The Earth

The textbook web resources for Chapter 5 are here.


The Earth...
  - is a sphere because it's gravity pulls it into this shape.
    - objects less than 350km in diameter won't do this
  - bulges around the equator due to its rotation (inertia)
  - is composed mainly of
    - Oxygen
    - Silicon
    - Aluminum
    - Iron
    - Calcium
    - Magnesium
    - Sodium
    - Potassium
    - Titanium
    - and other elements
  - has layers:
    - crust
    - mantle
    - outer core
    - inner core

Layers of the Earth
1. The Crust
  - about 20 to 70 km thick
  - made of solid, low-density rocks (mainly silicates -
     compounds of silicon and oxygen)
2. The Mantle
  - just below the crust
  - extends about halfway to Earth's center
  - hot, semi-solid rock - mainly silicates (most commonly olivine)
  - mantle flows slowly, like warm wax or butter before it's melted
3. The Outer Core (Liquid Core)
  - dense, liquid - most likely iron and nickel (maybe some sulfur)
  - this core is liquid because of the intense heat, which melts it
4. The Inner Core (Solid Core)
  - compressed iron and nickel
  - the great pressure in the center of the Earth keeps this from
Differentiation - the separation of material by their density (such as seen with the layers of the Earth).  You can check out an animation of this process

Heating of the Core
Theory #1
  - The Earth started off hot, and the outer layers have insulated
     the inner layers, preventing it from cooling off too quickly.
Theory #2
  - The Earth started off cool, but is heated from the center by
     radioactive decay.  This is the process by which some
     elements break down into other elements by giving off atomic
     particles and energy (often in the form of heat).
Most likely, it is a combination of the two theories.  The Earth probably started off hot, and radioactive decay is slowing down the cooling process.

The Age of the Earth estimated to be around 4.6 billion years. calculated by measuring the radioactive decay of certain materials in Earth's rocks.  The same process can be done for rocks from other planets, moons, asteroids, etc.

Radioactive decay is there process by which elements break down into simpler elements over a long period of time.  The time it takes for half of a sample of a given element to break down is called its half-life.  (See the attached PowerPoint.)  Some examples of elements' half-lives are:
  - Uranium decays into Lead.  Half-life: 4.5 billion years.
  - Potassium decays into Argon.  Half-life: 1.3 billion years.
  - Carbon-14 decays into Carbon-12.  Half-life: 5,800 years.

Earth's Internal Motions
The Earth's mantle flows by convection.  This means that the hotter material close to the core will tend to rise, and the cooler material near the crust will tend to sink.  This creates currents in the mantle, which move the Earth's plates.  You can check out an animation of this process

The ideas of continental drift and plate tectonics explain how the continents have separated since the time they were together as the supercontinent, Pangaea.

The Earth's Atmosphere
Earth's early atmosphere was mainly methane (CH4
) and ammonia (NH3).  Over Earth's history, it has evolved to be...
    78.08%  Nitrogen (N2)
    20.95%  Oxygen (O2)
      0.93%  Argon (Ar)
    Varying amounts of Water Vaopr (H2O) (btwn 0.1 and 3.0%)
      0.03%  Carbon Dioxide (CO2)
    Trace amounts of neon, helium, krypton, hydrogen, methane,
      carbon monoxide, and other pollutants.

How it got this way...
  - gasses given off by volcanic eruptions
  - gasses delivered by comets
  - UV light breaks down methane and ammonia
  - plants and cyanobacteria make O2 by photosynthesis
  - ozone (O3) is formed because UV light causes O2 to be split
     into individual O atoms.  These then combine with other O2
     molecules to form O3.

Some of the energy that the Earth receives from the sun is trapped in our atmosphere by gasses, such as carbon dioxide and water vapor.  This trapped energy is converted into heat and helps to warm the Earth.  This process is called the Greenhouse Effect.

The atmosphere is most dense at the surface of the Earth, where the weight of the air above compresses it.  As you go higher and higher, the air gets thinner until is fades out completely.  Thin air has been detected by satellites 100km about the Earth's surface - and the orbits of these satellites can be slowed by it, causing them to eventually fall towards Earth and burn up.

Earth's Magnetic Field
The Earth's magnetic field is caused by electric currents in the outer core (which is made of molten iron and nickel - both of which are magnetic elements).  The currents in the outer core are caused by convection and the motion of the Earth's rotation - this process is called the magnetic dynamo.
  - Moon - no magentosphere - too small to have a large,
     convecting core.
  - Venus - weak magnetosphere - rotates very slowly.
  - Jupiter & Saturn - rotate very quickly, strong magnetospheres.

Polarity is the property of a magnetic field that gives it a north pole and a south pole.  Two similar poles repel each other, but two opposite poles attract.  Earth's magentic poles do not line up with the true north and south poles - they are off by about 15 degrees - and they switch (reverse polarity) about every 10,000 years.

In the upper atmospere, the magentic field deflects most of the charged particles given off by the Sun.  These solar particles would damage living tissue, and could sterilize the Earth if they were not filtered out.

The solar particles can interact with nitrogen and oxygen in the atmosphere, exciting them and causing them to give off light.  We call this an aurora.  (Aurora borealis in the Northern Hemisphere, aurora australis in the Southern.)

The magnetosphere is the area where the shape of the upper atmosphere is controlled by the magnetic field.  Charged particles are trapped in two zones called Van Allen radiation belts.  The particles here will penetrate spacecraft and damage living tissue.

Earth Motions
Air in the Earth's atmosphere moves in a North-South direction as a result of convection and the uneven heating of the Earth's surface.  (In other words, the equator gets more heat than the poles.  This difference in temperature causes convection currents.)

The wind belts that are created by convection are deflected to the side (right in the Northern Hemisphere, left in the Southern) due to the rotation of the Earth.  This is the Coriolis Effect (see attached PowerPoint).  This causes storms and strong bands of high-altitude winds called jet streams.  We see similar effects on the gas planets in our solar system (Jupiter, Saturn, Uranus, and Neptune).

Precession is the change in direction of the axis of a spinning object.  The Earth's axis completes one cycle in about 26,000 years.  This means that, at the time the Great Pyramid was built in Egypt, Thuban was the North Star - not Polaris.  In about another 12,000 years, Vega will be the North Star.  About 13,000 years after that, the North Pole will point at Polaris again.


Homework from the Text:

  • Read Chapter 5 (pg 147-175).
  • Do Review Questions #1-8, 10-14 (pg 175).
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