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Plate Tectonics II
Geoscience 10: Geology of The National Parks
Unit 3
Plate Tectonics II
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Most students find this week to have more new
material than any other;
We always debate how much time to spend on the
“science”, “impacts” and “cute critter” material;
We usually work fast through the deep-earth and
tectonics parts to get to the cute-critter and
living-on-Earth parts;
So suck it up and power through, and things should
get easier.
Geoscience 10: Geology of The National Parks
Unit 3
SUBDUCTION
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We saw that sea floor is made at spreading ridges such
as the one through the Gulf of California that almost
reaches Death Valley;
Sea floor is basalt--just what you’d get if you melted a
little bit of mantle rock, and let the melt rise to the
top and freeze;
Although sea floor is generally less dense than mantle,
very old, cold sea floor can be dense enough to sink into
hot mantle;
As sea floor is made, Earth does not blow up like a
balloon, so sea floor must be lost somewhere.
Geoscience 10: Geology of The National Parks
Unit 3
SUBDUCTION (cont’d)
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Sea floor is lost where it sinks back into the
mantle at Subduction Zones.
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All sorts of things happen there:
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Moving rocks stick, then slip, giving earthquakes;
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As the rocks go down, they are squeezed until the
arrangement of atoms in the minerals changes to one
that takes up less space; sometimes this rearrangement
affects a lot of rock at once, giving an “implosion”
earthquake (the deepest quakes are of this type); …
Geoscience 10: Geology of The National Parks
Unit 3
SUBDUCTION
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(cont’d)
More things at Subduction Zones
Mud and rocks and even islands are scraped off the
downgoing slab, piling up like groceries at the end of a
check-out conveyor belt (that’s what makes up Olympic
National Park);
Some mud and rock are carried down a bit and then
squeezed back out, something like squeezing a
watermelon seed between your fingers until it squirts
out (you may find some of this at Olympic, too); …
Geoscience 10: Geology of The National Parks
Unit 3
SUBDUCTION (cont’d)
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Still more things at subduction zones:
Some mud and water are carried even farther down; the water
lowers the melting point of the rocks (just as adding water to
flour speeds cooking in the oven);
And a little melt is generated, rises, and feeds volcanoes (Crater
Lake, Mt. St. Helens, etc.) that form lines or arcs at continent
edges or offshore;
This melt is richer in silica and poorer in iron than the basalt it
comes from, and is called andesite, because the volcanoes in the
Andes were formed this way.
Geoscience 10: Geology of The National Parks
Unit 3
A Bit of REVIEW…
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Earth hot, soft and convecting deep (asthenosphere); colder, harder and
breaks in upper mantle and crust (lithosphere) floating on top;
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Lithosphere broken in few big plates and many little ones; most “action”
is at plate edges;
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Plates pull apart at spreading centers, splitting continents to make
basaltic sea floor;
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Plates come together at subduction zones, where cold sea floor dense
enough to sink into hot mantle, where scraped-off materials pile up to
make edges of continents (Olympic, etc.), and where water and sediment
taken down lower melting point of rock, feeding silica-rich (andesitic)
volcanoes;
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Plates also may slide past at transform faults, such as San Andreas;
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Stick-slip of moving rocks make earthquakes; in subduction zones,
collapse of minerals under rising pressure may make very deep quakes.
Geoscience 10: Geology of The National Parks
Unit 3
… Introducing Volcanoes
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Towers of rising rock from very deep (often core-mantle
boundary) feed “hot spots”;
Plates drift over the top, and hot spots occasionally punch
through to make lines of volcanoes, which often are oceanic
islands (seamounts);
Hotspots from mantle, basaltic, very similar to sea floor;
A new hotspot looks like a mushroom when rising; the head
feeds huge (state-sized) lava flows called flood basalts, and
the stem then feeds the lines of volcanoes;
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Hawaii is the classic example;
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Yellowstone also a hot spot; head of Yellowstone hot spot
covered eastern Washington and Oregon with basalt.
Geoscience 10: Geology of The National Parks
Unit 3
Volcanoes: Characteristics
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In melted rock, silicon and oxygen make SiO4
tetrahedra that try to polymerize (stick together in
chains, sheets, etc.) to make lumps;
Can break up lumps with great heat and much iron (in
basalt); makes lavas that flow easily and don’t explode,
so Hawaii is a shield volcano, much wider than high;
Can break up lumps with volatiles (water, CO2 , etc.),
but when these escape near surface, lava won’t flow
easily and plugs system, so next time trapped volatiles
explode; form steep stratovolcano with alternating
flows and pyroclastics (blown-up bits) from explosions.
Geoscience 10: Geology of The National Parks
Unit 3
Volcanic Hazards
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Pyroclastic flows, deadly hot, fast rock-gas mixes;
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Pyroclastics, big rocks that fall on your head, or smaller
ones that plug up jet engines;
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Poison gases, that kill many very quickly;
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Landslides and mudflows, that bury whole cities;
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Tsunamis, giant waves that devastate coasts;
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Climate change, especially cooling from particles blocking
the sun and frosting crops;
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Mostly subduction-zone; Hawaii flows mostly block roads
and burn houses, usually out-runnable.
Geoscience 10: Geology of The National Parks
Unit 3
Predicting Volcanoes
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Easy to figure out where they are likely;
Often but not always possible to figure out that eruption
will happen in next days or hours;
Valuable, has saved many lives, but imperfect;
And people get mad if you tell them to leave and then
nothing happens;
United States Geological Survey especially does in US;
highly valuable, and greatly underappreciated;
Lots of people continue to build where dangerous.
Geoscience 10: Geology of The National Parks
Unit 3