1. Plate Tectonics

2. Objectives
To be able to describe the history and supporting evidence for plate tectonic theory
To be able to explain the basic principles of plate tectonic theory
To be able to describe & draw the 3 types of plate boundaries
To be able to describe features on Earth explained by plate tectonic theory

3. Definition from the Greek τέκτων; tektōn, meaning “builder” or “mason”
The main features of plate tectonics are:
The Earth's surface is covered by a series of crustal plates.
The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated.
Convection currents beneath the plates move the crustal plates in different directions.
The source of heat driving the convection currents is radioactivity deep in the Earths mantle.

4. History
Alfred Wegener first proposed concept of continental drift (1915)
Wegener said all continents were joined together in one landmass called Pangaea
He used circumstantial evidence (puzzle-like fit of continents, fossil data, similar rock types and ages, glacial striations) to support his theory
He proposed a “pole-fleeing force” to explain Pangaea’s drift away from the South Pole toward the equator and gravitational forces of the sun and moon to explain the westward drift of North America
His theory was rejected; he died in Greenland in His body is presumed to be buried under >300 feet of ice.

5. Evidence Cited by Wegener

6. History (cont.)
Post World War II – technological advances led to detailed mapping of the seafloor; ridges and trenches discovered
Age dating of oceanic crust showed seafloor rocks are youngest at the ridges and oldest toward the trenches
Paleomagnetic data showed a record of reversals preserved in the volcanic rocks of the seafloor
Seafloor spreading proposed in by Howard Hess and R. Dietz
Convection model proposed as mechanism explaining crustal motion; HEAT drives the system
Plate tectonic theory accepted by the late 1960’s

7. Basic Principles
The lithosphere exists as separate and distinct plates (<10 major plates; up to a few dozen including microplates)
Plates ride on the fluid-like asthenosphere
Plate motions range from ~1-4 cm/yr (as fast as fingernails grow) to ~16 cm/yr (as fast as hair grows)

9. Compositional vs. Mechanical Layering

10. The Lithosphere and Earth’s Interior  
Oceanic crust
Continental crust
Average thickness
5-10 km
30-70 km
Density
3.2 g/cm3
2.7 g/cm3
Composition
Rich in Fe, Mg Poor in Si, Al
Rich in Si, Al Poor in Fe, Mg

11. Plate Tectonic Map

12. Tectonic Plate Motions

13. California Seismicity

14. Types of Plate Boundaries
Divergent (AKA constructive; e.g. Salton Sea in Southern California)
Transform (e.g. San Andreas Fault)
Convergent (AKA destructive)
ocean-ocean (e.g. Mariana Islands)
ocean-continent (e.g. Pacific Northwest  Cascadia Subduction Zone)
continent-continent (e.g. India-Asia)

15. Divergent

16. Transform

17. Convergent

18. Phenomena on Earth Explained by Plate Tectonic Theory
Mountains
Trenches
Midocean Ridges
Earthquakes
Volcanoes
Mineral/Ore Distribution
Resource Distribution (i.e. geothermal, etc.)
Island chains and arcs
Faulting and Folding of Rocks
Et Cetera…

19. Summary Alfred Wegener  continental drift led to plate tectonics
Supporting evidence includes: puzzle-like fit of continents; fossil, rock, glacial, and paleomagnetic data; seafloor spreading
Driven by convection
Explains most geologic phenomena on Earth