Google Classroom Questions
Below are questions you should be able to answer for our upcoming test on Plate Tectonics and things that result from tectonic plate movement. These are included in your Google Classroom assignment.
- What are tectonic plates?
- What is the difference between oceanic tectonic plates and continental tectonic plates?
- What is continental drift?
- What were Wegener's 4 pieces of evidence supporting his hypothesis.
- EXPLAIN what CAUSES continents to drift.
- List the 3 MAIN types of tectonic plate movement and draw arrows to show the direction of the movement.
- What is a ridge and what is a rift? How are they the same and how are they different?
- Which MAIN type of tectonic plate movement is subdivided into 2 types and what are these 2 subtypes?
- What is the reason that some converging plates collide and some converging plates subduct?
- What is a destructive plate boundary and what is a constructive plate boundary? Why are they called this?
- At which plate boundaries do volcanoes occur and where do they NOT occur? At which plate boundaries do earthquakes occur?
- What is the "Ring of Fire", where does it occur, and why is it called the Ring of Fire?
- What is a "hotspot?" Explain fully how island chains may be formed over hotspots?
VI. Plate Tectonics
A. The lithosphere (the crust + the uppermost part of the mantle) is broken into large pieces called tectonic plates.
1. There are 12 major tectonic plates and several minor ones 2. Oceanic plates are made of oceanic crust and underlying rocky uppermost part of the mantle a. Ex. Pacific Plate 3. Continental plates are made of continental and oceanic crust and underlying rocky uppermost part of the mantle a. Ex. North American plate includes North American continent and part of Atlantic Ocean crust |
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Tectonic Plate Movement & Continental Drift
B. Tectonic plates move or "float" on top of more dense mantle as it convects
1. Continental tectonic plates drift slowly with convection currents in the asthenosphere, causing continents to move over long periods of time a. Called continental drift |
Tectonic plates are pieces of lithosphere, which is composed of the "rocky" crust and uppermost part of the mantle. These tectonic plates float on the more dense mantle. We studied how the mantle has convection currents (much like our lava lamp demonstration on the previous webpage) that result from magma being heated by the Earth's core and rising towards the lithosphere. These currents, much like currents in the ocean or a river, cause the tectonic plates to slowly move like a boat on a river. Because some tectonic plates are composed of continental lithosphere, the movement of these continental tectonic plates cause what is called "continental drift."
Here's a really wild interactive link that lets you look at the Earth during those different time periods. |
4 types of evidence supporting Wegener's hypothesis of continental drift
Early on, people saw that the continents looked like they could fit together like puzzle pieces. The continents appeared to once fit together, but later seemed to come apart from each other. There were lots of ideas about how this might have happened (you may watch the optional video at the bottom of this webpage, called the History of Tectonic Plate Theory), but it was a German meteorologist named Alfred Wegener who first proposed the idea of continental drift. The teachers in the video below give a good summary of the 4 pieces of evidence that Wegener found to support his hypothesis, but it wasn't until much later (see Bill Nye video further down in this page) that scientists discovered evidence that support how continents drifted.
Look at the map on the left, which superimposes the boundaries of present day countries onto the Pangaea supercontinent 225 million years ago. "Think about this: You wake up in New York City, decide to go for a stroll, head east after breakfast, and a short time later, still on foot, you find yourself in Morocco." In an article written by Robert Krulwich, discover which countries were neighbors during the Paleozoic period when the supercontinent, Pangaea, existed. |
Supercontinent "Pangaea Proxima"
What if we looked 250 million years into the future? Here's what scientists predict our next supercontinent might look like based on what we know about tectonic plate movement.
What if we looked 250 million years into the future? Here's what scientists predict our next supercontinent might look like based on what we know about tectonic plate movement.
Volcanoes & Earthquakes
C. MOST of the world's earthquakes and volcanoes occur at the boundaries between tectonic plates...
1. And the majority of these volcanoes & earthquakes occur around the Pacific Plate boundary a. Called the “Ring of Fire” |
MOST of the world's earthquakes and volcanoes occur at the boundaries between tectonic plates. In particular, volcanoes occur at subduction plate boundaries and divergent plate boundaries (see what these are below). Volcanoes also occur over hotspots, which can occur on, near or far from plate boundaries. Earthquakes can occur at ALL plate boundaries. The Ring of Fire was so named because MOST of the world's active volcanoes occur around the enormous Pacific Plate boundary (see map on the left.) It is also where most of the earthquakes take place.
