1. Plate Tectonics
Liz LaRosa for use with my 5th Grade Science Class

2. Explain convection currents are the result of uneven heating inside the mantle resulting in the melting of rock materials, convection of magma, eruption/flow of magma, and movement of crustal plates
Describe how the movement of crustal plates can cause earthquakes and volcanic eruptions that can result in mountain building and trench formation

3. Earth’s Layers
The Earth's rocky outer crust solidified billions of years ago, soon after the Earth formed.
This crust is not a solid shell; it is broken up into huge, thick plates that drift atop the soft, underlying mantle.

4. The Crust Outermost layer 5 – 100 km thick
Made of Oxygen, Silicon, Aluminum

5. The Mantle Layer of Earth between the crust and the core
Contains most of the Earth’s mass
Has more magnesium and less aluminum and silicon than the crust
Is denser than the crust

6. The Core Below the mantle and to the center of the Earth
Believed to be mostly Iron, smaller amounts of Nickel, almost no Oxygen, Silicon, Aluminum, or Magnesium

7. Tectonic Plates

8. The core is believed to be made mostly of iron.
True
False

9. The crust contains most of the earth’s mass.
True
False

10. 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.
The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.
There are 12 major plates on Earth, each of which slide around at a rate of centimetres per year, pulling away from, scraping against or crashing into each other.
Each type of interaction produces a characteristic “tectonic feature”, like mountain ranges, volcanoes and (or) rift valleys, that we will discuss during this lecture.

11. 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).

12. 2 Types of Plates
Ocean plates - plates below the oceans
Continental plates - plates below the continents

13. 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.

14. Continental Drift Alfred Wegener 1900’s
Continents were once a single land mass that drifted apart.
Fossils of the same plants and animals are found on different continents
Called this supercontinent Pangea, Greek for “all Earth”
245 Million years ago
Split again – Laurasia & Gondwana 180 million years ago

15. Evidence of Pangea

16. The crust and the upper part of the mantle is made up of the asthenosphere.
True
False

17. Sea Floor Spreading

18. Sea Floor Spreading
Mid Ocean Ridges – underwater mountain chains that run through the Earth’s Basins
Magma rises to the surface and solidifies and new crust forms
Older Crust is pushed
farther away from the ridge

19. Convection Currents
Hot magma in the Earth moves toward the surface, cools, then sinks again.
Creates convection currents beneath the plates that cause the plates to move.

20. 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).

21. Different Types of Boundaries

22. Three types of plate boundary
Divergent
Convergent
Transform
Firstly, there are three types of plate boundary, each related to the movement seen along the boundary.
Divergent boundaries are where plates move away from each other
Convergent boundaries are where the plates move towards each other
Transform boundaries are where the plates slide past each other.
Presenter: See diagrams for each - it is important to remember the names of the boundary types and the motion involved.

23. Three Basic Types of Plate Boundaries
Using hands to show relative motion
Divergent
Transform
Convergent
Video: Plate Boundaries— Plates can have three kinds of motion across boundaries: [note video lecture on next slide]
They can move away from each other like at a divergent boundary (= spreading ocean ridge or a rift zone). Or use parallel hands like the transform beginning hands and move them apart allowing the magma (thumbs) to come up.
2. They can move toward each other to make a convergent boundary.
3. They can slide past each other horizontally at a transform boundary.
When two plates carrying continents converge, a continental collision occurs where continental crust piles up. Continental crust is lower density than mantle rocks, so continental rocks cannot be “subducted” into the mantle. If continental rocks are pushed into the mantle, they will soon pop up again. This is like trying to push a piece of styrofoam into a swimming pool. You can push the styrofoam into the water (with some force) but, when you let it go, it pops back to the surface because it is much less dense than the water on which it floats.
ACTIVITY: Teaching about Plate Tectonics Using Foam Models: Download FoamFaultModel_Activity.pdf from Animations Page:
Graphics from “This Dynamic Planet, World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics.” A Smithsonian, USGS, US Naval Research lab publication.
You can find this at .ﾊCopyright protected: The content may only be used for personal, educational or noncommercial purposes;
USGS Graphics

