Unit 3 Geoscience 10: Geology of The National Parks Plate Tectonics II

Geoscience 10: Geology of The National Parks Unit 3

Plate Tectonics II Plate Tectonics II


Most Most students find this week to have more new find this week to have more new material than any other;material than any other;

We always debate how much time to spend on We always debate how much time to spend on the “science”, “impacts” and “cute critter” the “science”, “impacts” and “cute critter” material; material;

We usually work fast through the deep-earth We usually work fast through the deep-earth and tectonics parts to get to the cute-critter and tectonics parts to get to the cute-critter and living-on-Earth parts;and living-on-Earth parts;

So suck it up and power through, and things So suck it up and power through, and things should get easier.should get easier.

Plate Tectonics II Plate Tectonics II


We saw that sea floor is made at spreading ridges We saw that sea floor is made at spreading ridges such as the one through the Gulf of California that such as the one through the Gulf of California that almost reaches Death Valley;almost reaches Death Valley;

Sea floor is basalt--just what you’d get if you Sea floor is basalt--just what you’d get if you melted a little bit of mantle rock, and let the melt melted a little bit of mantle rock, and let the melt rise to the top and freeze;rise to the top and freeze;

Although sea floor is generally less dense than Although sea floor is generally less dense than mantle, very old, cold sea floor can be dense mantle, very old, cold sea floor can be dense enough to sink into hot mantle;enough to sink into hot mantle;

As sea floor is made, Earth does not blow up like a As sea floor is made, Earth does not blow up like a balloon, so sea floor must be lost somewhere.balloon, so sea floor must be lost somewhere.

SUBDUCTIONSUBDUCTION


Sea floor is lost where it sinks back into the Sea floor is lost where it sinks back into the mantle at mantle at Subduction ZonesSubduction Zones..

All sorts of things happen there:All sorts of things happen there:

Moving rocks stick, then slip, giving earthquakes;Moving rocks stick, then slip, giving earthquakes; As the rocks go down, they are As the rocks go down, they are squeezed squeezed until the until the

arrangement of atoms in the minerals changes to arrangement of atoms in the minerals changes to one that takes up less space; sometimes this one that takes up less space; sometimes this rearrangement affects a lot of rock at once, giving an rearrangement affects a lot of rock at once, giving an “implosion” earthquake (the deepest quakes are of “implosion” earthquake (the deepest quakes are of this type); …this type); …

SUBDUCTION (cont’d)SUBDUCTION (cont’d)


More things at Subduction ZonesMore things at Subduction Zones Mud and rocks and even islands are scraped off the Mud and rocks and even islands are scraped off the

downgoing slab, piling up like groceries at the end downgoing slab, piling up like groceries at the end of a check-out conveyor belt (that’s what makes up of a check-out conveyor belt (that’s what makes up Olympic National Park);Olympic National Park);

Some mud and rock are carried down a bit and Some mud and rock are carried down a bit and then squeezed back out, something like squeezing then squeezed back out, something like squeezing a watermelon seed between your fingers until it a watermelon seed between your fingers until it squirts out (you may find some of this at Olympic, squirts out (you may find some of this at Olympic, too); …too); …



Still more things at subduction zones:Still more things at subduction zones: Some mud and water are carried even farther down; the Some mud and water are carried even farther down; the

water lowers the melting point of the rocks (just as adding water lowers the melting point of the rocks (just as adding water to flour speeds cooking in the oven);water to flour speeds cooking in the oven);

And a little melt is generated, rises, and feeds volcanoes And a little melt is generated, rises, and feeds volcanoes (Crater Lake, Mt. St. Helens, etc.) that form lines or arcs at (Crater Lake, Mt. St. Helens, etc.) that form lines or arcs at continent edges or offshore;continent edges or offshore;

This melt is richer in silica and poorer in iron than the basalt This melt is richer in silica and poorer in iron than the basalt it comes from, and is called andesite, because the volcanoes it comes from, and is called andesite, because the volcanoes in the Andes were formed this way.in the Andes were formed this way.



Earth hot, soft and convecting deep (asthenosphere); colder, Earth hot, soft and convecting deep (asthenosphere); colder, harder and breaks in upper mantle and crust (lithosphere) floating harder and breaks in upper mantle and crust (lithosphere) floating on top;on top;

Lithosphere broken in few big plates and many little ones; most Lithosphere broken in few big plates and many little ones; most “action” is at plate edges;“action” is at plate edges;

Plates pull apart at spreading centers, splitting continents to make Plates pull apart at spreading centers, splitting continents to make basaltic sea floor;basaltic sea floor;

Plates come together at subduction zones, where cold sea floor Plates come together at subduction zones, where cold sea floor dense enough to sink into hot mantle, where scraped-off materials dense enough to sink into hot mantle, where scraped-off materials pile up to make edges of continents (Olympic, etc.), and where pile up to make edges of continents (Olympic, etc.), and where water and sediment taken down lower melting point of rock, water and sediment taken down lower melting point of rock, feeding silica-rich (andesitic) volcanoes; feeding silica-rich (andesitic) volcanoes;

Plates also may slide past at transform faults, such as San Plates also may slide past at transform faults, such as San Andreas;Andreas;

Stick-slip of moving rocks make earthquakes; in subduction zones, Stick-slip of moving rocks make earthquakes; in subduction zones, collapse of minerals under rising pressure may make very deep collapse of minerals under rising pressure may make very deep quakes.quakes.

