Plate Tectonics Lecture Notes

Transcription

1 Plate Tectonics Lecture Notes (Topic 11A) page 1 Plate Tectonics Lecture Notes Learning Outcomes for the Plate Tectonics Unit 1. Students can identify and describe observations that were (and are) used to support the ideas of continental drift and seafloor spreading, and can articulate arguments for how and why these observations support the ideas. 2. Students can describe the layers of the Earth (including plates), specifically the composition of the lithosphere, the phase/state of the lithosphere and mantle, and the relative temperature and thickness of the lithosphere and mantle. 3. Students can explain how and why the plates move, and give an example of a typical plate speed. 4. Students can describe the motion of plates at divergent, convergent, and transform boundaries, and explain how and why the plate motion produces earthquakes, island chains, mountain chains, and volcanoes at these boundaries. 5. Students can explain how and why hotspot island-seamount chains form and their features (e.g., island size, location of active volcanoes in the chain). 6. Given a map showing features of the ocean floor (e.g., mid-ocean ridge, trenches, islandseamount chains) and land (e.g., mountain chains), students can identify the locations of plate boundaries, earthquakes, and volcanoes, and predict the direction of plate motion and subduction.

2 Observations (mainly) from the Continents Plate Tectonics Lecture Notes (Topic 11A) page 2 Continents Ocean Floor Kind of Rock Description Density Why are the continents so much higher than the ocean floor? Isostasy: The continents float in the magma inside the Earth than the ocean floor because are made of granite which has a lower density than basalt. Contractionism: The oceans are places that happened to a little more than other places, so they contracted more than other places, making them deeper. Thus, rainwater flowed into them, making them cool and contract even more than other places. Continental Drift Describe the idea called continental drift. Evidence for Continental Drift Observations: The edges of the continents fit together like the pieces of a puzzle. Observations: Where very old mountain ranges or mineral deposits like coal and diamonds are found along the coast of one continent, they are also found on the coast of the another continent at the place where the continents fit together. Observations: Identical fossils of plants and animals have been found on different continents, but today each continent has its own unique groups of plants and animals today adapted to each continents unique physical and chemical conditions: climate, soil, geography, etc. Hypothesis #1: The continents have always been in the same place. All these different ways in which the edges of the continents match (shape, mineral deposits, mountain ranges, fossil remains) just happen to match by chance. It is a coincidence. Hypothesis #2: The continents were once joined together in one large continent. Thus, they match in many ways along their edges, the places which were once together. Since the continents are not together now, they must have split apart and moved to their current locations. Which hypothesis seems more likely to you? Why was the idea of continental drift rejected by most scientists when it was proposed?

3 Plate Tectonics Lecture Notes (Topic 11A) page 3 New Evidence for Continental Drift Where did new evidence supporting continental drift come from? Mid-Ocean Ridge (What is the MOR? Where is it? What happens at the MOR or nearby?) Trenches (What are trenches? Where are they? What happens at trenches or nearby?) Rock Magnetism In lava, magnetic particles are constantly bumping and moving around since they are in a liquid form. As the lava cools down, the magnetic particles slow down, and their attraction towards the magnetic North Pole of the Earth is strong enough to orient them in the same direction as the Earth s magnetic field. When the lava becomes solid rock, the magnetic particles are frozen pointing towards the magnetic North Pole. This is what we observe in rocks that recently formed from lava in places like Hawaii. Evidence from Old Rocks: The magnetism in old rocks does NOT point towards the current magnetic north pole. What does this suggest about the location of the magnetic north pole of the Earth? ALL rocks of the same age (in other words, all rocks from lava that cooled down at the same time) point towards the same location.