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Tectonic Plate Movement
There are 3 main types of tectonic plate movement, divergent, convergent and transform.
1. Diverging plates move apart from each other, exposing mantle and creating volcanoes, which, in turn, "construct" new lithosphere in the form of volcanic mountains and new seafloor. This is why a divergent plate boundary is also known as a constructive plate boundary. As the new lithosphere moves away from the divergent plate boundary, this is called seafloor spreading. A great example is the mid-Atlantic ridge.
2. Transform plates slide past each other and create great stress on the rock plates as they grind past each other. When the rock finally breaks under the stress, earthquakes occur. Typically, there are no volcanoes at transform plate boundaries. The Pacific Plate is moving northward in California where it is going in the opposite direction of the North American plate. This explains why California has so many earthquakes.
3. Converging plates move towards each other, and, depending on their relative densities, one may "subduct" beneath the other, OR the two plates may "collide" and push up to build mountains.
3a. Subduction: When two plates of unequal density converge, the more dense plate sinks or subducts beneath the less dense plate. And when the more dense plate subducts beneath a less dense plate, the subducting plate slowly dives into the hot mantle, causing the lithosphere to melt and get destroyed. This is why a subduction plate boundary is also known as a destructive plate boundary. The subducting plate is oceanic plate, so as it dives into the mantle, it carries water and all the dead shells and bones that settled to the ocean floor with it. The carbon from the shells and the water mixed with the melting rock produce carbon dioxide and other gasses, which then produce particularly explosive subduction volcanoes. Subduction boundary volcanoes are the gassy "peanut butter" volcanoes described in the video "Ring of Fire - Abbreviated." Where the plate dives down, it creates extremely deep "trenches" in the ocean. The subducting plate also pushes against the overriding plate and causes buckling and mountain building. Subduction boundaries can occur between more dense oceanic plate and continental plate, but they can also occur between two oceanic plates of unequal density. Remembering that material that is cooler is more dense that warmer materials, an older, colder (more dense) oceanic plate will subduct beneath a warmer, younger (less dense) oceanic plate. Why would one be older and colder? On the opposite side of the plate is a divergent plate boundary that is making new seafloor, which is warmer and less dense since it's closer to the volcanoes that are producing the new seafloor. This is happening near Japan and also near the Aleutian Island arc. It's called an island arc because the islands are formed along the subduction boundary by underwater subduction volcanoes.
3b. Collision When two plates of equal density converge (such as two continental plates converging), they push upward to build mountains. The pressure of the two plates can build up and, once the pressure is too great, the rocks break and create earthquakes. But there is no magma rising up at a colliding plate boundary, so there are no volcanoes. An example of this is where the Indian Plate is colliding with the Eurasian Plate and creating the Himalaya Mountains. These mountains are continuing to rise up and get higher!
1. Diverging plates move apart from each other, exposing mantle and creating volcanoes, which, in turn, "construct" new lithosphere in the form of volcanic mountains and new seafloor. This is why a divergent plate boundary is also known as a constructive plate boundary. As the new lithosphere moves away from the divergent plate boundary, this is called seafloor spreading. A great example is the mid-Atlantic ridge.
2. Transform plates slide past each other and create great stress on the rock plates as they grind past each other. When the rock finally breaks under the stress, earthquakes occur. Typically, there are no volcanoes at transform plate boundaries. The Pacific Plate is moving northward in California where it is going in the opposite direction of the North American plate. This explains why California has so many earthquakes.
3. Converging plates move towards each other, and, depending on their relative densities, one may "subduct" beneath the other, OR the two plates may "collide" and push up to build mountains.
