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Plate tectonics

Boundaries and Hotspots



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





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)



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.



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



Boundary types

• Divergent – plates moving apart

• Convergent – three potential combinations:

– Oceanic vs. Oceanic

– Continental vs. Oceanic

– Continental vs. Continental

• Conservative – movement is lateral only



Distribution



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



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





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



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



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



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)



Age of the Oceanic Lithosphere



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?





Convergent Boundaries

• There are three types and you need to know thefeatures and hazards you would expect to find on each



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.



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)



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)



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



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)



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)



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



The Himalaya’s



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



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



Conservative Boundaries

• No subduction or

abduction

• Plates try to move

laterally past each other

• Most famous – the SanAndreas Fault

• No volcanism

•Generates massiveearthquakes



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



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



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



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

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