4 Plate Tectonics Lecture Notes (Topic 11A) page 4 New Evidence from the Ocean led to the idea called Sea-Floor Spreading Describe the idea called sea-floor spreading. Evidence for Sea-Floor Spreading Observations of Rock Magnetism: On land, the sides of a volcano have the same magnetism along all slopes of the volcano. On the ocean floor, rock magnetism changes in stripes of the same width along the slopes on both sides of the mid-ocean ridge (MOR). Volcano Ocean Floor MOR Convection Cells Explanation of the Observations of Rock Magnetism: At volcanoes on land, lava comes out the top and spills down the sides, leading to vertical layers with different magnetism but the same magnetism horizontally. Clearly, the mid-ocean ridge is not like a volcano on land that grows upwards since the rock magnetism is not the same all along both slopes of the mid-ocean ridge. Rock forms from cooling lava at the top of the mid-ocean ridge on both sides of the mid-ocean ridge at the same time, so the rock on both sides has the same magnetism. If rock then moved away from the ridge at the same rate on both sides, an identical magnetic pattern on both sides of the ridge would result. This is what is observed. Observations of the Age of the Ocean Floor The farther you get from the mid-ocean ridge, the the ocean floor is, and the sediments there are on top of it. Explanation of the Observations of the Age of the Ocean Floor A young rock formed from lava that cooled down and solidified recently. An old rock formed from lava that cooled down and solidified a long time ago. The lava erupting from the mid-ocean ridge cools down and solidifies into new ocean floor. The sea floor moves the mid-ocean ridge, making room for more lava to come up. Over time, the ocean floor moves farther and farther from the midocean ridge, getting older and older as more time passes. In addition, more and more pile up on the ocean floor as more and more time passes.

5 Plate Tectonics Lecture Notes (Topic 11A) page 5 Layers of the Earth The basic idea of plate tectonics is that the Earth is covered by a solid outer layer called the lithosphere which floats on a warmer, semi-solid layer which can move called the mantle. The lithosphere is broken into pieces called plates. The semi-solid rock down in the mantle moves due to the heat of the Earth s core and pushes the plates away from and into one another, resulting in the earthquakes, volcanoes, mountains, trenches, and so on where the plates meet. The lithosphere is made up of two parts. The upper part is made of granite or basalt. The lower part is made of solidified mantle rock. Granite has a lower density that basalt. Since the bedrock of the continents is made of granite while the ocean floor is made of basalt, continental lithosphere has a lower density than oceanic lithosphere. Label the lithosphere and the mantle in the picture of the interior of the Earth. Write warm or cold in each layer. Write solid or semi-solid in each layer. Examine the picture on right. Which layer of Earth is thinner, the lithosphere or the mantle? Which has a lower density, the lithosphere or the mantle? Explain your reasoning. The lithosphere floats on top of the mantle, so the lithosphere must have a density than the mantle. The mid-ocean ridge and trenches are found along the edges of the plates. In other words, the mid-ocean ridge and trenches separate the plates. Circle each plate in picture of the interior of the Earth above How many plates are shown in the picture?

6 Plate Motion and How it Creates and Destroys Plate Tectonics Lecture Notes (Topic 11A) page 6 The Earth s core heats the bottom of the mantle, lowering its density and causing it to rise. Cooler mantle material slides in to replace it, and the process repeats in giant, slow-motion convection cells. Examine the picture below. What causes the plates to move? In other words, what pushes the plates in the directions shown? Plates are pushed by the flowing beneath them. Describe the motion of the plates. Plates move away from and towards. Volcanoes at the Mid-Ocean Ridge and the Creation of New Oceanic Lithosphere Plates move away from one another at the mid-ocean ridge, creating a. The magma comes up to:. The magma cools when it comes into contact with ocean water, and hardens into new ocean floor. The Collision and Destruction of Plates at Trenches When the two plates collide, the -density plate sinks down into the Earth, making the ocean floor deeper and thus creating a. The diving plate is heated by the mantle, causing it to and thus the plate is destroyed. Some of melting plate becomes a low-density magma which rises up and melts its way through the plate above, creating volcanoes.

