Plate Tectonics II: Making Mountains & Volcanism by ckwyK4to

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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



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                                             GEOSC 10 - Geology of the National Parks
26 Dec 2004


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                                              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

								
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