Geology Reference
In-Depth Information
More than 4.5 billion years ago, the materials that constitute the sun, earth, and other
planets in the solar system were part of a big cloud of gaseous matter, called a
nebula,
swirling around in the Milky Way galaxy.
According to the
solar nebula hypothesis,
some of this matter collapsed into each other,
becoming the giant ball of light and energy we call the sun. After the sun formed, the re-
maining matter was left as particles in a spinning disk of cloudy material around it. Small
rocky particles in the cloud were attracted to one another by the force of gravity. As
they crashed together, they became larger rocky objects, or
planetesimals,
flying in or-
bits around the sun.
Over time these planetesimals continued to attract matter or crash into each other,
growing larger until they formed the rocky internal planets of our solar system: Mercury,
Venus, Earth, and Mars.
Shortly after the earth formed, another planetesimal crashed into it, knocking a large
amount of surface material off the earth and into orbit around it. This material eventu-
ally combined together to form the moon, which is held close to the earth by the pull of
gravity.
The newborn earth was very different from the earth you are familiar with. Almost all
the matter or elements that make up the modern earth were present as atoms and mo-
lecules, but they were all mixed up or
undifferentiated.
Under the influence of gravity's pull, the most dense materials (such as iron and nickel)
sank into the center of earth's swirling ball of matter and formed the core. Surrounding
the core, somewhat less dense elements collected, forming the mantle. The least dense
materials were left to form the outermost layer, the lithosphere. These layers are de-
scribed in detail in Chapter 4.
This process of separation based on density and other chemical characterist-
ics is called
differentiation,
and it created the distinct layers of varying composi-
tion observed in the modern earth. In fact, differentiation continues today, moving
less dense materials to the surface of the earth through the subduction, melting,
and eruption of rock materials (as I describe in Chapter 9).
Scientists believe that much of the surface water on earth today arrived with icy comets
that crashed into the young earth. Upon impact, the ice melted into water and remained
trapped on earth due to the pull of gravity. Another source of water on early earth may
have been water vapor erupting from volcanoes.
