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Doc Brown's Chemistry - Earth Science Revision Notes
9.
More on Plate Tectonics and subduction Revision KS4 Science
IGCSE/O level/GCSE
Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel
360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science
(revise courses equal to US grades 9-10)
based on a GCSE Geological & Earth Science TASK SHEET * Earth
Science Homepage * 5
multi-word fill GCSE worksheets + answers * GCSE
Earth Science Quiz: Foundation-easier m/c Quiz and
Higher-harder level
m/c Quiz ANSWERS-NOTES
1. The Evolution of the Earth's atmosphere and Carbon Cycle ...
2. The Rock Cycle and types of
rock (details 'evolve' through sections 3. to 9.) ...
3.
Weathering of Rocks ... 4. Igneous Rocks ...
5.
Sedimentary Rocks ... 6. Metamorphic Rocks ...
7.
The Structure of the Earth ... 8. Plates and their movement ...
9. Plate Tectonics ...
10. The Moon and Planets

9.
More on
Plate Tectonics and subduction (using
the basic ideas to explain all the effects)
9.1
9(a) When
plates move apart: New crust is formed mainly at mid-ocean ridges where magma breaks through a huge
fractures in the crust. ((2)
in Fig 9.1 above) This is known as
sea floor spreading and is happening
along oceanic ridges, including the mid-Atlantic ridge. This causes
cracks through which more molten magma material from deep below the lithosphere
can push through producing new rock. The magma from theses chains of
linked undersea volcanoes (or just long gashes of hundreds of kilometres!)
rapidly cools to form
basalt type rocks of the new crust spreading out on either side. (see also
evidence
for this mechanism) Sometimes a long central rift valley forms (4). All
in all, what is described below, is the detail of the
ultimate rock recycling machine!
9(b) When
plates collide [more in 9(c)]: Crust material is removed from the
tectonic plates whenever two plates collide head on because one plate descends
into the subduction zone to be melted and combined with the mantle material ((1)
oceanic-oceanic plates meeting
(e.g. Pacific Ring of Fire) and (3)
oceanic-continental plates meeting
(e.g. Andes Mountains) in Fig
9.1 above).
One plate descends into a deep ocean trench, and mud and sand pour into
these trenches and at (3)
can end up as bands of metamorphic rock in the 'fold' mountains - see 9(c).
9(c)
When continental plate meets oceanic plate
the thinner more dense oceanic plate
subducts below the continental plate, and partly melts under the thicker but less dense granitic plate.
Deep ocean off-shore trenches are formed and parallel mountain chains with
volcanoes and earthquake activity too. The geology can be
complex and the sediments of the continental crust get crunched up into fold
mountains. Metamorphic rocks can be formed due to the heat and pressure in the
processes (casing recrystallisation without melting), accompanied by considerable faulting, folding, igneous intrusions and
volcanoes. Some of the molten rock cools deep below the surface to form
course-grained grained rocks like granite. The magma which rises to the
surface cools rapidly to form fined grained rocks like basalt lava.
If continental plates meet
(i.e. after all the ocean has been squeezed out!),
the massive collision and compression can build up huge mountain ranges like the Himalayas. Even
the pre-existing sedimentary rocks, like limestone and sandstone from the seas originally
between the plates, can be squashed up and become part of the fold mountain
ranges (the top of Mount Everest is limestone!). They can also be heated to give
regions of metamorphic rock, more folding and compressional
faulting. The whole process goes on for millions of years! and these 'new' mountain
ranges replace 'older' ones worn down by weathering and erosion processes.
See
below for side-ways passing movement.
 9(d) Earthquake or Seismic Waves:
- When two plates meet e.g. at (1)
or (3) in
Fig 9.1 then
the rocks are compressed and the tension builds up even if one plate is
descending. Eventually a point comes were the strain in the rocks is too much
for the structure to maintain and the rock layers move suddenly to relieve the
tension. The release of energy is enormous and radiates out as 'shock waves'
or seismic waves. These can create fault lines which themselves can be centres of
seismic activity. Earthquake have enormous destructive power, not just on
land, but undersea they create giant tidal waves called 'tsunami'.
- Earthquake power can be measured on the:
- Richter Scale
based on shock
wave acceleration and energy.
- It is a logarithmic scale, meaning
an increase in 1 unit means 10x more powerful.
- An earthquake of magnitude 7 is
1000x more powerful than one of magnitude 4 on the Richter scale.
- Mercalli Scale
is based on a
succession of increasingly 'dramatic' observed events.
- It was devised before Richter's
Scale.
- What the geologist Richter did was
to give Mercalli's scale numerical values based on seismometer
vibration measurements. The bigger the vibration amplitude, the more
powerful the earthquake.
- Earthquakes can be detected with an
instrument called a seismometer, which detects vibrations in
the ground its placed on. It is even sensitive enough to detect the
minute vibrations in the earth's crust thousands of miles from the
epicentre of an earthquake. From the graph of the amplitude of the
wave (vibration) versus time (from a seismograph instrument
connected to the seismometer), the energy released by the earthquake
wave can be measured and a value assigned to it on the Richter
Scale. Also, from the time intervals of the graph, and compiling and
processing data from several seismographic stations around the world, it
is possible to work out where the earthquake took place as well as its
strength.
- Other uses of seismometers
- Seismographic data can also be used to
analyse the structure of the earth e.g. the density and thickness of the
crust, mantle and inner & outer layers of the mantle by analysing the
complicated wave patterns of the vibrations of earthquake waves.
