Wrap Up

Review of the main topics and ideas you encountered in Unit 2.

THE EARTH: It's HOT Inside!

• The deeper a mine or oil well is, the hotter it is at the bottom; volcanoes bring up heat from below.
• The earth’s heat made mostly by the decay of natural radioactive atoms in rocks.
• How materials (and people!) behave depends on what they are (iron, silica, etc.) and on the conditions they are placed in (heat, pressure).

THE EARTH: Layered by Composition & Behavior

• Iron core, mantle with silica added to iron, ocean crust with more silica, continental crust with still more silica (way more complex than this, but this is a start)—going up, each layer less dense and floats on layers below.
• Core has a solid inner part (higher pressure squeezes to solid) and liquid outer part.
• Crust plus upper mantle (called lithosphere) tend to break not flow; deeper in mantle tends to flow not break (asthenosphere, plus other names and layers we won’t worry about) (hot solids can flow--think of a chocolate bar in your pocket, or blacksmith making horseshoes).
• Mantle and crust solid, but with a little melt in a few places.

CONVECTION: Moving the Mantle's Heat

• Heating causes expansion & rising, cooling causes contraction & sinking; together form convection cells (in spaghetti pots on the stove, and in the soft part of Earth’s mantle).
• Lithosphere is broken into a few big plates that raft around on the convection cells like scum on a spaghetti pot.
• Where the lithospheric plates are pulled apart, they tend to break. The breaks often slant down, and one side slides down along the other, making an earthquake fault.
• Death Valley is a great example of convection!

NEVADA GETTING WIDER (with Death Valley)

• Lake Tahoe (California) and Snowbird (Utah) ski areas are moving apart about as rapidly as your fingernails grow (an inch or so per year)--can measure with GPS, etc.
• We find layers of rock offset as shown in the diagram, and earthquakes still happen and increase that offset.
• Lava may leak up the cracks to feed volcanoes.
• If this kept on, it could tear the west apart to make an ocean basin.

DEATH-VALLEY: Tear-Apart Makes Oceans

• That’s what the Gulf of California is.
• Baja California is drifting away from the mainland.
• The breaking of the rocks has focused along a crack down the middle of the Gulf.
• As the rocks move away from the crack, their weight no longer squeezes the hot mantle beneath the crack.
• For most rocks, squeezing tends to make solid, and “unsqueezing” (a drop in pressure) favors melting.
• So the hot mantle under the crack melts a little, and the melt leaks up the crack and freezes.
• The sea floor of the Gulf of California (and of all other oceans!) is made of the frozen crack-filling lava.
• The sea floor is hottest, and thus highest, near the crack, forming a mid-oceanic ridge.
• Such ridges wind through Earth’s oceans like the seam on a baseball.
• Ocean-floor rocks are youngest near the ridges, oldest farthest from the ridges.
• Sediment (wind-blown dust, fish poop, etc.) thickens away from the ridges because older rocks have had more time for fish to poop on them.
• Where ridges come up on land, they are ripping continents apart, as at Death Valley and in the East African rifts.

EARTHQUAKES!

• Ground shaking from any cause.
• Few big, deep ones, maybe from pressure squeezing minerals to new, smaller form.
• Most when one batch of rocks moving past another batch gets stuck for a while, bends, then breaks loose, and the bend snaps back like a broken rubber band.
• The break between the two batches of rocks is called a fault.

EARTHQUAKE: Seismic Waves

• Rocks shake neighbors, which shake neighbors, in waves moving from the quake.
• P (or push) waves are normal sound waves, go through solid, liquid, gas.
• S (or shear) waves slower, don’t go through liquids (know Earth’s outer core is liquid because P but not S waves go through).
• Shake buildings and may knock them down.

EARTHQUAKE SIZE

• Increase of 1 in Richter magnitude means an increase of 10 in ground motion, and an increase of 30 in total energy (magnitude-3 quake moves ground 10 times more than a magnitude-2 quake, which moves ground 10 times more than a magnitude-1 quake).
• Can just feel magnitude-1 quake, magnitude-5 damages buildings, biggest (about magnitude-9) far bigger than the atom bomb.

EARTHQUAKE OCCURRENCES

• For each increase of 1 in magnitude, quakes become about 10 times less common.
• But because an increase of 1 in magnitude means an increase of about 30 in energy, most energy release (and most damage) in a few big, rare quake.
• Quakes especially where big blocks of rock moving past each other.
• Not many in Pennsylvania.

EARTHQUAKE DAMAGE

• Can be HUGE (worst ever estimated to have killed 800,000 people, China in 1556).
• We can predict where big quakes are likely.
• Can’t (yet) predict a quake is about to happen.
• Maybe rocks give hints they’re about to break, but not sure.