Plate tectonics
• Plates are driven by cooling of Earth (convection)
• Gravity provides additional force to move plates.
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Modified from USGS Graphics
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.
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.
USGS Graphics
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!!
Three Basic Types of Plate Boundaries
Divergent
Using hands to show relative motion
Transform
Convergent
USGS Graphics
Three Basic Types of Plate Boundaries
Divergent
Transform
Convergent
USGS Graphics
Tectonic Plates
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?
Earthquakes
There are thousands of small earthquakes every day
“Strong” earthquakes (~M7) occur once a month. >M8 occur about once/year.
Where are the deepest earthquakes?
For earthquakes of the past 2 weeks, go to http://www.iris.edu/seismon/
Earthquakes & Plate Boundaries
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?
Create your own maps at http://www.iris.edu/quakes/maps.htm
Modified from USGS Graphics
Volcanoes & Plate Boundaries
This map shows that locations of volcanoes (ones above sea
level) also tend to occur along the plate boundaries
Modified from USGS Graphics
Tectonic Plates
How fast are the plates moving?
Plates move 1-10 centimeters per year (≈ rate of fingernail growth).
Fingernail growth plotted: http://jclahr.com/science/earth_science/thumbnail/index.html
Modified from USGS Graphics
Continental Drift
How fast are the plates moving?
Plates move 1-10 centimeters per year (≈ rate of fingernail growth).
Source: http://www.tectonics.caltech.edu/outreach/animations/index.html
Seafloor Spreading
Source: http://www.tectonics.caltech.edu/outreach/animations/index.html
Collision of ‘Drifting’ India with Eurasia
Side view of subduction, ‘drifting’ India, volcanoes, & mountain-building
Source: http://www.tectonics.caltech.edu/outreach/animations/index.html
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