T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 1

Name: Plate Tectonics Unit II:

Section: Plate Boundaries (3.5 pts)

Plate Boundaries

We will now discuss the different possible situations that can occur along the edges of plates (plate boundaries), and look at specific places in the world where these events are happening. There are three major kinds of plate boundaries: plates coming together (convergent boundaries), plates moving apart (divergent boundaries), and plates moving side-to-side in opposite directions next to one another (transform boundaries).

1. (a) What are convergent plate boundaries?

(b) What are divergent plate boundaries?

(c) What are transform plate boundaries?

T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 2

Divergent Boundaries

Divergent boundaries are places where plates are moving apart or diverging. There are two kinds of divergent boundaries: where oceanic lithosphere is separating and where continental lithosphere is separating.

Oceanic lithosphere is moving apart at the mid-ocean ridge. At the top of the mid-ocean ridge there is a rift valley where the plates are splitting apart. (The rift valley is not nearly as deep as a true ocean trench, which goes down several miles.) The motion of magma in the mantle beneath the plates rubs against the bottom of the plates, pulling them apart at the mid-ocean ridge (this is called rifting), and then magma rises up in between the plates to fill in the gap. This magma meets the cool ocean water and cools into new, solid lithosphere on each side of the ridge.

Initially, the lithosphere at the ridge has a relatively low density, so it floats higher than the neighboring lithosphere. The lithosphere ages as it moves away from the ridge. The lithosphere’s density increases as it ages, because more and more sediments pile on top of the lithosphere, making it heavier. The lithosphere’s density also increases, because the lithosphere cools more and more due to its contact with cold ocean water, making it contract (shrink, get smaller). As the lithosphere ages, it sinks down into the mantle (a bit) because it can no longer float as well on the mantle due to its higher density. The lithosphere is like a boat as more and more people get into the boat. The boat does not float as high in the water, but sinks down into the ocean a little as each person steps onboard. Thus, the increase in density over time makes the ocean floor move downwards (get deeper) as it moves away from the mid-ocean ridge.

T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 3

The motion of the semi-solid mantle can pull continental lithosphere apart too. The magma which rises up to fill in the resulting gap, though, is just like the magma which rises up at the mid-ocean ridge, so when it cools, it forms new ocean floor (oceanic lithosphere), not continental lithosphere. As the continents get farther and farther apart, new ocean floor – and thus a new ocean – grows in between the continents. This is happening right now at places where the mid- ocean ridge “runs into” land like the Gulf of California (between Baja California and the rest of Mexico) and the Red Sea (between Africa and the Arabian Peninsula). There are some places in the world where a continent is just beginning to split (like the East African Rift Valley in Africa).

2. Is the ocean floor coming together or moving apart at the mid-ocean ridge?

3. Why are there volcanoes at the mid-ocean ridge?

4. Why does the ocean floor become more dense as it ages (gets older)?

5. Why is the top of the mid-ocean ridge higher (taller) than the ocean floor on either side of it?

6. True or false? “Continents are splitting apart in places where the mid-ocean ridge runs into the continents.”

7. Give an example of a place in the world where continents have “recently” (in a geologic sense) begun to split apart. T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 4

Convergent Boundaries

Convergent boundaries are places where plates are coming together (colliding) or converging. There are three kinds of convergent boundaries: (1) where oceanic lithosphere meets continental lithosphere, (2) where oceanic lithosphere meets oceanic lithosphere, and (3) where continental lithosphere meets continental lithosphere.

Oceanic Lithosphere meets Continental Lithosphere

The upper part of continental lithosphere (continental crust) is made of granite, whereas the upper part of oceanic lithosphere is made of basalt, so continental lithosphere has a lower density than oceanic lithosphere. When they meet, the higher-density oceanic lithosphere is forced to dive down into the Earth (subduct), since there is not room for both at the surface of the Earth.

