Geography A2 Revision
PLATE TECTONICS AND ASSOCIATED HAZARDS
The Theory of Plate Tectonics
>Hot Spot: A point on the surface of the earth located above a plume of rising magma. E.g. Hawaiian Islands.
>Plate: Rigid slabs that float on the underlying semi-molten mantle (asthenosphere) and are moved by
convection current within it.
>Plate Tectonics: A theory that attempts to explain the formation and distribution of the Earth’s major
structural features in terms of plates that make up its surface.
>Seismic waves: Shock waves released by the rupture of rock strata at the focus of an earthquake.
>Tsunami: Sea waves, which can be very large, generated by shallow focus underwater earthquakes, volcanic
eruptions, underwater debris slides and large landslides into the sea.
>Volcano: An opening or vent through which magma, molten rock, ash or volatiles erupt onto the surface of
the Earth.
Alfred Wegener’s Theory: The splitting of continents over the ages ‘Continental Drift’.
Evidence supporting Wegener’s Theory:
Geological:
>The ‘jigsaw’ fit of South America and Africa.
>Evidence of glaciation of the late Carboniferous period – deposits found in South America, Antarctica & India.
>Striations along the sea bed – the continents must have scoured the sea bed as they drifted.
>Rock sequences in Northern Scotland agree with those found in eastern Canada.
Biological:
>Fossils found in India are comparable with similar fossils in Australia.
>Fossil remains of the same reptile were found in both South America and Southern Africa.
>Identical fossil remains found on different continents with no evidence indicating that the fossils could fly or
swim.
Development of the Theory:
>The Mid-Oceanic Ridge (MOR) was discovered.
>Magnetic Stripes: An alternating pattern of magnetic stripes, mirrored exactly on each side of the MOR. This
suggests that the ocean crust is slowly spreading away from this boundary.
>Sea floor Spreading: Plates must be being destroyed somewhere to accommodate the increase in their side
at MOR’s.
The Earth’s Layers:
1.
The Crust – Varies in thickness, thickest under continents, thinner under oceans.
2.
The Lithosphere – The crust and the rigid upper section of the mantle. Rigid & brittle.
3.
The Asthenosphere – The semi-molten layer, on which the plates float and move. ‘Rheid’ – plastic
properties.
4.
The Mantle – Made up of Molten and semi-molten rocks.
5.
The Outer-core – Molten/liquid, ‘Rheid’.
6.
The Inner-core – Solid, due to extreme pressure, composition is iron and nickel.
Continental Crust
Oceanic Crust
Thickness
30 – 70km
6 – 10km
Age
Over 1,500 million years
Less than 200 million years
Density
2.6 (lighter)
3.0 (heavier)
Composition
Silicic
Basaltic
>Hot spots: Generate thermal convection currents within the Asthenosphere, which causes magma to rise
towards the crust and then spread before cooling and sinking. This circular motion causes the plates to move.
Features of Plate Margins:
>Constructive (Divergent) Margins: Plates move apart. Oceanic plates – Form MORs. Continental plates –
Form Rift valleys.
>Ocean Ridges: Slow rate = 10-15mm per year, Intermediate rate = 50-90mm per year, Rapid rate = >90mm
per year. E.g. Mid Atlantic Ridge.
>Rift Valleys: Formed by constructive plate margins on a continental plate. E.g. East African rift valley.
>Destructive (Convergent) Margins: Plates move toward each other.
Scenarios: Oceanic Vs Continental, Oceanic Vs Oceanic, Continental Vs Continental.
Oceanic Vs Continental
>Oceanic Plate (denser) is subducted under
the Continental plate (more buoyant)
Oceanic Vs Oceanic
>The Denser of the two oceanic plates is
subducted.
Continental Vs Continental
>As plates meet, both plates are forced up into
fold mountains, no volcanic activity.
>Conservative margin: When two plates slide past each
other, causing no destruction or creation of crust. The
movement of the plates causes friction, which, when
released, can cause shallow-focus earthquakes.
Plate Margin
Tectonic Features
Constructive
Destructive
(1) Subduction
Movement of Plates
Divergent
Convergent
(2) Collision
Examples
New crust, MORs, Shield Volcanoes,
shallow-focus earthquakes, Median rift
valleys,
Continental rift valleys
Mid-Atlantic Ridge.
(1a) Oceanic Vs Oceanic: Trenches, island
arcs, strato volcanoes, earthquakes
(shallow, intermediate and deep)
(1b) Oceanic Vs Continental: Trenches, fold
mountains, strato volcanoes, earthquakes
(shallow, intermediate and deep)
(1a) Tonga trench.
(2) Continental Vs Continental: Fold
mountains, shallow-focus earthquakes.
(2) Himalayan type: Indian
plate colliding with the
Eurasian plate.
East African Rift valley.
(1b) Nazca subducting under
the South American plate.
Conservative
Two plates shearing
past each other
Shallow-focus earthquakes
San Andreas fault: Pacific
plate Vs North American
plate
Not at plate
boundaries
Hotspots: may be near
the centre of a plate
Plume volcanoes
Hawaiian islands: emperor
seamount chain
Volcanic Eruptions:
>Batholiths: Dome-shaped, large masses of cooled magma, usually granite.
