Transcript Continental Drift

Tectonic Plates and Their Motions
• How did the idea of continental drift lead to
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plate tectonics?
How do we know that plates move apart at
divergent plate boundaries?
How do we know that subduction occurs at
convergent plate boundaries?
How do we know that plates slide past one
another at transform plate boundaries?
What does the mantle-plume (hot spot)
hypothesis explain what plate tectonics cannot?
What forces cause plate motions?
How did the idea of continental drift
lead to plate tectonics?
• Alfred Wegener – Noted matching continental
boundaries by shape and geologic features
(1912, 1928). Developed the idea of
continental drift - continents move over
geologic time.
• Good evidence for fitting continents together.
• However, no good mechanism for movements.
Wegener suggested they move like icebreakers through
the oceanic crust.
This idea was dismissed by most of the geologic
community at the time, in fact Wegener was ridiculed!
However, it started a search for a mechanism that would
explain the motion of the continents.
Continental Drift – The Geologic Evidence
The plants and animals (fossils), the ice, the rocks all match.
Alfred Wegener proposed
Continental Drift in 1912
because….
but the climates and poles do not.
The shapes of continents match,
Fig 12.1
Evidence From The Rocks
Evidence From Fossils
Evidence From Paleoclimate Studies
How did the idea of continental drift
lead to plate tectonics?
Wegener saw the continents as a giant jigsaw puzzle that fit
together into a single continent in the past - Pangea.
Fig 11.2
How do we know that plates move apart at
divergent plate boundaries?
• Matching the edges of Africa and South
America requires three conditions:
• A plate containing the two existed before the
Atlantic Ocean existed.
• That plate split and separated the two continents,
producing the Atlantic Ocean.
• The Atlantic Ocean grows wider with time.
• Plate tectonics predicts that oceanic crust
forms where continents separate. Is the
geology of the seafloor consistent with this?
How do we know that plates move apart at
divergent plate boundaries?
A divergent plate
boundary: creation of
new oceanic crust
and lithosphere from
the upwelling mafic
magmas as the
ocean basin opens
up by extension.
Can we directly
observe this
happening anywhere
on Earth?
How do we know that plates move apart at
divergent plate boundaries?
Iceland - a mid-ocean ridge
is present above sea level.
The island is widening at
the rate of ~4 mm/year due
to the divergent plate
boundary.
We know the Earth’s
magnetic field has
reversed many times in
the past. Stacks of lava
flows on land record
this.
If the seafloor spreads
at divergent boundaries,
the periodic magnetic
reversals should be
captured in the cooling
mafic rocks and carried
away on both sides –
and rocks should get
older away from the
spreading center.
This was verified by
geologists in the 1960s
aboard ships taking
magnetic readings.
The magnetic record of
ocean crust:
The normal and reversed
fields are recorded as a
set of matched strips on
either side of the
divergent boundary.
Thus, the time periods of
Earth’s magnetic
intervals is recorded in
rock - younger in the
center of ocean basins,
and older towards the
edges.
Animation.
How do we know that plates move apart at
divergent plate boundaries?
Hundreds of isotopic dates on seafloor basalts have confirmed this.
How do we know that plates move apart at
divergent plate boundaries?
Heat flow data
measured on the
ocean floor: Midocean ridges are hot
areas and heat flow is
progressively lower
away from the ridge.
Mid-Ocean Ridges are Seismically Slow Areas Due to Heat
Mantle tomography – provides images similar to ultrasound.
Movement of GPS Stations
GPS station positions change as plates move.
Will the position of these stations change relative to one another?
Beginning in the early 1990’s GPS stations were set up globally, they now
number in the thousands worldwide.
We have nearly 20 years of measurements of plate motions.
Continental Rifts Are the Precursors to Ocean Basins
When tensional forces
act on a continent, rift
valleys form.
Continental Rifts Are the Precursors to Ocean Basins
With time the rift
widens, mafic crust
forms, it is covered
with eroding
sediment, and
sinks into the
asthenosphere due
to its high density,
becoming an
ocean.
Animation.
Continental Rifts Are the Precursors
to Ocean Basins
Fig 12.19
The East African Rift may
eventually form a new
ocean basin.
Forming a Divergent Boundary
How do we know that subduction occurs at
convergent plate boundaries?
• Convergent boundaries are characterized by
the following (contrast with divergent
boundaries):
• Plates move towards one another (rather than
apart).