3a. Subduction: When two plates of unequal density converge, the more dense plate sinks or subducts beneath the less dense plate. And when the more dense plate subducts beneath a less dense plate, the subducting plate slowly dives into the hot mantle, causing the lithosphere to melt and get destroyed. This is why a subduction plate boundary is also known as a destructive plate boundary. The subducting plate is oceanic plate, so as it dives into the mantle, it carries water and all the dead shells and bones that settled to the ocean floor with it. The carbon from the shells and the water mixed with the melting rock produce carbon dioxide and other gasses, which then produce particularly explosive subduction volcanoes. Subduction boundary volcanoes are the gassy "peanut butter" volcanoes described in the video "Ring of Fire - Abbreviated." Where the plate dives down, it creates extremely deep "trenches" in the ocean. The subducting plate also pushes against the overriding plate and causes buckling and mountain building. Subduction boundaries can occur between more dense oceanic plate and continental plate, but they can also occur between two oceanic plates of unequal density. Remembering that material that is cooler is more dense that warmer materials, an older, colder (more dense) oceanic plate will subduct beneath a warmer, younger (less dense) oceanic plate. Why would one be older and colder? On the opposite side of the plate is a divergent plate boundary that is making new seafloor, which is warmer and less dense since it's closer to the volcanoes that are producing the new seafloor. This is happening near Japan and also near the Aleutian Island arc. It's called an island arc because the islands are formed along the subduction boundary by underwater subduction volcanoes.
3b. Collision When two plates of equal density converge (such as two continental plates converging), they push upward to build mountains. The pressure of the two plates can build up and, once the pressure is too great, the rocks break and create earthquakes. But there is no magma rising up at a colliding plate boundary, so there are no volcanoes. An example of this is where the Indian Plate is colliding with the Eurasian Plate and creating the Himalaya Mountains. These mountains are continuing to rise up and get higher!
Divergent Plate Boundaries, Seafloor Spreading, Ridges & Rifts
E. Divergent Plate Movement
1. As 2 plates move apart, magma rises and creates new crust 2. Seafloor spreading - undersea divergence a. Newest crust is nearest volcanic vents b. Moves outward from volcanic vent i. ~ 3 cm/year (about as slowly as your fingernails grow) c. Called constructive plate boundary (because new seafloor is created) Divergent plate boundaries occur where two plates are moving apart. As they do, the mantle is under less pressure, which causes it to become more like a liquid (think of our Oobleck experiment). The magma rises up to the surface, creating volcanoes. Notice the image on the right, which shows the magma rising up. Under the cold ocean, the molten rock solidifies, which creates volcanic mountains and new seafloor. As the tectonic plate continues to move away from the boundary, more seafloor is created and the older seafloor is now further away from the divergent boundary. This is called seafloor spreading. Because new seafloor is created, we also call divergent plate boundaries a "constructive plate boundary." Notice also that the other side of the same plate is subducting below a continental plate and melting, thus destroying it. This explains why the Earth isn't getting larger. d. Ex. Mid-Atlantic ocean ridge i. 12,000 mile volcanic mountain range deep under the Atlantic Ocean, except…. ii. ...where the ridge rises above the ocean in Iceland and becomes a rift 3. Rift valleys- continental divergence a. Similar to seafloor spreading, except on land b. As the plates pull apart, water eventually fills in i. Ex. Iceland & “Rift Valley” in Africa Map showing plate boundaries where Great Rift Valley is splitting African continent in Kenya. Note that the island of Madagascar split off from the African continent about 65 million years ago. You can see how it fits like a puzzle piece just southwest of Madagascar, which also shows you the direction that the plate is moving. You may (but don't have to) click on the image to get more detailed & complicated information than you need to know.
CLICK HERE to watch a short National Geographic video about a huge crack that opened up overnight in Kenya in 2018.
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Seafloor Spreading
Notice how the part of the plate that is nearest to the magma rising up floats a little higher on the asthenosphere. This is because the hot magma is heating the lithosphere at that location, causing it to be a little less dense than the older, colder seafloor that is further away from the divergent boundary.
Seafloor is generally composed of basalt, which is more dense than continental lithosphere, which is largely made up of less dense quartz. Because it is made of basalt and because the lithosphere near the continental plate boundary is older and colder, the oceanic plate will subduct beneath the continental lithosphere. There are locations where an older, colder oceanic plate will subduct beneath a warmer, newer oceanic plate (Japan, Aleutian Islands).
Ridges are underwater mountain ranges that occur at divergent plate boundaries. Rifts also occur at divergent plate boundaries, but they occur on land and create valleys. The sequence of images above show how continental crust pulls apart, creating a valley, which eventually fills in with water. Volcanoes also occur along rift valleys. Just like Madagascar did about 65 million years ago, the part of Africa east of the African rift will eventually break free from the African continent and become an ocean island. Iceland, which lies on a divergent plate boundary, may also break into two pieces. When the rift eventually becomes submerged below water, it will form an underwater ridge. But there's something else going on in Iceland that is creating new island (see "Hotspots" later on).
