1. The Structure of the Earth and Plate Tectonics

2. Structure of the Earth The Earth is made up of 3 main layers: –Core –Mantle –Crust Inner core Outer core Mantle Crust

3. The Crust This is where we live! The Earth’s crust is made of: Continental Crust - thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old Oceanic Crust - thin (~7 km) - dense (sinks under continental crust) - young

4. How do we know what the Earth is made of? Geophysical imaging- when there is an earthquake it sends out seismic waves (shock waves) through the Earth. Seismologists can measure the time it takes for these waves to reach seismic monitoring stations set up around the globe. (The machine that measures seismic waves is called a seismometer). The different layers in the earth have been inferred using the time of travel of refracted and reflected ( bent backward angularly) seismic waves created by the earthquakes (see left diagram). That is, changes in the seismic velocity occur as the waves pass through different materials. Measuring these changes tell seismologists how many layers there are and the thickness and physical properties of each layer. We need not wait for earthquakes to occur, on a local scale on land (cheap but slow methods) and at sea (more expensive but quicker) explosions can be set to cause shock waves to pass through the crust (simulating an earthquake) that can be measured in the same way. Other geophysical methods, for example measuring different gravity, magnetic and electrical anomalies by air and (or) satellite can help to reconstruct shallow crustal features. We can also go and examine rocks at and near the surface of the crust, through fieldwork, drilling boreholes and mining. Geophysical surveys: seismic, gravity, magnetics, electrical –Acquisition: land, air, sea and satellite –Geological surveys: fieldwork, boreholes, mines

5. What is Plate Tectonics?

6. If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

7. Plate Tectonics The Earth’s crust is divided into 12 major plates which are moved in various directions. This plate motion causes them to collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic set of Earth structures or “tectonic” features-”, like mountain ranges, volcanoes and (or) rift valleys. The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

8. World Plates

9. What are tectonic plates made of? Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle.

10. What lies beneath the tectonic plates? Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.

11. Plate Movement “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

12. Practical Exercise 1 Supercontinents!

13. What happens at tectonic plate boundaries?

14. Divergent Convergent Transform Three types of plate boundary

15. Spreading ridges –As plates move apart new material is erupted to fill the gap Divergent Boundaries

16. Age of Oceanic Crust Courtesy of www.ngdc.noaa.gov

17. Iceland has a divergent plate boundary running through its middle Iceland: An example of continental rifting

18. There are three styles of convergent plate boundaries –Continent-continent collision –Continent-oceanic crust collision –Ocean-ocean collision Convergent Boundaries

19. Forms mountains, e.g. European Alps, Himalayas Continent-Continent Collision

20. Himalayas Example: India used to be an island, but about 15 million years ago it crashed into Asia (see map). As continental crust was pushing against continental crust the Himalayan mountain belt was pushed up. “Mountains” were also pushed down into the mantle as the normally 35 km thick crust is approximately 70 km thick in this region. Mt Everest is the highest altitude mountain on our planet standing 8,840 metres high. This means that below the surface at the foot of the mountain the crust is a further 61 km deep!!

21. Called SUBDUCTION Continent-Oceanic Crust Collision At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust. This is called a Subduction Zone.

22. Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes Subduction

23. When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches. –E.g. The Mariana Trench is 11 km deep! Ocean-Ocean Plate Collision

25. Where plates slide past each other Transform Boundaries Above: View of the San Andreas transform fault is built is an example of a transform boundary between the Pacific plate and the North American plate.

27. …what’s the connection? Volcanoes and Plate Tectonics…

28. Red dots= active volcanos. Volcanism is mostly focused at plate boundaries Pacific Ring of Fire

29. - Subduction - Rifting - Hotspots Volcanoes are formed by:

30. Pacific Ring of Fire Hotspot volcanoes

31. Hot mantle plumes breaching the surface in the middle of a tectonic plate What are Hotspot Volcanoes? Photo: Tom Pfeiffer / www.volcanodiscovery.com The Hawaiian island chain are examples of hotspot volcanoes. The source of this volcanism is a mantle plume of hot mantle material rising up from near the core- mantle boundary through the crust to the surface (see left diagram).

32. The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other.

33. …what’s the connection? Earthquakes and Plate Tectonics…

34. As with volcanoes, earthquakes are not randomly distributed over the globe At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Figure showing the distribution of earthquakes around the globe

35. Where do earthquakes form? Figure showing the tectonic setting of earthquakes

36. Plate Tectonics Summary The Earth is made up of 3 main layers (core, mantle, crust) On the surface of the Earth are tectonic plates that slowly move around the globe Plates are made of crust and upper mantle (lithosphere) There are 2 types of plate There are 3 types of plate boundaries Volcanoes and Earthquakes are closely linked to the margins of the tectonic plates