MODULE 2831 (A) GLOBAL TECTONICS & GEOLOGICAL STRUCTURES
REVISION NOTES
1.1
The Structure of the Earth
Candidates should be able to:
(a)
State the depths and composition of the Earth’s inner core, outer core, mantle,
continental crust and oceanic crust. Describe and explain the physical states and
composition of the Earth’s inner core, outer core, mantle, continental crust and
oceanic crust.

The Earth has a layered structure with obvious boundaries (discontinuities) between the
layers where there are abrupt changes in chemical composition or physical state.
Depth
0km
Crust – solid, brittle
Oceanic – basic – rich in Si (not as much as continental), Fe, Mg and Ca
(SIMA) – nearly all igneous + thin layer sediment, 5-10 km thick, average
7km, density 2.9g/cm3, maximum age = 200Ma, layered structure.
Continental – acid-intermediate – rich in Si, Al, K and Na (SIAL) – great
variety of sedimentary, igneous and metamorphic rocks, 0-90km thick,
average 33km, density 2.7g/cm 3, maximum age = 3,900Ma (3.9 billion),
deformed.
Av. 33 km (5-90km) - Mohorovicic Discontinuity (Moho)
Upper Mantle 33-700km – solid, rocky silicate material, ultrabasic
composition, plastic behaviour in asthenosphere, density 3.3g/cm 3.
Lower Mantle 700-2900km – solid, rocky silicate material – “pyrolite”,
ultrabasic composition, very high pressure – dense, compact minerals,
density 5.4g/cm 3.
2900 km - Taylor-Gutenberg Discontinuity
Outer Core – liquid, metallic – iron, sulphur, nickel, temperatures and
pressures lower than inner core, Earth’s magnetic field generated by
convection currents in liquid iron, density 10g/cm 3.
5000 km - Lehman Boundary
Inner Core – solid, metallic – iron-nickel alloy, very high temperatures
(~6000°C) and pressures (~3,600,000bars), density 15g/cm 3.
6370 – Centre of the Earth!
(b)
Describe the nature of the (rheid) plastic asthenosphere and (rigid) brittle
lithosphere.
Crust – 0-33km (average), maximum 90km
Acid-intermediate-basic composition,
density 2.7-2.9 g/cm3
Moho – at 33 km depth (average)
Mantle
Ultrabasic composition,
density 3.3g/cm3 (upper mantle)
Lithosphere – 0-100km depth
Solid, brittle – rigid
Includes crust and top of mantle –  mainly
acid-basic composition, density ~2.8 g/cm 3
Asthenosphere – 100-250km depth
Plastic due to 5-10% partial melting
Part of upper mantle below lithosphere – 
ultrabasic composition, density 3.3 g/cm 3
Low velocity zone – seismic waves slow
down due to decrease in rigidity
Subdivided by physical behaviour
Subdivided by chemical composition
1
(c)
State the depth and describe the nature of discontinuities between the layers in
the Earth; inner core, outer core, mantle, continental crust and oceanic crust.



Mohorovicic Discontinuity (Moho) – @ average depth 33km – crust-mantle boundary –
mirrors the shape of the surface of the Earth, deepest beneath mountain belts as crust is
thickest, seismic waves are strongly reflected/refracted.
Taylor-Gutenberg Discontinuity – @ 2900km – core-mantle boundary – distinct change
from solid, rocky mantle to liquid, metallic outer core. Transverse S waves are not
transmitted beyond this due to presence of liquid outer core (liquids have zero rigidity) →
S wave seismic shadow zone at epicentral angles >103°. Longitudinal P waves slow
down and are strongly refracted → P wave seismic shadow zone at epicentral angles
103-143°.
Lehman Boundary - @ ~5000km – inner-outer core boundary – not a distinct boundary
due to a gradual change from all liquid in outer core to all solid in inner core. P waves
speed up in solid inner core, S waves are generated by the P waves.
(d)
Explain how the internal structure of the Earth can be inferred from direct
observations of rocks seen in deep mines and rocks now at the surface which have
been brought up in volcanic vents and as ophiolites.






Rocks at surface – deep seated plutonic rocks from 20km+ depth can be exposed at the
surface by weathering and erosion – gives information about the crust.
Rock in deep mines – the deepest gold mines in South Africa reach depths of 6km – they
provide evidence for the geothermal gradient – the increase in temperature with depth.
Drilling holes – the deepest hole ever drilled is the “Mohole” drilled to a depth of 13km in
Russia – it didn’t reach the Moho! – gives information about the crust.
Volcanic vents – volcanoes may bring up fragments of the surrounding country rock
(xenoliths) ripped off the sides of the vents – in some cases these xenoliths are peridotite
nodules that have come from the upper mantle.
Kimberlite pipes – very deep-seated volcanic pipes that tap the upper mantle down to
depths of 250km – these may bring up diamonds formed at very high temperature and
pressures.
Ophiolite complexes – slices of oceanic crust thrust up (obducted) and preserved on the
continental side of destructive plate margins instead of being subducted. They provide a
complete cross-section through oceanic crust and may have upper mantle attached. E.g.
Troodos ophiolite complex, Cyprus.
(e)
Explain how the internal structure of the Earth can be inferred from indirect
observations using seismic wave velocities and shadow zones, whole Earth and rock
density data and meteorites from within the solar system.





