| EarthQuakes |
For Parents/Educators you are very welcome to download any or all of our Printable
Thematic Units and/or Lesson Plans for use in your homes, homeschool, classroom or
community centres. ►►Earthquake Home
One of the most frightening and destructive phenomena of nature is a severe
earthquake and its terrible aftereffects. An earthquake is a sudden movement of
the Earth, caused by the abrupt release of strain that has accumulated over a long
time. For hundreds of millions of years, the forces of plate tectonics have shaped
the Earth as the huge plates that form the Earth's surface slowly move over, under,
and past each other. Sometimes the movement is gradual. At other times, the
plates are locked together, unable to release the accumulating energy. When the
accumulated energy grows strong enough, the plates break free. If the earthquake
occurs in a populated area, it may cause many deaths and injuries and extensive
property damage.
Scientists have begun to estimate the locations and likelihoods of future damaging
earthquakes. Sites of greatest hazard are being identified, and definite progress is
being made in designing structures that will withstand the effects of earthquakes.
Where Earthquakes Occur
The Earth is formed of several layers that have very different physical and chemical
properties. The outer layer, which averages about 70 kilometres in thickness, consists
of about a dozen large, irregularly shaped plates that slide over, under and past
each other on top of the partly molten inner layer. Most earthquakes occur at the
boundaries where the plates meet. In fact, the locations of earthquakes and the
kinds of ruptures they produce help scientists define the plate boundaries.
There are three types of plate boundaries: spreading zones, transform faults, and
subduction zones. At spreading zones, molten rock rises, pushing two plates apart
and adding new material at their edges. Most spreading zones are found in oceans;
for example, the North American and Eurasian plates are spreading apart along the
mid-Atlantic ridge. Spreading zones usually have earthquakes at shallow depths
(within 30 kilometres of the surface).
Illustration of Plate Boundary Types
Transform faults are found where plates slide past one another. An example of a
transform-fault plate boundary is the San Andreas fault, along the coast of California
and northwestern Mexico. Earthquakes at transform faults tend to occur at shallow
depths and form fairly straight linear patterns.
Subduction zones are found where one plate overrides, or subducts, another,
pushing it downward into the mantle where it melts. An example of a subduction-
zone plate boundary is found along the northwest coast of the United States,
western Canada, and southern Alaska and the Aleutian Islands. Subduction zones
are characterised by deep-ocean trenches, shallow to deep earthquakes, and
mountain ranges containing active volcanoes.
Map of the Tectonic Plates
Earthquakes can also occur within plates, although plate-boundary earthquakes are
much more common. Less than 10 percent of all earthquakes occur within plate
interiors. As plates continue to move and plate boundaries change over geologic
time, weakened boundary regions become part of the interiors of the plates. These
zones of weakness within the continents can cause earthquakes in response to
stresses that originate at the edges of the plate or in the deeper crust. The New
Madrid earthquakes of 1811-1812 and the 1886 Charleston earthquake occurred
within the North American plate.
How Earthquakes Happen
Measuring Earthquakes
Volcanoes and Earthquakes
Predicting Earthquakes
Thematic Units and/or Lesson Plans for use in your homes, homeschool, classroom or
community centres. ►►Earthquake Home
One of the most frightening and destructive phenomena of nature is a severe
earthquake and its terrible aftereffects. An earthquake is a sudden movement of
the Earth, caused by the abrupt release of strain that has accumulated over a long
time. For hundreds of millions of years, the forces of plate tectonics have shaped
the Earth as the huge plates that form the Earth's surface slowly move over, under,
and past each other. Sometimes the movement is gradual. At other times, the
plates are locked together, unable to release the accumulating energy. When the
accumulated energy grows strong enough, the plates break free. If the earthquake
occurs in a populated area, it may cause many deaths and injuries and extensive
property damage.
Scientists have begun to estimate the locations and likelihoods of future damaging
earthquakes. Sites of greatest hazard are being identified, and definite progress is
being made in designing structures that will withstand the effects of earthquakes.
Where Earthquakes Occur
The Earth is formed of several layers that have very different physical and chemical
properties. The outer layer, which averages about 70 kilometres in thickness, consists
of about a dozen large, irregularly shaped plates that slide over, under and past
each other on top of the partly molten inner layer. Most earthquakes occur at the
boundaries where the plates meet. In fact, the locations of earthquakes and the
kinds of ruptures they produce help scientists define the plate boundaries.
There are three types of plate boundaries: spreading zones, transform faults, and
subduction zones. At spreading zones, molten rock rises, pushing two plates apart
and adding new material at their edges. Most spreading zones are found in oceans;
for example, the North American and Eurasian plates are spreading apart along the
mid-Atlantic ridge. Spreading zones usually have earthquakes at shallow depths
(within 30 kilometres of the surface).
Illustration of Plate Boundary Types
Transform faults are found where plates slide past one another. An example of a
transform-fault plate boundary is the San Andreas fault, along the coast of California
and northwestern Mexico. Earthquakes at transform faults tend to occur at shallow
depths and form fairly straight linear patterns.
Subduction zones are found where one plate overrides, or subducts, another,
pushing it downward into the mantle where it melts. An example of a subduction-
zone plate boundary is found along the northwest coast of the United States,
western Canada, and southern Alaska and the Aleutian Islands. Subduction zones
are characterised by deep-ocean trenches, shallow to deep earthquakes, and
mountain ranges containing active volcanoes.
Map of the Tectonic Plates
Earthquakes can also occur within plates, although plate-boundary earthquakes are
much more common. Less than 10 percent of all earthquakes occur within plate
interiors. As plates continue to move and plate boundaries change over geologic
time, weakened boundary regions become part of the interiors of the plates. These
zones of weakness within the continents can cause earthquakes in response to
stresses that originate at the edges of the plate or in the deeper crust. The New
Madrid earthquakes of 1811-1812 and the 1886 Charleston earthquake occurred
within the North American plate.
How Earthquakes Happen
Measuring Earthquakes
Volcanoes and Earthquakes
Predicting Earthquakes
| Thanks & References, Further Reading, Teaching Materials & Lesson Plans, Kids Stuff, Links & Resources Printables Colouring Pages FEMA USGS Earthquake Earthquakes kipedia |
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