Download Plate Tectonics - Mrs. Robbins Earth Science

Plate Tectonics
Continental Drift
Alfred Wegener’s theory:
Continents were once a
single land mass that
drifted apart.
• Fossils of the same plants
and animals are found on
different continents
• Called this supercontinent
Pangaea, Greek for “all
Earth”
• 245 Million years ago
• Split – Laurasia &
Gondwana about 180
million years ago
http://members.enchantedlearning.com/subjects/astronomy/planets/earth/Continents.shtml
Evidence of Continental Drift
Fossils – various fossil remains of the same prehistoric
organisms have been found in places now separated by
oceans.
•
Evidence (cont.)
• Landforms: Coal fields and glacial remains from some
continents seem to line up with the edges of other continents.
Evidence (cont.)
• Climate: Fossils and rocks associated with different climates
than present can be found in the layers of earth, and
resemble those of other continents from a similar time
period.
Plate Tectonics (cont.)
• Plates are pieces of the lithosphere that move around on top
of the asthenosphere.
• Fit together like jigsaw puzzles (or at least they did at one
time)
What causes plates to move?
• Heat from the earth’s core
(higher temperature and
pressure) causes convection
currents in the mantle.
• As fluid is heated in the
lower mantle it becomes less
dense, therefore, it rises
toward the upper mantle
where there is less heat.
• As the fluid cools it becomes more dense and subject to gravity,
causing it to move back into the lower mantle.
Sea Floor Spreading
• At the mid-ocean ridge, molten material (magma) rises from the mantle
and erupts. Forms the largest mountain chains in the world, completely
under water!
• Magma rises to the surface and solidifies and new crust forms.
• Older Crust is pushed farther away from the ridge.
• Process continually adds new material to the ocean floor.
Video
Mid-ocean ridges
Mid-ocean Ridges
Evidence at mid-ocean ridges
• Molten rock - The shape that basalt makes when it cools
rapidly (underwater). Seeing this all over the ocean shows
that eruptions have occurred over and over.
“pillow lava”
Drilling
• Scientists took core samples
from different sides of the midocean ridge and compared their
age.
• The Glomar challenge found that
the further you got from the ridge
the older the rocks were.
• This was enough evidence to
prove sea-floor spreading and
gave Wegener his mechanism for
the continental drift theory!
Glomar Challenger
• Magnetism - evidence shows that earth’s magnetic poles have
reversed themselves.
• Last happened 780,000 years ago (so magnetic north was actually south).
• The rock that makes up the ocean floor actually lies in patterns of
magnetized “strips”, which hold a record of reversals in earth’s magnetic
field.
Plates move in all directions
Types of Plate Boundaries
• Plate boundaries define the edges of different pieces of lithosphere.
Faults – breaks in the earth’s crust where rocks have slipped past each
other, form at these boundaries. The 3 types of plate boundaries are:
Convergent, Divergent, and Transform.
http://pubs.usgs.gov/gip/dynamic/understanding.html
Convergent Boundary
One plate dives under another in a process called subduction. Oceanic crust
(more dense) subducts under continental crust (less dense). The subducting
plate sinks back into the mantle over tens of millions of years. This creates a
deep ocean trench. Earthquakes and Volcanoes are common at these
boundaries.
http://pubs.usgs.gov/gip/dynamic/understanding.html & http://www.geology.com
The Pacific rim is commonly referred to as “The Ring of Fire”
• Subduction zones
• 452 volcanoes, over 75% of the worlds volcanoes in this
region.
• 90% of the world’s earthquakes occur here as well.
When two continental plates collide (both low density granite rock), the
crust buckles and forms mountain ranges. This is how the Himalayas formed,
and the Grand Tetons.
http://pubs.usgs.gov/gip/dynamic/understanding.html & http://www.geology.com
Indian and Eurasian Plates converged to form the Himalayas.
Rock folding can be observed at converging continental plates
When two oceanic plates collide the more dense plate will
subduct, creating a trench and volcanic island arcs.
Example: Aleutian Island Volcanic Arc – subduction of Pacific
plate beneath N.A plate.
trench
Divergent Boundary - oceanic
New crust is generated as the plates pull away from each other Results in the
formation of new oceanic crust at the mid-ocean ridge. Also associated with
volcanic activity.
http://www.geology.com
Divergent Boundary – Continental
Diverging continental plates result in the formation of rift valleys. Eventually,
when the rift is wide and deep enough, ocean will fill the space between.
http://www.geology.com
Arabian and African Plates diverging as the Red Sea floods
the space between.
Transform Boundary – 2 plates slip past each other, moving in opposite
directions. Earthquakes occur frequently along these boundaries.
www.geology.com
Volcanic hotspots – magma deep within the mantle melts
through the crust like a blow torch. As plates move over top, so
does the volcanic activity.
Yellowstone hotspots
•
•
•
Scientists have traced Yellowstone's origin to a hot spot in the mantle, one of a
few dozen such hot spots on Earth. Buoyant material from a hot spot rises through
the upper mantle, bringing heat from the Earth's interior closer to the surface.
The Yellowstone hot spot has interacted with the North American plate for
perhaps as long as 17 million years, causing widespread outpourings of basalt that
bury about 200,000 square miles in Washington, Oregon, California, Nevada, and
Idaho under stacks of lava flows half a mile or more thick. Some of the basaltic
melt, or magma, produced by the hot spot accumulates near the base of the plate,
where its heat melts rocks from the Earth's lower crust. These melts, in turn, rise
closer to the surface to form large reservoirs of potentially explosive rhyolite
magma. Catastrophic eruptions have partly emptied some of these reservoirs,
causing their roofs to collapse. The resulting craters, some of which are more than
30 miles (50 kilometers) across, are known as volcanic calderas. Because the plate
was moving an inch or so per year southwestward over the hot spot for millions of
years as the calderas formed, groups of calderas are strung out like beads on a
string across parts of Idaho and Wyoming.
The thermal energy of the presumed Yellowstone hotspot fuels more than 10,000
hot pools and springs, geysers (like Old Faithful), and bubbling mudpots (pools of
boiling mud).
• Remember, ALL of this occurs in Geologic
time!!! Our lifetimes do not compare… not
even close.
• Millions to tens of millions of years.
• Plates move and mountain ranges rise by only
mm to cm each year.
Plate Boundaries
Diagram
Type of
Boundary
Divergent
Features
Formed
Example
Rift Valley
Mid-ocean
ridge
Subduction Trench
Volcanoes
Folded
Convergent
Mountains
Transform Transform
Fault
Mt. Rainier
Mt. Everest
San Andres
Fault