Seismic wave velocities and shadow zones – P and S waves undergo refraction and
reflection at the various layer boundaries in the Earth and their behaviour depends on the
composition and state of the material through which they travel. See earthquakes section
1.2a for more detail.
At the Moho seismic waves are strongly reflected/refracted.
They speed up in the upper mantle, due to increasing rigidity and incompressibility, until
they reach the asthenosphere (low velocity zone) at 100-250km depth – here they slow
down due to a decrease in rigidity (5-10% partial melting). In the transition zone (400700km depth) they show a series of stepped increases in velocity due to minerals
changing to dense, compact forms. In the lower mantle, seismic wave velocities continue
to increase. As properties are continually changing, seismic wave velocities continually
change  they are continually refracted – follow curved paths.
At the Taylor-Gutenberg Discontinuity (2900km), S waves are not transmitted through the
liquid outer core (liquids have zero rigidity) → S wave seismic shadow zone at epicentral
angles >103°. P waves slow down and are strongly refracted → P wave seismic shadow
zone at epicentral angles 103-143°.
Beyond the Lehman Boundary (~5000km) in the inner core, P waves speed up and S
waves are generated by the P waves.
2


1.2
Whole Earth and rock density data – The crust has an average density of 2.8g/cm 3, the
whole Earth has an average density of 5.5g/cm 3,  there must be denser material at
depth and an iron-nickel alloy has a suitable density – also supported by the existence of
the Earth’s magnetic field (metallic liquid in outer core).
Meteorites from with the Solar System – meteorites are planetary fragments. The iron
meteorites “irons” are thought to represent the composition of the core. The stony
meteorites are thought to represent the composition of the lower mantle. One type of
meteorite – the carbonaceous chondrites (serpentine, olivine, sulphur and carbon) – are
thought to be the original material of the nebula from which the Solar System evolved.
Earthquakes
Candidates should be able to:
(a)
Describe the characteristics of seismic waves (P, S and L) generated by
earthquakes and the paths of body and surface waves.
When an earthquake occurs stored energy is released as seismic waves. There are 3 types:
1. P (primary) waves: Travel fastest. Longitudinal/compressional waves – wave vibrates in
direction of travel. Can travel through solids and liquids.
2. S (secondary) waves: Travel ~ half the speed of P waves. Transverse/shear waves – wave
vibrates at 90º to direction of travel. Can’t travel through liquids because liquids have zero
rigidity.
P & S waves are body waves – travel through Earth.
3. L (surface) waves: Travel the slowest. Roll around the surface of the Earth and cause the
most damage.
The paths and travel times of P and S waves depend on the properties of the material through
which they pass. Waves travel in straight lines – they can be reflected and refracted due to
changes in velocity.
 Reflection occurs when the critical angle of incidence is exceeded – tends to happen at
layer boundaries, e.g. Moho – big change in velocity, strong reflection. Law of reflection:
Angle of incidence = Angle of reflection.
 Refraction occurs as result of changes in velocity. When a wave slows down (enters a
low velocity medium) it is refracted towards the normal. When a wave speeds up (enters a
high velocity medium) it is refracted away from the normal.
Properties affecting seismic wave velocities:
 Density – As density increases, P and S wave velocities decrease.
 Compressibility – As incompressibility increases (becomes less compressible, more
incompressible), P wave velocity increases.
 Rigidity – As rigidity increases, S wave velocity increases.
As depth increases in the mantle, due to increasing pressure, it becomes more dense, more
rigid and less compressible. The effects of compressibility and rigidity outweigh the effects of
density so overall in the mantle seismic wave velocities increase with depth.
Continuous Refraction – the density, compressibility and rigidity are continually changing with
depth (due to pressure), therefore the seismic waves are continually changing velocity and
continually being refracted → seismic waves follow curved paths.
Seismic shadow zone – An area on the Earth’s surface where seismic waves are not
received. Up to epicentral angles of 103º P and S waves are received (P waves first).
 S wave seismic shadow zone – beyond epicentral angles of 103º
S waves are transverse waves – cannot be transmitted through the liquid outer core (liquids
have zero rigidity) reached at a depth of 2900km. Gives S wave seismic shadow zone at
epicentral angles beyond 103º.
 P wave seismic shadow zone – 103º-143º
P waves can travel through the liquid outer core, but they slow down dramatically so they are
strongly refracted and reflected. Gives P wave seismic shadow zone between epicentral
angles of 103º and 143º.
But, S waves can travel through solid inner core – they are generated by P waves.
3
(b)
Define the terms: foci, epicentres, isoseismal lines and how they are
determined or plotted.
 Focus – Point inside the Earth at which the earthquake originates. Can be classified as:
Shallow focus <70km (cause most damage)
Intermediate focus 70-300km
Deep focus 300-700km.
Earthquakes don’t occur at depths >700km – rocks behave plastically.
 Epicentre – Point on the Earth’s surface directly/vertically above the focus.
Usually (but not always) sustains the most damage. Depends on underlying geology – loose,
unconsolidated sediments amplify the size of seismic waves.
 Isoseismal lines – Lines on a map joining points of equal earthquake intensity.
Ask eyewitnesses in different locations to fill in a questionnaire – work out the intensity and
plot on a map. Can be used to locate the epicentre, as this should be the area of maximum
intensity.
(c)
Explain why earthquakes occur when stored stress is released and how
earthquakes are detected and measured using seismographs and seismograms.



Earthquakes occur along active faults. Most active faults are located along plate margins,
but earthquakes can occur anywhere where sufficient strain has built up. They are the
result of frictional forces and brittle behaviour. Elastic rebound theory – Movement along
faults is not uniform, prevented by friction. Stress builds up → rocks become strained/
deformed. Eventually the rock fails and snaps, stress is released and rock moves to a
new position. The stored energy is given out as seismic waves.
In some cases faults creep → strain is gradually released as small movements so large
earthquakes don’t occur.
Large earthquakes are likely to happen on “locked” sections of faults that haven’t moved
for some time – a “seismic gap” where strain is building up.

Seismic waves are detected by seismometers/ seismographs – these produce a
recording called a seismogram. As distance from epicentre increases – arrival times
increase, amplitude decreases, lag time between first arrivals of P and S waves
increases.

Seismograms – can be used to determine magnitude and locate the epicentre of an
earthquake. Magnitude is the maximum amplitude of S wave. The distance to epicentre is
determined by measuring the lag time between first arrival of the P waves and first arrival
of the S waves. A circle is drawn on a map centred on the seismometer station, with a
radius of the distance to the epicentre. Information from 3 seismometers is needed to pinpoint the epicentre – where the 3 circles intersect (triangulation).
(d)


Explain the nature and use of the Richter and Mercalli Scales.
Magnitude – Richter Scale: Measure of the amount of energy released, the amplitude
(height) of the seismic wave, the amount of ground shaking. A logarithmic decimal scale –
e.g. from magnitude 3 to 4 – height of wave increases 10 times, energy released
increases 30 times! The amplitude (height) of the S wave on a seismogram gives the
magnitude of the earthquake.
Intensity – Mercalli Scale: Measure of the amount of surface damage caused – the effects
on objects and people. A qualitative scale based on eye-witness accounts. Given in
Roman numerals – I to XII. The intensity should decrease with distance from the
epicentre (but depends on ground conditions). Intensities of VII upwards result in major
damage and disruption.
4
(e)
Appreciate the social and economic effects of earthquake activity - the type of
ground movement, damage to structures, liquefaction, landslips, tsunamis and
aftershocks.






