M A S T E R P A G E
Tectonics Background
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2.2a
To understand why earthquakes happen, we need to understand two basic facts about the planet we live on: that it
is made up of layers, and that its surface is broken into irregular pieces called plates. Much of what we know
about the composition of Earth has been learned by studying how earthquake waves travel in and through it.
Earth’s Layers
Just for a moment, imagine the Earth as if it were a hard-boiled egg. It has a thin crust (the shell); a thick middle
layer, or mantle (the white); and a core (the yolk). The crust and the uppermost portion of the mantle together
form the lithosphere. The plates are called lithospheric plates because they belong to this region. Scientists divide
the mantle into zones and the core into an inner and outer core. (See Master 2.2b.)
Crust and Lithosphere
Earth’s crust varies in thickness from about 30 km to between 70 and 80 km on the continents to only 6 km on the
ocean floor. Even at its thickest point, the crust is not nearly as thick in relation to the whole bulk of the Earth as
the shell of an egg is in relation to the egg. Remember that the crust is 70-80 km thick at its thickest point, and the
radius of the Earth is about 6,371 km.
The oldest rocks of the crust found so far have been dated by radioactive decay (isotopic dating) at about 4 billion
years. Earth scientists assume that the Earth was much hotter billions of years ago than it is today, and that the
lithosphere (crust plus upper mantle) broke into plates as it cooled and hardened.
The lithosphere extends to an average depth of about 100 km. It is deepest under the continents and shallowest at
the mid-ocean ridges.
Mantle
The mantle contains several zones, or layers with different properties. Its upper portion is a region with a plastic,
semisolid consistency, called the asthenosphere. The thickness of the asthenosphere is still a matter of debate.
Estimates of the distance to the base of the asthenosphere range from about 200 km to 700 km. The mantle
accounts for approximately 67% of Earth’s mass. Information from earthquake waves indicates that this region
generally behaves as a plastic; that is, it will bend and flow in response to pressure. Temperature and pressure
continue to increase as we move through the mesosphere to approach the core, at a depth of about 2,890 km.
Core
Both layers of the care are thought to be composed primarily of iron, with lesser amounts of nickel and possibly
silicon, sulfur, or oxygen. Scientists have measured the velocity of earthquake waves passing through the core,
and reason that such movement would be possible through materials with the physical properties of these
elements. The liquid outer core, which might be compared to the outer two-thirds of an egg’s yolk, reaches from a
depth of about 2,890 km to 5,150 km. The solid inner core goes the rest of the way to the center of the Earth.
Earth’s core is very hot. Its high temperatures are due to the tremendous pressure of the layers above it, heat
generated by the impact of other bodies during the formation of the planet, and radioactive decay. Evidence
collected from mines and deep wells shows that the average increase in temperature is about 1
°
C for every 40
meters of depth. If this rate held constant to the center of the Earth, theoretically, the temperature of the core
would be about 150,000
°
C. According to the evidence we have, however, the actual temperature is between 3,000
and 4,000
°
C at the core-mantle boundary, and about 5,000
°
C at the boundary of the outer and inner cores—
roughly the same temperature as the surface of the Sun!
Earth’s Plates
Most earthquakes and volcanoes are associated with large-scale movements of Earth’s plates, and occur at the
boundaries between plates. There are 12 major plates and a number of smaller ones. The plates are named after
continents (the North American Plate), oceans (the Pacific Plate), and geographic areas (the Arabian Plate).
