Movements on the Earth’s Surface

1. Movements on the Earth’s Surface Plate tectonics and the Earth’s Plate Boundaries

2. What is the Earth Made of? • If you could make a journey deep inside the Earth, you would find that it is made of several layers: • The crust • The mantle • The core

3. The Crust • The crust (or lithosphere) is the outer layer of the Earth which is between 7-50km thick • It is the thin outer coating of the planet • It is not smooth • 70% of it is covered by the ocean, so the crust is not just the land that we see • It is thickest under the continents and thinnest under the oceans

4. The Mantle • The mantle is below the crust and its about 2800km thick • The temperatures near the crust are 500°C and at the bottom of the mantle can reach up to 3000°C • The bottom of the mantle is solid, but the top rocks move slowly because of convection currents • It is the source of our volcanoes and earthquakes

5. The Core • The core is the centre of the Earth • It consists of an outer core and an inner core • The outer core is mainly metal, not rock; and is between 4000°C to 6000°C • The outer core gives the Earth its north and south poles and magnetic field • The inner core is almost 10 000°C, but it does not boil because of the weight of the rest of the Earth pushing on it

7. The Moving Crust • The crust is broken into a number of pieces called tectonic plates • These plates float on magma at the top of the mantle • The speed of movement can be between 1cm and 10cm per year

8. Plate Tectonics • Plate tectonics is a combination of two theories: continental drift and sea-floor spreading • Continental drift is the idea that the continents are continually moving and have significantly changed positions over millions of years • The theory of sea floor spreading proposes that the middle of the ocean is spreading apart, moving very slowly in opposite directions

9. Continental Drift • For many years people noted the similarities in shape between the coastlines of Africa and South America and that they seemed to fit together like a jigsaw puzzle • A German meteorologist named Alfred Wegener put this idea with a range of other evidence into a book outlining a theory we know today as continental drift • He proposed that all the continents fitted together into a giant continent known as Pangea

10. Pangea was a supercontinent that was proposed to have existed 220 million years ago • When it started to break up, the continents drifted apart as they moved through the oceanic crust • This was backed up with evidence coastlines that fit, similar fossils, rocks and landforms that were created by glaciers

11. Tectonic Plate Movement • We know that it is not the continents themselves that are moving but rather continental and oceanic crust moving • These areas are called plate tectonics • The movement of these plates explains the existence of continental shelves and deep trenches • It also explains earthquakes and volcano distributions

12. Sea Floor Spreading • The idea of the sea-floor spreading was suggested by Harry Hess • His evidence came from the discovery of the Mid-Atlantic Ridge, a continuous mountain range in the middle of the Atlantic Ocean • Hess suggested convection currents deep inside the mantle caused spreading

13. Hess proposed that new rocky crust was being formed at the ocean ridges and spreading outwards, later called seafloor spreading • Hess proposed that the crust was sinking down into the Earth in other places called ocean trenches • The process of the crust sinking down is called subduction

14. Magnetic Striping • When molten rock solidifies, all the magnetite particles of any size line up in the magnetic field pointing in the same direction • During WWII, the US navy discovered there were bands of alternating strong and weak magnetism on the sea floor • They found the bands were parallel to the mid-ocean ridges • These bands were pointing north, then changed to pointing south, showing the Earth’s magnetic field changed every few million years

15. These patterns of stripes of rocks with alternating magnetism are called magnetic striping • The patterns on either side of the ridge were symmetrical – rocks at a particular distance from the ridge on one side always had the same magnetic direction as rocks the same distance away on the other side

16. Convection Currents Practical Exercise

17. What happens at Plate Boundaries The types of plate movement

18. Transform Boundaries • One plate can slide past another along a single fault line • This is called a transform boundary • A fault is a fracture in rock where movement has occurred

19. The two plates involved can become jammed over a period of time until the pressure builds up and the plate slips • The slipping causes earthquakes • Example: The San Andreas Fault in California • Example: The fault line that runs through New Zealand

21. Converging Boundaries • At a converging plate boundary, two plates move towards each other • There are generally three types of converging plate boundaries, depending on the plates involved • Mountain ranges, volcanoes and trenches can all be formed

22. Ocean to Continent Collision • When oceanic crust collides with continental crust, the oceanic landform is pushed downwards into the mantle • This is known as a subduction zone • It creates a line of mountains and also volcanoes as heat rises through the cracks in the crust • An ocean trench will form at the line of plate contact

