Volcanic eruptions are fascinating manifestations of the Earth's
dynamic inte rior which has been cooling for the past several
billion years. The planets of the solar system originated some 4.5
billion years ago from the same gas and dust cloud created by the
big bang. Some of the gas collapsed by the gravitational force to
form the Sun at the center, while the whirling disk of gas and dust
around the Sun subsequently cooled and lumped together to form
larger and larger lumps of materials or planetesimals. These
planetesimals collided fre quently and violently and in the process
liberated heat that melted the material in them. With time this
material gradually cooled and formed the planets of the solar
system. During the second half of the twentieth century the theory
of plate tectonics of the Earth became established and demonstrated
that our planet is covered with six large and many small plates of
the lithosphere. These plates move over a highly viscous lower part
of the Earth's upper mantle and contain the continental and oceanic
crusts. The lower mantle extends below the upper mantle until it
meets the core that is more than half the diameter of the entire
globe (12,740 km). The inner core consists mostly of iron and its
temperature is about 5000 kelvin, whereas the liquid outer core is
turbulent, rotates faster than the mantle, consists primarily of
iron, and is the source of the Earth's magnetic field.
General
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