Transcript Plate Tectonics

Plate Tectonics
A
Unifying
Theory
http://www.calstatela.edu/dept/geology/HotSpots.htm
Mid-Atlantic Ridge, Iceland
Theory of Plate Tectonics
• The upper mechanical layer of Earth (lithosphere) is divided into rigid
plates that move away from, toward, and along each other
• Most (!) deformation of Earth’s crust occurs at plate boundaries
3 Main Boundary Types
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Convergent
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Divergent
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Ocean-continent (e.g., Andes, Cascades)
Ocean-ocean (Japan, Indonesia, Alaskan Islands)
Continent-continent (Himalayas, Alps)
Oceanic crust subducts (cold, dense), continental crust
is usually too bouyant (less dense) & does not subduct
New oceanic crust created (e.g., African rift zone, Red
Sea)
Transform
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Strike-Slip fault (e.g., San Andreas Fault)
Between mid-ocean ridge spreading centers
Often difficult to recognize
Ocean-Continent Collision
AndesAndes
Mountains
Mountains
Ocean-Ocean Collision
Japan
Continent-Continent Collision
Himalayas
The Himalayas—
Roof of the World
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During the Early Cretaceous,
India broke away from Gondwana and began
moving north
 oceanic lithosphere was consumed at a
subduction zone along the southern margin of
Asia
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India’s Northern Trek
http://pubs.usgs.gov/gip/dynamic/understanding.html
http://pubs.usgs.gov/gip/dynamic/himalaya.html
Divergent Boundary
1
2
East African
rift zone
3
Red Sea
4
Mid-Atlantic
Ridge
Mid-Atlantic Ridge
http://www.calstatela.edu/dept/geology/HotSpots.htm
http://pubs.usgs.gov/gip/dynamic/understanding.html#anchor5567033
Transform Boundaries
Mid-Atlantic
Ridge
1. Areas in-between midocean ridge spreading
centers
2. Areas such as the San
Andreas Fault
San Andreas
Fault
Transform
Boundaries
San Andreas
Fault
San Andreas Fault
Photo by L. Stone – Point Reyes National Seashore – June 2005
Fence split by 1906 earthquake along the
San Andreas Fault. Point Reyes moved
northwest up to 20 feet.
http://pubs.usgs.gov/gip/dynamic/San_Andreas.html
Plate
Tectonics ...
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Recent
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Unifying theme of solid earth sciences
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Framework on which we hang (test!)
observations about Earth’s geology and
geophysics
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Is a THEORY
What makes the plates?
Earth’s Layers
3 main layers
defined by
composition:
• Crust - Outer
• Mantle - Middle
• Core - Center
http://www.physicalgeography.net/fundamentals/10h.html
Why Do the Plates Move?
Got Heat?
Earth - 3 Heat Sources:
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Loss of original heat of
formation (geothermal /
core is cooling)
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Radioactive decay of
elements in Earth’s
materials
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The Sun - external; not
important to plate tectonics
Convection: Driving Force of Plate Tectonics
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Interior of Earth has
sluggish convection in
some regions
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Heat from core rises,
creates convection cells
in the mantle
NOT LIQUID!
Given an infinite amount of
time, solids flow
http://pubs.usgs.gov/publications/text/unanswered.html
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Rising hot material at mid-ocean ridges and midocean volcanic islands
Descending cooler material at trenches
Lithospheric plates “carried” with the convection cells
More than Convection?
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In addition to thermal convection cells, some
geologists think that movement may be aided by
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“slab-pull” - slab is
cold and dense and
pulls the plate
“ridge-push” - rising
magma pushes the
ridges up and
gravity pushes the
ocean floor toward
the trench
Alfred Wegener
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1912 Continental Drift
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Observations
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Fit of Continents
Geology
Paleontology
Climate belts
Pangaea 200 Ma
Breakup 180 Ma
Rigid bodies moving through
yielding seafloor – could not
provide mechanism of
movement
Jigsaw-Puzzle Fit of Continents
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Matching mountain
ranges
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Matching glacial
evidence
Matching Fossils
Mechanism for Plate Movement!
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Arthur Holmes (Early 1920’s)
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Interior of Earth has sluggish
convection (transport of heat
from core)
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New ocean crust injected into
ocean floor (where?)
Harry Hess and Seafloor Spreading
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Crust moves with
convection currents
New ocean crust at MOR’s
Ocean crust dragged down
at trenches; mountains
form here
Continental crust too light;
remains at surface
Earthquakes occur where
crust descends
“It explains everything….”
Seafloor Spreading - Observations
• Fit of continents - new material pushes them apart
• Topography of ocean floors - hot ridges, trenches
• Volcanism at ridge axes - hot mantle material
• Seismic zones near margins - descending plates
Magnetism – The Final Piece
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Earth has
magnetic field
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Similar to a giant
dipole magnet
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magnetic poles
essentially coincide
with the geographic
poles
may result from
different rotation of
outer core and
mantle
Magnetic Reversals
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Earth’s present magnetic field is called normal
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magnetic north near the north geographic pole
magnetic south near the south geographic pole
At various times in the past, Earth’s magnetic
field has completely reversed
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magnetic south near the north geographic pole
magnetic north near the south geographic pole
When magma cools, takes on signature of
Earth’s prevailing magnetic field
magnetic iron-bearing minerals align with Earth’s magnetic field
What does this have to do with seafloor spreading?
Mid-Atlantic Ridge
Evidence for Hess’s Hypothesis
Magnetic stripes and isotopic
clocks
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http://pubs.usgs.gov/gip/dynamic/stripes.html
An observed magnetic profile
(blue) for the ocean floor
across the East Pacific Rise is
matched quite well by a
calculated profile (red) based
on the Earth's magnetic
reversals for the past 4 million
years and an assumed
constant rate of movement of
ocean floor away from a
hypothetical spreading center
(bottom). The remarkable
similarity of these two profiles
provided one of the clinching
arguments in support of the
seafloor spreading hypothesis.
Oceanic Crust Is Young
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Seafloor spreading theory indicates that
oceanic crust is geologically young
 forms during spreading
 destroyed during subduction
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Radiometric dating confirms young age
youngest oceanic crust occurs at mid-ocean
ridges
 and the oldest oceanic crust is less than 180
million years old
 oldest continental crust is 3.96 billion yeas
old
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Age of Ocean Basins
Intra-Plate Volcanism
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Provides absolute
rates and motions
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Fixed “hot spot” in the
mantle; deep, longlived magma chamber
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If hot spot is fixed
then plates are
moving
• Hawaiian Ridge-Emperor Seamounts chain - 6,000-km-long
• Bend at 43 Ma indicates motion of Pacific Plate abruptly changed from
north to west (??related to collision of India and Eurasia??)
Plate Tectonics as the Unifying
Concept of Earth Science
Accumulation of
Observations Evidence
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Patterns of continents
 Paleontology
 Geology
Patterns of sea floor ages
Patterns of seafloor depth
Patterns of volcanoes
Patterns of earthquakes