7 Plate Tectonics Lecture Notes (Topic 11A) page 7 Plate Density and Motion As oceanic lithosphere gets older, it gets dense, because more have piled up on top of the lithosphere, and because the more time the lithosphere is in contact with ocean water, the it gets. If oceanic lithosphere gets old enough, its density can become higher than the density of the mantle! Continental lithosphere s density does not change (much) over time and always remains much than the density of basalt and the mantle, so it can never sink down into the mantle; it always floats on top of the mantle. Transform Boundaries: Where Plates Slide Side-by-Side The mid-ocean ridge is not a straight line. Instead, there are places where it jumps to the side. In other words, there are gaps between different parts of the mid-ocean ridge. Transform Faults A transform fault is a place where two plates are sliding alongside one another in opposite directions. Transform faults are found in the gaps between parts of the mid-ocean ridge. Earthquakes The magma of the mantle flows in convection cells which steadily rub against the bottom of the plates, pushing them slowly against other plates. Along their edges, the plates are squeezed and twisted where they collide or rub against one another. Eventually, the plates cannot deform any more, and the pressure to move builds up until it overcomes the resistance: then the plates move ( slip ) all at once, and snap back into their original shape. This is an earthquake.

8 Plate Tectonics Lecture Notes (Topic 11A) page 8 Observations related to the Theory of Plate Tectonics Practice We have discussed a large number of observations which support the theory of plate tectonics. Which of the following observations are consistent with and have been used to support the theory of plate tectonics? (Mark all that apply.) Fossils of identical plants & land animals are found on 2 or more continents separated by oceans. Fossils of tropical animals and plants are only found in the tropics. The layer of ocean sediments covering the ocean floor is thicker on the abyssal plains than on the mid-ocean ridge. A layer of red clay is found on top of a layer of calcareous ooze on the abyssal plains. Most trenches are found in the middle of the ocean, far from land (continents and islands). The oceanic lithosphere near the mid-ocean ridge is younger than the lithosphere farther from the ridge. The oceanic lithosphere far from a trench is older than the lithosphere closer to the trench. Some kinds of rocks and mineral deposits (e.g., coal, diamonds) are found near the coast and on opposite sides of the Atlantic ocean. Which of the following observations about earthquakes and volcanoes have been used to support the theory of plate tectonics? (Mark all that apply.) Earthquakes typically occur at or near: the abyssal plains continental shelves continental slopes the mid-ocean ridge trenches The locations of earthquakes do not support the theory of plate tectonics. Most volcanoes are found at or near: the abyssal plains continental shelves continental slopes the mid-ocean ridge trenches The locations of volcanoes do not support the theory of plate tectonics.

9 Plate Tectonics Lecture Notes (Topic 11A) page 9 Plate Boundaries A lot of the action in geology happens along the edges of the plates: their boundaries. where plates come together ( converge ) and move apart ( diverge ). Another way to think about the theory of plate tectonics is to identify the characteristics of each kind of plate boundary and what s happening at each kind of plate boundary. The Mid-Ocean Ridge: A Divergent Boundary A plate made of oceanic lithosphere moves another plate made of oceanic lithosphere. Possible Answers: towards, away from - The plates are pushed away from the MOR by. - The plates get deeper (go downwards) as they move away from the MOR because their density (A) over time. The plates density changes because: they get (B) the more time that they are in contact with the ocean water. This causes the plates to (C). get (D) as more and more pile up on top of them. A: increases, decreases C: expand, contract B: warmer and warmer, cooler and cooler D: heavier, lighter Earthquakes: The plates rub as they move in opposite directions at the transform faults in the gaps between each part of the MOR. (See the section on transform boundaries.) Volcanoes: Magma rises up at the mid-ocean ridge, because at the mid-ocean ridge, the plates are moving. This creates a in the ocean floor, and magma comes up to fill in the empty space. New Ocean Floor is Created at the MOR: When the rising lava comes into contact with ocean water, it, causing it solidify into new ocean floor.