Ground shaking – Causes buildings, bridges, roads and dams to collapse; breaks power
lines and pipelines. Loose, unconsolidated sediments (soil, drift, superficial deposits, etc.)
amplify the size of the seismic wave and ground shaking, so structures suffer more
damage.
Soil liquefaction – Loose, unconsolidated sediment may also undergo liquefaction
(behaves as a liquid) due to the ground shaking.
Landslides – May be triggered by the ground shaking, most likely on steep slopes.
Tsunamis – Submarine earthquakes may trigger tsunamis (tidal waves). The energy
released forms waves in the water – form a broad swell out to sea, only become a
problem in shallow, coastal waters when the wave begins to slow down and break. Travel
at speeds of 1000km/hour, reach heights of 20m+ above sea level!
Aftershocks – Earthquakes are followed by aftershocks, which themselves may be large –
further risk of building collapse, hampers rescue attempts.
Fires – May be more devastating than the ‘quake itself – result of ruptured gas mains and
electricity cables, made worse by the fact that water mains are also likely to be ruptured
(no means to fight the fires).
(f)
Describe methods of earthquake prediction: seismic gap theory, detailed
measurements of gases, stress in rocks, water levels, ground levels, magnetism and
animal behaviour. Understand the social consequences of earthquake prediction.
Describe measures designed to reduce the impact of the effects of earthquakes.

Predict = foretell, expect an event. The purpose is to give prior warning, to prevent an
extreme natural event becoming a disaster. It is unexpected events that are the most
dangerous.
 Modern, scientific predictions are based on an understanding of the causes of geological
hazards, in particular, a knowledge of plate tectonic theory and the fact that the majority of
earthquakes occur along plate margins. Predictions are based on the assumption that
catastrophic events are heralded by precursors and that if these signs can be read then
the disaster can be averted.
 A successful prediction must pinpoint spatial occurrence (location), timing & magnitude.
Methods of Earthquake Prediction based on Precursor Phenomenon:
(that may become important in the future)
 Seismic Gap theory – Long term records are studied for individual faults – some sections
of the fault may be immobilised or “locked” and have not moved for some time. These are
likely to be the sites where major strain is building up. E.g. The 1989 Loma Prieta
earthquake occurred on a section of the San Andreas fault that had been locked since the
1906 earthquake.
 Detailed measurements of gases dissolved in groundwater – The concentration of radon
gas dissolved in groundwater increases. As radon is radioactive this can be easily
measured.
 Stress in rocks – An increase in stress can lead to changes in the physical properties of
the rocks as they become strained. E.g. Seismic wave velocities may change. Electrical
Resistivity is observed to decrease for several months prior to an earthquake and then
shows a series of stepped increases immediately beforehand. In 1997, three Greek
scientists (Varotsos, Alexopoulos, Nomicos) claimed to have successfully predicted 3
earthquakes along faults in Greece using electrical methods (the VAN method).
 Water levels – Groundwater levels in wells may change and geyser/hot spring activity in
the area may increase.
 Ground levels/inflation – Ground swells up prior to the earthquake due to build up of
stress. Can be measured using tilt meters.
 Magnetism – Slight changes in the Earth’s magnetic field can be measured using a
magnetometer.
 Abnormal animal behaviour – Especially in ground dwelling animals, such as snakes, has
been recorded particularly by the Chinese.
5
Social consequences of earthquake prediction:
Positive
Official warnings can be given.
Successful evacuation – lives are saved.
Preparedness of emergency services.
Publication of risk/damage assessment
maps in advance.
Negative
Panic.
Decline in property values.
Decline in business activity and
employment opportunities.
Reduced availability of insurance.
Measures designed to reduce the impact of earthquakes (mitigation/protection):
 Building Design – Earthquake-proofing – The majority of deaths in earthquakes are due
to the collapse of buildings, bridges and freeways. Earthquake-proofing includes inclusion
of rubber shock absorbing foundations, “bird-caging” (interlocking steel frameworks
around buildings), and having counter-weights on the roof to counteract shaking.
Stringent building codes should be applied in earthquake-prone areas.
 Hazard Risk Mapping and Land Use Zoning – Mapping of the geology to locate faults and
seismic risk areas. Identification of areas of unconsolidated sediment prone to liquefaction
and steep ground prone to landslides.
 Public Awareness and Education Programmes – E.g. The United States Geological
Survey (USGS) publish educational materials advising people what to do in the event of
an earthquake.
 Emergency Planning and Preparation – Evacuation procedures and drills can be in place
and the emergency services prepared for the aftermath of an earthquake. E.g. Japan
holds an annual full earthquake drill on 1st September which marks the anniversary of the
1923 Tokyo earthquake, magnitude 8.3, in which 143,000 were killed.
 Earthquake-proofing of mains electricity, gas and water pipes – Fires are a major hazard.
Many are caused by damaged electricity cables and broken gas pipelines. If water mains
are damaged there is no means of fighting fires. Flexible pipelines can be built with valves
so damaged sections can be isolated.
(g)
Describe and explain the Earth’s seismicity and aseismicity: the origin, depth
and distribution of earthquakes in relation to major features of the Earth's surface;
mid-ocean ridges, major rift systems, deep-sea trenches, fold mountain ranges,
transform faults, continental shields and ocean basins.
Seismic areas:
 Mid-ocean ridges/Constructive plate margins – tensional forces and rising magma –
frequent shallow focus earthquakes occur along normal faults bounding ridge axis and
along transform faults offsetting the ridge.
 Rift valleys – first stage of formation of a constructive plate margin – tensional forces –
shallow focus earthquakes occur along normal faults bounding the rift valley.
 Deep-sea trenches/Destructive plate margins – mark position of subduction zone –
inclined zone of seismicity – “Benioff zone” – shallow, intermediate and deep focus
earthquakes occur as a result of friction preventing smooth movement of down-going slab
– reverse and thrust faults. Subduction zones are the only place where deep focus
earthquakes occur – the only place where cold, brittle crust occurs beyond a depth of 100
km.
 Fold mountain (orogenic) belts/Continental collision zones – compressive forces –
reverse and thrust faults and folds – crust up to 90km thick – shallow and intermediate
focus earthquakes.
 Conservative plate margins – shear forces – strike slip/transform faults – shallow focus
earthquakes only.
Aseismic areas:
 Continental sheilds/Cratons – oldest areas (> 1 billion years to 3.9 billion years old) of
continental crust in centre of continents – very stable – earthquakes rare – may occur
along old faults and in rift systems.
 Ocean basins – aseismic expect along mid ocean ridges, trenches and at hot spots. At
hot spots shallow focus earthquakes may be triggered by the pressure of rising magma.
6
1.3
Global Tectonics
Candidates should be able to:
(a)
Explain the evidence for the movement of continents over time using rocks,
fossils and major geological structures.


