1. Plate Tectonics: Earth's Plates and Continental Drift

2. Some questions we will answer:
How is the earth always changing?
What forces inside the earth create and change landforms on the surface?
What is the theory of plate tectonics and how does it work?
What two theories help make up the theory of plate tectonics?
What is continental drift and sea floor spreading?
What happens when the plates crash together, pull apart, and slide against each other?

3. The Earth’s Layers
The Earth is made of many different and distinct layers. The deeper layers are composed of heavier materials; they are hotter, denser and under much greater pressure than the outer layers.
Natural forces interact with and affect the earth’s crust, creating the landforms, or natural features, found on the surface of the earth.

4. Before we start to look at the forces that contribute to landforms,lets look at the different layers of the earth that play a vital role in the formation of our continents, mountains, volcanoes, etc.

5. Crust
Mantle
Outer Core
Inner Core
crust - the rigid, rocky outer surface of the Earth, composed mostly of basalt and granite. The crust is thinner under the oceans.
mantle - a rocky layer located under the crust - it is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. Convection (heat) currents carry heat from the hot inner mantle to the cooler outer mantle.
outer core - the molten iron-nickel layer that surrounds the inner core.
inner core - the solid iron-nickel center of the Earth that is very hot and under great pressure.

6. The Crust Continental 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
The Earth has two different types of crust: Continental crust and Oceanic crust. Each has different properties and therefore behaves in different ways.
Continental crust:
Continental crust forms the land (the continents, as the name suggests) that we see today.
Continental crust averages about 35 km thick. Under some mountain chains, crustal thickness is approximately twice that thickness (about 70 km thick).
- The mountains we see on earth have deep roots in the crust that we can’t see. The crust “floats” on the more dense mantle and, like how only the tip of an iceberg sticks up out of the water, we see only the tip of the continental crust - the mountain ranges.
Continental crust is less dense and therefore more buoyant than oceanic crust
Continental crust contains some of the oldest rocks on Earth.
- Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) [Ga = billion years ago] and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley in the USA ( billion years), in Swaziland ( billion years), and in Western Australia ( billion years).
Oceanic crust:
As the name already suggests, this crust is below the oceans.
Compared to continental crust, Oceanic crust is thin (6-11 km).
It is more dense than continental crust and therefore when the two types of crust meet, oceanic crust will sink underneath continental crust.
The rocks of the oceanic crust are very young compared with most of the rocks of the continental crust. They are not older than 200 million years.

7. 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.
Plates are made of rigid lithosphere – formed of the crust and the extreme upper mantle (point out these layers on the figure).

8. What lies beneath the tectonic plates?
Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.
The asthenosphere, beneath the lithosphere, is part of the upper mantle and is so hot that it is 1 – 5% liquid (I.e. 95 – 99% solid). This liquid, usually at the junctions of the crystals, allow it to flow – which is why ‘astheno’ means weak.’ Beneath the asthenosphere is the rest of the mantle, which is completely solid – but can also flow (on geological time scales) because of the intense temperatures and pressures involved.
The base of the lithosphere-asthenosphere boundary corresponds approximately to the depth of the melting temperature in the mantle.

9. Plate Movement
“Plates” of lithosphere are moved around by the underlying hot mantle convection cells
How and Why do tectonic Plates move around?
The question of how tectonic plates are moved around the globe is answered by understanding mantle convection cells.
In the mantle hot material rises towards the lithosphere (like hot air rising out of an open oven - ever opened an oven door and felt the blast of hot air coming past your face?). The hot material reaches the base of the lithosphere where it cools and sinks back down through the mantle. The cool material is replaced by more hot material, and so on forming a large “convection cell” (as pictured in the diagram).
This slow but incessant movement in the mantle causes the rigid tectonic plates to move (float) around the earth surface (at an equally slow rate).