10 Plate Tectonics Lecture Notes (Topic 11A) page 10 Trenches (Subduction Zones): Convergent Boundaries in the Ocean At trenches, the plates are pushed together by magma convection cells in the mantle, and one plate dives below the other plate and into the mantle. This is called subduction. Examine the side-view picture on the right. The plate that goes down into the mantle is always made of lithosphere. Possible Answers: continental (granite crust), oceanic (basalt crust) The plate that goes down into the mantle is the plate with the density. Possible Answers: higher, lower Trench: Since the ocean floor (oceanic lithosphere) is going (A), the ocean gets (B), producing a trench in subduction zones. A: upwards, downwards B: deeper, shallower Earthquakes: The top of one plate rubs against the bottom of the other plate as one plate dives below the other plate. Ocean Floor Destroyed: The plate going downwards melts due to the (A) magma of the mantle; and friction with the mantle rock. In addition, the diving plate contains water and sediments. When the water and sediments mix with the rock, they lower the melting point of the rock (the temperature at which the rock melts), making it (B) for the rock to melt. A: hot, cold B: easier, harder Volcanoes: The subducting plate contains water and sediments. The mixture of water, sediments, the melting plate, and some mantle rock produces hot, -density magma. This magma rises up and melts its way through the plate above it. Possible Answers: high, low

11 Plate Tectonics Lecture Notes (Topic 11A) page 11 There are two kinds of subduction zones, so there two kinds of subduction zone plate boundaries. Trench next to a Chain of Volcanic Islands A plate made of lithosphere moves towards a plate made of lithosphere. Possible Answers: continental (granite crust), oceanic (basalt crust) The density of oceanic lithosphere (A) over time because it gets (B) the more time that it spends in contact with ocean water. This causes the lithosphere to (C). In addition, oceanic lithosphere gets (D) over time as more and more pile up on top of it. A: increases, decreases C: expand, contract B: warmer and warmer, cooler and cooler D: heavier, lighter Older oceanic lithosphere has a (A) density than younger oceanic lithosphere, so when they collide, the (B) oceanic lithosphere is pushed down by the (B) oceanic lithosphere. A: higher, lower B: older, younger Islands: As more and more lava comes out and cools down into solid rock, the volcanoes grow higher and higher. If they reach the surface of the ocean, then they are islands. When they are still below the surface of the ocean, they are called seamounts. Trench next to a Continent with Volcanic Mountains along the Coast A plate made of lithosphere moves towards a plate made of lithosphere. Possible Answers: continental (granite crust), oceanic (basalt crust) The (A) lithosphere it pushed down by the (A) lithosphere because oceanic lithosphere contains (B) which has a (C) density than continental lithosphere which contains (B). A: continental, oceanic B: basalt, granite C: higher, lower Mountains: As more and more lava comes out and cools down into solid rock, the volcanoes grow higher and higher.

12 Plate Tectonics Lecture Notes (Topic 11A) page 12 Mountains away from the Coast (Inland): Convergent Boundaries on Land Continental Lithosphere collides with Continental Lithosphere A plate made of continental lithosphere moves towards another plate made of continental lithosphere. Mountains: India Asia Continental Lithosphere Neither plate can dive down below the other plate and deep into the magma of the mantle, because the mantle has a much (A) density than continental lithosphere which contains (B). As the plates are forced together, the rock has to go somewhere. Since the plates cannot go down very far, the only direction they can go is (C). A: higher, lower B: basalt, granite C: up, down Earthquakes: The plates push against one another as they collide.