Continental Drift (Alfred Wegener, 1915) – theory that over geological time the continents
have been slowly drifting/moving over the surface of the Earth.
The geology of many continents are similar. All the continents were once joined as a
supercontinent - Pangaea (~300Ma). This then started to break up (~160Ma) and split into
two – Laurasia (northern continents) and Gondwanaland (southern continents), separated
by the Tethys Ocean. These two supercontinents have since broken up further and the
continents drifted to their current positions.
(b)
Describe the evidence of age, type and distribution of rocks, fossils and major
geological structures for a geological fit of past continents and of the edges of
continental shelves for a geographical fit. Explain the palaeoclimatic and
palaeomagnetic evidence for the movement of continents.
Geological fit based on rocks, fossils and major geological structures
 Rocks – Similar rock sequences of the same age occur on different continents. E.g.
South America and Africa have matching areas of continental shield. E.g. South America
and Africa have similar sedimentary sequences of 140-100Ma.
 Fossils – Gondwanaland (Antarctica, Australia, S. Africa, India and S. America) – has
widespread similar fossils, E.g. Glossopteris (a type of fern), and 270Ma Mesosaurus (a
crocodile-like reptile).
 Trilobites of Britain and North America are evidence for the proto-Atlantic (Iapetus)
Ocean. Trilobites were shallow marine organisms – lived on the continental shelf, couldn’t
cross deep oceans. Trilobites of Scotland and Northern Newfoundland are the same
species, but are different to the trilobites of England, Wales and Southern Newfoundland
– this suggests that the two areas were separated by a deep ocean (the proto-Atlantic).
After 400Ma, the same species are found in both areas – suggesting the ocean had
closed by this time.
 Major geological structures – Orogenic (mountain) belts have the same age and trend –
they fit together if the intervening ocean is removed. E.g. Caledonian-Appalachian fold
mountain belt can be traced through E. USA, Greenland, Scotland and Scandinavia –
formed when the proto-Atlantic Ocean closed.
 Geographical fit based on edges of continental shelves
Continents appear to fit together like a jigsaw if intervening ocean is removed. Fit is even
better if younger sediments are removed and it is taken at the100m submarine contour/edge
of continental shelf. E.g. South America and Africa.
 Palaeoclimatic evidence
Rocks reflect the environmental/climatic conditions in which they formed. The climatic belts
have remained in approximately the same place over geological time – the continents have
drifted through them. E.g. Gradual northwards drift of UK –
700Ma – glacial deposits (boulder clay) in Scotland  close to South Pole.
300Ma – Carboniferous Period – coal measures – tropical, swampy conditions  Equatorial.
270Ma – Permian Period – desert deposits – dune sandstones and evaporites  20ºN of
Equator (same latitude as Sahara Desert today).
Britain has continued to drift north.
Recent 1.6 Ma - 10,000 year old Pleistocene glacial deposits.
E.g. Gondwanaland glaciation – There are widespread 250Ma glacial deposits (boulder clay)
on all the southern continents suggesting these continents were joined and situated over the
South Pole at this time.
 Palaeomagnetic evidence
Polar wandering curves can be used to show when continents have been joined – matching
shape = continents were joined at this time. See section 1.3c for more detail.
7
E.g. Europe and North America – polar wandering curves match between 400Ma and 100Ma.
This is when the two continents were joined after the closure of the proto-Atlantic Ocean
(Iapetus). At 100Ma, the current Atlantic Ocean started to open, and the polar wandering
curves diverge after this time.
(c)
Describe and explain palaeomagnetism and reversals in the Earth’s magnetic
field and the variation of inclination with latitude.
The Earth has a dipole magnetic field the same shape as that around a bar magnet. The
magnetic field lines point from the South to the North Pole. As opposite poles attract, the
Earth’s North Pole is actually the south pole of its internal magnet. The field is thought to
originate from convection currents operating in the metallic, liquid Outer Core, which are
sustained by the Earth’s rotation.
 Palaeomagnetism
When iron-bearing rocks (e.g. basic lavas) cool below the Curie temperature (which is the
below freezing point) they become magnetised in the direction of the Earth’s magnetic field at
that time. The magnetic domains in the minerals line up in the direction of the Earth’s
magnetic field lines. The magnetic inclination (angle of dip of Earth’s magnetic field from the
horizontal) preserved in the rock varies with latitude on the Earth’s surface and can be used
to determine palaeolatitude – the position of the rock with respect to the Pole at the time of
its formation.
The palaeolatitudes of rocks of different ages from the same continent can be used to plot a
polar wandering curve. Such polar wandering curves can be used to show when continents
have been joined (matching shape = continents were joined at this time).
 Magnetic Reversals
When iron-bearing rocks (e.g. basic lavas) cool below the Curie temperature (which is the
below freezing point) they become magnetised in the direction of the Earth’s magnetic field at
that time. Over geological time, the Earth’s magnetic field has reversed or flipped – the North
Pole has become the South Pole and vice versa. Vine and Matthews (1963) suggested this
caused the magnetic anomalies/stripes discovered on the ocean floor. The anomalies/stripes
are due to differences in the Earth’s magnetic field and may be positive (normal polarity) or
negative (reversed polarity).
(d)
Describe the characteristic features of continental shelf, continental slope,
ocean basins (abyssal plain), seamounts, mid-ocean ridges, major rift systems, deepsea trenches, fold mountain ranges and continental shields (cratons).





