Wrap Up

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.

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 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.

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 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.

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 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.

VOLCANIC HAZARDS

• 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.

PREDICTING VOLCANOES

• Easy to figure out where they are likely.
• Often, but not always, possible to figure out that an eruption will happen in the 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.