How the Earth Works
Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The diagram above shows the main components of the earth:
- A thin outer layer, the lithosphere, is about 100 km thick. In
comparison to the whole earth, it is about as thick as the skin on an apple.
It consists of a crust on the surface and an underlying layer that is
chemically like the mantle.
- A thick layer of solid rock, the mantle. A layer of nearly molten,
weak rock called the asthenosphere separates the mantle from the lithosphere.
Chemically the lower lithosphere, asthenosphere and mantle are pretty much
identical. What separates them is strength. The asthenosphere is the weak zone
over which the plates slide.
- A central core.
The domain of life, from several kilometers deep in the lithosphere to 10 km
or so above the surface.
The domain of liquid water.
The domain of frozen water, including ice caps and permafrost. Often
considered part of the hydrosphere.
- The bottom 10 km or so is the troposphere, from a Greek word for
"turning." The sun warms the surface, but warm air rises, and as
it rises, it expands and cools. Then the cool air sinks. This constant
churning creates the weather.
- Above 10 km or so, the air is too thin for warm air from the surface to
continue rising. The next layer, the stratosphere, is stable and
stratified, with cold air at the base and warm air above. Because it is so
stable, commercial aircraft fly at the lower boundary of the stratosphere.
Military aircraft and the supersonic airliner the Concorde fly within the
stratosphere. Near the top of the stratosphere, solar ultraviolet causes
oxygen to form ozone (O3). Ozone absorbs solar ultraviolet and
helps protect the surface.
- From 50-80 km, the temperature of the atmosphere again begins to fall.
This layer is the mesosphere. At about 80 km we find the coldest
temperatures in the atmosphere, about -95 C. In the mesosphere, solar
radiation and particles create electrically charged atoms and this screen of
charged atoms absorbs radio signals. However, at night most of the charged
atoms recombine with stray electrons and the remaining charged atoms settle
into fairly smooth layers that reflect radio signals. This is why at night
you can often pick up AM stations a thousand miles or more away.
- Above 80 km temperatures again increase. The air is so thin that atoms are
accelerated by sunlight and solar particles. They are moving very fast, so
their temperature is very high, but there are so few atoms that a spacecraft
passing through this layer, the thermosphere, feels no appreciable
heat. The thrmosphere tapers off into space, but traces of atmosphere extend
1000 km or more above the earth.
External Effects (Astronomical):
Can be predictable, like day and night and the seasons, or unpredictable like
meteor impact and nearby supernovae.
- Atmospheric Circulation: Driven primatily by unequal heating of the
- Oceanic Circulation: Surface currents are driven by wind, deep circulation
by density differences
- Hydrologic Cycle: Driven by evaporation from oceans and transport by wind
- Rock Cycle: Rocks change because of both surface and internal processes
- Plate Tectonics: Driven mostly by earth's internal heat.
Convection is transport of heat by bodily moving hot and cold material,
usually because cold material is denser and warm material lighter. It drives
a host of processes.
- Top left: a pot on a stove reaches a rolling boil. Hot water from the
bottom rises and very slightly cooler water (not enough to make a difference
if you stick your hand in!) sinks.
- Top center: some lakes reach a point where the surface is cooler than the
bottom. When this happens, often in the fall, the lake undergoes a
convective overturn. The surface water sinks and the bottom water rises.
- Top right: cumulus clouds on a summer afternoon result from warm air
rising, expanding, and cooling, so that clouds condense. In the clear air
between, cooler air sinks.
- The same thing can happen in late fall. The surface air can be cool,
but upper level air can be very cold. The warmer surface air rises and
creates cumulus clouds. The result can be a very dramatic,
turbulent-looking sky, but one without any violent weather.
- Bottom left: two scales are represented here.
- Hurricanes are fueled by warm air rising from tropical seas. Cold air
rushes in to replace it. Hurricanes generally die out when cut off from
their source of heat by crossing land or drifting into colder oceans.
- Hurricane-like storms occur in the Arctic. They are not called
hurricanes but have the same spiral structure and even a central eye.
The surface water is only 0 C, but the air may be -20C, and the
"warm" water supplies energy for the storm. "Warm"
is a relative term.
- On a larger scale, the earth's wind belts are driven by warm air
rising at the equator and sinking in mid-latitudes.
- Bottom center: Convection in the earth's mantle ultimately drives plate
- Bottom right: In the sun and most stars, convection is the final stage in
transporting heat from the deeper interior to the surface.
Earth and the Universe
- Rotation (Day-Night)
- Lunar (Tides)
- Annual (Seasons)
- Precession and Orbit Variations (Ice Ages?)
- Galactic (250 m.y. period)
- Unpredictable Events
- Nearby Supernovae
- Meteor Impacts
- Long-Term Evolution of Sun
Unequal Solar Heating
- Equator to Pole
- Day - Night
- Different Surfaces
- Adiabatic Heating and Cooling
- Coriolis Effect
- High and Low Pressure
- Fronts and Air Masses
- At the equator, warm air rises. Since much of the air movement is
vertical, winds are light. This band is the Equatorial Doldrums.
- Warm air above the surface moves poleward and cooler air moves toward the
equator. The Coriolis Effect causes this air flow to be deflected to the
west. Thus there is a belt of easterly winds on either side of the equator.
Because they were so useful for crossing oceans in the days of sailing
ships, they are called the Trade Winds.
