tectonics: plate tectonics introduction

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An overview of the key evidence, boundary types, volcanoes and hazards.

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  • 1. Plate tectonics
    Boundaries and Hotspots

2. 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
3. 4. How do we know this?Part 2 New evidence
The Mid Atlantic ridge (1948)
Paleomagnetism and the reversal of the earths magnetic field (1950s)
Sea floor spreading dating of the sea floor rocks (1960s)
5. 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.
6. 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
7. Boundary types
Divergent plates moving apart
Convergent three potential combinations:
Oceanic vs. Oceanic
Continental vs. Oceanic
Continental vs. Continental
Conservative movement is lateral only
8. Distribution
9. 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
10. 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
11. 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
12. 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
13. 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
14. 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
15. 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)
16. Age of the Oceanic Lithosphere
17. 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?
18. 19. Convergent Boundaries
There are three types and you need to know the features and hazards you would expect to find on each
20. 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.
21. 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)
22. 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)
23. 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
24. 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)
25. 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)
26. 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
27. The Himalayas
28. 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
29. 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
30. 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
31. 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
32. 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
33. 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
34. Todays 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

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