A Bit of REVIEW…A Bit of REVIEW…


Towers of rising rock from very deep (often core-mantle Towers of rising rock from very deep (often core-mantle boundary) feed “hot spots”;boundary) feed “hot spots”;

Plates drift over the top, and hot spots occasionally Plates drift over the top, and hot spots occasionally punch through to make lines of volcanoes, which often punch through to make lines of volcanoes, which often are oceanic islands (seamounts);are oceanic islands (seamounts);

Hotspots from mantle, basaltic, very similar to sea floor;Hotspots from mantle, basaltic, very similar to sea floor; A new hotspot looks like a mushroom when rising; the A new hotspot looks like a mushroom when rising; the

head feeds huge (state-sized) lava flows called flood head feeds huge (state-sized) lava flows called flood basalts, and the stem then feeds the lines of volcanoes; basalts, and the stem then feeds the lines of volcanoes;

Hawaii is the classic example;Hawaii is the classic example; Yellowstone also a hot spot; head of Yellowstone hot Yellowstone also a hot spot; head of Yellowstone hot

spot covered eastern Washington and Oregon with spot covered eastern Washington and Oregon with basalt.basalt.

… Introducing Volcanoes… Introducing Volcanoes


In melted rock, silicon and oxygen make SiOIn melted rock, silicon and oxygen make SiO44 tetrahedra that try to polymerize (stick together in tetrahedra that try to polymerize (stick together in chains, sheets, etc.) to make lumps;chains, sheets, etc.) to make lumps;

Can break up lumps with great heat and much iron Can break up lumps with great heat and much iron (in basalt); makes lavas that flow easily and don’t (in basalt); makes lavas that flow easily and don’t explode, so Hawaii is a shield volcano, much wider explode, so Hawaii is a shield volcano, much wider than high;than high;

Can break up lumps with volatiles (water, COCan break up lumps with volatiles (water, CO2 2 , , etc.), but when these escape near surface, lava etc.), but when these escape near surface, lava won’t flow easily and plugs system, so next time won’t flow easily and plugs system, so next time trapped volatiles explode; form steep stratovolcano trapped volatiles explode; form steep stratovolcano with alternating flows and pyroclastics (blown-up with alternating flows and pyroclastics (blown-up bits) from explosions.bits) from explosions.

Volcanoes: CharacteristicsVolcanoes: Characteristics


Pyroclastic flows, deadly hot, fast rock-gas mixes;Pyroclastic flows, deadly hot, fast rock-gas mixes;

Pyroclastics, big rocks that fall on your head, or Pyroclastics, big rocks that fall on your head, or smaller ones that plug up jet engines;smaller ones that plug up jet engines;

Poison gases, that kill many very quickly;Poison gases, that kill many very quickly;

Landslides and mudflows, that bury whole cities; Landslides and mudflows, that bury whole cities;

Tsunamis, giant waves that devastate coasts;Tsunamis, giant waves that devastate coasts;

Climate change, especially cooling from particles Climate change, especially cooling from particles blocking the sun and frosting crops;blocking the sun and frosting crops;

Mostly subduction-zone; Hawaii flows mostly block Mostly subduction-zone; Hawaii flows mostly block roads and burn houses, usually out-runnable.roads and burn houses, usually out-runnable.

Volcanic Hazards Volcanic Hazards


Easy to figure out where they are likely;Easy to figure out where they are likely;

Often but not always possible to figure out that Often but not always possible to figure out that eruption will happen in next days or hours;eruption will happen in next days or hours;

Valuable, has saved many lives, but imperfect;Valuable, has saved many lives, but imperfect;

And people get mad if you tell them to leave and And people get mad if you tell them to leave and then nothing happens;then nothing happens;

United States Geological Survey especially does in United States Geological Survey especially does in US; highly valuable, and greatly underappreciated;US; highly valuable, and greatly underappreciated;

Lots of people continue to build where dangerous.Lots of people continue to build where dangerous.

Predicting VolcanoesPredicting Volcanoes

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Unit 3 Geoscience 10: Geology of The National Parks Plate Tectonics II