Plate tectonics theory proposes that the Earth's crust is divided into plates that move over time. Evidence for this includes matching geological formations between continents that have since drifted apart. The Earth has a solid inner core, liquid outer core, and mantle below the crust. Plates meet at boundaries that are either constructive, where plates move apart, or destructive, where plates converge. Earthquakes are often caused by stresses building up at plate boundaries. Volcanic activity commonly occurs near plate boundaries as well.

1. Plate Movement
The theory of plate tectonics
1912, Alfred Wegener published theory of single continent (Pangaea) that existed 300 million
years ago
o Later split into 2 continents
 Laurasia in north
 Gondwanaland in south
Different evidences to prove single continent
o Geological evidence
 Bulge of south America fitting into indent below west Africa
 Evidence of glaciation of late Carboniferous period – deposits found in south
America, Antarctica and India
 Rock sequences in northern Scotland and Eastern Canada
o Biological evidence
 Fossil brachiopods found in Indian limestones – comparable with fossils in
Australia
 Fossil remains in South America and southern Africa
 Fossil remains in coal in India and Antarctica
Earth’s layers
o Core – made up of dense rocks containing iron and nickel alloys
 Divided into solid inner core / molten outer core
o Mantle – made up of molten / semi-molten rocks containing lighter elements such as
silicon and oxygen
o Crust – even lighter because of elements
 Most abundant = silicon, oxygen, aluminium, potassium, sodium
 Varies in thickness
Oceanic crust – 6-10 km thick
Continental crust – 30-40km thick
Under highest mountain ranges – 70km thick
o Lithosphere – consists of crust and the rigid upper section of the mantle
 Approximately 80-90 km thick
o Asthenosphere – below lithosphere
 Semi-molten
Continental Crust Oceanic Crust
Thickness 30-70 km 6-10 km
Age Over 1,500 million years Less than 200 million years
Density 2.6 (lighter) 3.0 (heavier)
Composition Mainly granite; silicon, Mainly basalt; silicon,
aluminium, oxygen (SIAL) magnesium, oxygen (SIMA)
o Hot spots – generate thermal convection currents within asthenosphere
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2.  Seen in Hawaii (see picture below)
Features of plate margins
Constructive (divergent) margins
o Plates move apart in oceanic areas
o RIDGE VALLEYS
 Longest continuous uplifted features
 Precise form influenced by rate at which plates move apart
Slow rate – 10-15mm/year, produces wide ridge axis (30-50km) and
deep central rift valley (3,000m)
Intermediate – 50-90mm/year, produces well-marked rifts (50-200m
deep) with smoother outline
Rapid rate – >90mm/year, produces smooth crest and no rift
Destructive (convergent) margins
OCEANIC / CONTINENTAL CONVERGENCE
 Where oceanic and continental plates meet,
denser oceanic plates is forced under continental
crust – known as subduction
 Downwarping of oceanic crust creates a trench
(deep part of sea)
 Sediments accumulate and continental crust is uplifted to from fold mountains
 Further the rock descends the hotter the surrounding becomes. Andesitic lava
then creates complex, composite, explosive volcanoes. If eruptions take place
offshore a line of volcanic islands form called island arc
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3. OCEANIC/OCEANIC CONVERGENCE
 Ocean trenches and island arcs are the features associated – takes place
offshore
CONTINENTAL/CONTINENTAL CONVERGENCE
 Plates forming continental crust have much lower density than underlying
layers, not much subduction where they meet
 Due to there being no subduction – no volcanic activity. Can create shallow-
focus earthquakes
Conservative margins
o Where 2 crustal plates slide past each other / movement of plates is parallel – no
destruction/creation of crust
o At these margins – no volcanic activity
o However creates stresses as they rub past each other – causes shall-focus earthquakes
such as San Andreas Fault, San Francisco
Hot Spots
In centre of pacific ocean, find Hawaiian islands
Hot spot – concentration of radioactive elements inside the mantle; plume of magma rises to eat
into plate above. When lava breaks through to the surface, active volcanoes occur above the hot
spot
Hot spot is stationary – only plates move above. Hence the row of Hawaiian islands
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4. Vulcanicity
Distribution
Most volcanic activity associated with plate tectonic processes, mainly located along plate
margins
Such activity therefore found:
o Along ocean ridges
o Associated with rift valleys
o On or near a subduction zones
o Over hot spots
Volcanic Eruptions
Vary in form, frequency and type of volcanic eruption
Intrusive volcanic landforms
o When magma forced the surface, only small amount of lava reaches that level
o Most magma is intruded into crust where it solidifies
o Often exposed after erosion
o Batholith – formed deep below surface where large amounts of magma cools and
solidifies. Large crystals then formed in rock (e.g. granite). Often dome shaped and
exposed by erosion
o Metamorphic Aureole – transformed from Batholiths
o Dykes -
o Dykes – vertical intrusions with horizontal cooling racks. Cluster of dykes called ‘dyke swarm’
o Sills – horizontal intrusions with vertical cooling racks
A diagram to show intrusive volcanic
landforms
Extrusive volcanic landforms
Involves two forms of lava
o Basaltic lava – formed when magma is low in silica. effusive
o Andesitic/rhyoltic lava – silica rick magma. More explosive
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5. Main types of extrusive volcanic landforms
o Basic/shield volcanoes – formed from free flowing lava. Have gentle sides and cover a
large area
o Fissure/lava plateaux – extensive lava flows are basaltic in nature, flow great distances
o Acid/dome volcanoes – steep sided convex cones, viscous lava that is rhyoltic
o Ash and cinder cones – formed from ash, cinders and volcanic bombs ejected from
crater
o Composite cones – classic pyramid shaped volcanoes. Consisting of layers of ash and
lava
o Calderas – occur when build-up of gases becomes extreme- huge explosions removes
summit of volcano, leaving a ‘crater’ at the top
Nature of volcanic eruptions
o Vulcanologists classify volcanoes according to nature of eruption
o Classification based on degree of violence of explosion
Minor volcanic forms
o Solfatara – small volcanic areas without cones, produced by gases (sulphurous) escaping
to surface
o Geysers – occur when water heated by volcanic activity, explodes onto surface
o Hot springs/boiling mud – sometimes water heated below does not explode on surface.