To dive, the oceanic plate must bend downwards, so this makes the ocean floor deeper where the two plates meet: a trench. As the oceanic plate dives down into the Earth, it melts in the hot mantle, creating low-density magma which rises up beneath the other plate. The rising magma melts its way through the continental lithosphere. More and more lava comes up, cools, and piles up, forming volcanic mountains at the edge of the continent. This is happening along the coast of South America, where it created the Andes Mountains. It also is happening along the coast of much of Central America and the Pacific Northwest of the United States, where it produced the Cascades mountain range and the famous volcano Mt. St. Helens. T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 5

Both shallow and deep earthquakes are common along this kind of plate boundary, because the plates grind against one another near the surface and down deep, stopping their motion. Eventually the pressure to move overcomes the friction between the plates and the diving plate slips forward. In other words, an earthquake occurs. The continental lithosphere is compressed (squeezed) by the oceanic lithosphere pushing into it; it “snaps back” into shape like a bent eraser will become straight once you release it.

8. Which of the following are found where oceanic lithosphere collides with continental lithosphere: earthquakes, the mid-ocean ridge, trenches, volcanic islands, and/or volcanic mountains?

9. Is continental crust made of basalt or granite? Is oceanic crust made of basalt or granite? Which one has the higher density, basalt or granite?

10. Where oceanic lithosphere collides with continental lithosphere, which plate dives down into the Earth, the one with the oceanic lithosphere or continental lithosphere? Why is this plate the one that dives down into the Earth?

11. Why is there a trench where oceanic lithosphere collides with continental lithosphere?

12. What is the source of lava for the volcanoes that occur where oceanic lithosphere collides with continental lithosphere? In other words, why is lava coming up in this spot?

13. Give an example of a place in the world where oceanic lithosphere is colliding with continental lithosphere. T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 6

Oceanic Lithosphere meets Oceanic Lithosphere

When two pieces of oceanic lithosphere meet, the plate with the higher density subducts (dives down into the Earth). However, in this case, both plates are made of basalt, so neither is one more dense than the other simply by virtue of their composition. Instead, it is the older plate that subducts. The older plate has cooled down more, because it has been a longer time since it formed along the mid-ocean ridge. In addition, there has been more time for sediments to pile on top of it, making it heavier. Both the cooling of the plate (reducing its size) and the weight of the sediments make the older plate more dense than the younger plate. Thus, the older plate dives down into the Earth, making the ocean floor deeper (a trench), and melts, producing magma that rises, melts its way through the other plate, and bursts out at the surface. The molten material cools into solid rock on the surface and piles up higher and higher as more and more magma comes up. While underwater, these volcanoes are called seamounts. If these volcanoes grow high enough to reach the surface of the ocean, they produce a chain of islands. This is happening at the Mariana Trench and Mariana Islands, and along the south side of Alaska at the Aleutian Islands. Note that the island and seamounts are found on the plate which does NOT dive down into the Earth (and melt).

Once oceanic lithosphere gets old enough, it must start diving into the Earth: it is simply too dense to float on top of the mantle. If the oceanic lithosphere is pushing continental lithosphere, the plate can fracture along the boundary. A trench forms between the continental lithosphere and oceanic lithosphere, splitting the plate into two pieces.

T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 7

14. Which of the following are found where oceanic lithosphere collides with oceanic lithosphere: earthquakes, the mid-ocean ridge, trenches, volcanic islands, and/or volcanic mountains?

15. Does oceanic lithosphere’s density increase or decrease as it gets older? Why?

16. Where oceanic lithosphere collides with oceanic lithosphere, which plate dives down into the Earth, the older one or younger one? Why is this plate the one that dives down into the Earth?

17. Why is there a trench where oceanic lithosphere collides with oceanic lithosphere?

18. What is the source of lava for the volcanoes that occur where oceanic lithosphere collides with oceanic lithosphere?

19. What do we call the volcanoes if they do not reach the surface of the ocean (if they are underwater)?

What do we call the volcanoes if they reach the surface of the ocean?