- Seismometers have been used in the past
(particularly in the West versus Russia 'Cold War') of the 1950's,
1960's and 1970's to detect and measure the power and geographical
locations of underground explosions from the testing of atomic weapons.
The task of seismograph technicians continues as other countries have
developed nuclear weapons.
|
Richter Scale |
Mercalli Scale |
 |
|
<
3.5 |
only
detected by seismometers, very sensitive people |
|
3.5-4.2 |
feels like a heavy truck
passing |
|
4.3-4.8 |
felt
by people walking, most sleepers wakened |
|
4.9-5.4 |
objects swing and overturn
causing damage, trees sway |
|
5.5-6.1 |
walls crack,
general alarm |
|
6.2-6.9 |
buildings damaged,
chimneys fall |
|
7.0-7.3 |
ground
cracks, buildings collapse, pipes break |
|
7.4-8.0 |
most
buildings and bridges collapsed, major services out; landslides |
|
>
8.1 |
total
destruction, objects thrown in air, ground
moves in violently in waves |
When plates move apart
where no magma breaks through, land between 'slips' down
'fault' lines and this causes seismic activity, see (4)
in
Fig 9.1. Also at
mid-ocean ridges, the new crust movement can trigger earthquakes, see (2)
in
Fig 9.1
-
- The plates can pass each other sideways
and the 'grinding action' causes
tension to build up in the rocks either side of the fault line. Occasionally,
and unpredictably the stored tension energy is released causing earthquake activity. An example of this is
infamous San Andreas fault in California USA.
Note: When plates pass sideways there is no loss or gain of plate material
and usually little volcanic activity but there are plenty of minor earthquakes
and every so often 'the big one' - ask the people of LA!
- There is good evidence of side-ways movement in Scotland
on the SE to NE 'line' along the Great Glen of northern Scotland, though thankfully, there
is no seismic activity to worry about!
-
Most earthquakes happen many km below the Earth's surface and
it is difficult to monitor and evaluate all the factors that
might help to predict when an earthquake might happen e.g. temperature, earth
tremors, gas emissions etc. So, unfortunately tragedies continue to happen,
even though scientists do their best, despite the uncertainties of the
situation, to make accurate predictions.
9(e)
Volcanoes tend to form where plates meet ((1)
(e.g. Pacific Ring of Fire) and
(3) (e.g. the east Pacific
ocean trench and the Andes Mountains on the South American plate) in
Fig 9.1). The
crust and mantle are disturbed in the subduction zone and extra heat is
generated from compression and friction. Some of the upper mantle becomes much
more fluid, 'gassy' and less dense. This results in hot magma working
its way upwards to break through as a volcano. The explosive force of volcanoes
is usually due to the rapid release of high pressure gas trapped in the
magma. This can throw out huge quantities of magma, rocks and volcanic ash to
form surrounding deposits which can be studied by volcanologists to research the
history of a volcanoes eruptions.
9(f)
Fig
9.2 Folds and Faults caused by
tectonic activity - plate movement

an anticline near Mizen
Head, West Cork, Ireland
Fig 9.2
- Folding shows the compression of
layers due to plate tectonic movement as plates meet head on! Along the
various layers of rock a curve down is called a
syncline, a curve in
an upwards is called an anticline.
- Sometimes large sections of rock
layers are tilted at extreme angles by the tectonic forces.
- Fault lines are huge 'cracks' down through
layers of rocks. They are caused by earthquake activity and for subsequence
earthquakes, the rock movement is often along these fault lines.
- In the diagram the sequence might be
interpreted as follows from 10 up to 1:
- layers from 10 up to 4 laid down in
that order with 10 first
- the folding occurs later, since newer
layers of sedimentary rock would tend to be laid on top and fill up
the fold.
- the faulting occurred after the folding
because all the folds are uniformly displaced
- the left folds have been
displaced downwards with respect to the middle section (or middle
folds upwards with respect to left folds)
- the more right linear sections may have
been moved upwards with respect to the middle section or the middle
section has slipped down.
- layers 3, 2 and 1 could be
the most recent sedimentary
rock layers laid down later on top of the eroded layers 4-6 (by weather
or glaciations) and have not
been subjected to major tectonic forces since there is no evidence of folding or
faulting.
- Folding and faulting can give information
on the magnitude and direction of the tectonic forces involved.
9(g)
A rift valley is formed on continental crust when two plates move away
from each other and the land in between falls as shown in (4).
This is exemplified by the Great Rift Valley of Africa but it can also be filled
with sea water e.g. the Red Sea between the African Continent and the Arabic
states.
9(h) In
Fig 9.1 above the loss of plate at (1)
and (3) is matched by the
creation of new crust at (2)!
9(i) In situation (2)
new crust is formed but at (1) and
(3) crust is being moved. So all new rocks have their start
at (1) and eventually end up, in whatever rock form, by returning
to the mantle at (1) or (3). Hence all mineral material is eventually
recycled in the 'big picture'
shown in Fig 9.3 below
Fig 9.3
and
Fig 9.1 above. Most of these
'answer notes' are looking at the details of all the primary and
secondary processes involved. Note in
Fig 9.1 above
the arrow ==> on the right could match up with the ==> on the left i.e. its
a 'balanced' global cycle both internally and externally! Any mountain ranges
not subducted still get worn away by weathering and erosion, so everything
gets recycled in the end!
Fig 9.4
A simpler approach to the "THE ROCK CYCLE"
to show the relationship between the three types of rocks - the "3rd Big Picture View"
Fig 9.4
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