On earth, the
distribution of elements consists of iron, oxygen, silicon, and magnesium.
Planets such as Jupiter and Saturn consist of mainly volatile gases. Some of
these volatile gases are water, methane, and ammonia.
It is believed
that during the first five hundred million years of the earth’s acceleration
from smaller bodies, a giant impact took place which resulted in the earth
tilting on its axis. The time between the acceleration of the earth and the
formation of the oldest existing rocks may be accounted for by the
stratification or differentiation of the earth. The original rocks may have
been melted and their components redistributed into the core, mantle, and crust
during this interval.
In the 1960s and
1970s, a new view of the earth’s internal structure began to emerge. This view
allowed geologists to integrate data and observations from many diverse fields
into a unifying theory of the earth’s behavior, called plate tectonics. This
theory now forms the basis of our understanding of the earth’s large-scale
internal behavior and also has many implications about the geological process
that we can observe at the surface as well. The new model considers the earth’s
molten outer core to be a very active zone of circulation.
magnetic field originated because of the circulation of molten metal in the
outer core. This occurred because metals conduct electricity. Magnetism of the
lithosphere support the model of plate tectonics by providing strong evidence
that new oceanic crust formed volcanism at the ridge crest in the prevailing
polarity of the earth’s magnetic field.
Gravity varies due
to the rotation of the earth, topography, and variations in density. Isostatic
equilibrium is implied in the theory of isostasy is the idea that crustal
masses establish a state of dynamic equilibrium with the mantle. Thus, the
dense basalts of the ocean basins lie at a low elevation in comparison with
lighter granitic mountain ranges, which float higher in the mantle.
The first piece of
evidence for plate tectonics is the theory of continental drift which proposed
all the continents were once assembled into large continental mass and then drifted apart to their present
position. The next piece of evidence for plate tectonics is the theory of
sea-floor spreading. The symmetrical bands of alternately magnetized volcanic
crustal rock provided important evidence for the sea-floor spreading theory,
which proposed that new crustal material was formed by volcanic eruptions at
the crests of the midoceanic ridges and that slow lateral movement of the crust
away from the ridges was occurring.
Divergent plate margins:
Occur where rising
convection currents in the asthenosphere nearly reach the earth’s surface.
Divergent plate boundary can be recognized at the earth’s surface as cracks
apart parallel to the boundary, large linear blocks subside downward along
faults to form rift valleys or grabens along the crest of the rift zone, one
example is the mid-atlantic ridge. Convergent
Mark the zone of
contact or collision where plates move toward each other. Convergent plates
boundaries can be recognized at the earth’s surface as a line of volcanoes,
examples includes the Cascade volcanoes and the Andes in South America.
plates are sliding past each other along vertical fractures, known as transform
faults, at these boundaries.
arriving at the earth is not equally distributed. Rays that arrive at and near
the equator strike the planet directly, which concentrates the heat contained
within a small area. In contrast, rays that arrive at the surface near the
poles strike the earth in an oblique fashion, which distributes the same amount
of energy over a larger area. Whenever there is a temperature difference, heat
will flow from areas of higher to lower temperature. Atmospheric circulation is
broken into a series of cells limited to specific latitude due to divergence
and convergence zones. As the air masses rises from the surface, air moves
along the surface from the north and south to replace the rising air masses,
this creates a convergence zone. At about 30º north and south, the air masses
begin to sink back toward the surface, producing a zone of high pressure. When
they descend to near the surface, the flow splits apart, creating a divergence
circulation is circulation of the air due to a heat imbalance on the earth’s
surface caused by rays hitting the earth at different angles. On the other
hand, oceanic circulation is circulation of the water on our planet that is
intricately linked to the biosphere, atmosphere, and lithosphere.