Module 5: Theme4 Lithosphere – Key Idea 2
Oceanic Lithosphere
Knowledge:
The ocean crust has a layered structure; seismic layers 1,2 and 3.
Ophiolites and ocean drilling provide evidence for the origin and composition of the
oceanic crust and upper mantle.
Rates and directions of seafloor spreading may be calculated from the dating of ocean
crust and the patterns of ocean magnetic anomalies caused by field reversals: use of
radiometric dating and ocean drilling to date magnetic anomalies.
Oceanic lithosphere cools by conduction as it moves away from a spreading centre,
leading to a thickening of oceanic lithosphere with age.
Oceanic lithosphere is resorbed into the mantle by sinking in subduction zones within
oceans and at active continental margin; island arc and cordilleran magmatism;
marginal ocean basins and back arc spreading.
Ocean basin evolution can be traced from continental rifts through narrow seas to
mature ocean basins; the J Tuzo Wilson cycle – ocean growth and destruction as a
periodic event; age and location of the oldest ocean floor.
Skills:
Interpretation of evidence from ophiolites and drilling for the presence of a layered
structure to the oceanic crust.
Investigation of an ophiolite complex.
Interpretation of magnetic anomaly profiles and maps.
Interpretation of ocean floor age distribution maps.
Calculation of seafloor spreading rates from magnetic anomaly and mantle plume (hot
spot) data. Use of plumes to show direction of plate movement.
Interpretation of heat flow variations across the ocean floor.
The ocean floor
Topographic profile
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Module 5: Theme4 Lithosphere – Key Idea 2
The floor of the oceans is not flat, it has a topography characterised by a number of
features.
Exercise 1
Each of the following terms were introduced in year 12, try to write a definition of
description of each:
Abyssal plain
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Continental shelf
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Continental slope
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Trench
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Seamount
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Guyot
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Continental rise
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Exercise 2
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Module 5: Theme4 Lithosphere – Key Idea 2
Sketch a topographic profile across the floor of an ocean basin. Your sketch should show
each of the features listed in exercise 1. On the left side of your sketch you should show
an passive continental margin, and the right side should be an active continental margin.
Evidence
Typically the oceans have a depth of 4 – 5 km. At this depth there are very few
submersibles capable of withstanding the enormous pressures. Evidence for the
behaviour and structure of the seafloor comes from drilling, dredging and geophysical
techniques such as seismic reflection profiling, magnetic and thermal studies.
Exercise 3
The profile you drew in exercise 2 would have two major thermal anomalies, one
positive, the other negative. Locate and explain these anomalies:
Positive anomaly:
Location = …………………………………………………………………………..
Reason = …………………………………………………………………………….
Negative anomaly:
Location = …………………………………………………………………………..
Reason = …………………………………………………………………………….
The oceanic crust
Seismic reflection profiling
This technique uses artificially produced shock waves to find different layers by
reflecting off them, rather like light reflecting from a mirror. This method has shown that
there are three distinct layers within the oceanic crust, called layers 1, 2 and 3.
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Module 5: Theme4 Lithosphere – Key Idea 2
The structure of the oceanic crust
Layer 1 is the uppermost part of the oceanic crust. It is composed of sediment and its
thickness varies, although as a general rule, the thickness increases with distance from the
ridge. Thickness of the sediment depends upon distance from the continent, the amount
of transport by deep currents and in certain parts of the ocean, (where organic material is
the main type of sediment) food supply and ocean depth. There may be submarine fans,
built up from the sediment deposited by major rivers entering the ocean basin. Near the
equator there are thick accumulations of sediment due to the high productivity in those
regions.
Layer 2 is between 1 and 1.5 km thick. Seismic wave speeds and drilling show that this
layer is made of basic igneous rock. The upper part is composed of basaltic pillow lava
which formed when magma cooled rapidly on contact with sea water. Below the pillow
lavas is a complex built up from many dolerite dykes side by side. Layer 2 is highly
fractured allowing seawater to percolate into the crust.
Exercise 4
Compare basalt and dolerite.
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What is a dyke?
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Describe the internal structure of a basalt pillow.