Continental shelf – Immediately offshore, gently sloping (up to 200 m deep), underlain by
continental crust, deposition of clastic and carbonate sediments. Usually passive
continental margins, not associated with seismic or volcanic activity or subduction.
Continental slope – Steeper angled slope representing the true edge of the continent. It is
cut by submarine canyons. These are formed by high density, sediment-carrying,
turbidity currents that periodically flow off the continental shelf. Turbidity currents
deposit characteristic muddy sandstones called greywackes or turbidites, which
commonly show graded bedding.
Continental rise – Another break in slope at the base of the continental slope. Beyond this
is the flat abyssal plain.
Abyssal Plain – Away from the mid-ocean ridges and the edges of continents, the ocean
basins consist of abyssal plains at a depth of ~5km underlain by oceanic crust. These are
dotted with seamounts rising more than 1km above ocean floor, which are volcanic in
origin. Chains of seamounts form at hotspots, where mantle plumes produce magma
which periodically punches holes through the overlying crust to form volcanoes, e.g.
Hawaii. Guyots are flat-topped seamounts – represent previously emergent seamounts
whose tops were planed off by wave action prior to the mountains sinking down. These
may be associated with the formation of coral atolls.
Mid-ocean ridges e.g. Mid-Atlantic Ridge – Mark the position of constructive plate
margins. Tensional forces cause rifting and basic magma wells up to fill the gap. Midocean ridges rise about 3 km above the surrounding sea floor. The ridges are not
continuous – segments are offset by transform faults. Shallow focus earthquakes occur
8




along the ridges and transform faults. Oceanic crust gets progressively older away from
the mid-ocean ridges.
Major rift systems e.g. Red Sea, East African Rift Valley – Occur in continental crust and
are the result of tensional forces pulling the crust apart – first stages of opening of a new
ocean/formation of a constructive plate margin. A series of parallel normal faults form and
a block of crusts drop down between the faults producing a linear rift valley or graben.
Eventually, the floor of the valley may become flooded by sea water and basic magma
wells up to form oceanic crust. Shallow focus earthquakes occur along the rift valley.
Deep-sea (ocean) trenches – e.g. Marianas Trench – Represent the surface expression
of subduction zones at destructive plate margins. Ocean trenches are the deepest part of
the oceans – exceed 10 km.
Fold mountain ranges (orogenic belts) e.g. Himalayas – Occur at destructive plate
margins and in continental collision zones. These represent the end product of subduction
when all the intervening ocean crust has been subducted. Compressive forces → folding
and thrust faulting causes the crust to shorten and thicken (up to 90km) giving rise to fold
mountains. No volcanoes occur – crust is too think. Large volumes of acidic magma are
intruded into the base of the mountain belt giving rise to granite batholiths. These “roots”
of the orogenic belts may be exposed by later erosion. Shallow and intermediate focus
earthquakes may occur. Regional metamorphism.
Continental shields (Cratons) e.g. Canadian Shield – Oldest areas (> 1 billion years to 3.9
billion years old) of continental crust in centre of continents, mainly crystalline (igneous
and metamorphic) basement rocks – very stable – aseismic. Once continental crust has
formed it cannot be easily destroyed in subduction zones because it is light and buoyant.
(e)
Explain the concept of sea floor spreading and be able to calculate the rate of
spreading from different data sources.


Sea floor spreading – concept that new ocean floor (oceanic crust) is being
created/erupted at the mid ocean ridges and, thus, the sea floor is spreading away from
the mid ocean ridges. The idea was suggested in the 1960s using various lines of
evidence.
You must be able to calculate rates of sea floor spreading using the equation speed =
distance/time and be able to convert kilometres into centimetres or millimetres and
millions of years into years if asked.
(f)
Describe the evidence for the process of sea floor spreading using the depth of
the ocean floor, the distribution of mid-ocean ridges, the age of oceanic crust, the
pattern of volcanic islands and hotspots, the depth and distribution of earthquakes,
transform faults, heat flow and gravity anomalies, the age, formation and distribution
of ocean sediments and patterns of magnetic anomalies.
1. Mid Ocean Ridge and all it features – Distribution, volcanoes, earthquakes, transform
faults, heat flow and gravity anomalies. See section 1.3h for more detail.
2. Features of the Ocean floor
 Age – Other than a thin layer of sediment, all the oceanic crust is generated at the mid
ocean ridge. Therefore, the crust gets older (up to a max. of 200Ma) away from the ridge.
 Age, formation and distribution of ocean sediment – Made up of dead plankton
(microscopic marine organisms) – gets thicker (longer to accumulate) and older away
from the ridge.
 Patterns of magnetic anomalies – Magnetic stripes have been discovered on the ocean
floor. As iron-bearing rocks (e.g. basic lavas) cool below the Curie temperature they
become magnetised in the direction of the Earth’s magnetic field at that time. Over
geological time the Earth’s magnetic field has reversed or flipped – the North Pole has
become the South Pole and vice versa. This has produced a pattern of alternately
normally and reversely magnetised stripes parallel to, and symmetrical about, the mid
ocean ridge. Matching stripes on either side are the same age and formed at the ridge at
the same time. The stripes are irregular due to different rates of spreading/production of
magma along the ridge and because of the transform faults.
9


Hot Spots e.g. Hawaii and Emperor Seamounts – Caused by a stationary mantle plume
punching a hole through thin oceanic crust to form a basic shield volcano – also evidence
for the movement of the oceanic plate away from the ridge.
Layered structure of the oceanic crust – Sediment, basic pillow lavas, sheeted dyke
complex, basic plutonic rock (gabbro) and ultrabasic rock (peridotite). All, except the
sediment, is generated at the mid ocean ridge.
(g)
Describe about the nature and distribution of continental and oceanic tectonic
plates. Understand the concept of plate tectonics.


