Introductory material -- main points

1. The best scientific estimate of the age of the universe (that is, the time since the "big bang") is approximately 13.82 billion years. This is based on the standard cosmological model, analysis of the cosmic background radiation (CBR) left over from the big bang, and data from the COBE, WMAP and Planck space probes that focused on the CBR.
2. The best scientific estimate of the age of the Earth is approximately 4.54+/- 0.05 billion years. The age of the oldest mineral constituents in meteorites that condensed in the solar nebula is a little bit older (~4.5695 +/- 0.0002 billion years; Baker and others, 2005, Nature v. 436(7054), p. 1127-1131). Earth accreted from this primordial material over a period of time estimated to be between a few million and a hundred million years.
3. The range or domain of questions that are appropriately addressed using the methods of science is limited to those matters about which reproducible observations can be made and testable hypotheses developed. Science is the best methodology for studying the natural world, its developmental history and its underlying processes.
4. The essential characteristic of an observation that allows us to refer to it as a scientific fact is that the observation must be reproducible.
5. When a scientific explanation of how certain scientific facts relate to one another has been tested by scientists repeatedly, and has not been demonstrated to be faulty or untrue, scientists might refer to that explanation as a theory.
6. When it is first proposed but before it has sustained rigorous testing, a scientific explanation of how certain scientific facts relate to one another is called a hypothesis.
7. The essential characteristic of a scientific explanation of a phenomenon or of the relationship between scientific facts is that the explanation must be testable or falsifiable.
8. The boundary at the bottom/base of the crust is called the Moho.
9. Oceanic crust has the average composition of the rock types basalt and gabbro. These rocks are formed through the partial melting of the upper mantle asthenosphere below mid-ocean ridges.
10. Continental crust has the average composition of the rock type granodiorite.
11. Oceanic crust has an average density of ~2.89 grams per cubic centimeter, and is denser than continental crust.
12. Continental crust has an average density of ~2.65 grams per cubic centimeter, and is less dense than the oceanic crust.
13. The average density of the upper mantle is ~3.3 grams per cubic centimeter, so it is denser than the crust.
14. The average bulk density of the whole Earth is ~5.5 grams per cubic centimeter, from which we infer (given the average densities of the crust and upper mantle) that the deeper interior of Earth must include materials that are significantly denser than near-surface materials.
15. Average continental crust is approximately 5-8 times as thick as average oceanic crust.
16. The oldest continental crust that we have dated so far is about 4 billion years old.
17. The oldest oceanic crust that is still in an ocean basin is about 180-190 million years old.
18. The average thickness of the oceanic crust is approximately 5-8 km.
19. The average thickness of the continental crust is approximately 35-40 km, although it varies from ~25 km to ~70 km under the Himalaya and southern Tibet.
20. The lithosphere is the outermost solid layer of Earth that is divided into all of the various plates and microplates.
21. The asthenosphere is the weak part of the upper mantle below the lithosphere across which the lithosphere is able to move.
22. The average composition of the upper-mantle lithosphere and the upper-mantle asthenosphere is the same, and is dominated by dense silicate minerals like olivine and pyroxene.
23. The upper-mantle lithosphere (at a given depth below Earth's surface) is stronger than the upper-mantle asthenosphere. The asthenosphere is weaker because it is hotter, in the same way that warm candle wax is weaker than cold candle wax. The asthenosphere is able to flow like Silly Putty, even though it is mostly solid.
24. The asthenosphere is typically ~97% solid, 3% liquid. Under mid-ocean ridges and at hot spots, the asthenosphere might be 75% solid and 25% liquid. In either case, it is mostly solid.
25. The average rate at which a plate moves relative to an adjacent plate is on the order of ~5 centimeters per year -- some interactions perhaps 2-3x faster, many slower.
26. We currently think that the most important mechanism moving plates across Earth's surface is slab pull.
27. Another important plate-motion mechanism is ridge push. This is an unfortunate name, because the lateral motion away from the ridge is related to the ridge's elevation relative to surrounding seafloor and the inclined boundary between the upper-mantle lithosphere and the asthenosphere near the ridge. The plates are really not being pushed apart -- they are sliding apart under the influence of gravity.
28. The lithosphere contains all or part of two of Earth's primary layers: the crust and upper-mantle lithosphere (i.e., the rigid-elastic uppermost part of the mantle).

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