T E C T O N I C H A Z A R D S:
1.1A: The global distribution and causes of earthquakes, volcanic eruptions and tsunamis.
Earthquakes
Distribution:
Most earthquakes are found at, or close to, tectonic plate boundaries, often in clusters. About 70% of all
earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean. The Oceanic Fracture Zone is a belt of activity
through the oceans along the mid-ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and
California. The Continental Fracture Zone is a belt of activity following the mountain ranges from Spain via the Alps
to the Middle East, the Himalayas to the East Indies and then circumscribing the Pacific.
Some earthquakes however (intra-plate earthquakes), do not occur at plate boundaries due to hotspots.
Causes:
Earthquakes are a sudden release of stored energy. As two plates move past each other, the friction can cause the
rock to temporary get stuck. This allows strain to build up over time, and the plates are placed under increasing
stress. Earthquakes occur due to the sudden release of this stress – ‘slip-stick behaviour’. A pulse of energy radiates
out in all directions from the earthquake focus, and this motion can displace the surface.
Volcanic Eruptions
Distribution:
Volcanoes also occur close to plate boundaries, with the exception of hotspots like Hawaii. They tend to occur
around the western coast of North and South America, along the Mid-Atlantic Ridge and on the eastern coast of East
Asia and Australia. They don’t, however, occur in every plate boundary (not in the Himalayan region or the Eurasian
belt)
Causes:
Volcanoes erupt when magma rises to the surface, which is formed when the earth’s mantle melts. Magma erupts
through openings of the Earth’s crust, before flowing on its surface as lava (which hardens and becomes igneous
rock).
Tsunamis
Distribution:
The global distribution of tsunamis is fairly predictable in terms of source areas, with around 90% of all events
occurring within the Pacific Basin, associated with the activity at the plate margins. Most are generated at
subduction zones (convergent boundaries) particularly off the Japan-Taiwan island arc, South America and the
Aleutian Islands.
Causes:
Tsunamis are generated when a sub-marine earthquake displaces the seabed vertically as a result of movement
along a fault line at a subduction zone. The violent motion displaces a large volume of water in the ocean water
column, which then moves outwards in all directions from the point of displacement. Sub-marine earthquakes that
occur close to the shoreline can generate intense ground shaking damage, followed by the damage from the
subsequent tsunami.
1.1B: The distribution of plate boundaries resulting from divergent, convergent and conservative plate movement.
(oceanic, continental and combined situations)
, Divergent
Oceanic-Oceanic e.g. Mid-Atlantic ridge
1. Rising convection currents bring magma to the surface resulting in small, basaltic eruptions
2. As the plates are moving away from each other, these eruptions of magma then cool and form new oceanic
crust
3. This causes minor shallow earthquakes as well as the oceanic volcanic eruption
Continental-Continental e.g. East African Rift Valley
1. Mantle plume rises and increases pressure beneath the continent, causing it to become thinner, beak and
ultimately split apart
2. New ocean crust erupts in the void, resulting in ridges
3. Basaltic volcanoes and minor earthquakes also occur from this – erupted basaltic lava has a low gas content
and high viscosity
Convergent
Oceanic-Oceanic e.g. Aleutian Islands, Alaska
1. Oceanic crust is subducted beneath another
2. This crust then melts in the magma, which eventually rises up to form a volcanic eruption
3. Results in frequent earthquakes and eventually a volcanic island arc
Continental-Continental e.g. the Himalayas
1. Collision of two continental landmasses causes a mountain belt as the landmasses crumple
2. Magma can be generated at depth, but eruptions are generally rare as magma cools and solidifies beneath
surface – no volcanic activity
3. Infrequent major earthquakes are distributed over the large area
4. E.g. Himalayas were formed in the collision of the Indo-Australian and Eurasian plate. As both continental
plates have the same low density, subduction is not possible. Therefore, the plates crumpled, creating
enormous tectonic uplift. The movement can generate shallow, high magnitude earthquakes like in Kashmir
in 2005 and in Nepal in 2015.