If water mixes with surface deposits, boiling mud is formed
Intrusive and extrusive volcanic activity in the UK
UK has no current volcanic activity
Granites / other examples of intruded rocks occur
across Grampians in Scotland, in Ireland and southwest
of England
o Exposed batholith in Dartmoor – known as a tor
Dykes and sills also common
o Dykes generally occur as small ridges in
landscape as more resistant than surrounding
rocks
Basaltic flows – when lava cools, vertical cracks in flow
result in hexagonal columns
Volcanic plug – build-up of magma that has solidified and blocked the top of the volcano
Impact of volcanic activity
Primary effects:
o Tephra – solid material of varying grain size – volcanic bombs to ash ejected into the air
o Pyroclastic flows – very hot (800C), gas-charged, high-velocity flows made up of a
mixture of gases and tephra. Usually have a rock avalanche too
o Lava flows
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6. o Volcanic gases – include carbon dioxide, carbon monoxide, hydrogen sulphide, sulphur
dioxide and chlorine
Secondary effects :
o Lahars – volcanic mud flows
o Flooding – melting of glaciers and ice caps
o Tsunamis – giant sea waves generated after violent caldera-forming events
o Volcanic landslides
o Climatic change – ejection of vast amounts of volcanic debris into atmosphere can
reduce global temperatures and is believed to have been an agent in past climatic
change
Volcanic effects become a hazard when they have an impact on the human and built
environments, killing and injuring people, burying and collapsing buildings, destroying
infrastructure and bringing agricultural activities to a halt
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7. Seismicity
Causes of earthquakes
As crust of Earth is mobile, tends to be a build up of stress within the rocks
When pressure is suddenly released part of the surface experience an intense shaking motion
Point of pressure release is known as focus – point immediately above on earth’s surface is
called epicentre
Depth of focus is significant and 3 broad categories of earthquake are recognised:
o Shallow focus (0-70km deep) – tend to cause the greatest damage / account for 75% of
all earthquake energy released
o Intermediate focus (70-300km deep)
o Deep focus (300-700km deep)
Seismic waves radiate from focus rather like ripples in water
Three main types of seismic wave, each travelling at different speeds:
o Primary (P) waves travel fastest and are compressional, vibrating in direction in which
they are travelling
o Secondary (S) waves travel at half speed of P waves and shear rock by vibrating at right
angles to direction of travel
o Surface (L) waves travel slowest and near ground surface. Some surface waves shake
ground at right angles to direction of wave movement / some have a rolling motion that
produces a vertical ground movement
P and S waves travel through interior of Earth and recorded on seismograph
Distribution
Vast majority of earthquakes occur along plate boundaries – most powerful at destructive
margins
Conservative margins, boundary marked by a fault – movement along which produces the
earthquake
Some earthquakes occur away from plate boundaries and are associated with reactivation of old
fault lines
Human activity could be cause of some minor earthquakes
Magnitude and frequency
Magnitude measured on 2 scales
o Richter scale – logarithmic scale
 Event measured on a 7 point scale
 Has an amplitude of seismic waves 10 times greater than once measured at 6 on
a scale
 Energy release is proportional to magnitude – for each unit increase in scale,
energy released increases by approximately 30 times
 Mercalli scale - Measures intensity of event and its impact (12 point scale)
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8. Seismic records enable earthquake frequency to be observed – records only date back to 1848
when instrument to record seismic waves was first developed
Effects of earthquakes
Initial effect is ground shaking
Severity depends on magnitude of earthquake, distance from epicentre, local geological
conditions
Secondary effects:
o Soil liquefaction when violently shaken, soils with high water content lose mechanical
strength / start to behave like fluid
o Landslides/avalanches slope failure as a result of ground shaking
o Effects of people and built environment
 Collapsing buildings
 Destruction of road systems / other forms of communication
 Destruction of service provision (gas, electricity)
 Fires from gas pipes and collapsed electricity lines
 Flooding
 Disease
 Food shortages
 Disruption to local economy
o Tsunamis giant sea waves generated by shallow-focus underwater earthquakes, volcanic
eruptions, underwater debris slides and large landslides into the sea
Tsunamis
Have a very long wavelength (sometimes more than 100km) and low wave height (under 1m),
travel quickly at speeds greater than 700km/h-1
On reaching shallow water bordering land they increase rapidly in height
When a tsunami reaches land, its effect will depend upon:
o Height of the waves / distance they have travelled
o Length of event that caused tsunami
o Extent to which warnings can be given
o Coastal physical geography, both offshore and in coastal area
o Coastal land use / population density
Effects of most tsunamis are felt at least 500-600m inland depending upon coastal geography
Buildings, roads, bridges, harbour structures, trees and soil washed away
Around 90% if all tsunamis are generated within Pacific basin / are associated with tectonic
activity taking place around its edges
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