10. Land List the continents in your notes.
The large landmasses in the oceans are called continents.
List the continents in your notes.
Landforms are commonly classified according to differences in relief. The relief is the difference in elevation between the highest and lowest points. Another important characteristic is whether they rise gradually or steeply.
The major types of landforms are mountains, hills, plateaus, and plains.

11. Most people know that Earth is moving around the Sun and that it is constantly spinning.
But did YOU know that the continents and oceans are moving across the surface of the planet?
Volcanoes and earthquakes as well as mountain ranges and islands all are results of this movement.
Why is this?

12. Plate Tectonics

13. Most of these changes in the earth’s surface takes place so slowly that they are not immediately noticeable to the human eye.
The idea that the earth’s landmasses have broken apart, rejoined, and moved to other parts of the globe forms part of the..
Plate Tectonic Theory

14. Plate Tectonic Theory
About forty years ago, scientists exploring the seafloor found that it is full of tall mountains and deep trenches, a single seafloor mountain chain circles Earth and contains some of Earth’s tallest mountains. Along this mountain chain is a deep crack in the top layers of earth. Here the seafloor is pulling apart and the two parts are moving in opposite directions, carrying along the continents and oceans that rest on top of them. These pieces of Earth’s top layer are called tectonic plates. They are moving very slowly, but constantly. (Most plates are moving about as fast as your fingernails are growing -- not very fast!) Currently Earth’s surface layers are divided into nine very large plates and several smaller ones.

15. According to the theory of plate tectonics, the earth’s outer shell is not one solid piece of rock. Instead the earth’s crust is broken into a number of moving plates. The plates vary in size and thickness.

16. The North American Plate stretches from the mid-Atlantic Ocean to the northern top of Japan. The Cocos Plate covers a small area in the Pacific Ocean just west of Central America.
These plates are not anchored in place but slide over a hot and bendable layer of the mantle.

17. How is the earth always changing?
What is the theory of plate tectonics and how does it work?

18. Continental Drift and Seafloor Spreading.
To really understand how the earth became to look as it does today, and the theory of plate tectonics, you also need to become familiar with two other ideas:
Continental Drift
and
Seafloor Spreading.

19. Less than 100 years ago, many scientists thought the continents always had been the same shape and in the same place.
A few scientists noted that the eastern coastline of South America and the western coastline of Africa looked as if they could fit together.
Some also noted that, with a little imagination, all the continents could be joined together like giant puzzle pieces to create one large continent surrounded by one huge ocean.

20. So, if my contintents fit together, why does the earth look like it does today?

21. Continental Drift Theory
When the tectonic plates under the continents and oceans move, they carry the continents and oceans with them.
In the early 1900s a German explorer and scientist proposed the continental drift theory. He proposed that there was once a single “supercontinent” called Pangaea.

23. Wegner’s theory was that about 180 million years ago, Pangaea began to break up into separate continents. To back this theory up, he found perserved remains and evidence from ancient animals and plants from South America, Africa, India, and Australia that were almost identical.

24. Plate motions also can be looked at into the future, and we can have a stab at what the geography of the planet will be like. Perhaps in 250 million years time there will be a new supercontinent.

25. Plate Tectonics
But this doesn’t actually tell me how the mountains or volcanoes were formed or how earthquakes happen, does it?

26. YES!
As mentioned earlier, those tectonic plates are always moving. They are always moving:
pulling away from each other
crashing head-on
or sliding past each other.
Depending on which way these plates are moving will decide what is happening on the earth you and I are standing on.

27. They’re Pulling Apart!
When plates pull away from one another they form a diverging plate boundary, or spreading zone.
Thingvellir, the spreading zone in Iceland between the North American (left side) and Eurasian (right side) tectonic plates. January 2003.

