bc science connections 8...concept 1: earth’s surface is made of huge rocky plates •earth’s...
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BC Science Connections 8
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UNIT 4The theory of plate tectonics explains Earth’s geological processes
TOPIC 4.2
What are tectonic plates and how is their movement linked to geological processes?
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Topic 4.2: What are tectonic plates and how is their movement linked to geological processes?
• Thingvellir region of Iceland
– Area of sea floor spreading
– North American plate and Eurasian plate are slowly moving apart
Thingvellir region of Iceland: the rocky features on each side of the diver are the North American and Eurasian plates.
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Concept 1: Earth’s surface is made of huge rocky plates
• Earth’s lithosphere: outer layer of solid rock composed of crust and part of the upper mantle
• Consists of large slabs of rocky material called tectonic plates
– Some tectonic plates are only oceanic crust (Pacific plate)
– Most tectonic plates are oceanic crust and continental crust (North American plate)
Figure 4.9: Earth’s lithosphere consists of tectonic plates.
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Movement of Tectonic Plates
Tectonic plates move slowly and float on the asthenosphere.
• Asthenosphere: a layer that is made up of extremely hot material that behaves like plastic
– Less rigid than lithosphere (flows like toothpaste)
– Enables Earth’s plates to move because the hot material beneath them can flow
Figure 4.9: Earth’s lithosphere consists of tectonic plates.
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Discussion Questions
• Tectonic plates are composed of which parts of Earth’s layers?
• What are two types of tectonic plates? Describe two features of each.
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Concept 2: Tectonic plates move relative to each other, causing certain geologic activities.
• Tectonic plates move in different directions and at different rates relative to one another
• Plates interact with each other at their edges (plate boundaries)
Convergent plate boundary: when two tectonic plates collide
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Plate Boundaries
• Three types of plate boundaries:
– Divergent plate boundary
– Convergent plate boundary
– Transform plate boundary
Types of plate boundaries (left to right): divergent, convergent, transform
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Divergent Plate Boundary
• Divergent plate boundary:
– Where tectonic plates move apart and create new oceanic crust
– Occurs where the sea floor spreads along a mid-ocean ridge
– Can also occur in the middle of continents (continental rifting)
Divergent plate boundary
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Divergent Plate Boundary: Example
• Example:
– The Endeavour hydrothermal vents were created as the Juan de Fuca plate moved away from the Pacific plate
Endeavour hydrothermal vents
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Convergent Plate Boundary
• Convergent plate boundary:
– Where two tectonic plates collide
– Denser crust eventually goes below less-dense crust in a process called subduction
– Causes deep sea trenches to form
– Volcanoes and mountains form at these boundaries; earthquakes are common
Convergent plate boundary
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Convergent Plate Boundary: Example
• Example:
– Mount Garibaldi was created as a result of the Juan de Fuca plate subducting under the North American plate
Mount Garibaldi
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Transform Plate Boundary
• Transform plate boundary:
– Where two tectonic plates slide past each other horizontally
– Earthquakes are common
Transform Plate Boundary
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Transform Plate Boundary: Example
• Example:
– Queen Charlotte Fault results from movement along a transform plate boundary
– Earthquakes originate from movement along faults
October 2012: An 7.7 magnitude earthquake originated from movement along the Queen Charlotte Fault
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Discussion Questions
• What is subduction? When and why does it occur?
• Describe the geological processes that occur at a divergent boundary. Relate it to what you learned about sea floor spreading.
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Concept 3: Mantle convection contributes to tectonic plate movement.
• Tectonic plates move at a rate of 1 to 15 cm per year.
Figure 4.10
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Mantle Convection Drives Tectonic Plate Movement
Earth’s mantle is partially melted material.
• Energy from radioactive decay of some elements in Earth’s interior and core heats up parts of the mantle
Figure 4.11: Convection currents are thought to drive tectonic plate motion.
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Mantle Convection Drives Tectonic Plate Movement
• Mantle convection:
– Warmer, less dense material rises
– Cooler, denser material sinks
– Causes large convection currents in the mantle
– As the mantle material moves, it drags the tectonic plates above with it
Figure 4.11: Convection currents are thought to drive tectonic plate motion.
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How does mantle convection affect the movement of tectonic plates?
Two processes that determine how mantle convection affects the movement of tectonic plates:
• Ridge push
• Slab pull Figure 4.12: Ridge push and slab pull are thought to move tectonic plates.
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How does mantle convection affect the movement of tectonic plates?
Ridge push:
• New material pushes older material aside, causing tectonic plates to move apart
– Rising material spreads out as it reaches the upper mantle
– Causes the lithosphere to lift and push tectonic plates apart at divergent plate boundaries
Figure 4.12: Ridge push
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How does mantle convection affect the movement of tectonic plates?
Slab pull:
• Pulling of a tectonic plate due to gravity and subduction
– As leading edge of a subducting plate sinks, it pulls the rest of the plate with it at convergent plate boundaries
– Gravity and convection assist this movement
Figure 4.12: Slab pull
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Discussion Questions
• Describe mantle convection.
• Describe how slab pull and ridge push each contribute to plate movement.
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Summary: What are tectonic plates and how is their movement linked to geological processes?
• Earth’s surface is made of huge rocky plates.
• Tectonic plates move relative to each other, causing certain geologic activities.
• Mantle convection contributes to tectonic plate movement.