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Plate tectonics
Boundaries and Hotspots
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How do we know this?
Part 1 – older evidence
• Biology – same fossils in different parts of the
world
• Geology – same rocks in different parts of the
world
• Climate – fossils of plants that live in tropical
conditions in Antarctica
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How do we know this?
Part 2 – New evidence
• The Mid Atlantic ridge (1948)
• Paleomagnetism and the reversal of the
earths magnetic field (1950’s)
• Sea floor spreading – dating of the sea floor
rocks (1960’s)
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Convection – driving forces
• Very slow convectioncurrents flow in theasthenosphere.
•
These currents providehorizontal forces on theplates of the lithospheremuch as convection in apan of boiling soup (mustbe Heinz tomato) causes a
crouton on the surface ofthe water to be pushedsideways.
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Re-cap
• 6 major pieces of evidence for plate
tectonics/continental drift
• Biological, Geological, fit of the continents,
MOR discovery, absolute dating of rocks,
mapping of the sea floor
• Two types of plates – Continental and Oceanic
• Structure of the Earth
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Boundary types
• Divergent – plates moving apart
• Convergent – three potential combinations:
– Oceanic vs. Oceanic
– Continental vs. Oceanic
– Continental vs. Continental
• Conservative – movement is lateral only
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Distribution
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Divergent Boundaries
•Start intra-plate(current e.g. the RiftValley)
• Upwelling of magma in
a plume – driven bythermo nuclearreactions at the coremantle boundary
• Plume rises andconvection starts at thebase of the lithosphere
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Rifting
• As convection commences the plate is thinnedout by a series of extension faults.
• As the plate becomes thinner, volcanoes andlakes start to form in the valley (NB East Africa)
• More volcanoes continue to form until acomplete ridge exists, the plate is forced apartand new oceanic (basaltic) crust begins to formon either side
• As the lakes connect, and the level of the landdrops, the ocean floods the valley and forms anelongate sea such as the Red Sea
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The Mid Ocean
Ridges• The lines of submarine
volcanoes that form a
continuous featurethroughout the worlds
oceans
• These are the centres of
spreading, and the
youngest oceanic rocks
are found closest to the
ridge
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Transform Faults
•
The spreading does notoccur at the same rate
along the ridge
• Transform faults offset
the ridge giving it a blockyappearance
• Distance between the
faults varies, and they are
responsible for manyundersea earthquakes,
but the lack of vertical
displacement rarely
generates Tsunamis
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Islands on the Ridges
• Particularly large volcanoescan become islands – theydo sink as they move awayfrom the ridge to becomeSeamounts and Guyots
• Iceland is the largest MORisland – it is believed there isa plume beneath the islandcausing intense volcanism
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Hazards• Frequent low grade seismicity
(earthquakes below 5 on the RichterScale)
• Intense and frequent volcanism
• Ultra slow spreading is 10mm/yr(Atlantic), ultrafast 100mm/yr (East
Pacific and Galapagos)
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Age of the Oceanic Lithosphere
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Opening and closing of Oceans
• Rifting is the control overopening and closing
• The world is a fixed size,as such if new crust is
created, crust somewhereelse must also bedestroyed
• Hence there aresubduction zones at the
edges of most continents• Where will the next one
open up?
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Convergent Boundaries
• There are three types and you need to know thefeatures and hazards you would expect to find on each
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Ocean vs Ocean
• Stage 1 – Subduction producespartial melting and theformation of a chain ofvolcanoes. An Island Arc
• Stage 2 – Build up of intrusive
and extrusive igneous material,mixed with sediments formslarger Islands
• Stage 3 – The Island Chains join
up and sediments form anAccretionary wedge or prismwhich builds on to the fore arcarea.
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Hazards
• Volcanoes – wide variety of types, explosive
and effusive
• Earthquakes – can be very powerful
• Tidal waves – offshore earthquakes generate
waves and the steep islands often suffer
inundation, the other cause is collapse of
oceanic islands generates regular waves
(Stromboli)
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Structure of Subduction Zones
• Island Arcs (Archipelago) are often curved – i.e.
the Aleutian Islands, the Philippines, Malaysia,
Indonesia and the Aeolian Islands (small scale)
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Oceanic vs.
Continental
• Stage 1 – Weight ofcontinental sediments causessubsidence of the crust
• Stage 2 - Subduction developsand an Island Arc forms
• Stages 3&4 Compression fromPlate movement causes thecrust to thicken and shorten
through Folding and ThrustFaulting. Fold Mountain Chainis formed. Ie The Andes
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3D view
• Mt St Helens is formed in this way, the Cascades
represent the curved line of volcanoes just inland
on the Western Seaboard of the US.
• Hazards: Very explosive Volcanoes (MSH,
Popocatepetl – no really)
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Characteristics of
Subduction
Zones• Trenches – generally 5-
8 km deep, up to
11km – deepest is theMariana Trench
• Belt of earthquakes,
shallowest closest to
the trench
• Island arcs
(archipelago)
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Continental
Collision
• Note: This process ofMountain Building isoften referred to asan Orogeny, and the
mountains as“Orogenic Belts”
• The best case studyof this type of
collision is in theHimalaya, where theprocess is still activetoday
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The Himalaya’s
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Brief History
• India has movednorthwards over the last100 ma.
• It started well below the
equator and an oceanexisted to its north calledthe Tethys
• The Subduction of the
oceanic lithospherethickened the plate to100km and forced up thefold mountains
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Hazards
• The plate has moved veryrapidly – 200mm/yr
• Earthquakes are a majorhazard
• The crust is too thick fordiapirs of magma to make itto the surface so there areno volcanoes
• A significant hazard is thepotential for massmovement. Given India'stropical/desert location, the
young mountains beingsteep and the likelihood ofearthquakes the risk is high,added to that is thedevelopment of low quality
housing on slopes andmonsoonal weather andevents such as Chittagongbecome high risk
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Conservative Boundaries
• No subduction or
abduction
• Plates try to move
laterally past each other
• Most famous – the SanAndreas Fault
• No volcanism
•Generates massiveearthquakes
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Hotspots
• These are notboundaries as such, buta surface representationof mantle plumes
• Plates do not always
split above mantleplumes, but as the platemoves over the magmasource, it generates achain of Islands
•
The orientation of theisland chains gives thedirection of platemovement relative tothe plume
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Hawaii
• Chain of islands, connectedto the Emperor Seamountchain (submerged)
• The Islands become older to
the North West indicatingthe plate is moving NW.
• This was anticipated beforethe discovery of continentaldrift as the islands become
shallower in gradient, moreadvanced in vegetationsuccession and have deepersoils to the NW
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Other Hotspots• Yellowstone Caldera is probably the biggest and highest risk (see
“Supervolcano” the thrilling docu-drama), Iceland is unusual being a large
plume underneath a divergent boundary. Note the overlap with populartourist destinations!
• Note: the mantle plume theory is still hotly debated and poorly understood – it is one of the few remaining mysteries of tectonic theory, which isotherwise now widely accepted
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Today’s activity
• A3 sheet – you should have taken notes on thedifferent types of boundaries, make sure you haveenough detail to revise from
• The sheet has boxes for all the major surface
expressions of plate tectonics – describe each one andthe use the atlases and textbooks in the room to locateour major case studies for the rest of the year. Enter avery short description into each box
• Plate movements – familiarise yourself with the plate
movements and label them onto your map with thenames of the plates, the large poster map will give youmost of the information