1. Plate Tectonics Chapter 17

2. Early Observations In the late 1800s, Eduard Suess, an
Austrian geologist, hypothesized that the present southern
continents had once been
joined as a single
landmass that he named Gondwanaland.

3. Continental Drift
In 1912, Alfred Wegener, proposed that in the distant past, Earth’s continents were all joined as a single landmass.
He called this supercontinent Pangea
He said that the continents have separated and collided as they have moved over Earth’s surface for millions of years.

4. Continental Drift
Wegener proposed that if the land areas were brought back together, the move would line up ancient mountain ranges, similar continental rock formations, and evidence of ancient glaciers.
There are even similar fossils on both sides of the Atlantic.

5. Animation: Plate Movement
Continental Drift
Animation: Plate Movement

6. Evidence: Rock Formations
Some of the rocks of the
Appalachian Mountains in the United
States shared
similar features
with rocks in
Greenland and
Europe.

7. Evidence: Fossils
Similar fossils of several different animals and plants that once lived on land had been found on widely separated continents.
Reptiles that are only found in fresh water would not have crossed ocean waters.
Land-dwelling animals would not have swum that far of a distance.
Fern plant had been found on many continents that today, have very different climates.

8. Evidence of Continental Drift

9. Evidence: Climate Change
Coal deposits had been found in Antarctica indicated that this land once had a temperate, rainy climate.
Glacial deposits in Africa, India, Australia, and S America suggested that these areas had once been covered by thick ice caps.

10. A Rejected Hypothesis Wegner’s hypothesis had two flaws:
1. He could not explain what was causing the continents to move.
He suggested that the rotation of Earth could be possible, but physicists were able to show that this force was not great enough.
2. He could not explain how the contents were moving.
He suggested that the continents were plowing through a stationary ocean floor, but geologists argued that continents could not push through the ocean floor without fracturing and there was no evidence of such fracturing.

11. The Earth’s Layers The Crust Varies from 5-60 KM
In most places, a thin layer of sedimentary rocks covers the mostly granite-like rocks of the continental crust
The oceanic crust, under layers of marine sediments, is composed of darker and denser rocks similar to basalt

12. The Earth’s Layers The Mantle Extends to a depth of about 2900 KM.
Earthquake waves travel faster in the mantle than they do in the crust.
Composed mostly of dense, the dark mafic minerals olivine and pyroxene.

13. The Earth’s Layers The Core Composed of iron and nickel.
Outer Core is thought to be liquid because S-waves are unable to pass through the outer core.
The Inner Core seems to be solid.

14. Seafloor Spreading
Advancement in technology allowed scientists to study the ocean floor
Sonar: use of sound waves to measure water depth
Magnetometer: device that can detect small changes in magnetic fields

15. Evidence from the Oceans
In the 1950’s both fossils and the analysis of radioactive material showed the age of the oceanic crust increases with distance from the mid-ocean ridges.
Some oceans were growing wider from the middle.
Scientists also used magnetic measurements of the oceanic crust.

16. Ocean Floor Topography
Ocean ridges: underwater mountain chains
Mid-Atlantic Ridge, East Pacific Rise
Trenches: narrow, elongated depression in the seafloor with very steep sides
Mariana Trench (deepest)

17. Ocean Rocks & Sediments
1. ages of rocks vary in different places
Age of oceanic crust consistently increases increases with distance from a ridge
Rock near ocean
ridge = younger
Rock near
trenches = older

18. Ocean Rocks & Sediments
2. the thickness of ocean-floor sediment is much less than expected
Ocean floor sediments = a few hundred meters thick
Continents = up to
20 kilometers thick

19. Critical Thinking Question: Answer: Why aren’t seafloor sediments
as thick as continental sediments?
Answer:
thickness of sediments increases with distance from an ocean ridge
Sediments are thicker closer to trenches b/c the sediment supply is greater than it is near ocean ridges, which are often located in the middle of the ocean (hence, not as thick).

20. Magnetism Paleomagnetism: the study of Earth’s magetic field record .
Basalt is rich in iron-bearing minerals, therefore, provides an accurate record of ancient magnetism.
As basalt cools, the iron-bearing minerals become oriented parallel to Earth’s magnetic field.

21. The Geomagnetic Time Scale
Earth’s magnetic field periodically reverses (magnetic reversal)
Different ages provide a record of when the Earth’s magnetic field reversed its polarity.

22. Magnetic Symmetry
Scientists noticed that the positive and negative areas formed a series of stripes that were parallel to ocean ridges.
Scientists were able to determine the age of the ocean floor from magnetic readings and create isochron maps.

23. Isochron Map
Isochron: a line on a map that connects points that have the same age.

24. Seafloor Spreading
Harry Hess proposed a theory called seafloor spreading which states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.
Magma is forced up toward the crust along an ocean ridge and fills the gap that is created.
When magma hardens, a small amount of new ocean floor is created

25. The Missing Link
Seafloor spreading was the missing link needed by Wegner to complete his model of continental drift.
Provides the answer of how these landmasses move

26. Plate Tectonics
The surface of Earth is composed of about a dozen major rigid, moving crustal plates and several smaller plates.
These plates contain areas of light continental rock and dense oceanic bottoms.

27. Plate Boundaries
Tectonic plates interact at places called plate boundaries.
Each type of boundary has certain geologic characteristics and processes associated with it.

28. Convergent Boundary Convergent boundaries occur from converging plates
plates that are moving toward each other.

29. Convergent Boundary
Subduction occurs when a dense oceanic plate dives beneath a lighter continental plate.
Subduction forms ocean trenches that are linear fractures and are the deepest parts of the oceans.

