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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
• 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
• 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
• 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
• 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.
‣ If it is warm and buoyant, we get an
accretion zone (Olympics)
GEOSC 10 - Geology of the National Parks
The Fate of the Seafloor
• 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.
‣ Most things melt better in the presence of water and
impurities.
GEOSC 10 - Geology of the National Parks
Volcanic Arcs...
• 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...
• 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
• 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...
• 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!
‣ Marianas Trench off the Philippines is 35,000’ deep.
GEOSC 10 - Geology of the National Parks
First (best!) Science-fiction
• 20,000 Leagues
Under the Sea by
Jules Verne
‣ 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...
• Continents are lowest density (“lightest”)
‣ Mainly silica, v. little iron. Light colored rocks.
• Seafloor is heavier. Basalt.
‣ Silica (SiO4) + iron + magnesium.
• Mantle is heavier.
‣ But cold seafloor is heavier than hot mantle. That’s
why seafloor sinks at subduction zones.
• Core is heaviest. Mostly iron.
GEOSC 10 - Geology of the National Parks
Accretion
• 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.
‣ 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...
• 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
• 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.
‣ The volcano is tall, steep, and symmetrical
‣ A stratovolcano.
GEOSC 10 - Geology of the National Parks
Subduction Volcanoes
• 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
• 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
• 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
• 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
• 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
‣ nuee ardente
• Ashes and cinder: pyroclastic flows
• Landslide/avalanche
GEOSC 10 - Geology of the National Parks
Hazards
• 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
• 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 neede d to see this picture.
GEOSC 10 - Geology of the National Parks
26 Dec 2004
QuickTime™ an d a
Motion JPEG A decompressor
are need ed to see this p icture .
GEOSC 10 - Geology of the National Parks
Shorelines are Inundated
• 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