Plate Tectonic Theory – the theory that the Earth’s crust is made of a series of thin plates
that “float” on top of the solid (but plastic) mantle. The lithospheric plates move very
slowly (cm/year) due to convection currents in the upper mantle (asthenosphere).
Volcanic activity mainly occurs at plate boundaries/margins.
There are two types of crust/plates – oceanic and continental. The major plates are: North
American, South American, Pacific, Eurasian, African, Antarctic, Indo-Australian, Nazca.
(h)
Describe and explain the characteristics and distribution of constructive,
destructive and conservative plate margins and their relationship to vulcanicity,
seismicity, gravity, heat flow, rock type, mid-ocean ridges, fold mountains and
trenches.

You must be able to draw fully annotated cross-sections of the various plate margins.
Constructive Plate Margins
E.g. Mid Atlantic Ridge (Iceland), East Pacific Rise.
 Constructive plate margins are marked by the mid ocean ridges – linear, submarine
mountain chains, rising 1-3km above the abyssal plain. Tensional forces → rifting along
the ridge axis – central rift valley.
 Active volcanism – basic magma is produced by partial melting of ultrabasic mantle →
basic fissure and shield volcanoes, gently effusive Icelandic style eruptions, pillow lavas –
eruptions are frequent and huge volumes of magma are emplaced.
 Along its length, the mid ocean ridge is offset by horizontal, strike-slip transform faults –
these faults accommodate different rates of spreading along the ridge axis.
 Frequent, low magnitude, shallow focus earthquakes occur along normal faults along the
ridge axis and along the transform faults.
 Along the ridge, there is high heat flow and a positive gravity anomaly (excess of mass)
due to the rising magma.
Destructive Plate Margins
Two types:
Ocean-Continent, E.g. Western side of South America (the Andes) – Fold mountain belt.
Ocean-Ocean, E.g. Japan, New Zealand, Aleutian Islands, Indonesia – Island arc.
 Destructive plate margins are marked by the ocean trenches – these linear trenches are
the deepest part of the oceans (exceed 10 km) and are the surface expression of
subduction zones. Compressional forces → plates moving together, one plate overrides
the other forcing it down into the mantle (subduction). Ocean crust is always subducted
because its denser than continental.
 As the subducted plate (basic) moves down into the mantle it partially melts – rising
magma picks up silica as it passes through the crust → acid-intermediate composite
cone/stratovolcanoes, highly explosive eruptions, products mainly pyroclastics. Also
intrusion of granite batholiths at depth.
 Shallow, intermediate and deep focus earthquakes occur - the foci of the earthquakes
mark the position of the down-going subducted slab and become steadily deeper on the
continental/island arc side – the Benioff zone.
 On the trench side there is low heat flow (cold water) and a negative gravity anomaly
(deficit of mass) due to the presence of lower density water in the place of rock. On the
continental/island arc side there is high heat flow and a positive gravity anomaly (excess
of mass) due to the rising magma.
10


As the ocean crust is subducted, sediments may be scraped off the top of the slab and
accumulate in the trench to form an accretionary prism. In some cases, whole slices of
oceanic crust are thrust up and preserved. This process is called obduction and the
preserved fragment of oceanic crust is called an ophiolite (e.g. the Troodos Mountains,
Cyprus).
When all the intervening oceanic crust is destroyed by subduction, continental collision
occurs forming a fold mountain belt, e.g. Himalayas, Alps.
Conservative Plate Margins
E.g. San Andreas Fault (California), North Anatolian Fault (Turkey).
 Shear forces → plates slide past each other.
 No mechanism to produce magma → no volcanoes.
 Strike slip/transform faults, with horizontal movement – shallow focus earthquakes.
 Average heat flow, no gravity anomaly.
(i)
Know about possible mechanisms for the movement of plates and the balance
between formation of oceanic crust at mid-ocean ridges and its subduction in deepocean trenches.




The mechanism for plate movements is thought to be convection currents in the mantle.
The heat is produced by the decay of radioactive elements (U, K, Th). Hot, low density
magma rises at the mid ocean ridges, flows off sideways and then cools and sinks back
down. There is reduced friction in the plastic asthenosphere and the partial melting and
produces magma that acts as a lubricant allowing movement.
At constructive plate margins/mid ocean ridges tensional forces allow magma to rise and
the intrusion of dykes may push the plates apart.
At destructive plate margins gravity may pull the oceanic crust down into the subduction
zone. This may limit the maximum size of ocean basins.
No area of oceanic crust is older than 200 Ma because in geological terms oceans are
temporary features that open and close due to the shifting balance between construction
at mid-ocean ridges and destruction at subduction zones. The complete cycle of opening
and closing of an ocean is called a Wilson Cycle after J. Tuzo Wilson who suggested the
idea.
(j)
Describe the evidence for hotspots from mantle plumes within plates and in
relation to plate movement.

1.4
Hot Spots e.g. Hawaii and Emperor Seamounts – within plate volcanic activity. A
stationary mantle plume (long-lived area of high heat flow in the mantle) generates basic
magma and punches a hole through overlying crust → shield volcano. As the crust moves
over the hot spot a chain of volcanic islands is produced, each one younger than the one
before. Hot spots are only found in oceanic areas as the crust is thinner.
Geological Structures
Candidates should be able to:
(a)
Define and be able to explain: dip and strike, apparent dip, beds and bedding
planes, inliers and outliers. Be able to measure dips and strike.




