The Earth & Plate Tectonics
Slide show prepared by:
Robert Butler
Jenda Johnson
Kip Ault
The continent is a complex
mosiac of all geologic history
How do
continents grow?
Why do
earthquakes
occur in nearly
every US state?
How does
deformation
occurring in one
place affect
another place?
Earth
vs.
Egg
Earth radius = 6370 km
Lithosphere (plate) thickness = 100 ~2%
km
What % of Earth radius is lithosphere?
Egg radius = 0.75 inch
Egg shell thickness = 0.015 inch
What % of egg radius is shell?
0.016 % of the Earth’s radius is lithosphere.
0.026 % of the egg’s radius is shell.
Plate tectonics
Plates are driven by the cooling of the Earth.
Gravity provides additional force to move plates.
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.
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?
Seismicity & Distribution of 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/
World Seismicity & Plate Tectonics
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
Seismicity
Tectonics
and
Volcanoes
This map shows that the locations of subaerial (above
sea level) volcanoes correlate with earthquake locations.
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
What are the tectonic plates?
AKA: Lithospheric plate
• Is the ~100-km-thick surface of the Earth;
• contains crust and part of the upper mantle;
• is rigid and brittle; and
• fractures to produce earthquakes.
See video links in notes
USGS Graphics
What is the asthenosphere?
The asthenosphere is
• the hotter upper mantle below the lithospheric plate;
• a viscoelastic solid (NOT liquid!!); and
• can flow like silly putty.
See video links in notes
Deforming Earth’s Crust
Types of stress: Extension, Compression, Shear
Undeformed beds: no stress applied.
Extension makes faults and regional
thinning. (Ex., Basin & Range.)
Compression makes faults and folds.
(Ex., Rocky Mountains.)
Shearing displaces layers horizontally
and can result in strike-slip faulting.
(Ex., San Andreas Fault, California.)
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
See video and animation links in notes
Divergent Plate Boundaries


New crust is generated as the plates pull apart;
Occur on ocean floors and continental interiors;
 Earthquakes are shallow and small.
Fast-spreading Ridge
Example:
East Pacific Rise (moving
apart at about 15 cm/year)
Slow-spreading Ridge
Examples:
Atlantic mid-ocean ridge
Basin and Range, USA
African Rift Valley
Northern Red Sea
Transform Plate Boundaries
Lithosphere is neither produced nor destroyed as the plates
slide horizontally past each other.
Strike-slip fault—
San Andreas Fault, California
Transform fault—
a strike-slip fault between two
spreading ridges allows the
two plates to move apart.
Next slide: What is stress?
http://www.iris.edu/hq/programs/education_and_outreach/animations/27
Beijing>Powerpoints>003 Gulf of CAlif_EarthquakesAndTectonics.mp4
Convergent Plate Boundaries
Plates push together.
A) The denser plate subducts, or
B) two continental plates crunch
together and form high mountains.
Ocean /Ocean convergence (Marianas)
Continent/Continent Collision (Himalayas)
Ocean /Continent convergence (Cascades)
Next slide: Why and where would earthquakes occur in convergent boundaries?
Young Subducting Plate
Old Subducting Plate
Earthquakes along Convergent Zones with
Subducting Oceanic Lithosphere
Shallow earthquakes:
The most destructive
of these occur between the
plates on the plate boundary.
Intermediate and Deep:
Occur only within the
subducting oceanic
lithosphere.
See animation & video links in notes
Island Arc Subduction Zone
Obduction?
Island Arc Subduction Zone
http://www.iris.edu/hq/programs/education_and_outreach/animations/11
003 IRIS Convergent Zone Ocean-Continent
Sichuan 7.8 Crustal Quake (Overthrust Fault) 2008
Eurasian Plate
Tibetan Plateau
Himalayas
Indian Plate