1. THE THEORY OF PLATE TECTONICS
Mr. Halfen
Jan. 2016

2. What is a theory?
theory - a scientific concept which explains a large range of observations and experiments and can be used to make predictions
This is vastly different from the “everyday” use of the word where it is used to mean a guess or hypothesis.

3. Wegener’s Evidence Wegener’s evidence for continental drift:
the general fit of the continents to one another
the continuation of mountain ranges across continents
Large bodies of granite are found in adjacent locations in So. America and Africa
Large deposits of coal are found in adjacent locations in No. America and Europe

4. Wegener’s Evidence Wegener’s evidence for continental drift:
the distribution of fossil ranges across continents
the distribution of glaciation across the continents
Fossil plants are found in the Arctic Ocean

5. Review - Continental Drift
Wegener’s Theory of Continental Drift stated that the continents were once joined in a large landmass called Pangea.
His theory explained numerous geologic and geographic observations
It was rejected because Wegener could not supply a reasonable means to move continents across the Earth.

6. Volcanoes Volcanoes erupt melted material from inside the Earth.
Volcanoes are not distributed randomly; they are found near ocean ridges, deep ocean trenches and “hot spots” where they make chains of volcanic islands.

7. Hot Spots
Hot Spots are locations of volcanoes that are long-lived and voluminous.
Associated with many hot spots are chains of inactive volcanoes.
The most famous chain is the Hawaiian Islands/Emperor Seamount chain.

8. Major Plate and Volcanoes

9. Earthquakes
Earthquakes release energy from built-up stresses in the Earth.
Earthquakes do not occur randomly across the surface of the Earth. They are found in the same places as volcanoes and along faults.

10. Deep Earthquakes
In deep earthquakes, the earthquakes follow a pattern of going deeper into the Earth away from the deep ocean trench.

11. Earthquake Epicenters

12. Ring of Fire
The Pacific “Ring of Fire” is the region running from New Zealand to Japan to Alaska to Chile where earthquakes and volcanoes are common.

13. Sea-floor Spreading
The sea floor seems to be moving apart at mid-ocean ridges.
Observation from sea floor explorations:
Volcanic ridges are found on ocean floor
The youngest rocks are found near the ridges and the oldest are furthest away.
Ocean floor basalts record Earth’s magnetic field; the alternating polarity of the magnetic field show parallel and symmetric patterns on either side of the ridge.
Deep ocean trenches are found near ocean/continent boundaries that have volcanoes.

14. Data Overload
Given the vast number of observations, geologists began to re-visit Wegener’s theory.
All the data seemed to fit his idea, but there still was no mechanism to move large sections of the crust.

15. Structure of the Mantle
Highly sophisticated analysis of earthquake data revealed the existence of a plastic-like layer about 200 km down into the mantle.
The mid-ocean ridges seemed higher than would be expected by their thickness. There had to be something occurring beneath them to raise their level.

16. Convection
Geologists realized that the mid-ocean ridges were being elevated by upwelling magma.
It seemed reasonable that the deep-ocean trenches were where the crust was being pulled down.
It was tentatively concluded that convection cells were present in the mantle and the plates were moving with the convection cells.

17. Plate Tectonics
With a mechanism for moving large bodies of crustal plates and the large volume of observational evidence, plate tectonics became the underlying theory in geology in the late 1960’s.

18. Theory of Plate Tectonics
The lithosphere is broken into 7 major plates and numerous minor plates.
Plates are composed of the uppermost mantle and oceanic and/or continental crust (aka, the lithosphere).
These plates interact in predictable ways and produce many geologic phenomena.
All of the observations cited in the previous slides, as well as the formation of mountain chains, are explained by this theory.

19. Plate Boundaries

20. Plate Boundary
A plate boundary is where the two or more of the Earth’s plate come in contact

21. Divergent Boundaries Movement of Plates: away from each other
Examples: Mid-Atlantic Ridge, East Pacific Rise, East African Rift Zone
Features: graben, voluminous basaltic volcanism, shallow earthquakes

22. Convergent Boundaries – continent-continent
Movement: towards each other
Examples: Appalachians, Alps, Himalayas
Features: long mountain chains, thrust faults, andesitic volcanoes, shallow & deep earthquakes

23. Convergent Boundaries – ocean-continent
Movement: towards each other
Examples: Andes, Appenines
Features: mountain chains, thrust faults, andesitic volcanoes, shallow & deep earthquakes

24. Convergent Boundaries – ocean-ocean
Movement: towards each other
Examples: Japan, Phillipines, Aleutian Islands
Features: thrust faults, basaltic and andesitic volcanoes, shallow & deep earthquakes

25. Strike-Slip Boundaries
Movement: side-by-side in opposite directions
Examples: San Andreas Fault, Alpine Fault in southern New Zealand
Features: hills, shallow earthquakes, extensive faulting