Transcript The Theory of Plate Tectonics

Chapter 2 Lecture
McKnight's
Physical Geography
Lectures
Chapter 14
Earth Internal Processes
Andrew Mercer
Mississippi State University
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Learning Goals of Chapter 14
• Explain Wegener's continental drift hypothesis.
• Explain the theory of plate tectonics.
• Identify and describe the three kinds of plate
boundaries.
• Explain hot spots, accreted terranes, four types of
volcanoes and types of volcanic eruptions and their
materials.
• Explain measures of earthquake energy and effects.
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From Rigid Earth to Plate Tectonics
• Until mid-twentieth century,
scientists believed Earth’s
continents rigid
• Continental drift – pangaea
• Evidence includes similar
geologic features on coasts
of different continents
• Continents actually once fit
together
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From Rigid Earth to Plate Tectonics
• Paleontology records
support continental drift
concept.
• Glaciated continents
reconstructed made sense.
• Rejected by most scientists
until the last 60 years.
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The Theory of Plate Tectonics
– Plate boundaries
• Earthquakes occur along major fracture boundaries
• Correspond with locations of trenches and ridges in the sea
floor
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The Theory of Plate Tectonics
Sea floor spreading
• Mid-ocean ridges formed by magma rising up from the
mantle
• New basaltic ocean floor created; moves away from ridge
• At trenches, older lithosphere descends into the
asthenosphere where it is recycled – subduction
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The Theory of Plate Tectonics
– Paleomagnetism
• Iron in cooled magma orients itself with the magnetic poles
of Earth
• Provides a record of past magnetic fields
• Magnetic field has changed orientation at least 170 times
• Should be symmetry
in magnetic
orientation
• Used to verify age of
ocean floor rock and
sea floor spreading
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The Theory of Plate Tectonics
• Plate tectonic theory
–
–
–
–
Theory behind motion of lithospheric plates
Plates float on asthenosphere (thin molten ‘blood layer’)
7 major plates, 7 intermediate plates, 12 smaller plates
Smaller plates are large plates that are being subducted
• Plate tectonic activity
– Slow convection in Earth’s mantle
– Convection can push plates away from each other
– Most motion results from plates pulled by subduction of
dense oceanic lithosphere
– Heavier subducting plates are like hot taffy sliding off the
table.
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Major Plate Boundaries
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Divergent boundaries
• Plates move away from each other
• Asthenosphere wells up in the plate opening
• Represented by a
midoceanic ridge
• Associated with
shallow-focus
earthquakes and
volcanic activity
• Constructive
• Continental rift valley,
proto-ocean
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Convergent boundaries
• Collisions between plates
• Destructive
• Three primary collisions:
– Oceanic–continental – oceanic plate sinks since more dense;
subduction
» Forms oceanic trench and continental mountains (e.g.,
Cascades, Andes)
» Earthquakes occur along margin
» Volcano formation along the plates – continental
volcanic arc
» Forms metamorphic rocks – blueschist (above)
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More about convergent boundaries
• Three primary collisions (cont.):
– Oceanic–oceanic
» Subduction results in undersea
trench formation
» Deep and shallow earthquakes
» Island volcanic arc
– Continental–continental
» No subduction since two plates
are highly buoyant
» Builds huge mountain ranges
» Volcanoes are rare
» Shallow earthquakes are
relatively common
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More About Plate Boundaries
Transform boundaries
• Two boundaries slip past each other laterally
• Transform faults
• Neither creates nor
destroys crust
• Commonly produce
shallow focus
earthquakes
• San Andreas fault
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The Theory of Plate Tectonics
• The rearrangement
– 450 million years ago, one
supercontinent existed
– Broke up 200 million years ago
• Laurasia
• Gondwanaland
– Arrangement to the current continental
configuration
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The Theory of Plate Tectonics
• The Pacific Ring of Fire
– Plate boundaries exist all around the Pacific Rim
– Primarily subduction zones
– 75 percent of all volcanoes lie in the Ring of Fire
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The Theory of Plate Tectonics
• Additions to basic plate tectonic theory
– Mantle plumes
•
•
•
•
localized hot areas not associated with plate boundaries
Move with the plate, so eventually become inactive
Hot spot trail
Hawaiian islands
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Volcanism
• Magma chemistry and styles of eruption
– Type of eruption is determined by:
• Amount of gas involved – gas pressure
• Stickiness of rocks – how much pressure the rock can hold back to
produce a large eruption – high-silicate rocks resist more pressure
than low-silicate rocks
– High silica eruptions – pyroclastic – explosive and dangerous
– Low silica eruptions – quiet, non-explosive
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Volcanism
– Relatively temporary features on the landscape
– Much of Earth’s water originated from water vapor from
volcanic eruptions
– Magma contains
major elements
required for
plant growth
– Provides soil fertility
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[Insert Fig. 14-28 p. 328]
Lava flows
– Lava generally flows horizontally, parallel to the surface
along which it flows.
