Geology Reference
In-Depth Information
Crustal rocks begin to deform anytime two plates come in contact with one another. At
convergent boundaries, where plates move toward each other, the energy and tension
between the two plates builds up over time. Then, SNAP! The energy is released and the
ground ripples beneath us as the rocks bounce back to their original shape. Sounds
pretty dramatic, and it is. This process is what we experience as an earthquake: the
elastic rebound of earth's crust when built-up energy from plate movement is suddenly
released.
In this section, I explain how convection-inspired plate movements result in earth-
quakes, how earthquakes generate waves that travel through the earth, and how we
measure an earthquake's magnitude.
Responding elastically
It may seem strange to think of rocks as elastic, but that is the best way to describe how
they respond to the applied pressure of plate movements. As the rocks are pressed to-
gether, they deform. Energy is stored until a
slippage
occurs, releasing the stored energy
suddenly and allowing the rocks to spring back to their previous shape. This response is
called
elastic rebound.
Following the initial release of energy comes a series of
aftershocks
as the rocks continue
to shift and settle into place. Similarly, as the pressure between plates builds up there
may be a series of small adjustments that create
foreshocks
before the major release of
energy producing an earthquake occurs.
Sending waves through the earth
If you have ever experienced an earthquake, you have felt the ground beneath you
move. When the slippage occurs, the energy that is released spreads out in seismic
waves. The word
seismic,
which appears frequently in earthquake discussions, comes
from the Greek word for “shaking.”
Observing earth's interior by proxy
In Chapter 4, I discuss the interior of the earth and scientists' ideas about the solid inner core, outer
liquid core, and solid (though flowing) mantle beneath the lithosphere. Did you ever wonder how sci-
entists decided what the earth's interior looks like? Their ideas are based on observations of how seis-
mic waves travel through the earth, so earthquakes have directly influenced what we know about the
earth's layers.
