Transcript Plate Tectonics II: Making Mountains & Volcanism

Plate Tectonics II:
Making Mountains & Volcanism
GEOSC 10: Geology of the National Parks
Presented by Dr. Sridhar Anandakrishnan
The Pennsylvania State University
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
The World’s Volcanoes
GEOSC 10 - Geology of the National Parks
The World’s Ocean Trenches
GEOSC 10 - Geology of the National Parks
The World’s Big Earthquakes
GEOSC 10 - Geology of the National Parks
Overview
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Ocean floor material made at spreading ridges
Moved off to the side by mantle convection
Collides with continental crust...
‣ Subduction & accretion
‣ Volcanoes/mountains
‣ Trenches
Hot spots
GEOSC 10 - Geology of the National Parks
Review
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Mantle made of hot, soft rocks
(asthenosphere)
Upper mantle + crust are rigid
(lithosphere)
Lithosphere broken into plates
Plates move on mantle convection cells
Convection cells bring up mantle material and
it freezes at pull-apart ridges
GEOSC 10 - Geology of the National Parks
Oceanic Crust
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Let mantle material rise and freeze: Basalt
‣ Basalt is Silica (SiO4) + Iron + Magnesium.
‣ Dark in color, relatively dense.
Ocean floor is mostly basalt (formed at ridges)
Earth isn’t getting bigger - oceanic crust has to
be destroyed/recycled.
GEOSC 10 - Geology of the National Parks
Subduction
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As oceanic crust moves away from the ridge, it
cools, grows more dense, starts to sink back
down. (think of the lava lamp...)
As it is moving sideways, it will run into a
continent.
‣ If it is cold enough, already sinking, we get a
subduction zone.
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If it is warm and buoyant, we get an
accretion zone (Olympics)
GEOSC 10 - Geology of the National Parks
The Fate of the Seafloor
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As it (ocean crust) sinks down, it carries
seawater and sediments (ocean bottom mud).
As it sinks, it gets hot again (remember, it’s hot
inside the earth).
BUT, the added water and sediments help it to
melt.
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Most things melt better in the presence of water and
impurities.
GEOSC 10 - Geology of the National Parks
Volcanic Arcs...
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That melted mix of ocean floor basalt, water,
and sediments is low-density, which rises,
creating volcanoes at the surface of the Earth.
This type of volcanic rock is called Andesite
(from the Andes Mountains).
GEOSC 10 - Geology of the National Parks
Earthquakes...
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Occasionally that downgoing slab gets stuck
and then breaks free with a huge earthquake.
‣ Sumatra, 2004; Peru 1960; Alaska, 1964
‣ The deepest earthquakes aren’t well understood:
not like the “stick-slip” we talked about. More like a
sudden rearrangement of the solid (phase change).
GEOSC 10 - Geology of the National Parks
Trenches
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Where the slab is subducting, it deforms the
over-riding plate - a linear trench is formed. If
these trenches are near land, they fill up with
sediments from land.
Those in midocean are the deepest places on
the planet.
GEOSC 10 - Geology of the National Parks
Trenches Near Land...
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Trenches near continents aren’t as deep as
midocean trenches. Sediment washed out by
rivers fills them up quickly.
Midocean trenches are deeper than Mt.
Everest is high!
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Marianas Trench off the Philippines is 35,000’ deep.
GEOSC 10 - Geology of the National Parks
First (best!) Science-fiction
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20,000 Leagues
Under the Sea by
Jules Verne
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The 20,000 leagues
(about 60,000 miles) is
the length of the trip, not
how deep it dives Verne knew better!
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Density...
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Continents are lowest density (“lightest”)
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Mainly silica, v. little iron. Light colored rocks.
Seafloor is heavier. Basalt.
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Silica (SiO4) + iron + magnesium.
Mantle is heavier.
‣ But cold seafloor is heavier than hot mantle. That’s
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why seafloor sinks at subduction zones.
Core is heaviest. Mostly iron.
GEOSC 10 - Geology of the National Parks
Accretion
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As seafloor runs into continents, the sediment
is scraped off and smeared on the continent.