28. Sea Floor Spreading
The theory of seafloor spreading suggests that molten rock (think of a melted chocolate bar that has been left in your pocket for too long)... This hot substance (lava) from the mantle rises under the underwater ridge and breaks through a split at the top of the ridge (the crust... Remember, the plate). The split is called a rift valley. The rock then spreads out in both directions from the ridge as if it were on two huge conveyor belts. As the seafloor moves away from the ridge, it carries older rocks away. Seafloor spreading, along with the continental drift theory, became part of the theory of plate tectonics.

29. Mid ocean ridge

30. Iceland: An example of continental rifting
Iceland has a divergent plate boundary running through its middle
Iceland is located right on top of a divergent boundary. In fact, the island exists because of this feature.
As the North American and Eurasian plates were pulled apart (see map) volcanic activity occurred along the cracks and fissures (see photographs).
With many eruptions over time the island grew out of the sea!
Question: Why don’t we have islands like Iceland where ever we get an Ocean Ridge?
Answer: Scientists believe that there is a large mantle plume (an upwelling of hot mantle material) located right underneath where Iceland has formed. This would mean that more material would be erupted in the Iceland area compared with if there was just the divergent boundary without the plume underneath it.

31. The Crash!
What happens when plates crash into each other depends on the types of plates involved.
Because continental crust is lighter than oceanic crust, continental plates ”float” higher.
Therefore, when an oceanic plate meets a continetnal plate, it slides under the lighter plate and down into the mantle. The slab of oceanic rock melts when the ends get to a depth which is hot enough. A temperature hot enough to melt it is about a thousand degrees! This process is called subduction. Molten material produced in a subduction zone can rise to the earth’s surface and cause volcanic building, mountains, and islands.

33. Subduction
Oceanic lithosphere subducts underneath the continental lithosphere
Oceanic lithosphere heats and dehydrates as it subsides
The melt rises forming volcanism
E.g. The Andes
The Andes mountain range along the western edge of the South American continent is an example of a mountain belt formed by subduction.
The continental crust of the South American plate has buckled under the compressional strain of converging with the Nasca and Antarctic plates. Additionally there are many volcanoes, the result of melting of the subducting slab and the production of new material that has risen through the crust to the surface.

34. When they Crash
When two plates of the same type meet, the result is a process called converging.
Depending on what type of plates these are, depends on what occurs.

35. Converging Oceanic Crust
When both are oceanic plates, one slides under the other. Often an island group forms at this boundary.

36. Ocean-Ocean Plate Collision
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!
When two oceanic plates converge, because they are dense, one runs over the top of the other causing it to sink into the mantle and a subduction zone is formed.
The subducting plate is bent down into the mantle to form a deep depression in the seafloor called a trench.
Trenches are the deepest parts of the ocean and remain largely unexplored.

37. Manned or unmanned submersible vehicles (top right photo) have explored small parts of trenches discovering new species (like the fish photographed here) and amazing ecosystems.

38. Converging Continental Plates
When both are continental plates, the plates push against each other, creating mountain ranges.

39. Converging Continental Plates
Earth’s highest mountain range, the Himalayas, was formed millions of years ago when the Indo-Australian Plate crashed into the Eurasian Plate. Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about 5 cm a year!

40. 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!!

41. When plates slide past each other Transform Boundary
Sometimes, instead of pulling away from each other or colliding with each other, plates slip or grind past each other along faults.

42. These areas are likely to have a rift valley, earthquake, and volcanic action.
For example: Here, the San Andreas Fault lies on the boundary between two tectonic plates, the north American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of 5 to 6 centimeters each year. This fault frequently plagues California wit hearthquakes.

43. Three Types of Boundaries

44. Sea-Floor Spreading and Plate Boundaries
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

45. Converging Margins: India-Asia Collision
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

46. Transform Faults and Seafloor Spreading
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

47. …what’s the connection?
Volcanoes and Plate Tectonics…
…what’s the connection?
What’s the connection between volcanoes and plate tectonics….?

