Plate Tectonics

1. Plate Tectonics

2. Objectives • Describe and draw the internal workings of the earth • Explain the structure of the earth’s crust • Describe and explain the tectonic cycle • Explain the theory of plate tectonics • Understand the formation of landforms (features) associated with plate margins.

3. The Earth Keywords • Crust • Mantle • Lithosphere • Asthenosphere • Outer core • Inner core

4. KEYWORDS STRUCTURE OF THE EARTH • Crust – outer part of the earth. Very thin • Mantle – middle part of the earth semi molten rock. Moves plates by convection currents. • Lithosphere- marks change from crust to mantle (very thin) • Asthenosphere – piece of the mantle nearest the crust. • Outer core – iron and nickel (liquid) • Inner core – iron and nickel (solid)

5. Structure of the Earth

6. Internal Structure of Earth • Crust – solid rock, aka lithosphere, 3-10km thick under ocean, 70km thick under continents • Mantle – semi-molten rock (magma), aka asthenosphere, convection currents cause plate movements • Outer core – molten rock mainly consisting of iron • Inner core – up to 5000°C, solid nickel & ironl.

7. The Earth’ s Crust • What do you know about the earth’s crust from Junior Cert??

8. The Earth’s Crust Keywords • Oceanic Crust • Continental Crust • Plates – When the crust is broken up and splits into large pieces of rock. • Plate Tectonics – The study of plates, their movements and the landforms that are created. • Tectonic cycle – when plates collide and separate.

9. Plate tectonics keywords • Wegner • Pangaea – name given to the earth. One large area of land. • Continental Drift – the moving of plates, land and oceans. • Sea Floor Spreading – new crust being created. • Hess

10. Theory of Plate Tectonics – Continental Drift • Alfred Wegener (1912) – single land mass called Pangaea, broke to form todays continents • Theory backed by similar rocks & fossils found in Brazil and west Africa • However, Wegener could not explain how the continents moved apart

11. Theory of Plate Tectonics – Sea Floor Spreading • Harry Hess (1960s) studied the age of rocks in the Atlantic Ocean • The newest rocks were found nearest the Mid-Atlantic Ridge, while the oldest were found closer to the US coast • Theory states that the sea floor is constantly expanding as new magma forces its way up between the plates

12. Tectonic Cycle • Crustal plates move due to convection currents in the mantle • These currents are powered by the Earth’s core, causing magma to heat, rise, cool and sink in a circular motion • This movement pulls the plates apart (diverging/separating) or together (converging/colliding) • New crust is created where plates separate • When plates collide, the front of the heavier plate gets dragged underneath the lighter plate due to gravity – this is known as SUBDUCTION

13. Video Clips • http://www.youtube.com/watch?v=QDqskltCixA&sns=em • http://www.youtube.com/watch?v=QDqskltCixA&sns=em • Continental Drift http://www.youtube.com/watch?v=GYVS_Yh6dTk&feature=fvwrel

14. PLATE BOUNDARIES KEYWORDS • Destructive Plate Boundary (Convergent) • Constructive Plate Boundary (Divergent) • Conservative (transverse or transform) plate boundary

15. Plate Boundaries There are three types of plate boundaries. • Destructive/convergent (land destroyed) • Constructive/divergent (land created) • Conservative/transform (land is not destroyed or created)

16. 1. Destructive Plate Boundaries Plates move together and collide destroying the crust. This happens in 3 situations. • Between oceanic and continental plates • Between two oceanic plates • Between two continental plates

17. Oceanic – Continental Plate Collision= Subduction zones • Subduction occurs • The melted crust turns to magma and moves up through the continental crust to create some of the most explosive volcanoes in the world e.g Mount St. Helen in the USA • Where the two colliding plates meet a deep trench is created. (trenches mark all subduction zones). • The Pacific Ring of Fire is so named because many active volcanoes are created at subduction zones mark the edge of the Pacific plate. Eg. Mount Fuji.

18. Oceanic – oceanic plate collision = trenches and island arcs • When two oceanic plates collide one is usually subducted beneath the other. When one plate goes beneath the other deep trenches form. These are the deepest part of the ocean. • Marked by curved zones of volcanic islands known as island arcs. These volcanoes form when the subducted plate descends and melts, creating rising bubbles of magma that break through the crust above. • These active volcanoes have built up on the sea floor over millions of years until they appear above the surface of the water. E.g Japanese islands.

19. Continental – ContinentalFold Mountain • When 2 continental plates collide fold mountains are created. • Himalayas and the Alps. • Fold mountians are often composed of sedimentary rocks such as limstone that formed beneath the sea. • When two continental plates collide subduction does not occur . Colliding continental plates tend to move upwards or sideways, this can sometimes cause devasting earthquakes e.g Pakistan 2005.

20. Activity • Draw a diagram of a destructive plate boundary

21. 2. Constructive (divergent) plate boundaries = mid ocean ridges • These occur where plates are pulled apart by the convection currents below. • As the crust splits, a rift valley is formed allowing magma to rise and fill the gap creating new crust. Shallow earthquakes and volcanoes also occur e.g. Iceland 2010. • As magma becomes solid it forces the plates apart even further allowing more magma to rise. • Over millions of years the ocean floors have been created this way. (Sea floor spreading) • Eg. Mid Atlantic Ridge. The crust is moving about 5cm per year. 25km per million years. • E.g. Sea floor spreading in Iceland. Iceland formed where the mid atlantic ridge appears above the sea. Iceland is getting wider due to the process of sea floor spreading. It has over 200 active and dormant volcanoes.

22. Activity • Draw the diagram of a constructive plate boundary

23. 3. Conservative (transform or transverse) = fault lines • Where two plates slide past each other. • These boundaries are marked by fault lines. They are thousands of kilometers long and up to 8km deep. E.g San Andreas Fault in California. (1,300km long). • At this fault the Pacific plate and the North American fault are both moving in a north west direction. • The Pacific plate is moving at a faster pace (5cm per year) this is not smooth and the plate may stick . • This leads to a build up of pressure between the two plates until they suddenly go past each other causing an earthquake. • In the 1906 San Francisco earthquake the Pacific plate moved over 6m in one minute. • The Haiti 2010 earthquake occurred on a transform boundary between the Caribbean and North American plate.

24. Activity • Draw the diagram of a conservative boundary.

25. Tectonic Activity in the middle of plates = hotspots • Volcanoes can be found thousands of miles from the edges of plates. • Occur where high temps in the mantle produce a rising fountain (plume) of a molten rock known as a hotspot. • Hotspots are extremely hot areas deep in the upper mantle (asthenosphere). The magma rises up and pushes through the plate above. This creates a volcanoe. E.g Hawaiian Islands

26. The Hawaiian Islands • A chain of volcanic islands in the middle of the Pacific plate located over 3,200km from the nearest plate boundary. • As the pacific plate moves over the hotspot volcanoes grow, become dormant and eventually become extinct. • Hawaiian hotspot is about 500 – 600km wide and approx 86million years old.

27. Summary Video • http://www.youtube.com/watch?v=KCSJNBMOjJs&sns=em 55 mins long.