Oceanic-Continental e.g. The Andean Mountains
1. Mantle convection pulls plates towards subduction zones
2. Dense oceanic crust is subducted beneath less dense continental plate
3. The descending crust melts at depth (the Benioff zone) by a process called wet partial melting. This
generates magma with a high gas and silica content, which erupts with explosive force
4. The boundary between the oceanic and continental crust creates a deep trench
Conservative
When plates slide past each other along zones known as transform faults. Tend to have frequent earthquakes, with a
shallow focal depth, so they can be very destructive if they are high magnitude. However, there is no volcanic
activity.
Oceanic-Oceanic e.g. Haiti
Oceanic-Continental e.g. San Andreas Fault (Pacific plate and North American Plate)
1.1C: The causes of intra-plate earthquakes, and volcanoes associated with hot spots from mantle plumes.
Intra-plate Earthquakes
Earthquakes can occur in mid-plate settings, usually associated with major ancient fault lines being re-activated by
tectonic stresses. For instance, the New Madrid Seismic Zone on the Mississippi River generates earthquakes up to
magnitude 7.5 but is thousands of miles from the nearest plate boundary.
, Hotspot Volcanoes
Some volcanic eruptions are intra-plate, meaning that they are distant from a plate boundary at locations called mid-
plate hotspots, such as Hawaii and the Galapagos Islands.
At these locations:
1. Isolated plumes of heat convection, called mantle plumes, rise towards the surface, generating basaltic
volcanoes that tend to erupt continuously
2. A mantle plume is stationary, but the tectonic plate above it moves slowly over it
3. Over a long period of time, this produces a chain of volcanic islands, with extinct ones most distant from the
plume’s location
1.2A: The theory of plate tectonics and its key elements, (the earth's internal structure, mantle convection,
palaeomagnetism, and sea floor spreading, subduction and slab pull.)
1.2B: The operation of these processes at different plate margins (destructive, constructive, collision and
transform.)
Earth’s Internal Structure
The Earth is made up of multiple zones, such as the:
- Crust: denser oceanic crust, less dense continental crust
- Lithosphere: top part of upper mantle that moves with crust, it’s strong but brittle and snaps
under pressure, but is also flexible and can distort to continuous pressure, 2% is melted so it
glides on this lubricant
- Asthenosphere: bottom part of upper mantle
- Mesosphere, or lower mantle
Mantle Convection
All tectonic hazards are caused by the Earth’s internal heat engine. Radioactive isotopes like uranium 238 in the
Earth’s core and mantle generate huge amounts of heat which flow towards the Earth’s surface. This heat flow
generates convection currents in the plastic mantle. The interior of the Earth is therefore dynamic rather than static.
Palaeomagnetism
Palaeomagnetism is the study of the record of the Earth’s magnetic field in rocks, sediment or archaeological
materials. This record provides information on the past behaviour of Earth’s magnetic field and the past location of
tectonic plates. For instance, it reinforced the continental drift hypothesis and its transformation into plate tectonics.
Moreover, palaeomagnetic signals can prove that new ocean crust is created by the process of seafloor spreading at
mid-ocean ridges (seafloor spreading and palaeomagnetism occur at constructive margins)
Processes
Subduction: the process of one plate sinking beneath another at a convergent plate boundary. Earthquakes at
subduction zones can occur at a range of focal depths from 10km to 400km, following the line of the subducting
crust. It occurs at destructive margins, where an oceanic plate subducts under a continental plate, or oceanic plate
under another oceanic plate.
Slab pull: Cold dense oceanic plate is subducted beneath less dense continental plate; the theory is that because of
the contrast in density, the plate begins to sink into the mantle. This force and slab suction account for almost all of
the force driving plate tectonics. This occurs at destructive margins.
Gravitational sliding: Constructive margins have elevated altitudes because of the rising heat between them, which
creates a ‘slope’ down which oceanic plates slides. This occurs at destructive margins.
