when changing to more stable
Z is the core: is composed mainly of iron, nickel and
other metals. Its diameter is about half that of the Earth (3500 km radius) and its is very hot
and dense. It is believed that the core consists of an outer liquid layer (outer core) and a
solid inner layer
It is the iron core generates a magnetic field through and around the Earth.
Some general points:
The overall density of the Earth is
much greater than the average density of the rock of the crust. This is evidence
inner layers of the Earth are made of different more denser materials from that
of the crust e.g. the metallic core.
The lithosphere is the rigid, relatively cool crust,
and the outer or upper part of the mantle. It is split into sections called
plates - the base of tectonic theory.
The age when rocks where formed?
- The age at which rocks were formed, in, or on the
crust can be estimated
in various ways ..
- Fossils: As plants and animals evolve, species die
out and new ones emerge. The sequence and type of fossils can be worked out and the
timescale estimated. Therefore the fossils present in a layer can be used to estimate
the age of the sedimentary rocks. This dating method is not absolute like
radioisotope studies of igneous rocks but its the most useful for
isotope dating: This is a more accurate method
for dating very ancient igneous rocks. As certain isotopes, with VERY long
half-lives, decay to form more stable atoms, there is a change in the
isotope ratio of less stable / more stable. This ratio gets
smaller, and by knowing the rate of change from the half-life of the more
unstable atom, the age at which the magma cooled to give igneous rock can be
- For example: potassium-40 decays to Argon-40 with a half-life of 1300
(1.3 x 109y).
- The potassium-40/argon-40 ratio can be measured in an
analytical instrument called a mass spectrometer.
- If 50% of the potassium-40 remains, the rock is 1.3
x 109y old
- if 25% is left the age is 2.6 x 109
- if 12.5% is left the age is 3.9 x 109 years etc.
- Age of the Earth: Using this method it is estimated
to be 4.5 x 109 years.
- The radioisotope carbon-14,
14C, is of new use for
- Carbon-14's half-life is far too small at only 5700 years
to be of any use for the geological dating of rocks.
- 5700 years is
not very long in terms of geological time because most rocks are at
least hundreds of thousands, or millions of years old, from their date of
- However, carbon-14 is very useful to
archaeologists for dating artefacts of organic origin like wood and bone.
- For more details see
Radioisotopes and dating rocks and archaeological finds
7C How to we get our knowledge of the cross-section of the Earth?
How far can we drill into the
Earth's crust and mantle?
How do we know there is a crust, mantle,
inner and outer cores?
How can we use seismic waves (earthquake
waves) to help us
answer these questions?
At the most, we can only drill
down into the Earth's crust to a depth of 12km, which is not even through to
Therefore, we must find other
scientific methods to investigate the inner structure of the Earth since we
can't even penetrate the crust to get to the mantle!
When an earthquake happens in the Earth's
crust it results in the spreading out of seismic waves. They result from the
huge amounts of potential energy stored in the stressed layers of rock
resulting from plate tectonic movement. These earthquake waves can be
detected all around the world using an instrument called a seismometer.
The speed of seismic waves depends on the
material they are travelling through, in particular the density of the rock
layers. When the waves meet a boundary they may be partially reflected,
completely reflected, absorbed, continue in a direct line with a different
speed or the waves might be directed and change direction.
Because the density of the rock changes
gradually in a particular layer, so does the speed of the wave. If
refracted, the waves follow curved paths (see the diagram below). However,
at a boundary, the speed may change more abruptly giving a bigger change in
direction (just as you see with light ray experiments with prisms.
Scientists (seismologists) study the
properties and pathways of seismic waves to deduce the internal structure of
From the speed, absorption and
refraction of seismic waves scientists have worked out the number and depth
of the four layers of the internal structure of the Earth.
These seismologists calculate the time it
takes for these shockwaves to reach every seismometer around the world and,
importantly for a specific earthquake, observing the parts of the Earth's
surface where you don't detect the waves.
The monitoring and recording of
earthquake waves has proved the most fruitful direct scientific probing of
the Earth's structure and these seismic shock waves have given us most of
the detailed scientific knowledge we have of the total structure of the
Powerful earthquake waves (seismic
waves, shock waves) always
emanate from the central point of an earthquake, called the epicentre, and
they spread throughout much of bulk of the Earth if powerful enough.
These shock waves are detected by
seismologists who use instruments called seismometers to monitor
how long it takes for the shock waves to travel from one point near the
earth's surface to another point on the earth's crust.
These seismic waves are recorded all over
the Earth's surface using seismographs.
Scientists called seismologists
measure the time it takes for an earthquake to travel from the
earthquake epicentre to the detection system - the seismometer.
There are different types of waves are
not all are detected in particular zones of the Earth's surface.
P-waves which are longitudinal
waves, the vibrations are in the same direction as the waves.
A sort of 'push and pull' effect -
compression and decompression vibrations of the rock material.
P-waves travel through liquids and solids
and are faster than S-waves.
P-waves are no different in principle to
S-waves are transverse, the
vibrations are at 90o to the direction the wave is travelling.
L-waves are transverse
waves that travel on the Earth's surface moving the ground 'up and
down' in a wavelike form.
The waves can change speed and direction
as the properties of the mantle and core change.
The changes tend to be gradual giving
curved paths, but sometimes you get more abrupt changes (e.g. from mantle to
core) and sometimes a waves is absorbed and stops.
So, the behaviour of these waves is very
complex but inputting this data into sophisticated computer models enables scientists to
work out much of the inner structure of the earth.
I hope the diagram and
accompanying notes explain the basic ideas how violent earthquakes help us
to understand the structure of the Earth.
A brief guide to the detailed
arguments to follow.
P-waves (primary waves) and S-waves
take curved paths because of the ever changing density of the Earth's
layers producing a gradual refraction effect.
The longitudinal P-waves can pass
right through the centre of the Earth but due to refraction give two
small shadow zones (marked black on the diagram). They travel faster than
The transverse S-waves are absorbed by
the outer core and give one much larger shadow zone (marked blue + black
on the diagram). They travel slower than P-waves.
13. Earthquake Waves, S waves & P waves - study carefully!
To summarise before going into all the
refract as the density changes, and pass right through the core,
but the refraction does create narrow P-wave shadow zones which overlap with
the S-wave shadow zone.
can't go through the core, so are not detected on the other side
of the Earth, creating a broad S-wave shadow zones.
Fig 13. S and P earthquake
waves, shows the paths
of two types of earthquake waves (P, blue on
diagram) and S, green on diagram) as they travel through the
Earth from an
Longitudinal P-waves can pass
through all the layers of the Earth because the 'push and pull'
oscillation energy isn't absorbed as easily as it is for the S-waves.
Earthquake waves shadow zones
The diagram shows the zones on
the Earth's surface where the S and P earthquake waves can both be
detected by a seismometer (purple zone),
AND, just as importantly, where the S and P waves cannot be detected (no S waves can reach the black and
zones and no P waves can reach the black zone).
The reason for these shadow
zones is explained below (see also the 'physics' section at the end).
BUT, from the shadow zones you can
work out the depth of the mantle and the inner and outer layers of the
Around the world are
seismographic stations fitted with seismometers that can detect the P and S
earthquake waves and portrayed as a seismogram (seismographic trace).
Seismologists monitor the pattern of
seismic wave activity and crucially, when the waves meet a boundary between
layers some of the waves are reflected.
The waves may also change speed as the
density changes and cause the waves to change direction by refraction.
Within a layer most waves follow a curved
path, but the significant deviations occur at the boundaries of the layers.
By analysing the reflection, refractions
and shadow zones (where no waves are detected on the surface), it is
possible to work out the internal structure of the Earth.
