Overview of the main topics you will encounter 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.
Most of the Earth’s heat is made 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, except cold, old ocean crust can sink into hot mantle.
Core has a solid inner part (higher pressure squeezes to solid) and a 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 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, and cooling causes contraction & sinking; together forms convection cells (in spaghetti pots on the stove, and in soft parts of Earth’s mantle).
The 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. Breaks, in places such as Death Valley, often are angled rather than vertical, and one side slides down along the other, making an earthquake fault.
Death Valley is a great example of the effects 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.
This has offset layers of rock across earthquake faults, and earthquakes still happen and increase that offset.
Lava may leak up the faults to feed volcanoes.
If this kept on, could tear the west apart to make an ocean basin.
DEATH VALLEY: Tear-Apart Makes Oceans
The Gulf of California was formed by spreading, tearing Mexico apart.
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 frozen crack-filling lava.
The sea floor is hottest, and thus highest in elevation, 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 and oldest farthest from the ridges.
Sediment (wind-blown dust, fish poop, etc.) is thicker 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.
Ground shaking from any cause.
Few big, deep ones may be from pressure squeezing minerals to a new, smaller form.
Most earthquakes happen 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, or an earthquake fault.
EARTHQUAKE: Seismic Waves
During an earthquake, rocks shake neighbors, which shake neighbors, making waves moving away from the quake.
P (or push) waves are normal sound waves and go through solid, liquid, or gas.
S (or shear) waves are slower, and don’t go through liquids (know Earth’s outer core is known to be liquid because P but not S waves go through).
Seismic waves shake buildings and may knock them down.
An increase of 1 in Richter magnitude means an increase of 10 in ground motion, and an increase of 30 in total energy (a magnitude-3 quake moves the ground 10 times more than a magnitude-2 quake, which moves the ground 10 times more than a magnitude-1 quake).
Can just feel a magnitude-1 quake, magnitude-5 damages buildings, biggest (about magnitude-9) is far bigger than atomic bombs.
For each increase of 1 in magnitude, quakes become about 10 times less common.
But because the increase of 1 in magnitude means an increase of about 30 in energy, most energy is released (and most damage) in few, big, rare quakes.
Quakes occur especially where big blocks of rock are moving past each other.
Not many in Pennsylvania, and other central regions of lithospheric plates; mostly near plate edges.
Can be HUGE (worst ever estimated to have killed 800,000 people, in China in 1556).
We can predict where big quakes are likely.
Can’t (yet) predict whether a quake is about to happen.
Maybe rocks give hints they’re about to break, but not sure.