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PLATE TECTONICS Boundaries and Hotspots

Tectonics: Plate Tectonics

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Page 1: Tectonics: Plate Tectonics

PLATE TECTONICS

Boundaries and Hotspots

Page 2: Tectonics: Plate Tectonics

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

Page 5: Tectonics: Plate Tectonics

Divergent Boundaries Start intra-plate

(current e.g. the Rift Valley)

Upwelling of magma in a plume – driven by thermo nuclear reactions at the core mantle boundary

Plume rises and convection starts at the base of the lithosphere

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Rifting

As convection commences the plate is thinned out by a series of extension faults.

As the plate becomes thinner, volcanoes and lakes start to form in the valley (NB East Africa)

More volcanoes continue to form until a complete ridge exists, the plate is forced apart and new oceanic (basaltic) crust begins to form on either side

As the lakes connect, and the level of the land drops, the ocean floods the valley and forms an elongate sea such as the Red Sea

Page 7: Tectonics: Plate Tectonics

The Rift Valley and Associated Features

Eventually a new plate will form (it will be called the Somalia Plate, and the rest of Africa, the Nubia Plate.

The sea will flood the valley and connect to the Red Sea

Ensure you label these directions of movement onto your maps

Page 8: Tectonics: Plate Tectonics

The Mid Ocean Ridges

The lines of submarine volcanoes that form a continuous feature throughout the worlds oceans

These are the centres of spreading, and the youngest oceanic rocks are found closest to the ridge

Page 9: Tectonics: Plate Tectonics

Transform Faults The spreading does

not occur at the same rate along the ridge

Transform faults offset the ridge giving it a blocky appearance

Distance between the faults varies, and they are responsible for many undersea earthquakes, but the lack of vertical displacement rarely generates Tsunamis

Page 10: Tectonics: Plate Tectonics

Islands on the Ridges

Particularly large volcanoes can become islands – they do sink as they move away from the ridge to become Seamounts and Guyots

Iceland is the largest MOR island – it is believed there is a plume beneath the island causing intense volcanism

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Hazards Frequent low grade seismicity

(earthquakes below 5 on the Richter Scale)

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 over opening and closing

The world is a fixed size, as such if new crust is created, crust somewhere else must also be destroyed

Hence there are subduction 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 the features and hazards you would expect to find on each

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Ocean vs Ocean Stage 1 – Subduction

produces partial melting and the formation of a chain of volcanoes. An Island Arc

Stage 2 – Build up of intrusive and extrusive igneous material, mixed with sediments forms larger Islands

Stage 3 – The Island Chains join up and sediments form an Accretionary wedge or prism which builds on to the fore arc area.

Page 16: Tectonics: Plate Tectonics

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)

Page 17: Tectonics: Plate Tectonics

Oceanic vs. Continental

Stage 1 – Weight of continental sediments causes subsidence of the crust

Stage 2 - Subduction develops and an Island Arc forms

Stages 3&4 Compression from Plate movement causes the crust to thicken and shorten through Folding and Thrust Faulting. Fold Mountain Chain is formed. Ie The Andes

Page 18: Tectonics: Plate Tectonics

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)

Page 19: Tectonics: Plate Tectonics

Characteristics of Subduction

Zones

Trenches – generally 5-8 km deep, up to 11km – deepest is the Mariana Trench

Belt of earthquakes, shallowest closest to the trench

Island arcs (archipelago)

Page 20: Tectonics: Plate Tectonics

Continental Collision

Note: This process of Mountain Building is often referred to as an Orogeny, and the mountains as “Orogenic Belts”

The best case study of this type of collision is in the Himalaya, where the process is still active today

Page 21: Tectonics: Plate Tectonics

The Himalaya’s

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Brief History

India has moved northwards over the last 100 ma.

It started well below the equator and an ocean existed to its north called the Tethys

The Subduction of the oceanic lithosphere thickened the plate to 100km and forced up the fold mountains

Page 23: Tectonics: Plate Tectonics

Hazards

The plate has moved very rapidly – 200mm/yr

Earthquakes are a major hazard

The crust is too thick for diapirs of magma to make it to the surface so there are no volcanoes

A significant hazard is the potential for mass movement. Given India's tropical/desert location, the young mountains being steep and the likelihood of earthquakes the risk is high, added to that is the development of low quality housing on slopes and monsoonal weather and events such as Chittagong become high risk

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Conservative Boundaries No subduction or

abduction Plates try to move

laterally past each other

Most famous – the San Andreas Fault

No volcanism Generates massive

earthquakes

Page 25: Tectonics: Plate Tectonics

Hotspots These are not

boundaries as such, but a surface representation of mantle plumes

Plates do not always split above mantle plumes, but as the plate moves over the magma source, it generates a chain of Islands

The orientation of the island chains gives the direction of plate movement relative to the plume

Page 26: Tectonics: Plate Tectonics

Hawaii

Chain of islands, connected to the Emperor Seamount chain (submerged)

The Islands become older to the North West indicating the plate is moving NW.

This was anticipated before the discovery of continental drift as the islands become shallower in gradient, more advanced in vegetation succession and have deeper soils 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 popular tourist 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 is otherwise now widely accepted

Page 28: Tectonics: Plate Tectonics

Today’s activity

A3 sheet – you should have taken notes on the different types of boundaries, make sure you have enough detail to revise from

The sheet has boxes for all the major surface expressions of plate tectonics – describe each one and the use the atlases and textbooks in the room to locate our major case studies for the rest of the year. Enter a very short description into each box

Plate movements – familiarise yourself with the plate movements and label them onto your map with the names of the plates, the large poster map will give you most of the information