13 Plate Tectonics Lecture Notes (Topic 11A) page 13 Hotspots & Island Chains A hotspot is a place where the mantle is unusually hot and magma is melting through the plate above it. Volcanoes grow as more and more lava comes out on top of the plate. These underwater volcanoes are called seamounts. If the volcano reaches the surface of the ocean, it is an island. The causes of hotspots are not well understood. One hypothesis is that a last remaining unmelted piece of subducting oceanic lithosphere hits the core, gets superheated, and rises upwards. Hotspot Island-Seamount Chains More and more magma melts through the lithosphere, cools when it comes into contact with ocean water, and hardens into solid rock, piling up higher and higher. While it is underwater, the volcano is called a, but when and if it reaches the surface, it is called an. Plate Motion #1 Mantle Plume The volcano does not stay on the hotspot, because it is growing on top of a plate of the lithosphere. As the plate, the volcano is slowly carried off the hotspot. A new volcano then begins to grow over the hotspot on top of the plate until it too is carried off the hotspot by the motion of the plate. The only active volcanic island or seamount in the chain is the island or seamount located at ; all the others are extinct volcanoes (they do not have a source of lava). Changes in the Volcanoes of Hotspot Island-Seamount Chains The islands tend to SINK over time, because they (A) after they leave the hot spot. As a result, the island (B) and becomes (C) dense, so the lithosphere it is on top of does not float as well in the mantle below it. A: warm up, cool down B: expands, contracts C: more, less

14 Plate Tectonics Lecture Notes (Topic 11A) page 14 Some of the seamounts in the island chains are tablemounts (guyots): seamounts with flat tops. As we learned earlier in the semester, erosion by produces flat surfaces, so the existence of tablemounts suggests that some seamounts once reached the surface and then sank. The existence of coral atolls also supports the idea Coral Reefs Corals that islands sink. Coral atolls are coral reef islands Grow Upward that are not close to land. However, coral reefs can only grow close to land, because corals live on the ocean floor and need to grow in shallow water so that the algae living inside them can get sunlight. The algae living inside the coral use the sunlight for, and give some of the that they make to the corals. We think that the original coral reef did grow close to land, and as the island sank, the corals grew upwards to stay close the Sun by adding layers of white beneath them. This is how and why a coral reef grows. Hotspot Island-Seamount Chains as Evidence for Sea-Floor Spreading The motion of the sea floor is necessary to produce a chain of islands and seamounts. If the sea floor did not move, then at a hotspot we would find volcanic island or seamount; there would not be a chain of extinct volcanic islands and seamounts extending away from the hotspot and mid-ocean ridge. In addition, coral atolls in hotspot island-seamount chains are found in places where the water is too cold for corals to grow well. Corals do not grow well in cold water, because their reefs are made of and this substance in cold water. Nonetheless, some coral reefs are found in cold water, because they live on volcanic islands which grew on top of plates and the plates have slowly from the tropics towards the Poles.

15 Plate Tectonics Lecture Notes (Topic 11A) page 15 Plate Tectonics Review The Density of the Lithosphere and the Mantle i. Which has a higher density, granite or basalt? a. Granite has a higher density than basalt. b. Basalt has a higher density than granite. How is their appearance different? How could you tell them apart? ii. Which has a higher density, granite or the mantle? a. Granite has a higher density than the mantle. b. The mantle has a higher density than granite. iii. Which has a higher density, the mantle or basalt? a. The mantle has a higher density than basalt. b. Basalt has a higher density than the mantle. Where are most trenches found, close to land (e.g., continents, islands) or far from land (e.g., middle of the ocean)? a. Close to Land. b. Far from Land. Why are trenches found in this place? Where is rock of the sea floor younger, close to the mid-ocean ridge or close to trenches? a. Close to the Mid-Ocean Ridge. b. Far from the Mid-Ocean Ridge. Why is the sea floor younger in this place?

