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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

o Hot spots – generate thermal convection currents within asthenosphere

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,

aluminium, oxygen (SIAL) Mainly basalt; silicon, magnesium, oxygen (SIMA)

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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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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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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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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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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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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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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