23. Continent to Continent Collision • When two continental plates collide, they have similar densities, so no subduction takes place • Instead, the edges of the two plates crumple and fold into high mountain ranges

24. Ocean to Ocean Collision • When two oceanic plates collide, the older denser crust will subduct below the newer crust, creating a deep ocean trench • The subduction also creates a line of undersea volcanoes that may reach above the ocean surface as an island arc

25. The Mariana Trench • It is the deepest part of the world’s ocean • Its current depth is estimated at 10 971m • The deepest part of the trench is called Challenger Deep and is a small slot-shaped valley in its floor • It forms the boundary between two tectonic plates: the Pacific Plate and the small Marina Plate

26. Diverging Boundaries • Diverging boundaries or spreading plat boundaries form different features to converging and transform boundaries • These spreading boundaries can occur in the middle of the ocean or in the middle of land • When the plates separate, there is a rift between them • Magma rises up and cools to form a new crust

27. Plates and Currents Teacher Demonstration

28. Mars Bar Prac Plate tectonics

29. Volcanic Eruptions • Volcanoes form where there are weak spots in the Earth’s crust and where extremely hot molten rock called magma has gathered below the weak spots • This magma occasionally pushes upwards under great pressure into the volcano • When magma reaches the surface it is called lava which changes colour as it cools from white through to orange and red, until it becomes black and becomes a rock.

30. The eruption can be explosive when the magma: • is viscous (flowing very slowly) • contains a lot of water and gas • The explosion can throw out “volcanic bombs” of rock called scoria • Occasionally there can be a pyroclastic flow, where a cloud of ash, rock and gas at about 500°C rushes down the volcano like an avalanche at over 100km/h

31. Ring of Fire

32. The large series of volcanoes encircling the Pacific Oceanare referred to as the Ring of Fire, and notorious for frequent earthquakes and volcanic eruptions.  • The Ring of Fire contains over 450 volcanoes and is home to approximately 75% of the world's active volcanoes. • Nearly 90% of the world's earthquakes occur along the Ring of Fire; most recently, the devastating quakes in Chile, Japan and New Zealand.  • Volcanoes and Earthquakes of note:Christchurch Earthquake, New Zealand Mount Saint Helens, Washington, USA Mount Pinatubo, Philippines Mt. Fuji, Japan Paricutin Volcano, Mexico Santiago Earthquake, Chile Sendai Earthquake, Japan

33. What causes an Earthquake? • An earthquake is the rapid movement of the ground, usually back and forth and up and down in a wave motion • It is caused by the rapid release of energy as the tectonic plates move • Friction between the plates must be overcome before they can move. • When the force is great enough, the plates suddenly move as friction can no longer hold them • This sudden movement sends out waves of energy through the rock and the water, which then shake as waves of energy pass through them

34. Detecting earthquakes • Earthquakes are measured using an instrument called a seismometer • The trace of a seismometer is called a seismograph • The movement of the ground in an earthquake occurs in a shaking back-and-forth motion called a wave. • These waves in the Earth caused by the earthquakes are called seismic waves

36. Primary waves (P waves) are longitudinal waves that travel fast through the Earth • Secondary waves (S waves) are transverse waves that travel slightly slower than P-waves through the Earth • Surface waves are the slowest waves and cause the most destruction • S-waves and P-waves travel deep under the ground and then bend upward to reach the surface of the crust • P-waves shake the ground up and down • S-waves shake the ground sideways, back and forth • They are destructive if the earthquake is near the Earth’s surface

37. Epicentres and Foci • Earthquakes happen at particular places under the ground where the Earth slips, usually along a fault • The place where the quake starts is called the focus, which may be hundreds of kilometres deep in the Earth • The point on the Earth’s surface directly above the focus is called the epicentre • Buildings near the epicentre are usually the most heavily damaged

38. The severity of an earthquake is calculated in several different ways • One early method, still used in some cases today, is measured on the Richter scale • This scale goes between 1 and 9 • Each successive number is thirty times more energy released than the previous number • An earthquake measuring less than 2.0 is known as a microquake and is rarely detected by people • So an earthquake measuring 5.0 is thirty times more energy than one measuring 4.0

39. The Effects of Earthquakes • Earthquakes can cause damage to buildings on land, and can cause landslides • They can also cause destruction in the ocean • An earthquake in the ocean can cause a huge wave called a tsunami • These waves can be 100m high and cause massive destruction if they collide with the land near where people are living