Download Getting to Know: Why Earthquakes Occur

Getting to Know: Why Earthquakes Occur
You’ve probably seen the devastation caused by large earthquakes in photographs or on the news. Dramatic images of collapsed buildings, injured people, and widespread destruction are hard to forget. When we begin to talk about earthquakes, those are the images that often come to mind. These disturbing images may lead people to believe that earthquakes are rare and incredibly destructive.
The first thing that people may not realize is that earthquakes are not rare. Small earthquakes happen every day. In fact, some earthquakes are so small that people cannot feel them. These minor earthquakes are only “felt” by sensitive instruments that can detect seismic waves. Seismic waves are waves that transmit the energy of the earthquake.
What are earthquakes?
To answer that question, you need to recall what you have learned about Earth’s structure. If you remember, Earth is made up of layers. Earth’s center, the inner core, is surrounded by the outer core. The outer core is surrounded by the mantle, which contains molten rock. On top of this molten rock layer is the crust—the layer on which we live!
Large earthquakes damage
structures such as roads,
buildings, and bridges.
The crust is made up of many large pieces, called
tectonic plates. Tectonic plates fit together like a giant
jigsaw puzzle. Some parts of this puzzle are above
sea level and form the continents, and other parts are
below sea level, comprising the ocean floor. Because
tectonic plates rest on partially molten mantle rock,
they are always in motion and their boundaries
interact with each other. Convergent boundaries form
when plates are moving toward one another, divergent
boundaries form when plates move away from each
other, and transform boundaries form when plates
slide sideways past one another. This movement can
cause earthquakes.
Earth is made up of layers, including
the crust, mantle, outer core, and inner
core.Tectonic plates are parts of Earth’s
crust that move on the mantle.
Concept: Why Earthquakes Occur
Getting to Know
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Are tectonic plates the same
as continents?
Tectonic plates and continents are not the same. The continents are the parts of tectonic plates that are above sea level. However, tectonic plates also make up the ocean floor. For example, the North American Plate, which is the tectonic plate that the United States rests on, does not end at the shoreline—it continues far out into the ocean. In fact, the edge of the North American Plate is on the sea floor in the middle of the Atlantic Ocean! Sometimes people mistakenly think that continents and islands float on top of the oceans. The truth is that both the oceanic crust and the continental crust “float” on top of the mantle.
How is tectonic plate motion related to
earthquakes?
Seismic waves carry energy away
from an earthquake’s focus.
Tectonic plate movement is the primary cause of
earthquakes. As tectonic plates move, they grind
against each other or push into each other. The edges of the plates rub against each other
with a lot of friction, and the edges can become stuck for awhile. If this happens, pressure
builds up in the crust. Sometimes this pressure is released in a burst of seismic waves that
we call an earthquake. An earthquake’s focus, which occurs at the point where the pressure
is released when the two plates break loose again, can be near the surface or deep in Earth’s
crust. Seismic waves carry energy out in all directions from an earthquake’s focus.
Earthquakes can occur anywhere in Earth’s crust. Many earthquakes are so small that
they can only be detected using sensitive scientific instruments. Large earthquakes that
damage buildings and change the landscape are more common in areas near tectonic plate
boundaries.
Misconception: I saw an earthquake on TV, and it seemed like the earthquake
made cracks in Earth’s crust.
Earthquakes can cause smaller areas of ground to crack. However, these cracks do not
go through the entire thickness of Earth’s crust and they are different from the large
cracks in Earth’s crust, called faults, which are the focus of earthquakes. If the crack
formed by an earthquake broke through the entire depth of Earth’s crust, magma would
pour out and a volcano would form.
Concept: Why Earthquakes Occur
Getting to Know
www.discoveryeducation.com
2
© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.