1. Drifting Continents Chapter 17.1
Plate Tectonics
Drifting Continents
Chapter 17.1

2. Vocabulary
Continental Drift – the continents were joined as a single landmass that broke apart 200 mya
Still drifting
Pangaea – ancient landmass made up of all the continents
Alfred Wegener – found evidence to support the theory of continental drift

8. Evidence of the Drift Rock formations
Large geologic structures, such as mountain ranges fractured as the continents split
There should be similar rock types on opposite sides of the Atlantic
Rocks on the Appalachians are identical to rocks in Greenland and Europe

9. Evidence of the Drift Fossil formations
Similar fossils of several different animals and plants that lived on or near land had been found on several different continents
Land dwelling animals could not possibly have swum the great distances between continents
Trilobites
Ages of fossils predated the breakup of Pangaea

10. Evidence of the Drift Climatic evidence
Fossils of plants indicating the same type of climate have been found
On different continents
In current climates where they wouldn’t have survived

11. Evidence of the Drift Coal deposits Glacial deposits
In Antarctica show that the land must have been at one time closer to the equator
Glacial deposits
290 million year old deposits found in warm climates
Land must have at one time been located near the south pole

12. Evidence of the Drift Wegener’s idea was generally rejected
Most scientists believed in the early 1900’s that the continents were fixed
2 flaws in the theory
What force was strong enough to move the continents?
How could the continents move through solids?

13. Plate Tectonics
Seafloor Spreading
17.2

14. Vocabulary
Sonar – use of sound waves to detect and measure objects under water

15. Magnetic reversal – when Earth’s magnetic field changes polarity between normal and reversed
Magnetic field demo

17. Magnetometer – used to map the ocean floor by detecting small changes in magnetic fields

18. Sometimes the magnetic field of the earth completely flips.
The north and the south poles swap places.
Such reversals, recorded in the magnetism of ancient rocks, are unpredictable.
They come at irregular intervals averaging about 300,000 years;
the last one was 780,000 years ago. Are we overdue for another? No one knows.

19. Isochron – imaginary line on a map that shows points of the same age; formed at the same time
Seafloor spreading – ocean crust is formed at mid-ocean ridges and destroyed at deep-sea trenches
Continuous cycle of magma intrusion and spreading

21. Ridges and Trenches

22. Mid-Atlantic Range – chain of underwater mountains that run throughout the ocean basins
Lenth of 65,000 km
Contains active and extinct volcanoes

26. Topography – change in elevation in the crust

28. Tectonic Plates
Plate Boundaries
17-3

29. Tectonic plates
Huge pieces of crust and upper mantle that fit together at their edges to cover Earth’s surface
12 major plates and several minor plates
Move slowly
In different directions and at different speeds in relation to each other
Edges are called boundaries

31. Types of Boundaries Divergent (divide) Boundary
Convergent (collision) Boundary
Transform Boundary

32. Divergent Boundaries
Definition – place where two of Earth’s tectonic plates are moving apart
Associated with volcanism, earthquakes, and heat flow
Found primarily in the seafloor

33. Divergent Plate Boundaries

34. Rift valley – long, narrow depression that forms when continental crust begins to separate at divergent boundaries

35. Convergent Boundaries
Three types
Oceanic – Oceanic
Oceanic – Continental
Continental – Continental
Subduction – process by which one tectonic plate slips beneath another tectonic plate

36. Oceanic – Oceanic Convergent Boundary

37. Oceanic – Continental Convergent Boundary

38. Continental – Continental Convergent Boundary
Subduction zone

39. Transform Boundaries – most likely to cause earthquakes

40. Transform Boundaries

45. Convection
Chapter 17.4

46. Back to Wegener
Remember the two flaws to his theory of continental drift?
What type of force could possibly move the continents?
How do the continents move through solids?

47. Convection is the answer!!!
Large scale motion in the mantle
Transfer of thermal energy by the movement of heated material

48. Let’s talk about States of Matter
As matter cools
It contracts
Becomes denser
Cooled matter than drops due to gravity
Warmer matter
Is displaced
And then rise

51. So how’s it work in the Earth?!?
Up and down flow produces a pattern of motion called a convection current
Convection currents aid in the transfer of thermal energy
From warmer to cooler regions

52. Earth’s mantle is composed of partially molten material
Radioactive decay heats up the molten material in the mantle
Causes enormous convection currents to move material throughout the mantle

53. Convection in the Mantle
Driving mechanism of plate movements
Stiff part of mantle attached to the crust (cool)
Farther below, the mantle is hot and pliable
So…
The cool drops and is heated
The warm rises and cools
And the cycle continues

54. So… how does it all get started?
Set in motion by subduction
Move just a few centimeters per year

55. So how are convergent and divergent movement related to mantle convection?
Rising material spreads out as it reaches the upper mantle
Causes both upward and sideways forces
Downward part of convection occurs where sinking force pulls tectonic plates downward