1. Plate Tectonics
Chapter 7
Earth Science

2. Bellwork Ch7 L.1
Pangaea: one supercontinent that all the continents were once part of
Continental drift: suggests that continents are in constant motion on the surface of Earth

3. Launch Lab p. 217 Carefully peel clementine (set fruit aside)
DO NOT EAT
Try to refit the peel back together like a puzzle
Trade with your neighbor and try to reassemble each other’s peel
Which peel was easier to reconstruct? Why?
Look at a map, do the continents seem to fit together?
What additional evidence would you need to convince yourself that they were once together?
TURN IN ANSWERS. Now, you may eat your fruit.

4. Pangaea Theory proposed by Alfred Wegener
all continents are constantly moving
all continents were once part of a supercontinent

5. Evidence for continental drift
Climate clues: study sediments deposited by glaciers on different continents
Glacial grooves= scratches in rocks made where glaciers were moving
found on neighboring continents

6. Evidence for continental drift cont’d
Fossil clues
Look at animals present on different continents, some are specific to a single continent (kangaroos in Australia)
Similar fossils found on separate continents!
Glossopteris- fossil of a plant found on five different continents.

7. Evidence for continental drift cont’d
Rock clues
Determine where rocks formed
Rock formations on different continents had similar origins

8. Missing evidence HOW did the continents move?
What were the forces?
Continental drift not accepted until decades after Wegener death.

9. Homework Ch7 L.1 p. 222 #1-8 Study for quiz
OUTLINE OF THE CHAPTER IS NOW THE EXTRA CREDIT ASSIGNMENT

10. Bellwork Ch7 L.2
Mid-ocean ridge: mountain ranges in the middle of oceans
Seafloor spreading: process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge
Normal polarity: a state in which magnetized objects, such as compass needles, will orient themselves to point north.
Magnetic reversal: reverses the magnetic field direction
Reversed polarity: the state in which magnetized objects would reverse direction and orient themselves to point south

11. Mapping the ocean floor
Echo sounder used to determine depth of ocean floor
Created topographical map
Saw vast mountain ranges
Mid-ocean ridges longer than any mountain range on land.

12. Seafloor spreading Steps
Seafloor spreads and mantle below melts forming magma
Magma rises through cracks and erupts to become lava
Lava cools on ocean floor to form basalt
rounded structures called pillow lavas
Seafloor spreads apart and crust moves away from ridge
closer to the ridge means younger
IF sea floor spreads then continents must be moving

13. Topography of the sea floor
Sediment accumulates over crust (over time)
close to ridge have almost no sediment
far from ridge have thick layer of sediment
smooth seafloor
Called abyssal plain

14. Moving continents around
Continents move as the sea floor spreads along a mid-ocean ridge
NOT through solid mantle or the sea floor

15. Magnetism
Iron-rich outer core= magnet that creates Earth’s magnetic field
Normal polarity= compasses point north
Polarity changes every few hundred thousand to a million years

16. Rocks and magnetism
Basalt on ocean floor contains elements that are magnetic
minerals act like little magnets
Lava cools and permanently records direction and orientation of the magnetic field at the time of eruption

17. Evidence to support theory
Scientists used magnetometer to measure and record magnetic signature of rocks from seafloor
Parallel magnetic strips on either side of mid-ocean ridge confirm that crust is carried away from ridge in opposite directions

18. Homework Ch7 L.2
p.230 #1-11
Study for quiz

19. Bellwork Ch7 L.3
Plate tectonics: Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other.
Lithosphere: cold and rigid outermost rock layer
Divergent plate boundary: forms where two plates separate
Transform plate boundary: forms where two plates slide past each other
Convergent plate boundary: forms where two plates collide
Subduction: where the denser plate sinks below the more buoyant plante in a collision
Convection: circulation of material caused by differences in temperature and density
Ridge push: force that pushes plates away from each other. Caused by rising mantle material at mid-ocean ridge.
Slab pull: force caused by a sinking slab that pulls on the rest of the plate

20. Surface area conundrum
If crust continues to form and is never destroyed then the area of the Earth should keep getting bigger and bigger
IT DOESN’T!

21. Plate tectonics theory
Two points
Earth’s surface is divided into large plates
Each plate moves over Earth’s surface
Plates separate in ocean = earthquake and mid-ocean ridge formation
Plates come together= (one plate dives under another one) earthquake and chain of volcanoes form
Plates slide against one another= earthquake

22. Tectonic plates Plates float on hot mantle (called asthenosphere)
Pacific plate is the largest
Boundaries can mark locations of mid-ocean ridges
Plates are large pieces of lithosphere
Lithosphere: cold and rigid outermost rock layer of Earth
made of crust and upper mantle
thin below mid-ocean ridges
thick below continents

23. Plate Boundaries Exists where plates meet Three kinds: Divergent
Transform
Convergent

24. Divergent plate boundary
Forms where two plates separate
Mid-ocean ridges located at divergent plate boundaries
Rift valleys formed by divergent plates on land

25. Transform plate boundaries
When two plates slide past one another
Can get stuck and stress breaks rock apart and then keeps moving
rapid movement results in earthquakes

26. Convergent plate boundaries
Form where two plates collide
the denser plate sinks below the more buoyant plate (called subduction)
the denser plate sinks into the subduction zone
Oceanic x Continental collision
Denser Oceanic subducts under continental
Forms volcanoes above subduction plate
Continental x Continental collision
mountains form from uplifted rocks. No subduction.

27. Density lab p.233 Work in pairs Which liquid has the highest density?
Each pair gets a set of 4 test tubes with different liquids in them
Drop a bead of equal density into each test tube
Observe what happens to each bead
Which liquid has the highest density?
How did you determine which liquid has the highest density?
What happens when rocks of opposite densities collide?

28. Evidence for plate tectonics
Scientists can now measure how fast continents are moving
Wegener couldn’t do this
Explains why earthquakes and volcanoes occur in certain positions
Where do you think they occur?

29. Plate Motion Explains the HOW of Wegener’s theory Convection Currents
circulation of material caused by differences in temperature
Convection in the mantle (radioactive elements heat mantle)
Heated elements expand and contacts Earth’s crust transferring thermal energy
Heated elements then cool and sink back down. This expansion and sinking creates a current.

30. Plate motion cont’d Forces causing plate motion Basal Drag Ridge Push
caused by convection currents, they drag lithosphere similar to a conveyor belt moves groceries.
Ridge Push
is the force that moves the plates away from the ridge by using the potential from the rising mantle at the mid-ocean ridge. Pushes the lithosphere in opposite directions.
Slab Pull (slab= denser plate that sinks in subduction zone during collision)
as the slab sinks it pulls the rest of the plate with a force

31. A theory in progress Unanswered questions:
Why is the Earth the only planet (in our system) with plate tectonics?
Why do earthquakes and volcanoes occur far away from plate boundaries?
What forces dominate plate motion?

32. Homework Ch7 L.3 p.241 #1-8 Study for quiz STUDY FOR TEST
Extra Credit: outline of Chapter 7