Transcript Plate Tectonics Lecture

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Evidence for Plate Tectonics
Chapter 9.4
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Continental Drift - Review
Wegener
4
- Continental drift hypothesis
Strong Pieces of Evidence:
Continental
puzzle
Matching Fossil records
Matching mountain ranges
Altered Ancient climates
 Why
wasn’t his hypothesis accepted?
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Continental drift hypothesis
 The
Main objection to Wegener's
proposal was its inability to
provide believable mechanism
for movement of continents.
 He
felt the earths spin or ocean
currents caused the continents
to plow through the ocean floor
similar to the way an iceberg
moves.
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Theory of Platetectonics
 Theory
states: The earths lithosphere is fragmented
into a dozen or more large and small plates which
ride a top the asthenosphere.
 Earth
largest plate is the Pacific plate. Oceanic plates
like this one are created at divergent boundaries and
recycled at subduction zones.
 The
water and carbon compounds they pull down
with them increases the fluidity of the magma and
leads to chains of volcanoes.
 The
Asthenosphere exists beneath the lithosphere
and is hotter, weaker and more fluid than lithosphere
which allows the lithosphere to move.
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4 Key Pieces of Evidence give rise
to Plate Tectonics Theory
 1)
Ruggedness and youth of the ocean floor
 2) Tectonic & Volcanic activity along trenches
and mountain ranges
 3) Paleomagnetism pattern on either side of
divergent boundaries
 4) Hot Spots & seafloor spreading
+1. Roughness and youth
of the ocean floor
Before
the 19th century the ocean floor was
thought to be flat.
The
development of echo-sounding methods
or SONAR around 1955 allowed scientists to
study the ocean floor in great detail.
These
maps showed underwater mountains
chains in middle of oceans.
Evidence emerges as
Technology improves
 Sonar uses sound
waves to measure
water depth by
measuring the time it
takes for sound waves
to travel from the
device and back to
a receiver.
Relative youth of the ocean floor
from deep Ocean Drilling
Ships drilling for oil found
that the amount and
maximum age of sediment
decreased as they got closer
to the mid-ocean ridges
indicating the ocean floor had
just formed at these locations.
 Also the oceanic crust was much younger
than the continental crust.
Magnetism
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2. Earthquakes and volcanic
activity along plate boundaries
 Seismographs
are tools used to measure the
shacking of the Earth and can be used to locate the
epicenter of an earthquake.
 Our
Knowledge was greatly advanced in 1960’s from
the world wide standardized seismograph network
or WWSSN.
 This
was established to help monitor activities
related to the 1963 treaty banning above ground
testing of nuclear weapons.
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Major Tectonic Plates
+ Mapping Earthquakes and Volcanoes taught us that
most (90%) occur in narrow bands at Plate
boundaries.
Earthquake locations for events between 1965 and 1995.
The red dots are shallow earthquakes, the green are
intermediate depth, and the blue and purple are deep.
+ Triangles represent the location of a recently active volcanoes.
Some volcanic regions such as the Hawaiian Islands are
isolated but most correspond well to the map to Plate
boundaries.
+ Earthquake patterns at subduction zones
showed use the outline of the descending
oceanic plate.
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3. Repeated changes of Earth’s
magnetic field
A
magnetometer is a
device that can detect
small changes in
magnetic fields, similar
to a compass.
 In
the 1960’s scientists
using magnetometers
on military subs began
noticing odd magnetic
stripped patterns in
the ocean floor.
Magnetism
 It was known that rocks like basalt containing iron-bearing
minerals provide a record of Earth’s magnetic field as iron
they solidify.
 Surprisingly these patterns of magnetism in the rocks
indicated that earths magnetic field had reversed many
times in the geologic past.
Magnetism
 The normal and revered
polarity of the seafloor
forms a series of stripes
that run parallel to ocean
ridges.
 Additionally the same
mirror image pattern of
paleomagnetism was seen
on either side of a midocean ridge giving more
evidence of Seafloor
Spreading.
4. Seafloor Spreading
 An American scientist named
Harry Hess proposed the
theory of seafloor spreading
which states that new oceanic
crust is formed at ocean ridges
and destroyed at deep-sea
trenches.
 Convection currents in the
mantel drive Magma toward
surface along the ocean ridge
where it cracks and spreads
apart.
Seafloor Spreading
 Seafloor spreading was the missing piece that Wegener could
have used to complete his model of continental drift if only the
technology had been available.
 Continents are not pushing through ocean crust, as Wegener
proposed; they ride with ocean crust as it slowly moves away
from ocean ridges.
4. Hot Spots
 Maps made by sonar show that some volcanic
island chains form as a series of progressively
older islands.
 Radiometric dating of the rocks later confirmed
this progressively older trend of the Hawaiian
Islands as you move west.
 Seismographs are able to plot the
locations of earthquakes and from an
enormous magma hot spot
underneath the newest of the
volcanic islands.
 The Hawaiian Islands are more
evidence showing that the plate
underneath the hot spot is moving
NOT the hot spot as was previously
thought.