‣ Most of that sediment came from the continent, so
the continents grow slowly if at all.
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Sometimes, an oceanic mountain/volcano runs into
the continent, and the continent grows.
Oceans are never really old (oldest is 160
million years old).
Continents are old! 4 billion years old.
GEOSC 10 - Geology of the National Parks
Review...
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Mantle is hot and flows in convection cells,
called the asthenosphere.
Upper mantle and crust is rigid and broken into
a few plates. This is called the lithosphere.
Plates meet at pull-apart, push-together, and
slide-past boundaries. Mountains built here
(mostly).
Heat (from radioactive decay) drives
the whole thing.
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Subduction Volcanoes
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When basalt + water + sediments heat up,
they melt and rise, forming andesitic
volcanoes.
The magma tends to polymerize. It makes
stringy, lumpy rocks. As the magma comes
out the top of the volcano, it solidifies right
away.
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The volcano is tall, steep, and symmetrical
A stratovolcano.
GEOSC 10 - Geology of the National Parks
Subduction Volcanoes
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The magma doesn’t polymerize inside the
earth because the water and CO2 in the
magma keeps the magma fluid.
When it gets to the surface of the earth, the
water and gas escapes and the rock quickly
solidifies.
Sometimes, the rock forms a cap.
The pressure builds.
GEOSC 10 - Geology of the National Parks
Mt. St. Helens, 20 May, 1980
GEOSC 10 - Geology of the National Parks
Mt. St. Helens, 20 May, 1980
GEOSC 10 - Geology of the National Parks
Stratovolcano Eruption
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Magma, containing water, CO2 rises.
Earlier flows have capped the volcano.
Pressure builds in the magma pocket.
Something (small earthquake?) cracks the top
of the volcano.
‣ Pressure drops... releasing gas/water... which
cracks open the top some more... which drops
the pressure... releasing gas/water...
GEOSC 10 - Geology of the National Parks
Hot Spots
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Sometimes a plume of magma will rise up from
deep inside the asthenosphere.
‣ As deep down as core/mantle boundary?
The magma has lots of iron, so it doesn’t
polymerize. At the surface, the magma
spreads out making a broad gently-sloped
mountain.
Hawaii is a shield volcano. Broad, gently
sloped (like a gladiator’s shield).
GEOSC 10 - Geology of the National Parks
Hot Spots
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The location of the hot spot is fixed in the
asthenosphere... but because the plate moves
over it, the surface location of the hot spot
moves in a straight line (plates move straight usually).
GEOSC 10 - Geology of the National Parks
Hot Spots
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
Hazards
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Don’t be on the volcano.
Gasses are hot (300+); cloud blast is fast
(100+ miles/hr); gasses are heavy, so they
flow along the ground
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nuee ardente
Ashes and cinder: pyroclastic flows
Landslide/avalanche
GEOSC 10 - Geology of the National Parks
Hazards
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Lake Nyos, Cameroon is on a hot spot.
CO2 seeped into the lake and built up on the
bottom of the lake.
Something (small earthquake?) disturbed the
lake and the CO2 escaped.
Suffocated 100s of people living downhill from
the lake.
Now they pump the CO2 out
GEOSC 10 - Geology of the National Parks
Tsunamis
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Move a big volume of water, and it will create a
wave that travels across the ocean, eventually
striking land.
Landslides, volcano eruptions, even meteorite
strike
In subduction zone, during an earthquake, the
overlying plate will “snap” upwards, moving
water.
GEOSC 10 - Geology of the National Parks
26 Dec 2004
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
GEOSC 10 - Geology of the National Parks
26 Dec 2004
QuickTime™ and a
Motion JPEG A decompressor
are needed to see this picture.
GEOSC 10 - Geology of the National Parks
Shorelines are Inundated
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As the wave gets closer to shore, the sea
bottom is rising (water is getting shallower).
The wave gets bigger and bigger.
Strikes with great force, as well as flooding
and washing out villages & people.
Roads, infrastructure gone... disease, hunger...
GEOSC 10 - Geology of the National Parks