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THEORY OF PLATE TECTONICS
1. To understand what is meant by the theory of plate tectonics.
2. To be able to describe the characteristics of the 2 types of plates (oceanic and continental plates) involved
in plate movements.
3. To be aware that there are four different types of plate boundaries (destructive, constructive, conservative
and collision).
4. To be able to link the theory of plate tectonics to the world distribution of volcanoes and earthquakes.
5. To be able to explain the idea of convection currents in the mantle and how this relates to the theory of
plate tectonics.
1. To understand what is meant by the theory of plate tectonics
The theory of plate tectonic states that the lithosphere (the Earth’s crust and the rigid upper part of the
mantle) is made up of 7 large and several smaller plates. These plates float like rafts on the underlying semimolten asthenosphere (mantle) and are moved by convection currents that derive their heat from the earth’s
interior. The slow movement of the plates in relation to each other accounts for the distribution and shape of
the landmasses as well as the occurrence of volcanoes and earthquakes.
The theory of plate tectonics has four main aspects;
a. Location of tectonic plates
b. Tectonic plate characteristics
c. Movement of plates (plate boundaries)
d. Convection currents
2. To be able to describe the characteristics of the 2 types of plates (oceanic and continental plates) involved
in plate movements.
There are 2 types of tectonic plates; CONTINENTAL CRUST (Sial) and OCEANIC CRUST (sima). However, there
terms do not refer to actual continents and oceans but to different types of rock.
3. To be aware that there are four different types of plate boundaries (destructive, constructive, conservative
and collision).
The area where 2 tectonic plates meet is termed a plate boundary or plate margin. It is at plate boundaries
that most of the world’s major landforms occur, and where earthquake, volcanic and mountain building zones
are located.
Key points related to plate boundaries:
a. Due to its relatively low density, continental crust does not sink and so is permanent; being denser,
oceanic crust can sink. Oceanic crust is being formed and destroyed continuously.
b. Continental plates, such as the Eurasian plate may consist of both continental and oceanic crust.
c. Continental crust may extend far beyond the margins of the landmasses.
d. Plates cannot overlap. This means that either they must be pushed upwards on impact to form
mountains or one plate must be forced downwards into the mantle and destroyed.
e. No ‘gaps’ may occur on the Earth’s surface so if two plates are moving apart, new oceanic crust
originating from the mantle must be being formed.
f. Because the Earth is not getting larger or smaller, the rate at which new crust is being created must
be equal to the rate of crustal destruction. This is a state of ‘dynamic equilibrium’ i.e. change, is
occurring but the overall balance remains the same.
There are 4 types of plate boundaries; Constructive (diverging), Destructive (converging), Conservative and
Collision.
a. Constructive (diverging) plate boundary
Where 2 plates move apart, they leave a gap through which magma
(molten rock) oozes up. This type of boundary can occur where twp
oceanic plates move apart at sea and this leads to the development
of a mid-ocean ridge. When divergent plate boundaries occur on
land they form a rift valley.
b. Destructive (converging) plate boundary
Where 2 plates move towards each other and collide. If a continental
plate moves towards an oceanic plate, due to the difference in
densities of the plate (oceanic is heavier than continental plate), the
oceanic plate is subducted into the mantle. As the oceanic plate sinks
it melts. This creates gases and molten rock which move upwards
through weaknesses in the continental crust above and forms
volcanoes. (e.g. The Andes have been formed as the Nazca oceanic
plate has pushed into and under the South American plate).
The same process occurs where plates made of oceanic crust converge. The resulting volcanoes form
an island arc. (e.g. This happens near Japan where the Pacific plate pushes into and moves under the
edge of the Eurasian plate).
c. Collision plate boundary
Where two plates formed of continental crust collide, fold mountains
are formed as neither crust is dense enough to sink beneath the other.
Sediments (.e. layers of rock deposited in seas and oceans) are
squeezed into folds and slowly pushed up by the steady advance of
the two plates, eventually forming thick layers. (e.g. Indo-Australian
plate moved northwards towards the Eurasian plate forming the
Himalayas).
d. Conservative plate boundary (transform fault)
Where plates slide past each other in a transform margin, crust is
neither created nor destroyed. The friction caused as the plates grind
past each other causes cracks or faults in the crust. Earthquakes can be
caused by friction and the build-up of pressure between the moving
plates – the plates ‘stick’, then suddenly jerk. (e.g. San Andreas Fault,
the boundary lies between the North American and the Pacific plate).
4. To be able to link the theory of plate tectonics to the world distribution of volcanoes and earthquakes.
Earthquakes
The vast majority of earthquakes occur along plate boundaries, the most powerful being associated with
destructive margins. At conservative margins, the boundary is marked by a fault movement along which produces
the earthquake. Perhaps the most famous of these is the San Andreas fault of California where the fault line
represents the boundary between the North American and Pacific plates. Some earthquakes occur away from
plate boundaries and are associated with the reactivation of old fault lines. It has also been suggested that man
could be the cause of some minor earthquakes by building large reservoirs where the water puts pressure on the
surface rocks or by the subsidence of deep mine workings.
Volcanoes
Most volcanic activity is associated with plate tectonic processes and is mainly located along plate margins. Such
activity is therefore found:
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Along ocean ridges where plates are moving apart. The best example is the mid-Atlantic Ridge where Iceland
represents a large area formed from volcanic activity
Associated with rift valleys. The East African Rift Valley has a number of volcanoes along it, including Mount
Kenya and Mount Kilimanjaro
On or near subduction zones. The line of volcanoes, the Ring of Fire. that surrounds the Pacific Ocean is
associated with plate subduction. This tends to be the most violent of all activity
Over hot spots such as that which occurs in the middle of the Pacific Ocean and gives rise to the Hawaiian
Islands.
Hot spots (volcanic activity away from plate boundaries)
Vulcanicity is associated with plate margins in the centre of the Pacific Ocean; the volcanic Hawaiian Islands occur
which are not connected with any boundary. It is believed that this volcanic area is caused by a localised hot spot
within the Pacific plate. Inside the mantle it is possible that a concentration of radioactive elements causes a hot
spot to develop. From this, a plume of magma rises to eat into the plate above, and finally allow lava to be poured
onto the surface and active volcanoes to occur above the hot spot. The hot spot is a stationary feature, so as the
Pacific plate moves across it, a line of volcanoes is created, the one over the hot spot being active at the present
time. The rest form a chain of islands of extinct volcanoes. The oldest volcanoes have put so much pressure on
the crust that subsidence has occurred. This, together with marine erosion, has reduced these oldest volcanoes to
seamounts below the level of the ocean. From this evidence, it is clear that the Pacific plate is moving towards the
north west and is further proof that the Earth’s crust is moving.
5. To be able to explain the idea of convection currents in the mantle and how this relates to the theory of
plate tectonics.
The higher temperatures at the earth’s core and heat released by radioactive decay of elements within the
mantle help to create convection currents. A convection current is a circulatory pattern driven by the rising of
hot material and/or sinking of cold material. Hot material has a low density, so it rises; cold material has a
high density and sinks. These circulatory motions cause the crustal plates to move. They pull crust apart at
spreading ridges and rift zones, and pull slabs of oceanic crust back down into the mantle at subduction
zones.
How do convection currents move tectonic plates? (in diagrams0: