Three main sources of energy exist on Earth: solar, geophysical,
and nuclear. In a sense nuclear is the most basic, for fusion processes operate
the Sun and fission heats the Earth’s interior. Nuclear and geophysical working
together define the heating of the Earth’s interior, which pushes the
continents apart in “continental drift” and causes “hot spots” where there are
volcanoes and earthquakes. Africa fits nicely against South America, as was
obvious to me looking at a world map in the 7th grade, and indeed it was once
lodged there. Huge sources of radioactively- formed heat cause upward movements
of the Earth’s magma (hot rock), often in the middle of the oceans, and push
the adjacent lithosphere (“plates”) away from the heat source. Iceland with its
many volcanoes is the Northern limit of the mid-Atlantic rift. The movements of
the plates cause different pieces of the Earth to move and often smash into
each other, generating earthquakes and volcanoes along the way.
The results can be most impressive: the Himalayan Mountains,
highest on Earth, were formed when India sailed across the equator and smashed
into the rest of the Asian Continent. California was formed by at least three
land masses crashing into what is now Nevada. If the plate moves downward, back
into the Earth, it creates a “subduction zone” and this is often associated
with volcanoes and earthquakes. The Pacific Ocean is surrounded by a “ring of
fire” due to these collisions and subductions, which includes the frequent
earthquakes and volcanoes, alive or dormant, that are found along the West
Coasts of the Southern Andes Mountains and the United States, including Alaska,
the Aleutians, Kamchatka in Eastern Russia, much of Japan, New Guinea and New
Zealand. All of this was started by hot spots in the middle of the Pacific
Ocean, fueled initially by radioactive activity deep in the Earth.
The second main source of energy is another type of geophysical,
which includes the inertia remaining from the formation of the solar system,
but also the tides generated from this movement (seen mostly in ocean tides,
but also in slight deformations of the Earth’s crust such as the “bulge” at the
equator) and also the crustal movements mentioned above. The most obvious
inertial components include the daily rotation of the Earth about its own axis
and the annual revolving of the Earth about the Sun. We get seasons because the
Earth’s axis is tilted at 23.5° relative to the plane of its revolution about
the Sun, so in summer in the respective hemispheres the poles are pointed much
more directly at the Sun and hence each square meter gets a larger photon flux
or input of energy. Because the Earth is slowing down due to tidal friction (at
milliseconds per century, so don’t worry), like a slowing top, it is wobbling
slightly. These wobbles, as well as other cyclical shifts in the relation of
the Earth to the Sun, occur at three distinct thousands of year cycles, are
called Milankovitch cycles and are thought the principal cause of long-term
climate cycles, such as ice ages. The Earth has had something like 12 distinct
ice ages in its long history, and the relative warmth we enjoy now is not the
normal situation over geological time.
The Sun is the source of energy for most of what we see from day to
day on Earth.
It warms the atmosphere, operates the hydrological cycle and allows
and feeds life. Eventually essentially all of this high-quality input of
photons is transferred—at each work step and each transformation—to low grade
heat in accordance to the second law of thermodynamics. The heat is then
reradiated back to space as low-grade heat, maintaining the Earth at a relatively
constant temperature. When the Sun’s energy strikes the Earth’s surface that
portion that is not reflected does many types of work on the Earth’s surface.
We can feel the effects in the heating of dark surfaces, but the largest amount
of work that sunlight does on Earth is to evaporate water. This is because it
takes a great deal of energy to transform water from a liquid to a gaseous
phase. Wind and more generally weather is caused by the uneven heating of the
Earth’s surface by the Sun. At a larger scale, the Sun heats the Earth more at
the equator than toward the poles because the incident radiation is more
intense there because the land is perpendicular to the photon flux.
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