Plate tectonics
Geology 101
Fall 2013
Why does the Earth have
mountains and basins?
• Basic question asked by many: erosion
is evident everywhere, so why haven’t
all mountains eroded and filled in all
basins?
• Alfred Wegener proposed continental
drift in 1912.
Evidence for continental drift
• Continental “fit” – South America and
Africa separated by the Atlantic
• Similar rocks separated by the Atlantic
• Similar climate markers (glacial
striations) separated by the Atlantic
• Similar fossils separated by the Atlantic
Unifying theory of geology
• Extension of Wegener’s continental
drift theory from the early 20th century
• Needed not only information about
rocks but also breakthroughs in
geochronology and geophysics
• Continental drift failed to provide a
sufficient mechanism (tides and wind
were not enough); plate tectonics does
Mechanism(s) of tectonics
• First, and foremost, the second law of
thermodynamics says that heat is
transferred from hot bodies to cold
bodies (e.g., Earth’s core to space)
Mechanism(s) of tectonics
• Heat transfers in three ways: radiation,
conduction and convection. It is the last
of these that occurs within the mantle.
• Convection (and internal heating)
seems to keep the mantle hot and
plastic enough for plates to sink into it -may not be enough to move plates
Mechanism(s) of tectonics
• Slab pull seems to be the major “drag”,
ridge push and plate sliding are more
minor forces
Tenets of plate tectonics
• Plates exist
Plate materials
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Remember: lithosphere, not crust
Continental plates are made of granite
Oceanic plates are made of basalt
Granite and basalt are types of igneous
rocks
Tenets of plate tectonics
• Plates move
Hotspots
• Focused convection plumes from midmantle
Hotspots show plate movement
Problems with motion
• Hotspots may not be fixed in position
and they are certainly not permanent
• GPS does a better job of measuring
annual rates, though over the short-term
only
Tenets of plate tectonics
• Plate boundaries are areas of
geological phenomena such as
earthquakes, volcanoes and mountain
building
Plate interactions
• Convergent
Types of convergent boundaries
• Continental plate - continental plate
convergence results in non-volcanic
mountain ranges
Types of convergent boundaries
• Oceanic plate - oceanic plate
convergence results in a volcanic
oceanic island arc
• Subduction occurs
Types of convergent boundaries
• Continental plate - oceanic plate
convergence results in a volcanic
continental arc
Plate interactions
• Divergent
Types of divergent boundaries
• Between two oceanic plates
Sea floor spreading
• Oceanic divergent boundaries provided
evidence for plate tectonics.
Paleomagnetism (the measurement of
“stripes” of different polarity minerals in
ocean floor rock of different ages)
allowed Hess and others (1950s) to
postulate the movement of the sea floor
itself.
Types of divergent boundaries
• Between two continental plates
Plate interactions
• Transform
Tenets of plate tectonics
• Plate interiors are geologically quiet,
except for hotspots
Hotspots are areas of magma upwelling from the mantle
They can occur in the middle of a tectonic plate (e.g.,
Hawaii) or at a plate boundary (e.g., Iceland)
As shown below for Hawaii, the plate onto which the hotspot
magma erupts continues to move, leaving a “trail” of extinct
volcanoes
Hotspots remain more or less stationary during their
lifetime, though there is some controversy about this.
Grander synthesis: the Wilson
(supercontinent) cycle
• J. Tuzo Wilson (1970) suggested that
plate tectonics allow supercontinents to
rift apart and reform over and over on a
roughly half billion year cycle
• Evidence for this: evidence of prePangea supercontinents, such as
Rodinia
Summary of plate tectonic theory
• Plates exist
• Plates move
• Plates interact and generate geologic
phenomena
• Interiors of plates are geologically quiet*
Synthesis
• All three types
Nearby!