Dip and Strike – used to measure the orientation of any plane, e.g. bedding planes, joints,
faults, fold axis and dip of limbs, foliation and cleavage.
Strike – horizontal line on a plane surface, trend in which there is zero dip, measured with
a compass, given as 3 figure compass bearing, e.g. 090° = E-W strike.
(Use right hand rule – right hand along strike thumb pointing down dip direction).
Dip – maximum inclination from the horizontal, at 90° to strike, measured with a
clinometer, given as 2 figure angle, e.g. 45ºN.
Apparent dip – If the dip is measured at less than 90° to the strike = apparent dip, less
than true dip.
11





(b)
Beds – sedimentary rocks are laid down in beds. One bed/stratum = one period of
deposition and is laid down horizontally (law of horizontality). If beds are not horizontal
they must have been tilted or folded by forces. A single bed is uniform throughout (grain
size, shape, sorting, composition), except graded bedding.
Bedding planes – plane surfaces that separate beds, represent a break in deposition.
Inlier – an outcrop of older rocks completely surrounded by younger rocks, e.g. a hollow.
Outlier – an outcrop of younger rocks completely surrounded by older rocks, e.g. top of a
hill.
Inliers and outliers may be the result of erosion, faulting or folding – seen as a circularshaped outcrops on maps.
Recognise and explain the origin of an angular unconformity.
Unconformity – a plane surface representing a break in time when no deposition occurred and
erosion may have occurred. The plane of the unconformity represents a palaeo-land surface
and may show evidence of weathering. The rocks below may be much older than the rocks
above the unconformity.
Types of Unconformities:
 Angular unconformity – beds above and below the unconformity dip at different angles.
Sometimes called a T junction.
 Parallel unconformity/disconformity – beds above and below are parallel, but the plane of
the unconformity represents a palaeo-land surface and may be uneven.
Sequence of formation of an angular unconformity
1. Older sediments are deposited as horizontal beds.
2. Deposition stops. Rocks are uplifted and folded or tilted.
3. A period of erosion planes off the rocks.
4. Deposition resumes. Younger sediments are deposited as horizontal beds.
5. Further uplift and erosion exposes the unconformity.
Recognising unconformities
 Beds above and below will have different dips and strikes.
 Older beds are truncated (cut off) by plane of unconformity.
 Older dykes, minerals veins are truncated.
 Older faults are truncated.
 Older fold axial planar traces are truncated.
 There may be a basal conglomerate immediately above the plane of the unconformity in
the younger beds (containing clasts of the older rocks).
 Rocks below may be much older than rocks above (fossil evidence) and be very different
(deposited in different environments).
 Plane of unconformity is a palaeo-land surface – may be uneven, weathered (reddened)
and burrowed.
(c)
Explain the origin and nature of tectonic joints, cooling joints, unloading joints.
Joint – a fracture in rock along which there is no observable movement. Result of brittle
behaviour – develop in competent rocks. Often form at 90° to bedding. Most result from
tensional forces.



Tectonic joints – associated with folding. There are three types:
1. Tension joints – develop parallel to fold axis, at 90° to maximum compression, form on
crests of anticlines where rock is stretched, result of tensional forces.
2. Cross joints – develop parallel to maximum compression, at 90° to fold axis.
3. Shear/oblique joints – develop at 45°, result of shear forces.
Cooling joints – form in igneous rocks as they cool and contract – sets up tensional forces
resulting in hexagonal-shaped cracks developing called columnar jointing.
Unloading/sheet joints – form as a result of removal of overlying rocks by weathering and
erosion. The rocks “spring up” as the stress is released – joints open up parallel to
surface (horizontal).
12
(d)
Describe the formation of slaty cleavage by compressive forces and its relation
to folds.


Slaty cleavage – an alignment of flat, platy minerals (e.g. mica) at 90° to pressure –
cleavage planes develop in the rock. Result of plastic behaviour – develops in
incompetent rocks. In folds, the slaty cleavage develops at 90° to the pressure and
parallel to the fold axis – axial planar cleavage.
Minor/parasitic folds may also develop when incompetent beds are folded.
(e)
Describe stress and strain in rocks, and their variation due to temperature,
pressure and time. Understand the effects of tension, compression and shear forces
causing deformation of competent and incompetent rocks and the deformation of
fossils and ooliths.











(f)
Stress – internal pressure in a rock subjected to external forces. Causes:
Strain – change in shape or volume of a rock subjected to stress.
Tension – pulling, stretching force.
Compression – pushing, squeezing force.
Shear – twisting, tearing force – one side of the rock is moving relative to the other.
Competent rocks – strong, brittle rocks, e.g. igneous, metamorphic, sandstone, limestone
– develop joints and faults. In sandstones – hard, round quartz grains cannot align under
pressure they merely rotate.
Incompetent rocks – weak, plastic rocks, e.g. shale, clay – develop folds, slaty cleavage
and parasitic folds. In clays and shales – flat, platy minerals (clay minerals and mica) align
at 90° to the pressure.
Temperature – as temperature increases rocks become more plastic.
Pressure – may be confining/load or directed/compressive – at high pressures the rock
will tend to fold rather than fault.
Time – if stress is applied rapidly → brittle behaviour, faulting; if stress is applied slowly
→ plastic behaviour, folding.
Ooliths (oolites) and round fossils (corals, etc.) – act as strain markers – become
deformed/elliptical in shape when subjected to stress. Can be used to determine the
amount of strain the rock has undergone.
Define and recognise: fault plane, throw, fault dip, hanging wall, footwall.
Fault – a fracture in rock along which there has been observable movement. Result of brittle
behaviour – develop in competent rocks.








Fault plane – plane along which the movement occurred.
Throw – the amount of vertical displacement, measured by finding a reference bed or
feature on either side of the fault and measuring the vertical difference between them.
Fault dip – the angle the fault plane makes with the horizontal, measured away from the
horizontal using a clinometer in the field or a protractor on paper.
Hanging Wall – Block above the fault plane – when stood on the fault plane, this is above
your head.
Footwall – Block below the fault plane – when stood on the fault plane, this is below your
feet.
Upthrown Block – side of the fault that has gone up (dip-slip faults only).
Downthrown Block – side of the fault that has gone down (dip-slip faults only).
Always place arrows on faults to indicate direction of movement.
(g)
Describe and recognise: dip-slip faults (normal and reverse), graben (rift) and
horst, step faults, strike-slip faults (tear [wrench] faults and transform faults), thrusts
and explain their formation.
Dip-slip faults – movement along dip (vertical).
 Normal fault – dip-slip fault – tensional forces – constructive plate margins and rift valleys
– hanging wall has moved down relative to footwall, i.e. hanging wall is downthrown block.
13

