1. Evidence for Plate Tectonics
Earthquake & Volcano Activity
Teacher note: This slide is intended to show how nearly all of the world’s volcanoes and earthquakes are located at a plate boundary. These phenomena occur because of the stresses of colliding or diverging plates. The dark region on the map simply shows which part of the planet was under nighttime when the image was captured.

2. Evidence for Plate Tectonics
Changes in Sea Floor Magnetism
Evidence for Plate Tectonics
Continents fit together like a puzzle
Teacher note: Although the mechanism for magnetic pole reversals isn’t entirely understood yet, many scientists believe it occurs when large convection cells reverse their flow direction. Reversals occur every 500,000 years on average. The last reversal occurred 780,000 years ago, so the next is overdue. Recent evidence suggests that we are in the initial stages of a reversal. Studies suggest that a field reversals occurs quickly, taking only years to occur. This is considered very fast in Geologic time! Since rocks move away from the mid-ocean ridge, the black-white pattern is symmetric on either side of the ridge.

3. Evidence for Plate Tectonics
Changes in Sea Floor Magnetism
Evidence for Plate Tectonics
Fossils of plants and animals of the same species found on different continents.
Teacher note: Although the mechanism for magnetic pole reversals isn’t entirely understood yet, many scientists believe it occurs when large convection cells reverse their flow direction. Reversals occur every 500,000 years on average. The last reversal occurred 780,000 years ago, so the next is overdue. Recent evidence suggests that we are in the initial stages of a reversal. Studies suggest that a field reversals occurs quickly, taking only years to occur. This is considered very fast in Geologic time! Since rocks move away from the mid-ocean ridge, the black-white pattern is symmetric on either side of the ridge.

4. Evidence for Plate Tectonics
Changes in Sea Floor Magnetism
Evidence for Plate Tectonics
Rock Sequences in several continents were the same.
Teacher note: Although the mechanism for magnetic pole reversals isn’t entirely understood yet, many scientists believe it occurs when large convection cells reverse their flow direction. Reversals occur every 500,000 years on average. The last reversal occurred 780,000 years ago, so the next is overdue. Recent evidence suggests that we are in the initial stages of a reversal. Studies suggest that a field reversals occurs quickly, taking only years to occur. This is considered very fast in Geologic time! Since rocks move away from the mid-ocean ridge, the black-white pattern is symmetric on either side of the ridge.

5. Evidence for Plate Tectonics
Sea Floor Spreading
Teacher Note: This picture is meant to illustrate the symmetry of age for oceanic rocks on either side of a mid-ocean ridge spreading center. Note the hot-colored rocks near the spreading center are youngest and become older as one moves toward a continent. This observation would be impossible to substantiate without plate tectonic theory, but is a simple consequence of the theory once we begin to explore its implications.

6. Seafloor Spreading
In the 1960’s, a scientist named Henry Hess made a discovery about the ocean floor aboard the research ship Glomar Challenger
Using radar, he discovered the bottom of the ocean is not flat, it has trenches and ridges.
His team discovered that the youngest rocks are located at the mid-ocean ridges.
Picture from USGS

7. Age of the ocean floor (Mueller et al., 1996)

8. Evidence for Plate Tectonics
Changes in Sea Floor Magnetism
Teacher note: Although the mechanism for magnetic pole reversals isn’t entirely understood yet, many scientists believe it occurs when large convection cells reverse their flow direction. Reversals occur every 500,000 years on average. The last reversal occurred 780,000 years ago, so the next is overdue. Recent evidence suggests that we are in the initial stages of a reversal. Studies suggest that a field reversals occurs quickly, taking only years to occur. This is considered very fast in Geologic time! Since rocks move away from the mid-ocean ridge, the black-white pattern is symmetric on either side of the ridge.

9. About 200 million years ago, Earth’s tectonic plates were grouped together into a a supercontinent known as Pangea
most scientists agree on the general configuration of Pangea
well-constrained by the seafloor magnetic anomaly data
oldest anomalies in NW Pacific ~160 Ma

10. CONTINENTAL DRIFT
Alfred Wegener in the early 1900’s proposed the hypothesis that continents were once joined together in a single large land mass he called Pangea (meaning “all land” in Greek).
He proposed that Pangea had split apart and the continents had moved gradually to their present positions - a process that became known as continental drift.

11. CONTINENTAL DRIFT
According to the hypothesis of continental drift, continents have moved slowly to their current locations.

12. Pangaea about 200 million years ago, before it began breaking up.
Wegener named the southern portion of Pangaea Gondwana, and the northern portion Laurasia.

13. The continents about 70 million years ago
The continents about 70 million years ago. Notice that the breakup of Pangea formed the Atlantic Ocean. India’s eventual collision with Eurasia would form the Himalayan Mountains.

14. The position of the continents today
The position of the continents today. The continents are still slowly moving, at about the speed your fingernails grow. Satellite measurements have confirmed that every year the Atlantic Ocean gets a few inches wider!

15. Seafloor Spreading
Wegener was a meteorologist and his theory was not well accepted at the time he was publishing his ideas. (He died on an expedition in Greenland collecting ice samples)
One reason scientists had a hard time with Wegener’s theory is that there was no mechanism for the continents motion.

16. Seafloor Spreading Henry Hess proposed the sea-floor spreading theory.
Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges.
Then, it flows sideways, carrying the seafloor away from the ridge in both directions.
Picture from

17. Seafloor Spreading
As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created.
The older seafloor moves away from the ridge in opposite directions.
This helped explain how the crust could move—something that the continental drift hypothesis could not do.
Picture from

18. Mechanism for Plate Tectonics
Seafloor Spreading provided insight to the mechanism for how the continents moved.
The magma which pushes up at the mid-ocean ridge provides the new land pushing the plates, and the subduction zones gobble up the land on the the other side of the plates.
Picture from
The mechanism was convection currents!

19. Plate Tectonic Theory
Both Hess’s discovery and Wegner’s continental drift theory combined into what scientists now call the Plate Tectonic Theory.
Theory of plate tectonics:
The Earth’s crust and part of the upper mantle are broken into sections, called plates which move on a plastic-like layer of the mantle

20. Plate Tectonic Theory Plate Tectonics explains Earthquakes Mountains
Volcanoes