Plate Tectonics III:
Making Mountains, Obduction, & Tsunamis
GEOSC 10: Geology of the National Parks
Presented by Dr. Sridhar Anandakrishnan
The Pennsylvania State University
Go Dog Go
PJ Eastman, 1961
GEOSC 10 - Geology of the National Parks
Appalachians
GEOSC 10 - Geology of the National Parks
Sideling Hill, West VA
GEOSC 10 - Geology of the National Parks
Review
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Tectonics is driven by heat.
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Oceanic plates are basaltic.
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The plates (8 major ones, few small ones)
move on the surface of the Earth.
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Initially hot and buoyant... later cool and sink
Continental plates are silica-rich, low-density,
and buoyant.
GEOSC 10 - Geology of the National Parks
Review (2)
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When cold ocean plates collide with
continental plates, they “dive under” the
continents.
Subduction leads to
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Stratovolcano chains (Andes, Cascades, Aleutians)
Trenches (unless filled by sediments)
Deep earthquakes
Tsunamis
GEOSC 10 - Geology of the National Parks
When Continents Collide?
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Appalachian mountain range from
Newfoundland to Alabama, and again in
Oklahoma.
Continents are rarely destroyed - so the story
gets very complicated.
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A colleague’s office - he never throws anything
away, so the piles of papers and books get jumbled
up
Oceanic crust is created, then destroyed...
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Susquehanna Valley from
Space
Image courtesy NASA, MISR
GEOSC 10 - Geology of the National Parks
Appalachians Complicated
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About 300 Million years ago, N. America
collided with Europe/Africa to create a chain of
mountains (perhaps 15,000 ft high?).
Similar to what is going on today with India
and Asia.
GEOSC 10 - Geology of the National Parks
Continent-continent Collision
GEOSC 10 - Geology of the National Parks
Mt. Everest, looking N
Image courtesy USGS, photo by Gimmy P Li
GEOSC 10 - Geology of the National Parks
How to Shorten a Continent...
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Thrust fault
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Shorten up a continent
by sliding one part up
and over another part.
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This is what happened
in Great Smokies area.
GEOSC 10 - Geology of the National Parks
How to Shorten...(2)
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Susquehanna valley from space,
image courtesy NASA, MISR
Farther north (around
here), the rocks
“wrinkled up” like a
kicked rug.
The “rug” is layered
(hard and soft layers).
As time goes by, the
soft layers get eroded
and the hard layers
form the ridges.
GEOSC 10 - Geology of the National Parks
Cross Section Through SC
GEOSC 10 - Geology of the National Parks
Eventually the Collision Stopped
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This area spread apart (similar to Death Valley
pull-apart.
Atlantic Ocean formed... still spreading.
Mountains stopped being pushed up.
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Erosion scrapes away the tops of mountains and
deposits the remains (sediment) in the low areas or
in the ocean.
GEOSC 10 - Geology of the National Parks
Appalachians Still High...
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But, mountains have
“deep roots.”
By the principle of
isostasy, mountains
that stick up high
above the landscape
also have a thickened
crust below them that
sticks down into the
mantle.
GEOSC 10 - Geology of the National Parks
Isostasy, Erosion, Icebergs...
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As these mountains are eroded, they remain
high because material from below is rising up.
Like an iceberg floating in water.
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Chop off the part above water
Part below water will rise up.
- It will be almost as high.
- Depends on the density difference.
GEOSC 10 - Geology of the National Parks
Plate Collisions...
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Pull-apart (saw that first week - Death Valley)
Subduction (saw that last time - Crater Lake)
Obduction (this time)
Slide-past (San Andreas fault)
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Saw this briefly when we talked about earthquakes
GEOSC 10 - Geology of the National Parks
Slide-past Tectonics
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Sometimes 2 plates slide past each other.
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Usually not smoothly (stick slip... and offset of
fences, etc.)
If there is a “kink” in the boundary, then the
sliding-past behavior can make mountains.
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The 2 plates push together at the kink rather than
sliding past (as they do on the straight bits)
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Or the 2 plates pull apart at the kink...
GEOSC 10 - Geology of the National Parks
Project Habakuk
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During WW2,
proposal to build an
aircraft carrier from an
iceberg.
Mixture of wood pulp
and ice.
Small one built on
Lake Louise in
Canada.
GEOSC 10 - Geology of the National Parks
Rocky Mtns - Leading Idea
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Collision of N. America with oceanic crust.
Normally oceanic crust will subduct under
continental crust.
But, if the oceanic crust is hot, it will slide
under the continent, but scrape along the
bottom.
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Continent gets deformed way inland
Rockies are 1500 miles from the Pacific
GEOSC 10 - Geology of the National Parks
Rocky Mtns - Leading Idea
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Collision of N. America with oceanic crust.
Normally oceanic crust will subduct under
continental crust.
But, if the oceanic crust is hot, it will slide
under the continent, but scrape along the
bottom.
‣
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Continent gets deformed way inland
Rockies are 1500miles from the Pacific.
GEOSC 10 - Geology of the National Parks
Cross Section of West
Push up
far inland
Used to
be far
offshore
Warm
Cold
GEOSC 10 - Geology of the National Parks
Metamorphic Rocks
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Some rocks in the Rockies and Great Smokies
are metamorphic rocks.
‣ Means they have been changed from their original
form.
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Under the heat, pressure, and chemical action, they
change from (usually) sedimentary rocks to a
harder, more-resistant rock.
- They have been “cooked”
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The look different - folded, squeezed, with pretty
colors (and funny names).
GEOSC 10 - Geology of the National Parks
Metamorphic Rocks
GEOSC 10 - Geology of the National Parks
Igneous Rocks
Pele’s Hair, courtesy USGS
Andesite, courtesy USGS
GEOSC 10 - Geology of the National Parks
Why are They at the Surface?
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As the overlying rocks get eroded and
removed, the deeper rocks rise up.
The deeper rocks rise up because of isostasy.
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Like the iceberg rising up as the part above water is
lopped off, and the alien emerging...
GEOSC 10 - Geology of the National Parks
Rock Cycle
Courtesy USGS
GEOSC 10 - Geology of the National Parks
Types of Rocks
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Igneous rocks
‣ Formed by melting and then solidifying of magma
‣ Or during a volcano
Sedimentary rocks
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Formed by erosion of other rocks and then
deposition
Metamorphic rocks
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Formed by “cooking” rocks
GEOSC 10 - Geology of the National Parks
Hazards
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Most of us will die of old age.
Wear a
About 1% or fewer die of “outside causes”
seatbelt
‣ Car crashes and handgun deaths.
The main geologic hazards.
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Tsunamis, volcanoes, earthquakes, landslides.
GEOSC 10 - Geology of the National Parks
Tsunami Warning
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Because the water wave can take minutes to
hours to get across the ocean to land, there is
time for warning.
The earthquake/landslide/meteor strike can be
detected almost immediately, so we can send
out warning by radio/internet/smoke signals.
GEOSC 10 - Geology of the National Parks