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 convection currents flow in the asthenosphere.

• These currents provide horizontal forces on the plates of the lithosphere much as convection in a pan of boiling soup (must be Heinz tomato) causes a crouton on the surface of the water to be pushed sideways.

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 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

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

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

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

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

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

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)

Age of the Oceanic Lithosphere

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?

Convergent Boundaries

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

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.

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 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

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 the Mariana Trench

• Belt of earthquakes, shallowest closest to the trench

• Island arcs (archipelago)

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

The Himalaya’s

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

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

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

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

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

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

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