1. The Theory of Plate Tectonics
Based on two hypotheses
Continental Drift
Seafloor spreading

2. Continental drift hypothesis: evidence?
Proposed in 1911
All evidence found on the continents
Mechanism for moving the continents was not known

3. Continental Drift Hypothesis
Alfred Wegener proposed the hypothesis of continental drift in 1911
He gathered information from many different sources and used it as evidence for his hypothesis

4. Continental Drift Hypothesis
Fit of the continents
Fossil evidence
Ancient mountain ranges
Past climate evidence
Swamps formed in tropical regions
Glaciers formed at the south pole

5. The continental drift hypothesis proposes that the continents were assembled to form the super continent Pangaea.
Moved through time

6. Fit of Continents
Antonio Snider-Pelligrini (1858), a geographer cut out a map of Africa and South America suggesting they were connected at one time
Other physical evidence based on observation was used by Wegener

7. Fossil Evidence
Similar terrestrial species were found on many continents now separated by oceans.
Information collected by paleontologists

8. Ancient Mountain Ranges
The same sequence of rocks is found in North America, Great Brittan, and Norway. The pattern does not make sense with the continents in their current configuration.

9. Evidence of Ancient Glaciers
Glaciers carve the rock as they move.
Scientists can determine the direction of movement (notice the direction of movement noted in South America)
As South America sits today, the pattern would not make sense. (glaciers do not move from sea level to higher elevations)

10. Lack of mechanism to explain continental movement.
Evidence implied that the continents were at one time assembled
Wegener’s idea was not accepted by the scientific community because he could not explain how the continents moved
With advances in technology, new information was gathered in association with WWII

11. Exploration of the Ocean Basins
What information do you know regarding the seafloor that may be used to help understand the hypothesis of seafloor spreading?

12. Exploration of Ocean Basins
Ocean basins contain a variety of landforms not discovered until WW II.

13. Seafloor features were discovered using sonar.
Support
Seafloor features were discovered using sonar.
4:26
Echo sounding was used to map the seafloor

14. Oceanic crust is composed of volcanic rock.
Pillow basalts
Basalt is volcanic rock with a mafic composition.
Higher percentages of iron and magnesium, lower percentages of silicon and oxygen.

15. Seafloor Spreading
In the 1963, Harry Hess developed the idea of seafloor spreading to explain the seafloor’s formation
Continental movement is the result of ocean floor movement

16. Seafloor Spreading
Mafic lava is extruded along the mid-oceanic ridge to produce ocean floor.
New material is added, the cooler material is split and is “pushed” away from the ridge

17. Driving Mechanisms Convection cells within the mantle
Push
Pushing at the mid-oceanic ridges
Pull
Pulling at the subduction zones
Convection

18. The theory of plate tectonics
Explains that the Earth’s lithosphere is broken into distinct units that move as a coherent package. Where the tectonic plates meet, produces distinct physical characteristics on the Earth’s surface.