16 Plate Tectonics Lecture Notes (Topic 11A) page 16 Where are plates being destroyed? a. At the Mid-Ocean Ridge. c. At Volcanoes. b. At Trenches. d. Wherever earthquakes are common. How or why are plates destroyed in these places? Which of the following plates are growing larger? (Mark all that apply.) a. African Plate c. Nazca Plate b. Caribbean Plate d. South American Plate Why are these plates growing larger? Why aren t the other plates growing? A single plate CAN be made of large amounts of BOTH continental lithosphere (crust) and oceanic lithosphere (crust). Which of the following describes the relationship between the continental & oceanic lithosphere? a. The continental lithosphere is on top. b. The oceanic lithosphere is on top. c. They are attached side-by-side. d. A plate is only made of ONE kind lithosphere, NOT BOTH. Why do the plates move? a. Dense end of a plate sinks, pulling the rest of the plate with it. b. Earthquakes c. Earth's magnetic field d. Lava upwelling at the MOR pushes the plates apart. e. Magma flowing beneath the lithosphere rubs against the bottom of the plates.

17 Plate Tectonics Lecture Notes (Topic 11A) page 17 Which island chain is a hotspot island chain in map on the right? a. Chain A b. Chain B Why? How can you tell? There is a trench along the coast of Southern California. This is related to why we have so many earthquakes. a. True b. False Neither plate can dive beneath the other plate because both are made of GRANITE so they have the SAME density. What is wrong or misleading about this statement? How can it be corrected?

18 Plate Tectonics Lecture Notes (Topic 11A) page 18 Summary of How the Motion of Plates Creates Volcanoes, Mountains, and Island Chains Describe the three different ways that volcanoes form according to the theory of plate tectonics. Near the Mid-Ocean Ridge: Magma rises up from below to fill in the created when the plates move the mid-ocean ridge. Near Trenches: The diving plate in the hot mantle, producing some low-density magma which rises up and melts its way through the plate above it. Hot Spots: At some special places, magma rises up from deep in the mantle and melts through the lithosphere above. Describe the two different ways that mountain chains form according to the theory of plate tectonics. Volcanic Mountains, Close to Coast next to a Trench: The diving plate in the hot mantle, producing some low-density magma which rises up and melts its way through the plate above it. Non-Volcanic Mountains, Inland (Away from the Coast): Two plates made of collide. Neither can sink down into the mantle, because their density is much than the density of the, so rock rises upwards to make room for the rock that is being pushed together. Describe the two different ways that island chains form according to the theory of plate tectonics. Islands are ALL next to a Trench and ALL Islands are potentially Volcanic: The diving plate in the hot mantle, producing some low-density magma which rises up and melts its way through the plate above it. Lava comes up all along the edge of the trench, creating many volcanic islands. Islands are NOT near a trench and only ONE Island is Volcanic: At some special places, magma rises up from deep in the mantle and melts through the lithosphere above it. A volcanic island grows above this spot, but is carried away from the spot because the plate it has grown on. Another island begins to grow, and the process repeats.

19 Plate Tectonics Lecture Notes (Topic 11A) page 19 Plate Boundaries: All 3 kinds of plate boundaries summarized in one picture Mark the locations of the mid-ocean ridge, transform faults, and the trench in the picture below. Put the letter e in locations where earthquakes should typically occur, and put the letter v in locations where volcanic activity is likely. Mid-Ocean Ridge Transform Fault Trench Lithosphere Mantle Plates Tectonics Maps Activity On the maps on the next page: (a) Highlight the mid-ocean ridge in red, the transform faults in blue, and the trenches in green. (See the legend at the bottom of this page. It will help you identify them.) (b) With a grey pencil, highlight the edges of the plate that contains the letter P. (c) Put an arrow (or arrows) on plate P showing the direction that the plate is probably moving in. (d) Put small letter e s in places where you think that earthquakes will be common in the map. (e) Put small letter v s in places where you think that volcanoes will be found on the map.

20 Plate Tectonics Lecture Notes (Topic 11A) page 20 Maps for the Plate Tectonics Maps Activity Map #1 N W E P S Map #2 N W E P S

21 Plate Tectonics Lecture Notes (Topic 11A) page 21 Plate Tectonics Practice Maps You can practice identifying the directions of plates and the locations of earthquakes and volcanoes using the maps below. Use the same legend as the plate tectonics maps activity (previous two pages) to identify features in the maps below. Plate A Plate C Plate B Plate D

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