48. Pacific Ring of Fire Volcanism is mostly focused at plate margins
This map shows the margins of the Pacific tectonic plate and surrounding region. The red dots show the location of active volcanism. Notice how the majority of the volcanism is focused in lines along the plate boundaries? For this region this area is known as the “Pacific Ring of Fire”.
But why are all of the volcanoes located at the plate margins?
Volcanism is mostly focused at plate margins

49. Volcanoes are formed by:
- Subduction - Rifting - Hotspots
Volcanoes can be formed in three ways:
Via subduction. The subducting slab dehydrates to form new melt that will rise through the crust to be erupted at the surface.
Via rifting. When two plates pull apart magma rises, producing volcanic eruptions at the surface.
At “Hotspots”….hotspot do not necessarily occur along a plate boundary. So hotspot volcanoes can form in the middle of tectonic plates….Click for example.

50. Pacific Ring of Fire Hotspot volcanoes
Hotspot volcanoes in the middle of the Pacific plate.
Can anyone guess where they are?
Answer: Hawaii

51. What are Hotspot Volcanoes?
Hot mantle plumes breaching the surface in the middle of a tectonic plate
Firstly, what are hotspot volcanoes and how do they form?
A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time.
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).
A mantle plume may rise at any location in the mantle, and this is why hotspot volcanoes are independent from tectonic plate boundaries.
The Hawaiian island chain are an example of hotspot volcanoes (see right photograph).
The Hawaiian island chain are examples of hotspot volcanoes.
Photo: Tom Pfeiffer /

52. The volcanoes get younger from one end to the other.
The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.
Hotspot’s commonly form volcanic island chains (like the Hawaiian islands). These result from the slow movement of a tectonic plate over a FIXED hotspot.
Persistent volcanic activity at a hotspot will create new islands as the plate moves the position of the “old” volcanic island from over the hotspot.
Therefore at one end of the island chain you see the youngest, most active volcanic islands (directly over the hotspot) and along the island chain the extinct volcanoes become older and more eroded (see diagram).
This way geologists can use hotspot volcano chains to track the movement of the tectonic plate through time.
The volcanoes get younger from one end to the other.

53. 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 (Continental & Oceanic)
There are 3 types of plate boundaries (Convergent, Divergent, Transform)
Volcanoes and Earthquakes are closely linked to the margins of the tectonic plates
The take home messages from this lecture:
See if the students remember what each layer, type of plate and type of boundary are called.
Can they describe the motion of the three plate boundary types?
Where does volcanism occur in relation to plate tectonics?
Where do earthquakes occur in relation to plate tectonics?

54. …what’s the connection?
Earthquakes and Plate Tectonics…
We have seen the connection between volcanoes and plate tectonics.
Is there a connection between earthquakes and plate tectonics?
…what’s the connection?

55. 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
The black dots on this map of the world depict where earthquake activity is occurring. You can see that, as with volcanoes, the earthquakes are NOT randomly distributed around the globe. Instead they occur in linear patterns associated with plate boundaries.
(Note this diagram of earthquake distribution closely resembles the Pacific Ring of Fire distribution of volcanism).

56. Where do earthquakes form?
We know there are three types of plate boundaries: Divergent, Convergent and Transform. Movement and slipping along each of these types of boundaries can form an earthquake.
Depending on the type of movement, the earthquakes occur in either a shallow or deep level in the crust.
The majority of tectonic earthquakes originate at depths not exceeding tens of kilometers.
In subduction zones, where old and cold oceanic crust descends beneath another tectonic plate, “Deep Focus Earthquakes” may occur at much greater depths (up to seven hundred kilometers!).
These earthquakes occur at a depth at which the subducted crust should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep focus earthquakes is faulting.
Earthquakes may also occur in volcanic regions and are caused there both by tectonic faults and by the movement of magma (hot molten rock) within the volcano. Such earthquakes can be an early warning of volcanic eruptions.
Figure showing the tectonic setting of earthquakes