Plate tectonics describes the large rigid plates that make up the Earth's surface and their movement over time. Convection currents in the Earth's mantle provide the driving force that causes plates to move at rates of 1-10 cm per year. Plates interact at boundaries which can be divergent, convergent, or transform. Features associated with plate boundaries include volcanoes, earthquakes, mountains, and ocean trenches.

1. Plate Tectonics
• What are the “plates”
• What causes them to move
• What are the different types of
plate boundaries
• What features are associated
with plate boundaries

2. Convection is like a boiling pot. Heated soup rises to the surface, spreads and begins to
cool, and then sinks back to the bottom of the pot where it is reheated and rises again.
Plate tectonics
• Plates are driven by cooling of Earth (convection)
• Gravity provides additional force to move plates.
Modified from USGS Graphics
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3. What are the tectonic plates?
AKA: Lithospheric plate
• The ~100-km-thick surface of the Earth;
• Contains crust and part of the upper mantle;
• It is rigid and brittle;
• Fractures to produce earthquakes.

4. What is the asthenosphere?
Asthenosphere:
• Is the hotter upper mantle below the lithospheric
plate;
• Can flow like silly putty; and
• Is a viscoelastic solid, NOT liquid!!
USGS
Graphics

5. Three Basic Types of Plate Boundaries
Divergent
Convergent
Transform
USGS Graphics
Using hands to show relative motion

6. Three Basic Types of Plate Boundaries
Divergent
Convergent
Transform
USGS Graphics

7. Plate Boundaries -Iceland

8. There are a dozen large lithospheric plates (smaller plates not shown).
Some plates have continents; some don’t. All are in motion.
Question: What evidence is there for these plate boundaries?
Tectonic Plates

9. Subduction
•http://www.tectonics.caltech
.edu/outreach/animations/su
bduction.html

10. Tsunami
•http://www.tectonics.cal
tech.edu/outreach/anim
ations/tsunami.html

11. What causes plates to move?

12. What causes plates to move?

13. There are thousands of small earthquakes every day
“Strong” earthquakes (~M7) occur once a month. >M8 occur about once/year.
Earthquakes
Where are the deepest earthquakes?
For earthquakes of the past 2 weeks, go to http://www.iris.edu/seismon/

14. Notice that the earthquakes coincide with plate boundaries,
and the deepest quakes (blue) are in subduction zones.
Question: Where would you expect to see volcanoes?
Modified from USGS Graphics
Earthquakes & Plate Boundaries
Create your own maps at http://www.iris.edu/quakes/maps.htm

15. Modified from USGS Graphics
This map shows that locations of volcanoes (ones above sea
level) also tend to occur along the plate boundaries
Volcanoes & Plate Boundaries

16. How fast are the plates moving?
Plates move 1-10 centimeters per year (≈ rate of fingernail growth).
Tectonic Plates
Modified from USGS Graphics
Fingernail growth plotted: http://jclahr.com/science/earth_science/thumbnail/index.html

17. How fast are the plates moving?
Plates move 1-10 centimeters per year (≈ rate of fingernail growth).
Continental Drift

18. Collision of ‘Drifting’ India with Eurasia
Source: http://www.tectonics.caltech.edu/outreach/animations/index.html
Side view of subduction, ‘drifting’ India, volcanoes, & mountain-building

19. Note on Source: First 8 slides are modified from a slide show prepared by Dr. Robert
Butler, University of Portland, and Jenda Johnson,
…the full source of which can be found at:
http://www.iris.edu/hq/programs/education_and_outreach/videos#B