24. Divergent Boundaries Spreading ridges
In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents or on the ocean floor.
As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity.
Presenter: Reiterate the process by going through the diagram, including the presence of mantle convection cells causing the plates to break apart and also as a source for new molten material.
Where a divergent boundary forms on a continent it is called a RIFT or CONTINENTAL RIFT, e.g. African Rift Valley.
Where a divergent boundary forms under the ocean it is called an OCEAN RIDGE.
Spreading ridges
As plates move apart new material is erupted to fill the gap

25. Features of Divergent Boundaries
Mid-ocean ridges
rift valleys
fissure volcanoes

26. Convergent Boundaries
There are three styles of convergent plate boundaries
Continent-continent collision
Continent-oceanic crust collision
Ocean-ocean collision
Convergent boundaries are where the plates move towards each other.
There are three types of convergent boundary, each defined by what type of crust (continental or oceanic) is coming together.
Therefore we can have: continent-continent collision, continent-oceanic crust collision or ocean-ocean collision….

27. Continent-Continent Collision
Forms mountains, e.g. European Alps, Himalayas
When continental crust pushes against continental crust both sides of the convergent boundary have the same properties (think back to the description of continental crust: thick and buoyant). Neither side of the boundary wants to sink beneath the other side, and as a result the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges. For example, the European Alps and Himalayas formed this way.

28. 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!!

29. Continent-Oceanic Crust Collision
Called SUBDUCTION
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.
The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram).
When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram).
The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes.
Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.

30. 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.

31. 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.

32. Transform Boundaries Where plates slide past each other
The third type of boundary are transform boundaries, along which plates slide past each other.
The San Andreas fault, adjacent to which the US city of San Francisco is built is an example of a transform boundary between the Pacific plate and the North American plate.
Above: View of the San Andreas transform fault

33. Earthquakes & Plate Boundaries
Notice that the earthquakes coincide with plate boundaries, and the deepest quakes (blue) are in subduction zones Question: Where would you expect to see volcanoes?
This slide shows the relationship between plate tectonics and earthquake location. There are thousands more earthquakes every year than shown here.
Many are in the middle of plates. Why would there be earthquakes in the middle of plates? Remember that the plates are sliding over a round globe and they crack, crumple, and creak as they adjust to the new position. All these adjustments cause earthquakes.
For earthquakes of last 2 weeks, go to
INTERACTIVE FLASH rollovers : ﾊﾊﾊﾊDynamicPlanet-Earthquakes & Volcanoes
Create your own maps at
Modified from USGS Graphics

34. Volcanoes & Plate Boundaries
This map shows that locations of volcanoes (ones above sea level) also tend to occur along the plate boundaries
This image shows a smattering of the most prominent and active volcanoes that occur above sea level. There are thousands of submarine volcanoes not shown here.
Next slide shows plate boundaries.
Where do volcanoes occur?
Volcanoes occur on divergent boundaries (mid-ocean ridges are continuous submarine volcanic mountain ranges) and on convergent boundaries where an oceanic plate subducts beneath either another oceanic plate or beneath a continental plate. (A later slide shows the different convergent boundaries). They can also occur in the middle of a plate due to “hotspot” processes or to continental rift areas which haven’t opened enough to define separated plates.
It is an oversimplification to say that ALL earthquakes and volcanoes occur at or near plate boundaries but there is a very strong concentration of earthquakes and volcanoes near plate boundaries. If you understand how plate motions produce earthquakes and volcanoes, you can explain 80% of earthquakes and volcanoes.
INTERACTIVE FLASH rollovers : ﾊﾊﾊﾊDynamicPlanet-Earthquakes & Volcanoes
Modified from USGS Graphics

35. Tectonic Plates
How fast are the plates moving? Plates move 1-10 centimeters per year (≈ rate of fingernail growth).
The Nazca (beneath S.American plate) and Pacific Plates are the fastest.
Rate of plate motions are typically an inch or two per year (the rate of fingernail growth.) Although this seems slow on the human time scale, the movement over 100s of millions of years builds and destroys land masses.
Dr. John Lahr, seismologist, graphed his fingernail growth relative to the plates:
Graphics from “This Dynamic Planet, World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics.” A Smithsonian, USGS, US Naval Research lab publication.
You can find this at .ﾊCopyright protected: The content may only be used for personal, educational or noncommercial purposes;
Fingernail growth plotted:
Modified from USGS Graphics