Review the Unit 3 Introduction
You have reached the end of Unit 3! Double-check the list of requirements on the Unit 3 Introduction page and the Course Calendar to make sure you have completed all of the activities listed there.
Review of the main topics and ideas you encountered 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.
- 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.
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 are of this type).
- 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).
- 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 hot, soft and convecting deep (asthenosphere); colder, harder and breaks in upper mantle and crust (lithosphere) floating on top.
- Lithosphere broken in 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 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 make earthquakes; in subduction zones, collapse of minerals under rising pressure may make very deep quakes.
- 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.
- Hawaii is the classic example.
- Yellowstone also a hot spot; head of Yellowstone hot spot covered eastern Washington and Oregon with basalt.
- 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.
- Pyroclastic flows, deadly hot, fast rock-gas mixes.
- Pyroclastics, big rocks that fall on your head, or smaller ones that plug up jet engines.
- Poison gases, that kill many very quickly.
- 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.
- Mostly subduction-zone; Hawaii flows mostly block roads and burn houses, usually out-runnable.
- 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 under appreciated.
- Lots of people continue to build where dangerous.
Following are some supplementary materials for Unit 3. While you are not required to review these, you may find them interesting and possibly even helpful in preparing for the quiz!
- Website: Crater Lake National Park
- Website: Olympic National Park
- Website: Mount St. Helens National Volcanic Monument
Comments or Questions?
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