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Plate Tectonics

• Introduction• Predecessor of plate tectonics: Continental drift hypothesis• What is the theory of plate tectonics? • What is a plate? • How do we find their edges? • How many plates are there?• Why is plate tectonics important?

• Types of relative plate movements (and related stresses)

• Types of plate boundaries

• Plate boundaries and earthquake depths

• Measuring plate motions

• Why do plates move?

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News, articles, etc. on Plate Tectonics https://www.diigo.com/outliner/6vks13/Plate-Tectonics?key=hrh02jmyit This Dynamic Earth - USGS

Continental Drift

In 1912, Alfred Lothar Wegener (1880-1930), a German meteorologist, published the hypothesis of continental drift with multiple lines of evidence to back it up. Wegener proposed that all of the continents were once joined together as one supercontinent that he called Pangea.

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This Dynamic Earth - USGS

Fossil evidence for once joined continents

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Related puzzle activity: http://volcanoes.usgs.gov/about/edu/dynamicplanet/wegener/This Dynamic Earth - USGS

Later studies have allowed the positions of the continents at different times in Earth history to be determined, and have documented the breakup of Pangea.

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http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?PlateMoTime

Plate Tectonics

Today, we know that it isn’t just the continents that have moved. Rather, the Earth’s surface is broken into pieces of continental and/or ocean crust called

plates. Plate tectonics deals with the nature of these plates, what happens at their boundaries, how and why they move, etc.

Fig 20.3 - Understanding Earth

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Plate tectonics also explains the locations of earthquake faults, volcanoes, and major mountain ranges.

Photos from: This Dynamic Earth (USGS) and Understanding Earth

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Patterns in the locations of earthquakes and volcanoesActive continental margins (plate boundary)

vs.Passive continental margins (no plate boundary)

The western margin of North America is an active area with earthquakes (yellow) and volcanic activity (red). In contrast, the eastern margin of North America is a quiet passive area with no active volcanoes and few earthquakes. We now know that the active zones coincide with the boundaries between plates (blue).

USGS

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http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?PlateBound

The discovery of extensive ocean ridges and young ages for the ocean floor (in the 1940s and 1950s) provided another important clue.

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Pioneering woman who mapped the ocean floorhttps://www.theguardian.com/science/2016/mar/13/terrawatch-kate-ravilious-marie-tharp-mid-atlantic-ridge?CMP=twt_a-science_b-gdnscience

Marie Tharp, Pioneering Mapmaker of the Ocean Floorhttp://www.ldeo.columbia.edu/news-events/remembered-marie-tharp-pioneering-mapmaker-ocean-floor

Sea floor magnetism was a key part of mapping ocean floor age and figuring out the processes responsible for the pattern

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This Dynamic Earth - USGS

Zones of progressively deeper earthquakes revealed the existence of subduction. This led to an understanding of how ocean crust is recycled at ocean trenches.

Geology 2nd ed. -Chernicoff

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This Dynamic Earth - USGS

What is a plate? – The brittle outer part of the earth (crust and uppermost mantle) is called the lithosphere. The lithosphere is broken into plates that move on the asthenosphere, a part of the mantle which is plastic (able to flow).

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Extension

Compression

Shear

Normal faults

Stretching and thinning

Strike-slip faults

Shearing

Reverse and thrust faults

Folding

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Types of relative plate movements and boundariesand related stresses and fault types

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This Dynamic Earth - USGS

13The mid-ocean ridge (shown in red) winds its way between the

continents much like the seam on a baseball This Dynamic Earth - USGS

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This Dynamic Earth - USGSFig 1.15 - Understanding Earth

Mid-Atlantic Ridge at Iceland

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East African Rift

This Dynamic Earth - USGS

Continental Rifting16

Continental rifting: The Afar Triangle

Photo from Space Shuttle:STS061-079-024 - NASA

East African Rift

Gulf of Aden

Red Sea

Africa

SOUTH

Next: 20-Plate Divergence.mov(Red Sea rift)

17http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?DivergBoundFig 20.3 - Understanding Earth Geology 2nd ed. - Chernicoff (originally from Bullard,

1969, The Origin of the Continents: Scientific American)

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This Dynamic Earth - USGS

Transform Boundaries

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http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?TranFault

Transform plate boundarySan Andreas fault, California

Fig 1.17 - Understanding Earth

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Ocean-ocean convergent boundary This Dynamic Earth - USGS21

Japan, an example of an oceanic island arcPhoto from Space Shuttle: STS059-218-044 - NASA

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Ocean-continent convergent boundary This Dynamic Earth - USGS23

http://www.npr.org/templates/story/story.php?storyId=124361777(Charles Darwin witnessed 1895 Chilean quake, deduced that the Andes required millions of years to build)

Intrusive igneous rocks formed along an ancient volcanic arc: Part of the Sierra-Nevada batholith in Yosemite National Park

Next: 20-Ocean-Continent.mov (Andean subduction zone)

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Continent-continent convergent boundary This Dynamic Earth - USGS25

Formation of the highest mountains on earth by continent-continent collision

This Dynamic Earth - USGS

Next: 20-Continent-Continent.mov(India-Asia collision zone)

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PT Summary: http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?MotionatPlate12

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Fig. 10-16 Chernicoff - Geology

All types of plate boundaries can produce shallow earthquakes. The deepest earthquakes (and some of the largest earthquakes) occur in subduction zones.