30. Three Types of Convergent Boundaries
1) Oceanic-Oceanic
One of the two plates is subducting beneath the other
Creates a deep-sea trench
The subducted plate descends into the mantle and melts which is then forced back to the surface
forms and arc of volcanic islands
ie: Mariana Trench/Islands & Aleutian Trench/ Islands

31. Oceanic-Oceanic Examples
Mariana Trench
Philippine Plate subducting under the Pacific Plate
Aleutian Trench
Pacific Plate is being subducted under the North American Plate

32. Three Types of Convergent Boundaries
2) Oceanic-Contintnetal
Denser oceanic crust is subducted
Creates trench & volcanic arc… insead of arc of volcanic islands, a series of volcanoes erupt along the edge of the continental plate
Mountain range with many volcanoes
ie: Andes Mountains & Peru-Chile Trench

33. Oceanic-Continental Example
Peru Chile Trench Nazca Plate is being subducted under the South American Plate

34. Three Types of Convergent Boundaries
3) Continental-Continental
Colliding edges of the continents are crumpled and uplifted to form a mountain range
ie: Himalayas
Indian Plate is
colliding with the
Eurasian Plate.
(no subduction)

35. Convergent Boundaries
Plate Boundary Movement

36. Divergent Boundary
A divergent boundary (rift) is found at the mid-ocean ridges where upwelling material creates new crust that moves away from the ridge in both directions.

37. Divergent Boundary
Over millions of years, seafloor spreading along a divergent boundary may cause an ocean basin to grow wider.
Atlantic Ocean = 2-3 cm/yr
Atlantic Ocean Growing Animation
Some divergent boundaries may form rift valleys: when continental crust begins to separate, the stretched crust forms long, narrow depression.
East Africa -

38. East African Rift Valley

39. Plate Boundaries
When a plate slides horizontally past another plate, they meet at a transform boundary.

40. Transform Boundary
Characterized by long faults, sometimes hundreds of km long, and by shallow earthquakes
Most offset sections of ocean ridges
Rarely do transform boundaries occur on
continents.
San Andreas Fault is the exception

41. San Andreas Fault

42. What Moves the Plates?
The transfer of thermal energy by the movement of heated matter is called convection.
The heating of matter causes it to expand and to decrease in density.
The warmed matter then rises.
The cooler part of the matter sinks.
This up-and-down flow produces a pattern of motion called a convection current.

43. Convection Currents

44. Push & Pull
How is mantle convection related to the movement of tectonic plates?
The weight of the uplifted ridge is thought to push an oceanic plate toward the trench formed at the subduction zone called ridge push.
The weight of a subducting plate helps pull the trailing lithosphere into the subduction zone in a process called slab pull.

45. Earthquakes
Any vibrating, shaking, or rapid motion of the Earth’s crust.
Most occur when stress builds along a zone of weakness or a break in the rock known as a fault.
When the crust shifts, energy is released.
The energy radiates in all directions through vibrations.

46. Hot Spots
In several places on earth, hot plumes of magma pierce the crust.
As a crustal plate moves over this source of magma, volcanoes form at the hot spot.
This movement of a plate leads to the formation of a chain of volcanoes of differing ages.
One example is the Hawaiian Islands.

47. Volcanic Hazards
When volcanoes erupt they may spew hot lava, hot ash, and/or toxic gases.
The lava and ash can bury cities, and the toxic fumes can suffocate people.
Volcanoes can also provide fertile soil that is composed of weathered volcanic material.

48. Earthquakes
The place underground where the break occurs is the focus of the earthquake.
The epicenter is the location at the Earth’s surface just above the focus.

49. Measuring Earthquakes
Magnitude (Richter Scale)
Seismographs are the most reliable measures of earthquakes.
Each increase in one unit of magnitude means a ten-fold increase in shaking.
Intensity (Mercalli Scale)
Based upon the reports of people who experienced the earthquake and observed the destruction.

50. Mercalli vs. Richter Scale

51. Seismic Waves
P-Waves
Primary (they arrive first), Pressure, or Push-Pull.
Material expands and contracts and particles move back and forth in the path of the wave.
Sound waves that travel through solids, liquids or gases.

52. Seismic Waves
S-Waves
Secondary (arrive later), Shear, or Side-to-side.
Material shears out of shape and snaps back.
Travels only through solids.

53. Seismic Waves Surface Waves Travel along the earth's surface.
The slowest waves but the ones that damage in large earthquakes.

54. An Earthquake’s Epicenter
Seismologists
Scientists who study earthquakes
Use the difference in the speeds of P and S-Waves from three seismic recording stations to locate the epicenter

55. Earthquake Origin Time
To find the origin time, a seismologist needs to know the arrival and travel time of the P-waves.

56. The Earth’s Layers

57. Earthquake Shadow Zones
When a major earthquake occurs, both P-waves and S-waves are received over most of the earth.
The opposite side of the earth will receive P-waves but no S-waves.
S-waves cannot penetrate the liquid outer core.
There is also a region where neither P-waves or S-waves are received.
Refraction (bending) of the waves at the mantle-core boundary causes this ring-shaped region known as the shadow zone.

58. Earthquake P-wave Shadow Zones

59. Earthquake P-wave Shadow Zones
A P-wave traveling through the outer core is labeled K.
A bounce off the core is labeled c.
A P-wave in the inner core is I.
S-waves do not pass through the core, because the outer core is fluid.

60. The “Ring of Fire”
A large number of the world’s volcanoes and seismic events occur around the edges of the Pacific Ocean.
Japan, the western coast of the United States are on the Ring of Fire.
These areas are damaged frequently by earthquakes and volcanoes.

61. Seismic Hazards
Earthquakes can cause damage by shaking, movement of the crust, or large waves in oceans, called tsunamis.
Seismic Risk Level Map for the U.S.
Probability of damage in 100 years. Blue = none green = minor yellow = moderate red = major