Robert Butler Jenda Johnson Kip Ault The Earth & Plate Tectonics Slide show prepared by:
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Transcript of Robert Butler Jenda Johnson Kip Ault The Earth & Plate Tectonics Slide show prepared by:
How do continents grow?
Why do earthquakes
occur in nearly every US state?
How does deformation
occurring in one place affect
another place?
The continent is a complex mosiac of all geologic history
Earth vs. Egg
Earth radius = 6370 kmLithosphere (plate) thickness = 100 km
What % of Earth radius is lithosphere?
Egg radius = 0.75 inchEgg shell thickness = 0.015 inch
What % of egg radius is shell?
~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 the cooling of the Earth. Gravity provides additional force to move plates.
Modified from USGS Graphics
There are a dozen large lithospheric plates (smaller plates not shown).Some plates have continents; some don’t.
All are in motion.
Tectonic Plates
.
There are thousands of small earthquakes every day “Strong” earthquakes (~M7) occur once a month. >M8 occur about
once/year.
Seismicity & Distribution of Earthquakes
Where are the deepest earthquakes?
For earthquakes of the past 2 weeks, go to http://www.iris.edu/seismon/
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
World Seismicity & Plate Tectonics
Create your own maps at http://www.iris.edu/quakes/maps.htm
Modified from USGS Graphics
This map shows that the locations of subaerial (above sea level) volcanoes correlate with earthquake locations.
Seismicity Tectonics and Volcanoes
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
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
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.
USG
S G
raph
ics
See video links in notes
Deforming Earth’s Crust
Types of stress: Extension, Compression, Shear
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.)
Undeformed beds: no stress applied.
Three Basic Types of Plate Boundaries
Divergent
Convergent
Transform
USGS Graphics
Using hands to show relative motion
Three Basic Types of Plate Boundaries
Divergent
Convergent
Transform
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 RidgeExample: East Pacific Rise (moving apart at about 15 cm/year)
Slow-spreading RidgeExamples: Atlantic mid-ocean ridge Basin and Range, USAAfrican Rift ValleyNorthern 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
Ocean /Ocean convergence (Marianas)
Ocean /Continent convergence (Cascades)
Continent/Continent Collision (Himalayas)
Plates push together. A) The denser plate subducts, or B) two continental plates crunch together and form high mountains.
Next slide: Why and where would earthquakes occur in convergent boundaries?
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
http://www.iris.edu/hq/programs/education_and_outreach/animations/11
003 IRIS Convergent Zone Ocean-Continent