As the solar system formed,
the swirling material that was rotating around the sun cooled
and solidified. Little bits of rock began to form near the sun.
Farther out, where there was little heat, even gases like nitrogen
froze into ice. These fragments moved in their own orbits and
collided with each other. The force of gravity pulled them together.
There were many collisions. The larger clumps gradually swept
up the smaller ones. The planets began to form.
Gravity causes objects to attract
one another. The greater the mass of the objects, the more strongly
gravity pulls them together. As the mass of the planets increased,
gravitational forces increased. The atoms were pulled more and
more strongly toward each other. The centers of the planets began
to heat up: if the planet was large enough, the rocks melted
and the planet became a spinning ball of molten minerals.
Now let's think about this for
a minute. When you put objects into a liquid, the heavy ones
sink and the light ones float. If the whole earth is liquid rock,
where can the heavier elements sink to? Down is toward
the center of the earth, and the heavier elements sank down through
the liquid toward the center.
Eventually the inner planets
cooled enough so that their surfaces became solid. The interiors,
however, remained, and remain, very hot. Scientists have developed
theories about the structure of the earth, based on information
gathered through studying earthquake waves and the earth's magnetic
field. So far as we know, this is how the earth is structured.
The earth has a solid inner
core made mostly of iron and nickel. It is solid because it is
under so much pressure: all the rest of the earth is pressing
down on it. Its temperature is about 7000 degrees
Kelvin (about 12,000 degrees Fahrenheit) -- hotter than the
surface of the sun!
The outer core is also made
of nickel and iron, but is liquid. Currents in this liquid metal
generate the earth's magnetic field. The temperature of this
molten metal is about 3000-6500 degrees Kelvin (roughly 5000-
11,250 degrees Fahrenheit). The lower temperature is above the
boiling point of iron on the surface of the earth. This is still
too hot for us to really imagine. The pressures holding this
hot layer down are still immense.
84% of the volume of the earth
is in the part called the mantle. This is above the hot core,
and made mostly of iron (yes, more iron!) magnesium, aluminum,
silicon and oxygen. The rock in the mantle is hot, and somewhat
plastic, like very, very thick cookie dough. This area does not
conduct heat well, which explains why the interior of the earth
is still so very hot.
We live on the crust. The crust
is very thin compared to the rest of the earth, thinner than
the skin on an apple by comparison. It is made of granite and
basalt, minerals that are lighter than iron. It is rich in silica,
a light weight element. The rock on the deep sea bottom is only
about 5 to 7 kilometers thick. The rocks that make up the continents
are about 30 to 70 kilometers thick, thicker than the deep sea
bottoms. The continents are made mostly of granite, which is
light compared to the rest of the materials that make up the
earth. I think of the continents as floating like islands of
soap bubbles floating on water.
Even the rocks that make up
the surface of the earth are still pretty warm. When miners go
down as deep as a mile, heat from the earth becomes a problem
for the miners, making work there very uncomfortable.
Every time that you go down
a mile, the termperature
increases by 72 degrees
The crust of the earth is not
all one piece, like the outside of a tennis ball. As the surface
cooled, the rocky surface cracked. (Think of how the surface
of a pan of brownies cracks as it cools.) It cracked into twelve
main plates. These plates float on top of the mantle. The continents
are on top of these plates. The plates move very slowly as the
plastic rock in the mantle moves under them. Where they collide
we get mountains, volcanoes, and earthquakes. Sometimes one plate
will slide down underneath another plate. This is called subduction.
Subduction is an important process because it allows elements
locked in the rocks to be heated up and returned to the surface
by volcanoes. Of course, this takes millions of years.
Here you see the crust floating
on top of the mantle. You can see the weight of mountain ranges
pushing down on the mantle rocks. It is hard to imagine rock
being plastic, but we are thinking about enormous forces here.
In this picture the mountains
on the continents are much higher in proportion to the curve
of the planet than mountains really are. If you could hold the
planet in your hand it would seem quite smooth.
Other planets in our solar system
have magnetic fields, so it seems reasonable to assume that they
also have iron cores. In our solar system, rocky planets are
near the sun, and the big gas giants are further out, where gases
will solidify in the cold temperatures of space. We can guess
that rocky planets, being made from the same elements as earth,
and having been formed by similar processes, will probably have
much in common with our own world.
from NASA, Earth from Space
Copyright ® 1999, 2003. Elizabeth Anne Viau and her
licensors. All rights reserved. This material may
be used by individuals for instructional purposes but not sold.
Please inform the author if you use it at