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Convergent Plate Boundaries
F. Convergent plate boundary
1. Two subtypes: subduction & collision 2. Subduction: where the more dense plate moves beneath the less dense plate a. Typically occurs between oceanic (more dense) and (less dense) continental plates i. Can also occur between two converging oceanic plates (i.e. Japan, Aleutian Islands) b. The subducting plate carries rock, dead organic material & water into the asthenosphere i. Destructive plate boundary – melts rock, water vaporizes ii. Less dense gas & molten rock rises towards surface iii. Causes mountain building, earthquakes & volcanoes iv. Ex. Mt. St. Helens in Washington state Subduction volcanoes result near subducting plate boundaries because, as the plate melts, the less dense molten rock, the dead organic material (shells & bones) and water rises to the surface. Subduction volcanoes are particularly gassy and can erupt quite violently. 3. Continental collision a. Occurs between 2 converging continental plates i. Equal density plates collide and move upward ii. Causes mountain building & earthquakes, but NOT volcanoes (no magma rises at colliding plate boundaries) iii. Ex. Himalaya mountains |
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G. Transform Plate Boundaries
1. Two tectonic plates slide past each other
a. Ex. San Andreas fault: Pacific plate slides past the North American plate b. Creates fractures or breaks known as "faults" c. Not volcanic d. Called conservative plate boundary (plate is neither constructed nor destroyed) |
H. Hotspots
How Tectonic Plates Move (not notes!)
Scientists don't know exactly how the plates move, but evidence suggests 3 possibilities - perhaps all working together
a. Convection currents in the asthenosphere drag the tectonic plates like a ship floating on a current b. Slab pull: The weight of the more dense oceanic crust pulls the plates down into the mantle at subduction zones c. Ridge push: Magma coming up into the ridge pushes diverging plates apart |
Convection currents, tectonic plate movement, volcanoes,... oh,my
The diagram above has lots of words we learned. See if you can spot the part on the diagram illustrated by these words:
There's a place where seafloor spreading is occurring. It's not labeled, but can you find it?
- Convection current
- ocean ridge (is it diverging or converging?)
- hot spot volcano
- transform fault (where two plates slide past each other)
- rift volcano (just like in Iceland)
- subduction volcano
There's a place where seafloor spreading is occurring. It's not labeled, but can you find it?
Some interesting information & interactives....
The History of Tectonic Plate Theory
The video on the right talks about how the theory of plate tectonics came about.
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Map of earthquakes from 1975 -1995
Here's a map of where earthquakes occurred from 1975 to 1995. Compare this map to the tectonic plate boundaries. What do you notice?
Did you hypothesize that earthquakes occur at ALL tectonic plate boundaries? The evidence supports your hypothesis - mostly! Think about it.....tectonic plates are always on the move. This causes earthquakes. Earthquakes are happening all the time, but sometimes they are so minor that you cannot feel them. When pressure builds up between two tectonic plates as they move apart (divergence) or collide (subduction and collision) or transform (slide past each other), it eventually breaks the crust. When the pressure is very high, it can break rocks and create an earthquake.
Volcanoes and earthquakes: When pressure from rising magma and gases build up before a volcanic eruption, this, too, can cause an earthquake. Many times earthquakes and tremors will happen before the volcano actually erupts.
Did you hypothesize that earthquakes occur at ALL tectonic plate boundaries? The evidence supports your hypothesis - mostly! Think about it.....tectonic plates are always on the move. This causes earthquakes. Earthquakes are happening all the time, but sometimes they are so minor that you cannot feel them. When pressure builds up between two tectonic plates as they move apart (divergence) or collide (subduction and collision) or transform (slide past each other), it eventually breaks the crust. When the pressure is very high, it can break rocks and create an earthquake.
Volcanoes and earthquakes: When pressure from rising magma and gases build up before a volcanic eruption, this, too, can cause an earthquake. Many times earthquakes and tremors will happen before the volcano actually erupts.
Map of Volcanoes
Here's a map of where Earth's volcanoes occur. Compare this to your tectonic plates map. Do you notice anything?