Seismologists use seismometers to produce
seismographs (seismograms) to monitor and record the minute vibrations in
the Earth's crust caused by an earthquake.
Above is a seismogram, a
seismographic recordings of the vibrations experienced in the UK from an
earthquake on the other side of the world!
You only need to know about P-waves
(arrive 1st) and S-waves (arrive 2nd), but as you can see there are other
waves and the recording is quite complex!
The difference between the arrival times
of the P-waves and the S-waves can be used to calculate how far away the
earthquake was. From different geographically located seismometer readings
you can then calculate where the epicentre of the earthquake was.
The main changes in the seismic wave
behaviour occur about half-way to the centre of the Earth.
P waves, longitudinal waves, can travel right through
the Earth, passing through the mantle and core in the process.
The wave paths are curved
because the waves are gradually refracted by the gradual change in density
as you go deeper into the Earth.
The refraction and absorption
effects causes the formation of zones where one or more of the earthquake
waves cannot be detected.
S waves, transverse waves, travel through the
mantle showing that it is almost solid and certainly not liquid.
The transverse S-waves can only travel through
solids and are slower than P waves.
The S-waves travel through the
mantle which suggest its quite solid, but think of it as being a
very viscous material, which can melt to form magma in 'hot spots' which
can form huge slowly rising plumes of magma.
The fact that S-waves are absorbed
by the outer core, shows that the outer core is liquid.
However, the S waves are absorbed by the
outer core and are not detected on the other side of the Earth from where
the earthquake occurred, creating the
S wave shadow zone (which overlaps with a smaller P wave shadow zones).
As well as travelling through
the crust and mantle, P waves can also travel right through the core layers
and be detected on the other side of the Earth.
However because of the refracted
curved paths of the P waves, there are two smaller zones where P waves are
By collating lots of data from
different seismometers around the world on the Earth's crust, it is possible
to work out from the speed and path patterns of the P and S wave detected
the structure of the Earth, that we cannot in any way investigate directly.
The earthquake wave data is put
into a mathematical computer model, from which you can get the thickness of the
crust and the rocky plastic mantle, the thickness of outer liquid metal core
and the radius of the solid inner core of metal.
For example, the P waves speed up in the
inner core suggesting that not only is the density greater, but that the
inner core is also solid.
Physics notes and my Fig 13.
The size of the arc created by S-wave
detection depends on the size of the core.
The size of the arcs of the different
shadow zones of the S-waves and P-waves depend on the size of the inner and
Therefore, from the pattern, you can
deduce the size radius of the inner core and the depth of the outer core.
So, you can then calculate of the
depth of the mantle (plus the crust).
P waves, the primary
waves, are longitudinal waves and can pass through liquids and solids
(blue paths on the diagram).
The oscillation/vibration of
longitudinal waves is in the direction of wave motion, remember the 'push &
pull' of the 'slinky spring' in your physics lessons.
This compression and
decompression of the material helps retain the energy of the wave and so can
pass through liquids or solids (rather like sound can pass through any
The energy from P waves is much
more gradually absorbed by the medium than in the case of S waves.
S waves, the secondary
waves, are transverse waves and can only travel through solids
(green paths on the diagram).
In transverse waves, the
oscillation/vibration is a right angles to the direction of wave motion and
in liquids or 'plastic' material, the energy of the wave is quite quickly
dispersed and the amplitude drops to zero, i.e. the wave is no longer
This is why you get the shadow
zone beyond the liquid outer core below the mantle.
In the diagram Fig 13., you
should notice all the paths of the earthquake waves are curved.
The curvature is due to the
waves being continually refracted as the density of the mantle and
The refraction is more prominent
at the mantle - outer core boundary and the outer core - inner core boundary
where the density changes the most abruptly.
Technically, as with light, when
the waves pass into a more dense material they bends towards the normal and
when the waves pass into a less dense medium they bend away from the normal.
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