Plate boundaries and earthquake depths 28

Map of part of the Pacific ocean floor showing the 6,000-km-long Hawaiian Ridge-Emperor Seamounts chain – This volcanic trail of the Hawaiian “hotspot” could be related to a mantle plume This Dynamic Earth - USGS

Measuring plate motions - using hot spots29

dia.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?HotSpot12

Hotspots and plate movements

Geology 2nd ed. - Chernicoff

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Hot spots and the opening of the south Atlantic Oceanhttp://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/hotspots/index.htm

http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/index.htm

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Measuring plate motions - using sea floor age 31

Fig 20.11 - Understanding Earth

A GPS ground receiver at Augustine Volcano (Cook Inlet, Alaska) recording signals sent by GPS satellites. Artist's conception of a Global Positioning System satellite in orbit shown in inset. Source: USGS

32How we know: http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/index.htmNASA GPS Time Series plate movements: http://sideshow.jpl.nasa.gov/post/series.htmlGPS time series: http://en.wikipedia.org/wiki/File:Global_plate_motion_2008-04-17.jpgMore GPS http://www.dpc.ucar.edu/VoyagerJr/gpsplatebound.html

How do we know plates move today? From Earthquakes, GPS, VLBI, & SLR

Major plate boundaries and their rates of movement33

Since 1994, Australia has moved about 5 feet! http://newatlas.com/australia-gda2020/44674/

Fig 20.12 - Understanding Earth

Projected future positions of the continents - 100 million years from now: If current plate movements continue unchanged, part of California will become an island off the coast of Washington, the Mediterranean Sea may close, and eastern Africa may become an island. Geology 2nd ed. - Chernicoff

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The general scientific consensus is that plate movements are related to convection.

Convection in the Earth results from the escape of heat from the interior and involves the pull of gravity on rocks of different densities.

Essentially, colder, more dense rocks sink while hotter, less dense rocks rise.

Plate Tectonics:

Why do plates move?

Fig 1.13 - Understanding Earth

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Fig 20.25 - Understanding Earth

Suggested mechanismsto drive plate motions

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Best fit to evidence for major force causing

plate movements

Doesn’t match rates

of movement

Partly true, but convection not as organized as this

Deep plumes exist, but are not a major

cause of plate movements

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Slab pull – probably the most important driver of plate movements

Image source: This Dynamic Earth – USGS http://pubs.usgs.gov/publications/text/dynamic.html

- cold, relatively dense lithosphere is pulled downward at subduction zones and pulls the rest of the plate along behind it

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http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?ConvecTect

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Image source: http://www.gly.uga.edu/railsback/DynamicPlateTectonics.html

Not only do subducting plates sink, the boundary can also “roll back” over time. This causes the subduction zone to pull on both plates.

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travels with west-moving plate.ck of trenchCompression of continent because of subduction

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New sea floor made at

spreading ridge

Subduction with rollback pulls on

plates

New sea floor made

Plate boundary locations change over time

This Dynamic Earth - USGS

Subduction of an ocean ridge led to formation of the San Andreas fault

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This Dynamic Earth - USGS

Subduction of an ocean ridge led to formation of the San Andreas fault

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This Dynamic Earth - U.S. Geological Survey

http://pubs.usgs.gov/publications/text/dynamic.html

Paleomap Project - Continent locations of the past, present and future.

http://www.scotese.com/

Plate Tectonics - U.S.G.S. Cascades Volcano Observatory

http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/framework.html

What is Plate Tectonics? – USGS and National Park Service

http://www2.nature.nps.gov/geology/usgsnps/pltec/pltec1.html

Plate Tectonics Animations – USGS and National Park Service

http://www2.nature.nps.gov/geology/usgsnps/animate/pltecan.html

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Plate Tectonicshttp://www.ucl.ac.uk/EarthSci/people/lidunka/GEOL2014/Geophysics1-%20Plate%20tectonics/PLATE%20TECTONICS.htm

How we know plates move – GPS, VLBI, etc.http://www.see.leeds.ac.uk/structure/dynamicearth/plates_move/index.htm

NASA Space Geodesy Project – What is VLBI?http://space-geodesy.nasa.gov/techniques/VLBI.html

Digital Tectonic Activity Map (combines topography, plate boundaries, movements, etc.)

http://denali.gsfc.nasa.gov/dtam/

Wegener's Puzzling Evidence Exercise (6th Grade Activity)

http://volcanoes.usgs.gov/about/edu/dynamicplanet/wegener/

CT scan of Earth links deep mantle plumes with volcanic hotspots

http://www.sciencedaily.com/releases/2015/09/150902134939.htm

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