Types of normal faults:
Step faults – a series of normal faults all downthrown in same direction.
Block faults – a series of upthrown (horst) and downthrown (graben) blocks bounded by
normal faults.
 Reverse fault – dip-slip fault – compressional forces – destructive plate margins and
continental collision zones – hanging wall has moved up relative to footwall, i.e. hanging
wall is upthrown block.
 Thrust fault – low-angle reverse fault – fault dip is less than 45° – has all the same
characteristics as a reverse fault. May occur along the basal planes of nappes.
Strike-slip faults – movement along strike (horizontal) .
 Tear/wrench fault – strike-slip fault – shear forces – conservative plate margins –
movement is horizontal, features on either side of fault are displaced laterally. Sense of
movement may be sinistral (opposite side has moved to left) or dextral (opposite side has
move to right).
 Transform faults – strike-slip fault – offset segments of the mid ocean ridge – sense of
movement is opposite to that expected – it is the sea floor that is spreading away from the
ridge axis, transform faults accommodate different rates of spreading along the ridge.
(h)
Describe and recognise: slickensides, fault breccia and mylonite and explain
their formation.




Slickensides – parallel scratches/grooves along fault plane – formed as one surface
moves over the other and grinds/polishes the rock. Can be used to show direction of
movement – parallel to slickensides.
Fault breccia – large, broken, angular fragments of rock formed during movement of the
fault. May be cemented by a mineral such as calcite.
Mylonite – dynamic metamorphic rock associated with faulting – hard, crystalline, broken/
crushed rock, with crude foliation developed parallel to fault plane.
Other features developed along faults include: fault gouge (pulverised rock – clay),
hydrothermal mineral veins and drag folds. Strata and older features on either side of the
fault will be displaced.
(i)
Define and recognise: fold limbs, hinge, crest, trough, axial plane, axial plane
trace, plunge, antiform, synform.
Fold – a bend or flexure in rocks, where there is a change in the dip on either side. Result of
compressional forces and plastic behaviour – develop in incompetent rocks. Form in fold
mountain belts at destructive plate margins/continental collision zones.












Hinge – line along which there is maximum curvature, dips change on either side.
Limb – the section of the fold between successive hinges – dip of limbs can be measured
using a clinometer in the field or a protractor on paper (measured from the horizontal).
Core – centre of the fold.
Crest – highest point on a bed in an anticline.
Trough – lowest point on a bed in a syncline.
Axial plane – plane that joins hinge lines through a series of beds – trend can be
measured using a compass in the field.
Axial plane trace – intersection of the axial plane and the topographic/land surface.
Antiform – any up-fold. Give striped outcrop patterns on maps, beds dip outwards.
Synform – any down-fold. Gives striped outcrop patterns on maps, beds dip inwards.
Attitude – orientation and inclination of the fold, its shape.
Plunge – if the hinge line is not horizontal it is said to plunge – the direction and angle of
plunge can be measured. Gives V-shaped (curved) outcrop patterns on maps.
Fold style – determined by amount of pressure – measured using the interlimb angle –
120-70° = open, 70-30° = closed, 30-0° = tight, 0° = isoclinal (extreme pressure from both
sides).
14
(j)
Describe and recognise the formation of symmetrical and asymmetrical
anticlines and synclines, plunging folds, monoclines, overfolds, isoclinal folds,
recumbent folds, nappes, domes, basins and explain their formation.












Anticline – antiform in which the beds get older into the centre (core). Give striped outcrop
patterns on maps, beds dip outwards.
Syncline – synform in which the beds get younger into the centre (core). Gives striped
outcrop patterns on maps, beds dip inwards.
Symmetrical folds – formed by equal pressure from both sides, fold axis is
vertical/upright, limbs dip by the same amount (have same outcrop width on maps).
Asymmetrical folds – formed by unequal pressure from the sides, fold axis is inclined,
limbs dip by different amounts (limb with steeper dip will have narrower outcrop width on
map).
Overfolds – as the amount of pressure increases from one side the fold becomes an
overfold – fold axis is inclined, one is limb inverted (can be determined using sedimentary
structures).
Recumbent folds – formed by extreme pressure from one side – axial plane is subhorizontal, one limb is inverted.
Nappes - formed by extreme pressure from one side – recumbent fold fails brittly and a
thrust fault develops along the base. The top part of the fold may be thrust many
kilometres over the underlying rock. These are found in fold mountain belts and are some
of the largest tectonic structures.
Plunging folds – fold hinge line is not horizontal, gives V-shaped outcrop pattern on map.
Monoclines – step-shaped fold – formed by pressure from one side, only has one limb.
Isoclinal folds – formed by extreme pressure from the sides – has vertical limbs and an
interlimb angle of 0°.
Domes – formed by pressure from all directions – dips outwards in all directions. Give
circular outcrop patterns on maps.
Basins – formed by pressure from all directions – dips inwards in all directions. Give
circular outcrop patterns on maps.
(k)
Recognise the age relationships between structures on simplified geological
maps, cross-sections and photographs.







In order to deduce the geological history/recognise age relationships, we need to use our
knowledge of structural geology. The following key ideas are used:
Principle of Uniformitarianism – Processes that operate today also operated in the past –
“The present is the key to the past.”
Principle of Superposition – In an undisturbed sedimentary sequence, the oldest beds are
at the bottom. The beds get younger upwards.
Law of horizontality – Sedimentary beds are laid down horizontally.
Law of cross-cutting relationships – If a rock (e.g. a dyke) or structure (e.g. a fault) cuts
across another rock then it must be younger.
Law of included fragments – The fragments must be older than the rock containing it (e.g.
pebbles in a conglomerate are older – must have come form somewhere else).
Ensure you know all the necessary geological map symbols and can recognise horizontal
and dipping beds; unconformities; inliers and outliers; dykes, sills and other igneous
intrusions; metamorphic aureoles; different fault and fold types on maps and cross
sections.
(l)
Describe the age relationships of beds using cross-cutting features: beds,
faults, folds, unconformities and igneous features.

Law of cross-cutting relationships – If a rock (e.g. a dyke) or structure (e.g. a fault) cuts
across another rock then it must be younger. Can be used to determine age relationships.
© D. Armstrong, 2004.
15