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Exercise 5
Explain how hydrothermal circulation through the ocean crust produces black smokers.
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Module 5: Theme4 Lithosphere – Key Idea 2
Layer 3 is about 5 km thick on average and composed of basic igneous rock. The upper
part is gabbro, this gradually changes to a sequence of cumulates; olivine-rich gabbro and
peridotite.
Exercise 6
Explain what cumulates are.
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Ophiolites
Ophiolite sequences
In certain parts of the world, there are sequences of rocks which show a characteristic
series of layers. The top layer is a sedimentary rock called chert. This is a rock rich in
silica which formed from the accumulation of the shells of tiny marine organisms. Below
this there is a layer of pillow basalt, then a sheet of dykes, gabbro and then cumulates.
The lowest part of the sequence is a rock called serpentinite. This is an ultrabasic rock
which has undergone some degree of alteration and metamorphism, producing an
assemblage of metamorphic minerals including serpentine and chlorite.
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Module 5: Theme4 Lithosphere – Key Idea 2
Examples of ophiolite sequences
The most famous example of an ophiolite sequence is found in the Troodos Mountains in
Cyprus. This was studied in detail during the 1960’s and it provided evidence for the
structure of the oceanic crust and hence for plate tectonics. Other famous examples
include the Lizard peninsula in Cornwall and the Mona complex in Anglesey.
Comparison of ophiolite sequences and what we know of ocean crust
Subduction zone processes
Types of subduction zone
As you already know, if two plates collide (at a convergent plate margin) one of them
may be forced down into the mantle. This process is called subduction.
Exercise 7
Using Japan and the Andes in South America as examples, sketch two types of
destructive plate boundaries.
Japan-type
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Module 5: Theme4 Lithosphere – Key Idea 2
Andean-type
Benioff zones
As the plate descends, earthquakes occur along its length. These earthquakes are
common at shallow depths, near the surface and decrease in number down to a depth of
about 300 km. Between 300 and 450 km there are few earthquakes but below 450 km
there are many more. The deepest earthquake foci occur at about 670 km.
The earthquakes occur for a number of reasons. The shallow earthquakes near the
surface are caused by normal faults in the subducting oceanic lithosphere. As the plate is
subducted, it is forced to bend, so the upper part is put under tension, this causes the
normal faulting. Another reason why shallow earthquakes occur is due to the process of
“underthrusting”, where the subducting plate is pushed below the overriding plate.
Earthquakes at greater depths may occur due to compression. As the plate is subducted it
seems to encounter resistance, so the rocks are put under compression, causing faulting
and earthquakes. Other subduction zones seem to show that these deeper earthquakes are
caused by tension, as if the end of the subducting plate is pulling on the rest of it, causing
it to break up.
Finally, some earthquakes, particularly the deepest ones, may be caused by mineral
transformations. As the rocks descend, they encounter different pressure and temperature
conditions under which the minerals become unstable. For example, the mineral
serpentine loses its water at depths between 100 and 300 km, this causes a volume change
and resulting in seismic waves. Other mineral transformations include olivine changing
to spinel (at about 400km) and spinel changing to perovskite at about 670 km.
Magma production
Subduction zones produce andesitic magma. This is probably due to partial melting of
the overlying asthenosphere, triggered by the presence of water brought in by the
subducting plate. (The subducted plate becomes mixed into the mantle, this is called
resorption). This magma rises, eventually producing volcanoes at the surface. In the
case of a Japan-type subduction zone, these volcanoes form a chain of volcanic islands
called an island arc, parallel to the trench. Below these volcanoes there are large plutons
intruded into the crust.
In an Andean-type margin, the magma produced is also andesitic. This rises through the
continental crust, intruding it and producing volcanoes at the surface. The edge of the
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Module 5: Theme4 Lithosphere – Key Idea 2
continental crust is crumpled to form a mountain chain parallel with the edge of the
continent. This is called a cordillera.
Accretionary prisms
As subduction occurs, some of the sediment is scraped off onto the continent in a series
of slices, called accretionary wedges. These build up to form an accretionary prism.