Divergent/Constructive Margins
Destructive/Convergent Margins
Movement of Plates
Plate Margin
Continental
crust
Oceanic
crust
Type of Magma
Basaltic
Mafic
Lava
Characteristics
Low viscosity – Runny
High viscosity – thick
Type of Eruption
Effusive – gas escapes easily
Explosive – gas released at once
Material Erupted
Mainly lava
Lava bombs, ash, dust
Frequency of
eruption time
Regular and can be continuous
From time to time: long dormant periods
Form of the
Volcano
Intrusive Volcanic Landforms:
>Dykes: Vertical intrusions with horizontal cooling cracks. They cut perpendicular to the bedding planes.
>Sills: Horizontal intrusions with vertical cooling cracks. They lie parallel to the bedding planes.
Extrusive Volcanic Landforms:
>Lava Plateaux: Formed from fissure eruptions. Generally flat and featureless.
>Shield/Basic Volcanoes: Mafic lava, gentle sides and cover a large area. E.g. Mauna Loa, Hawaii.
> Acidic/Dome Volcanoes: Steep sided, viscous lava. E.g. Puy region, central France.
>Ash & Cinder cones: Formed from ash, cinders and volcanic bombs. Steep sided and symmetrical. E.g.
Paricutin, Mexico.
>Composite cones: Pyramid-shaped volcanoes, consisting of layers of ash and lava. E.g. Mt Etna, Sicily.
>Calderas: A cauldron-like volcanic feature, formed by the collapse of land following a volcanic eruption. E.g.
Yellowstone National park, US.
Minor Volcanic forms:
>Solfatara: Small volcanic areas without cones, produced by gases escaping to the surface.
>Geysers: A fountain of heated water, exploding onto the surface of the earth.
>Hot springs/boiling mud
The Impact of Volcanic Activity
Primary Effects:
>Tephra: Solid material, from volcanic bombs to ash particles, ejected into the atmosphere.
>Pyroclastic flows: Very hot, gas-charged, high velocity flows of gases and tephra.
>Lava flows
>Volcanic gases
Secondary Effects:
>Lahars: Volcanic mud flows. Caused by mixing hot ash particles with water.
>Flooding
>Tsunamis: Giant sea waves.
>Volcanic Landslides
>Climatic Change: The ejection of vast amount of volcanic debris into the atmosphere can reduce global
temperatures.
Volcanic effects become hazard when they impact upon the human and built environments, killing and injuring
people, burying and collapsing buildings, destroying the infrastructure and bringing agricultural activities to a
halt.
Mt Nyiragongo, Congo, 2002
>Lava had low viscosity – fast travelling downhill.
>Authorities announced a ‘Red Alert’ – full evacuation – minimizing death toll.
>Lava flows destroyed at least one-third of Goma, a town with over 200,000 inhabitants.
>Also destroyed water and power supplies and many of the medical facilities.
>Death toll – 147.
>350,000 Fled.
>Sulphurous water entered the lake, polluting the lake – which was a major source of drinking water.
>Several earthquakes.
>Looting of homes.
>2 days after eruption, UN – Humanitarian aid.
>UN – set up camps - $15 million.
>The lava flows destroyed many businesses – massive increase in unemployment.
Mt Etna, Sicily, 1991 – 93
>Strato Volcano.
>Thousands of people live on the slopes – fertile soils.
1991
>Large earth barrier was constructed.
1992
>The accumulated lava began to spill over the earth barrier.
>Smaller barriers were destroyed by rapidly advancing lava.
>Decision – cut off the flow by blocking the primary feeder channel – dropping concrete blocks in it.
>Engineers blasted opening into the lava tube – redirect the lava flow – successful!
Criticism of intervention
>Success of intervention depends on low effusion rates during the eruption, higher elevation of eruptive vents,
the possibility of diverting the lava flow into uninhabited areas.
The Soufriere Hills, Montserrat, 1995 – 97
>Early activity – ash emissions, steam explosions and numerous earthquakes, small pyroclastic flows.
1997
>Extensive pyroclastic flows – damaged houses and killed some inhabitants.
>The Airport and Docking facilities were destroyed.
Aid – British Govt – Montserrat is still a dependent territory
>Setting up Exclusion zones.
>Evacuating 7,000 of islands 11,000 inhabitants.
>Resettlement of some of the population.
Re-establishment of air and sea links.
>Providing farming areas for those resettled.
>Setting up the Montserrat volcano Observatory.
The UK govt spent more than £100 million in AID.
SEISMICITY
Causes of Earthquakes:
As the crust of the Earth is mobile, there tends to be a slow build up of stress within the rocks. When this
pressure is suddenly released, parts of the surface experience an intense shaking motion that lasts for just a
few seconds.
>Focus: The point at which this pressure release occurs within the crust.
>Epicentre: The point immediately above the focus, on the Earth’s surface.
Three Categories of Earthquake:
>Shallow-Focus: (0-70km deep) Cause the greatest damage.
>Intermediate-Focus: (70-300km deep).
>Deep-focus: (300-700km deep).
Seismic waves:
>Primary (P): Travel the fastest and are compressional (vibrate in the direction they travel).
>Secondary (S): Travel at half the speed of P waves vibrating at right angles to the direction of travel.
>Surface (L): Travel the slowest and near to the ground surface. Some waves shake the ground at right angles
to the direction of wave movement, some have a rolling motion that produces vertical ground movement.