20. Give an example of a place in the world where oceanic lithosphere is colliding with oceanic lithosphere. T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 8

Continental Lithosphere meets Continental Lithosphere

The upper layer of continental lithosphere (continental crust) is made of low-density granite, so continental lithosphere is never dense enough to sink down into the much higher-density mantle. Thus, when two pieces of continental lithosphere collide, the continental crust does not dive down deep into the Earth. Instead, the rock is squeezed upwards (and downwards), building tall mountains. Each time the plates slip forward towards one another, the resulting earthquake lifts the mountains higher and higher. For example, the subcontinent of India is being pushed northeast into Asia, producing the Himalayas, the mountain range with the tallest mountain in the world, Mt. Everest. Another example is the Alps in Europe, where the Italian peninsula is being forced north into the rest of Europe.

The collision does push some continental crust down a bit lower. However, it does not sink deep down into the mantle. The part of the lithosphere beneath the continental crust can still dive down into the Earth. This can actually cause neighboring continental and oceanic lithosphere to break into separate plates. The oceanic lithosphere will then begin subducting beneath the continental lithosphere, creating a trench along the coast.

T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 9

21. Which of the following are found where continental lithosphere collides with continental lithosphere: earthquakes, islands, the mid-ocean ridge, mountains, trenches, and/or volcanoes?

22. Does a piece of continental lithosphere ever sink deep into the Earth when it collides with another plate? Why or why not?

23. Which best explains the mountains found where continental lithosphere collides with continental lithosphere: (a) land rises because the plates are forced together more and more and the rock has to go somewhere, or (b) the land rises as more and more lava comes up from below and cools and hardens into solid rock?

24. Give an example of a place in the world where continental lithosphere is colliding with continental lithosphere.

T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 10

Growing and Shrinking Plates

Plates always move away from the mid-ocean ridge. However, some plates have the mid-ocean ridge on both sides (for example, the African plate). How can the plate be moving in opposite directions on opposite sides? How can the plate be moving both east and west? The answer is that the plate is growing (getting bigger); new material is being added along both boundaries.

It is not entirely clear whether it is the plate that is moving or the location of the mid-ocean ridges relative to the plate, or both. The red arrows in the map on the right show the direction from newer rock to older rock. They cannot show the absolute motion of the plate relative to some fixed, permanent reference point because the entire surface of the Earth is in motion, including the mid-ocean ridges and trenches.

At least one plate next to a trench is moving towards the trench. At the trench, one plate is subducting (diving down into the Earth) and melting. Some plates are completely surrounded by trenches (for example, the Filipino plate), so they are slowly being pushed down into the earth. They will shrink and shrink until they are completely destroyed.

25. Where are plates growing: at the mid-ocean ridge or at trenches? In other words, where is material being added to plates?

26. Where are plates being destroyed (being made smaller): at the mid-ocean ridge or at trenches?

27. Which plate will get larger and larger over time, one with the mid-ocean ridge along is edges or one with trenches along its edges?

T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 11

Transform Boundaries

The mid-ocean ridge is not one, continuous ridge, but instead is broken into segments or “pieces.” There are “jumps” (gaps) between the segments, and these jumps or gaps are called transform faults. A transform fault is a place where two plates slide next to one another in opposite directions horizontally.

The lithosphere on either side of the fault moves away from the segment of the mid-ocean ridge where it was “born” (where it cooled from lava into solid rock). The plates on either side of the fault rub against one another, so friction between them keeps them from moving until the pressure becomes too much and they slip, causing an earthquake. (The pieces or parts of the mid-ocean ridge are where the volcanoes happen. The plates are spreading apart, and magma rises up from below to fill in the hole or gap that is opening between the plates.)

We live near one large transform fault called the San Andreas Fault that runs between a segment of the mid-ocean ridge in the Gulf of California and a segment off the coast of Northern

California.

T. James Noyes, El Camino College Plate Tectonics Unit II: The Plate Boundaries (Topic 11A-2) – page 12

28. True or false? “The mid-ocean ridge is not one continuous line of volcanoes. There are gaps or “jumps” in the mid-ocean ridge where there is little or no volcanic activity.”

29. Does the ocean floor move towards, away from, or parallel to the mid-ocean ridge?

30. Does the ocean floor move towards, away from, or parallel to transform faults?

31. Where are the volcanoes, at the mid-ocean ridge or at the transform faults?

32. Where are the earthquakes, at the mid-ocean ridge or at the transform faults?