Part 4: Earth and Moon
John P. Pratt
The Earth as a Planet
The Earth's Age
- The earth appears to be 4.6 billion years old.
- Radioactive measurement support estimates made from assumed rates of sedimentation.
- Radioactive measurements are based on several assumptions and are not always reliable.
- In any case, the earth appears to be very old, just from looking at the rock strata.
The Earth's internal structure
- The earth's gravitational force on the moon shows it has a heavy core, twice as heavy as rock.
- We think the core is iron-nickel because that is right mass and such meteorites fall from the sky.
- Seismic waves from earthquakes show that the outer core is liquid, the inner is solid.
- The thick rock outer half of the earth is called the mantle.
- The asthenosphere is the partially melted layer just under the crust.
- The crust, or lithosphere, is very thin (a few km) and floats on top of the asthenosphere.
The Earth's interior is like a smelter.
- The hot metals sink to the center like metals sink to the bottom of a vat.
- The mantle floats on the metal like they float in the upper half of the vat.
- The crust is like the thin scum of slag that floats on the very surface.
- The continents are on large plates that float on the asthenosphere.
- As the asthenosphere moves, it drags the plates with it, causing earthquakes.
- The mid-Atlantic ridge is spreading, causing the U.S. to move westward, over the San Andreas
fault in California.
- Underground molten rock is called magma, when it reaches the surface it is called lava.
- Entire mountains are volcanic.
- Many planets have huge volcanic areas.
The Earth's Magnetic Field
- The earth is like a huge, but weak, magnet which causes compasses to point approximately north.
- The earth's core is belived to be too hot to be a magnet.
- The earth's magnetism is thought to come from currents in the liquid nickel-iron core.
The Earth's Atmosphere
- The earth's early atmosphere is thought to have been steam (H2O) and carbon dioxide (CO2).
- The steam condensed into the oceans and the CO2 became limestone rocks.
- The atmosphere's 23% oxygen content apparently comes entirely from plants.
- Because the earth is rotating, anything moving in the northern hemisphere tend to drift right.
- That causes low pressure areas to circle counter-clockwise, high pressure counter-clockwise.
Impacts on the Earth
- Until recently, geologists ignored most catastropic events in earth history
- Huge meteorites have made large craters, like the Winslow crater in Arizona
- It is now believed that the extinction of the dinosaurs were caused by such an impact.
- After geologists witnessed a comet impact Jupiter, they have found many more craters on earth.
- The moon is about 1/4 the radius of the earth.
- The moon's density is about half that of the earth, like one big rock (no iron core).
- The moon had no atmosphere, so stars are visible in the daytime.
- There is much less earthquake activity than on earth.
- The moon's surface has thousands of huge impact craters.
- The side facing the earth also has huge plains called mares ("seas") covered with dark lava flows.
- Bright rays extend out from large craters.
- It is believed that the craters were mostly formed early in the formation of the solar system.
- Tidal forces are caused by gravity pulling harder on the near side of a body than the far side.
- Tidal forces have caused the moon to be in a synchronous orbit, so it always faces the earth.
- The tides on earth are caused by these tidal forces, with the moon pulling harder on near side.
- The reaction of the tides on the moon is to accelerate it into a slightly more distant orbit.
- The moon also slows the earth's rotation very slightly.
- Roche's limit is the distance from a planet at which a small body will be torn apart by tidal forces.
The Origin of the Moon
- It is still a mystery where the moon came from.
- Its composition is without iron, but much like the mantle rocks of the earth.
- The best theory is the unlikely giant impact hypothesis, that something broke it from the earth's mantle.