• Subduction destroys lithosphere at convergent
boundaries (rather than creating it).
• Convergent boundaries are very asymmetrical, with
one plate angled downward beneath another
(divergent boundaries are generally symmetrical).
How do we know that subduction occurs at
convergent plate boundaries?
Mountain building and volcanoes are the expected result of
convergence and subduction of one plate beneath another.
How do we know that subduction occurs at
convergent plate boundaries?
• Characteristics of subduction zones:
• Deep-sea trenches mark the plate boundary.
• Plate collision causes compressional stress, and
related folding, faulting, and earthquakes.
• Deep earthquakes occur within the subducted
plate, which retains it’s brittle character at depth
because it is much cooler than the mantle
surrounding it.
• Addition of water to the mantle above the
downgoing slab causes melting and volcanic
activity over the subduction zone.
How do we know that subduction occurs at
convergent plate boundaries?
How do we know that subduction occurs at
convergent plate boundaries?
on next slide
Calculated depths to
earthquake foci show
progressive increase of
depth to the subducted
slab as distance
increases from the
trench.
How do we know that subduction occurs at
convergent plate boundaries?
Earthquakes in relation to a subducting plate - The planar area of
earthquakes is known as the Wadati-Benioff zone.
Mantle tomography – hot
material in red (slower
seismic wave velocity),
cold material in blue
(faster seismic wave
velocity).
Earthquake locations
shown by white dots.
Clearly shows the
subducting oceanic
lithosphere (cold) beneath
the Japan volcanic arc
system.
Zhao, 2004, Global tomographic images of mantle plumes and
subducting slabs: Insight into deep Earth dynamics, Physics of
the Earth and Planetary Interiors, v.146, p.3-34.
More detailed image of subduction zone beneath Japan.
Types of Convergent Plate Boundaries
1) Oceanic-Continent
The western margin of South America is this type of boundary. Denser oceanic
lithosphere subducts under the less dense, much older, continental crust. Density
of the continental crust = 2.7-2.8 g/cm3, density of oceanic crust = 3.1-3.3 g/cm3.
Types of Convergent Plate Boundaries
2) Oceanic-Oceanic
Most of the western Pacific typifies this type of boundary, e.g. the area around
Japan. Older, colder, denser ocean crust will subduct beneath younger, hotter,
more bouyant crust.
Types of Convergent Plate Boundaries
3) Continent-Continent
The Himalayas are this
type of boundary, due
to the collision of India
with SE Asia.
How do we know that plates slide past one another at
transform plate boundaries?
Strike-slip faults show obvious and measurable displacement over time. The rate of
motion on the San Andreas, ~5.6 cm/yr, is similar to spreading ridge velocities.
How do we know that plates slide past one another at
transform plate boundaries?
Transform boundaries occur where there are strike-slip faults between spreading
ridges. Earthquakes occur dominantly along the transform between spreading ridge
segments.
Plate motions and velocities are
directly measured with GPS
Confirms plate tectonic motions over periods of just a few years.
What does the mantle-plume (hot spot)
hypothesis explain that plate tectonics cannot?
The mantle-plume
hypothesis has two
main components:
1) Hot spots form where
narrow columns
(plumes) of unusually
hot mantle
convectively rise from
the core-mantle
boundary, and
2) Plume locations are
considered to be
stationary in the
mantle.
What does the mantle-plume (hot spot)
hypothesis explain that plate tectonics cannot?
Fig 12.33
The trails of volcanic seamounts in the Pacific fit well in the
mantle-plume model.
Most data indicates
that plumes move
slowly, if at all.
Plumes appear
stationary, whereas
the plates move
laterally over them.
Hot Spot Volcano Tracks
What forces cause plate motions?
• Plate motions and plumes are caused by
convection in Earth’s mantle.
• Downwelling at convergent boundaries:
• Controls the speed and direction of plate motions.
• Gravity pulls the dense, subducted slab down, this
is called slab pull.
• Mid-ocean ridges are high areas, plates created
here “slide” downhill away from the divergent
boundary, this is called gravity sliding.
• Convection cells in the mantle may also act to drive
plate motion via traction forces at the base of the
lithosphere.
Forces Which Drive Plate Motion
1) Slab Pull at trenches of convergent boundaries.
2) Ridge push at divergent boundaries.
3) Convection cells in the mantle.