Oceanic Ridges
Oceanic ridges occur at divergent plate margins. It is an undersea mountain range, rising
2 to 3 km above the ocean floor. In the centre of the ridge is a rift valley, produced by
tension. The ridge is characterised by high heat flow and shallow seismic activity (to a
depth of 20 km.) The ridge is offset by transform faults. Transform faults occur because
of the curved shape of the plate boundary. The rigid behaviour of the rocks do not allow
the boundary to be curved, so instead it breaks up into a series of segments divided up by
transform faults (like a series of right-angle bends).
Magma production
As the plates move apart at the rate of a few cm per year, the mantle material underneath
rises and eventually partially melts. The partial melting of the dry peridotite produces a
magma of basaltic composition. Magma reaching the surface in the rift valley forms
pillow basalt.
Possible causes of plate movement
There are three possible causes for plate movement.
Exercise 8
Describe the following processes:
Convection
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Module 5: Theme4 Lithosphere – Key Idea 2
Ridge push
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Slab pull
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Age – thickness – depth
As the oceanic crust moves away from the ridge, the site of its formation, it cools. This
causes it to contract and so the depth of the ocean increases away from the ridge, until it
reaches equilibrium at the abyssal plain. The thickness of the crust increases, however,
because as time goes on, more and more sediment accumulates, increasing the thickness
of seismic layer 1.
Oceanic Basins
Roll-back
The point at which the subducting slab bends is called the hinge point.
The old oceanic crust is cold and dense, so when it is subducted, it descends readily. In
fact the speed at which the slab descends may be greater than the speed the plate is
moving (slab pull effect). This makes the hinge point shift backwards, this is called rollback.
Roll-back has an effect on the overriding plate – it stretches it. The result is that the
continental crust becomes extended and a basin is formed, called a back arc basin.
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Module 5: Theme4 Lithosphere – Key Idea 2
Back-arc basins and fore-arc (marginal) basins
The collision of two plates creates a lot of bending and stretching of the lithosphere. The
back arc basin is formed because the continental crust becomes stretched, creating normal
faults . It is possible for the stretching to pull the continent apart to make a new ocean.
The leading edge of the overriding plate is also flexed to make a fore-arc (marginal)
basin.
The J Tuzo-Wilson Cycle
Plate tectonics represents a cyclic process. New crust is created at the ocean ridges and
old crust is destroyed at the subduction zones. The Tuzo-Wilson cycle describes this
idea. Very simply, the cycle is:
stretching to form a new basin  new oceanic crust forms to make ocean wider 
subdiction occurs, ocean closes up.
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Module 5: Theme4 Lithosphere – Key Idea 2
Exercise 9
These are steps in the Tuzo-Wilson cycle. For each step draw a cartoon to represent the
processes described.
Step 1
A piece of continental lithosphere is stretched. It undergoes extension, causing normal
faulting, a rift valley is formed. Volcanoes occur in the rift valley due to magma
generation.
Give an example. ……………………………………
What type of magma is produced? ……………………………………
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Module 5: Theme4 Lithosphere – Key Idea 2
Step 2
The continental lithosphere continues to stretch, more faults form allowing the central
block of the rift valley to drop further. The surface falls below sea level and sea water
invades forming a shallow sea.
Give an example. …………………………………..
What characteristic igneous features form on the sea bed? …………………………….
Step 3
The sea widens as more basaltic oceanic crust is formed. The edges of the ocean are both
passive continental margins.
Give an example. ……………………………………
Step 4
A fracture occurs in the oceanic lithosphere (on the right of your picture). A subduction
zone forms as one plate (the left one) is pushed down into the mantle. The plate is
resorbed and partial melting occurs to produce an island arc.
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Module 5: Theme4 Lithosphere – Key Idea 2
Step 5
Spreading ceases at the ridge and another subduction zone forms, (this time at the left of
your picture). This is an Andean-type active continental margin.
Step 6
Eventually the two continents are brought back together as the ocean closes up.
ESTA GEOTREX The Geology Teachers Resource Exchange Contributor: Owain Thomas
School Date: 22:04:05
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