Overview of the main topics you will encounter in Unit 3
PLATE TECTONICS II
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.
SUBDUCTION
We saw that the 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.
Sea floor is lost where it sinks back into the mantle at Subduction Zones. All sorts of things happen there:
Moving rocks stick, then slip, giving earthquakes;
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 may be of this type);
Mud and rocks and even islands are scraped off the down going 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 squeeze-back-out stuff at Olympic, too);
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.
A BIT OF REVIEW
Earth is hot, soft and convecting deep (asthenosphere); colder, harder and breaks in the upper mantle and crust (lithosphere) that are floating on top.
Lithosphere is broken into a few big plates and many little ones; most “action” is at plate edges.
Plates pull apart at spreading centers, splitting continents to make basaltic sea floor.
Plates come together at subduction zones, where cold sea floor is 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.
Plates also may slide past at transform faults, such as San Andreas.
Stick-slip of moving rocks makes earthquakes; in subduction zones, collapse of minerals under rising pressure may make very deep quakes.
INTRODUCING VOLCANOES
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 are from the 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.
Hawaii is the classic example.
Yellowstone also a hot spot; the head of Yellowstone hot spot covered eastern Washington and Oregon with basalt, but Yellowstone's lava is modified coming through the crust so that more silica is erupted than for Hawaii.
VOLCANO CHARACTERISTICS
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 the surface, lava won’t flow easily and plugs the system, so the next time trapped volatiles explode; form steep stratovolcanoes with alternating flows and pyroclastics (blown-up bits) from explosions.
VOLCANIC HAZARDS
Include pyroclastic flows, deadly hot, fast rock-gas mixes;
And pyroclastics, big rocks that fall on your head, or smaller ones that plug up jet engines;
Also, poison gases, that kill many very quickly;
And landslides and mudflows, that bury whole cities;
Tsunamis, giant waves that devastate coasts;
Climate change, especially cooling from particles blocking the sun and frosting crops;
Hazards especially dangerous from subduction-zone volcanoes; Hawaii flows mostly block roads and burn houses, usually out-runnable.
PREDICTING VOLCANOES
Easy to figure out where they are likely;
Often, but not always, possible to figure out that an eruption will happen in next days or hours;
Predictions are valuable and have saved many lives, but are imperfect;
And people get mad if you tell them to leave and then nothing happens;
United States Geological Survey especially handles predictions in US; highly valuable, and greatly under-appreciated;
Lots of people continue to build and live where dangers await.
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