- Around 30 degrees from the equator, cool air sinks. Again, since much of
the air movement is vertical, winds are light. This band is the Horse
Latitudes, supposedly because sailing ships becalmed here often had to
throw overboard the bodies of horses that died of starvation or thirst.
- Air sinking in the horse latitudes flows equatorward in the Trade Winds
and also poleward. The poleward flow is deflected east by the Coriolis
Effect, so in mid-latitudes there is a belt of westerly winds, the Westerlies.
In the Southern Hemisphere, south of Africa and Australia, there is nothing
but ocean around the globe. With nothing to block or divert the winds, they
get very intense. These latitudes are called the Roaring Forties.
- Cold air flowing outward from the poles is, like the Trade Winds,
deflected westward. This zone is called the Polar Easterlies.
The boundaries between wind belts are not fixed but fluctuate and have
Surface currents, shown below, are driven by the winds. Warm water is red and
cold water is blue. The Trade Winds propel ocean water westward along the
equator, and when it strikes a continent, it is diverted poleward. However, a
narrow return flow also occurs along the equator. In mid-latitudes the currents
are driven eastward by the Westerlies. The opposing wind belts cause currents in
all the ocean basins to form gyres, or giant loops.
Thermohaline (Deep) Circulation
- Evaporation makes water more saline and denser
- Freezing makes water more saline and denser
- Cold water is denser than warm water
A combination of surface and deep flow creates a giant global heat conveyor.
The coldest and densest water forms off Antarctica and flows along the ocean
floors until it reaches an obstacle. Then it rises and joins the surface
circulation. As water loops around the North Pacific gyre, it becomes extremely
warm. Even though the amount of water that passes from the Pacific to the Indian
Ocean is not great, the amount of heat it carries is. It cools a bit rounding
Africa, then warms in equatorial latitudes and carries water up the
Atlantic into the Arctic. Finally the water cools and sinks, mixes with cold
bottom water, and begins the cycle again.
- Evaporation from Oceans
- Precipitation on Land
- Infiltration into Ground (Ground Water)
- Runoff (Erosion)
Water: The Wonder Liquid
- Principal Agent in Modifying Earth’s Surface
- Medium for Storing and Distributing Global Heat
- The Universal Solvent
- Essential for Life
- Destructive to Rocks
- Lowers Melting Point of Rocks
- Reduces Strength of Rocks Under Pressure
The Rock Cycle
- New Rocks Exposed by Erosion
- Rocks Broken Down Mechanically and Chemically (Weathering)
- Components Transported by Erosion
- Components Cemented into Sedimentary Rocks
- Burial and Heating creates Metamorphic Rocks
- Melting Creates Igneous Rocks
- Outer Crust of Earth Moves a Few cm/yr
- Driven by Convection in Earth’s Interior
- Accounts For:
- Mountain-Building (Orogeny)
- Configuration of Continents
- Configuration of Continents Governs:
- Oceanic Circulation
- Weather and Climate Patterns
- Mountains and Rain Shadows
- Pathways and Barriers for Migration
- Ecological Niches
Stuff That Just Doesn't Happen
- The earth's axis cannot suddenly flip or change orientation. Its
orientation can change due to the gravitational influence of other
solar system bodies, but that process is very slow. Furthermore, the moon
exerts a powerful stabilizing influence on the earth's axial tilt.
- The earth's rotation cannot suddenly and dramatically change.
- Mass movements on the earth like ocean currents, weather systems and
large earthquakes can produce small changes
- Tidal friction, mostly generated by the moon, does slow the earth's
- Earthquakes do not cause volcanic eruptions, or vice versa. Earthquakes
associated with volcanic eruptions are due to magma movement and caldera
collapse and are relatively small.
- Meteor impacts do not trigger volcanism, either at the site or far away.
Large impacts can melt huge volumes of rock, but that is not volcanism.
- Changes in the earth's magnetic field do not seem to have any
significant impact on the biosphere. The magnetic field does intercept
charged particles from the sun but has no effect on light, ultraviolet,
radio waves, or any other electromagnetic radiation.
- There is no known or foreseeable way or triggering large earthquakes or
volcanic eruptions artificially, even on site, let alone by remote control.
Conceivably, pumping water into a fault zone to lubricate it might trigger
the fault (and has on a small scale) but it would be a massive operation to
trigger a large event.
- Drilling deep into the earth does not trigger volcanic eruptions.
- An earthquake cannot be stopped once in progress
- The earth cannot be completely flooded although you can make bad movies
about it (Waterworld)
Fallacies About Risk
- Nothing in the real world ever has a probability of one or zero
- Risk can never be eliminated, only minimized
- Good outcomes can never be guaranteed
- The chances of a particular rare event happening in a given place
or time are low. The chance of some rare event happening in a given
place or time are very high. Your chance of winning the lottery is
very small but somebody will win it.
- If there are 1000 significant localities for events to happen, the
odds are that one of those localities will experience a thousand-year
event every year.
- A few unusual events does not make a pattern
- A cluster of earthquakes does not mean earthquakes are getting more
- A cluster of cancer cases does not mean there's a common cause
- An exception to a pattern does not disprove the pattern
- A cold winter does not disprove global warming
- A drop in gasoline prices does not mean energy shortages don't exist
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Last Update February 28, 1997