– It eventually cools in horizontal orientation, strata.
– Streams flowing
through lava flows
result in irregular
or fragmented
surface.
– Uniform cooling
results in hexagonal
structure (see figure).
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[Insert Fig. 14-30 p. 329]
Flood basalt
– Most extensive lava flows come from hot spots.
– Flood basalt is a vast accumulation of lava build up.
– It is correlated with mass extinctions.
Yellowstone Park
contains a “hot spot”
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Volcanic peaks
The Hawaiian Islands and Yellowstone Park are examples of ‘hot spots’.
The Hawaiian version is a very large, but mild shield volcano – not explosive:
less sticky silicate rock and less gas.
Hawaiian Islands
are actually the
tallest mountains
on earth, rising
from the bottom
of the ocean to
13,500 feet
above sea level.
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Volcanism
Composite Volcano – ‘strato’ (layered)
• More silica lavas (andesite lava)
• Form symmetric, steep-sided volcanoes
• Pyroclastics from explosive lava
• Pyroclastic flows
produce steep
slopes, lava holds
it together
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More volcanic peaks
Lava domes
• Masses of very viscous lava that do not flow far
• Lava bulges from the vent, dome grows by expansion from
below and lava within
• Some lava domes form inside of composite volcanoes
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Cinder Cones
•
•
•
•
Smallest volcanic mountains
Basaltic magma is common
Slopes form from pyroclastic materials
Generally found in association
with other volcanoes
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Calderas
• Result from a volcano that
explodes, collapses, or both
• Immense, basin-shaped
depression; larger than
original crater
• Crater Lake in Oregon is
an example
• Yellowstone Park is a major
example – a ‘hot spot’ that
exploded, forming a caldera basin.
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Volcanic hazards
– Volcanic gases – mainly water vapor, but can cause acid
rain and alter global climate
– Lava flows – cause
immense property damage
– Eruption clouds – gas and ash
material clouds that extend up
to 16 km into the atmosphere,
drop large rock fragments
called “bombs”
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Volcanic hazards
– Pyroclastic flows – avalanche of hot gases and material,
up to 100 mph
– Volcanic mud flows (lahars) – result from heavy rain
and/or snow melt during an eruption
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Monitoring volcanic hazards
– Research to locate previous
pyroclastic flows and lahars
– Tiltmeters, measure the slope of
a volcano to look for swelling
– Monitor earthquake activity
The U.S. west coast
portion of the “Ring of
Fire” exhibits many
volcanoes.
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Volcanism – Igneous intrusion
Some volcanic activity does not reach the surface – cools as
dense, hard rock crystals like granite and rhyolite.
Sometimes, the earth
Erodes down to expose
these intrusions that
never made it to the
surface on their own
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[Insert Fig. 14-49 p. 339]
Plutonic intrusions later exposed above ground
– Dikes – vertical sheet of magma thrust upward into
preexisting rock; long and narrow
– Veins – Molten
material forces
itself into smaller
fractures in
preexisting rock;
take irregular
shapes
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Folding
Tectonic force can squeeze or stretch layers of rock,
creating rough surfaces that erode in complex ways.
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Faulting
• Occurs when rock breaks accompanied by
displacement
• Occurs along zones of weakness in the crust,
fault zones
• Begin as sudden ruptures or slow creep after
breaking
Call these
“block” faults
or “dip-slip”
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Common block faults come in many
varieties
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Faulting
– Horst – uplift
of a land block
between two
parallel faults
Graben - collapsed area
of fractured rock
– Graben – downthrown
land block between
two parallel faults
Horst - lifted rock
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Transform or slip-strike fault zones
Slide past each other in opposite directions
Example: San Andreas Fault – a major fault and quake zone
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Earthquakes – sudden slip along a fracture
line
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Quake magnitude measured exponentially
– Logarithmic scale, 32nd power
– Richter scale
– Strongest recorded
earthquake –
9.5 in Chile
• Shaking intensity
– Intensity of ground
shaking not consistent
during an earthquake
– Mercalli intensity scale
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[Insert Table 14-2 p. 347]
Shaking and damage are worse on softer, wetter earth
Liquefaction - very wet soils can ‘liquify’
when shaken – buildings fall apart or move
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• Hard rock is best
• Sand is shaken easily
• Mud is the worst - very hazardous when shaken