Geology of the National Parks

Wrap Up


Review the Unit 2 Introduction

You have reached the end of Unit 2! Double-check the list of requirements on the Unit 2 Introduction page and the Course Calendar to make sure you have completed all of the activities listed there.

Unit 2 Overview

Click here to review the Unit 2 Overview and make sure you understand all the main topics.

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.


  • 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.


  • 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.


  • 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.


  • 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.

Reminder - Continue to work on Exercise #1. See Course Calendar for specific dates.

Supplemental Materials

Following are some supplementary materials for Unit 2. While you are not required to review these, you may find them interesting and possibly even helpful in preparing for the quiz!

Comments or Questions?


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