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Until the 1950s, the various branches of geology, including the study of volcanoes
and earthquakes, appeared to have no real connection, and the progress of the
science was toward continued divergence. What pulled everything back together
was the radical concept of plate tectonics that came into fruition in the 1960s
and 1970s. Plate tectonics is sometimes referred to as “the glue that holds geol-
ogy together” to reflect this power. It explains virtually all volcanoes and the lion’s
share of earthquakes and even relates them to each other.
The following review of plate tectonics is a good place to start for anyone
needing a refresher.
Earth Architecture
The Earth is a sphere that is flattened at the poles and bulging at the equator.
Internally, it is like a hard-boiled egg. In the center, instead of a yolk, it has a core.
The core is composed of iron-nickel and contains an inner core that is solid and
an outer core that is liquid. The spinning of the Earth causes the two to inter-
act as a self-exciting dynamo that gives Earth its strong magnetic field. The egg
white is equivalent to the Earth’s mantle, which encases the core. The mantle is
composed of dense minerals that are rich in iron and magnesium. It has several lay-
ers reflecting different minerals and mechanical properties. The shell of the egg is
equivalent to the Earth’s crust, a thin layer of light rock upon which humans live.
Unlike the shell, which is uniform, there are two types of crust. Thin, dense, young
crust is pulled toward the center of the Earth by gravity and sinks down, where-
as thick, light, old crust floats higher. The deeper crust is covered by oceans and
called oceanic crust, whereas the lighter crust forms the continents and is called
continental crust. The concept of isostasy is the balance between them.
If a person dropped the egg and the shell cracked into fragments that remained
stuck to the egg, these would be the plates of the Earth. However, the plates are
not only composed of crust. If the egg was not placed in cold water after it was
boiled, a thin layer of egg white would be stuck to the shell. This sandwich of shell
and white is equivalent to the sandwich of crust and rigid mantle and is called the
lithosphere. Plates are considered lithospheric plates because they are not only
composed of crust. The complication arises below the lithosphere because rather
than the mantle staying rigid throughout, there is a gummy layer of mantle beneath
the lithosphere called the asthenosphere. (Imagine a layer within the egg in which
the egg white remained runny.) It is the floating, moving, and interacting of the
lithospheric plates that defines the science of plate tectonics.
PREFACE:
An Essay on
Plate Tectonics
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