plate tectonics - o'mara's science...
TRANSCRIPT
Plate Tectonics
Sir Francis Bacon, 1620, noted that the continental coasts on
opposites sides of the Atlantic fit together like puzzle pieces.
Could North and South America once have been joined to Europe
and Africa?
Fossil evidence from Western Africa and Eastern South America
supports this assertion (Mesosaurus, Glossopteris)
Alfred Wegener• 1915 - German geophysicist
• Tried to explain climate patterns
indicated by fossils
• Suggested all continents might have
been a single super continent – Pangaea
– Said that Pangaea began to break apart
180 million years ago
Although rejected, Wegener’s idea did not die…
1940 - Hugo Benioff plotted locations of deep earthquakes at edge
of Pacific (Pacific Ring of Fire)
Seismographs revealed that earthquakes and volcanoes do not occur
at random - but in zones
Other support:
Radiometric dating - oceanic crust much younger than Earth
Echo sounders used to map sea floor (ocean ridges match shorelines)
1960 - Harry Hess suggests that new seafloor develops at ocean
ridges and spreads outward with trenches along edges of ocean
basins, powered by convection currents in the mantle
Hypothesis: Seafloor Spreading!
94% of energy comes from radioactive atoms breaking down releasing energy,
must be areas of cooling—near trenches!
Seafloor spreading was an idea
proposed in 1960 to explain the
features of the ocean floor. It
explained the development of the
seafloor at the Mid-Atlantic Ridge.
Convection currents in the mantle
were proposed as the force that
caused the ocean to grow and the
continents to move.
(right) The Mid-Atlantic Ridge
conforms to the shape of the
adjacent continents. The inset
shows the central rift.
Seafloor Spreading - A Key Idea
Evidence for Seafloor Spreading…
• 1963 - Frederick Vine & Drummond
Matthews
– Magnetic patterns in ocean floor were
mapped
– Noticed that there were magnetic patterns
on either side of an ocean ridge
– Crust is moving like a large conveyer belt
away from ocean ridges!– http://www.wwnorton.com/college/geo/egeo/f
lash/2_3.swf
• 1965 - Sir Edward Bullard used a computer to
compare coastlines at the continental shelf edge
Theory which describes the movement of
crustal plates on the mantle – Plate Tectonics!
Work of John Tuzo Wilson - lithospheric plates
floating on the asthenosphere
Types of Plate Boundaries
Some plates move fast ~ (16 cm/year) and some
slowly ~ (2-3 cm/year)
All plate motions can cause earthquakes at plate edges
(mostly ridges & trenches)!
Plate Boundaries…
1. Transform - locations where crustal plates move past
one another (ex. San Andreas Fault)
Translation at transform boundaries causes shear.
2. Divergent - boundaries where plates move apart; new ocean floor is
created, spreading rates can differ at ridges (slower, steep slopes) and
rises (faster, gentler slopes)
•Divergent Oceanic Crust - the Mid-Atlantic Ridge, East Pacific Rise
•Divergent Continental Crust - the Rift Valley of East Africa
Extension of divergent boundaries
causes splitting and rifting.
http://www.wwnorton.com/college/geo/egeo/flash/2_7.swf
3. Convergent Plate Boundaries - regions where plates are
pushing together, trenches have the largest earthquake
magnitudes, can be further classified as:– Island Arc Trench Systems - subduction of oceanic crusts, lithosphere is
heated, rises back to surface as volcanoes (islands), ex. Japanese Islands
– Continental Arc Trench Systems - oceanic plate will subduct under continental
plate, volcanoes will form on continents (ex. Peru-Chile Trench & Andes)
– Continental Mountain Systems - two continents collide & sedimentary rock is
folded to form mountains (ex. Himalayas)
http://www.wwnorton.com/college/geo/egeo/flash/2_6.swf
Evidence for Plate Tectonics:
• Ancient Life & Climates (fossils)
– Coral reefs in eastern North America, western Europe, Alps, &
Himalayas rocks that are 350 myo
• Ocean Sediments - sediments are younger & much thinner than
should be (must be recycled over time)
• Continent Fit
Paleomagnetism: most igneous rocks contain magnetite -
becomes permanently fixed as rock cools (mini-compass)
1.Strips of alternating magnetic polarity at spreading regions
2. Apparent Polar
Wandering: plate movement
causes the apparent position of
the magnetic poles to have
shifted
The paleomagnetic fields in
the rocks would indicate a
single pole until the
continents drift apart.
Hot Spots: surface expression of plumes of magma (ex. Hawaii, Iceland,
Yellowstone)
6% of energy for plate tectonics comes from hot material (magma) rising
up from the core
A volcanic island chain can form when a plate passes
over a hot spot and a stationary mantle plume.
show Exploring the Earth
video clip
Website for plate tectonics
worksheet
• http://geoinformatics.sdsc.edu/doe/student
/tectonics/continental_drift.html