Main Topics: Module 2

Evidence showing the Earth is hot inside includes higher temperatures in deeper parts of mines and oil wells, and volcanoes bringing up heat from below.
Most of the Earth’s heat is made by the decay of natural radioactive atoms in rocks, but some of the heat is made in other ways.
How materials behave depends on what they are made of (their chemistry - iron, silica, etc.) and on the conditions they are placed in (heat, pressure).

The Earth has an iron core, a mantle with silica added to iron, ocean crust with more silica, and continental crust with still more silica (the details are way more complex, but this is a start)—going up, each layer is less dense and floats on layers below, except that cold, old ocean crust can sink into hot mantle.
The core has a solid inner part (the very high pressure deep in the Earth squeezes the iron to solid) and a liquid outer part.
The crust plus the upper mantle together make a layer called the lithosphere, which tends to break, not flow; deeper in the mantle, the rock tends to flow rather than break (Just below the lithosphere is the asthenosphere, as well as deeper layers with other names we won't worry about) (hot solids can flow—think of the behavior of a chocolate bar in your pocket, or of a horseshoe that a blacksmith is shaping).
The mantle and crust are almost entirely solid but with a little melted rock in a few places.

Heating causes soft rocks as well as liquids and gases to expand and rise, and cooling causes contraction and sinking; together the rising and sinking flows organize to form 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!

Lake Tahoe (California) and Snowbird (Utah) ski areas are moving apart about as rapidly as your fingernails grow (an inch or so per year)— this is measured with GPS and other techniques, as well as reconstructed from the geology.
This motion 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 process continues long enough into the future (many millions of years), it could tear the western U.S. apart to make an ocean basin.

The Gulf of California was formed by spreading and 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 them solid, and “unsqueezing” (a pressure drop) 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 and wind to drop dust on them.
Where ridges come up on land, they are ripping continents apart, as at Death Valley and in the East African rifts.

An earthquake is the ground shaking from any cause.
A few big, deep ones may be from pressure-squeezing minerals until they suddenly switch to a new, smaller form.
Most earthquakes happen when one body of rocks moving past another body gets stuck for a while, bends, then breaks loose, and the bend snaps back like a broken rubber band.
The break between the two bodies of rocks is called a fault, or sometimes an earthquake fault.

During an earthquake, rocks shake neighboring rocks, which shake their neighbors, making seismic waves moving away from the quake.
P (or push) waves in the rocks are the same as sound waves that go through air and liquids.
S (or shear) waves are slower and don’t go through liquids or gases (the liquid outer core was discovered because P but not S waves go through it).
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).
You can just feel a nearby magnitude-1 quake, a nearby magnitude-5 quake is big enough to damage buildings, and the biggest natural quakes from moving rocks (about magnitude-9) release far more energy than atomic bombs.

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 is released (and most damage is done) in a few big, rare quakes.
Quakes occur especially where big blocks of rock are moving past each other.
There are not many big quakes in Pennsylvania or other central regions of lithospheric plates; big quakes mostly occur near plate edges.

Damage from the worst earthquakes can be HUGE (the worst ever is estimated to have killed about 800,000 people, in China in 1556).
We can predict where big quakes are likely.
We can’t (yet) predict whether a quake is about to happen.
Rocks may give hints they’re about to break, but we're not sure we can "read" them reliably.