tsunami. we now know that, directly or indirectly, plate tectonics influences nearly all geologic...
TRANSCRIPT
TSUNAMI
We now know that, directly or indirectly,
plate tectonics influences nearly all geologic processes,
past and present.
With little or no warning, an earthquake or volcanic
eruption can unleash bursts of energy far more powerful
than anything we can generate.
Tsunamis can be one such effect.
While we have no control over plate-tectonic
processes, we now have the knowledge
to learn from them.
In geologic terms, a plate is a large, rigid slab of
solid rock. The word tectonics comes
from the Greek word "to build."
Plate tectonics refers to how the Earth's surface
is built of plates.
The theory of plate tectonics states that the Earth's
outermost layer is fragmented into a dozen or more large and small plates that are moving against one
another as they ride atop hotter, more mobile material.
About two thirds of the Earth's surface lies beneath
the oceans. Data gathered by oceanographic surveys led to
the discovery that a great mountain range, called the
global mid-ocean ridge, virtually encircles the Earth.
Rising an average of about 4,500 meters above
the sea floor, the mid-ocean ridge overshadows all the
mountains in the United States except for
Mount McKinley in Alaska.
Computergenerated topographic
map of a segment
of the Mid-
Oceanic
Ridge…
During the 20th century, greater use of earthquake-
recording instruments (seismographs) enabled scientists to learn that earthquakes tend to be
concentrated in certain areas, most notably along the oceanic trenches and
spreading ridges.
Tsunamis are usually caused by underwater earthquakes.
These often occur offshore at subduction zones
(places where a tectonic plate is gradually slipping under a continental plate).
Part of the sea floor can snap upward abruptly,
while other areas sink downward,
when sections of the plates that have been
locked together for a while move suddenly
under the strain.
In the instant after such an underwater earthquake,
the shape of the sea surface mirrors the new contours of the sea floor--some areas of water are pushed upwards,
and others sink.
This starts a series of waves that rush outwards--the beginning of a tsunami.
These waves travel very far and very fast
(more than 500 miles/hour, or the speed of a
jet airplane).
At first, out at sea in deep water, the waves
are very far apart--sometimes
hundreds of miles—and their crests are not very high, perhaps only a few feet above the rest
of the surface …
Seen from a passing ship or low-flying plane,
they would probably not even be noticeable…
But, when a tsunami leaves deep water and approaches the shore, it slows down and its
height grows.
The wave crests also squeeze closer together.
Depending on the shapes of the sea floor off the coast,
a tsunami hitting the coast may appear as a series
of towering walls of water that can level buildings.
Contrary to many beliefs, most tsunamis do not result
in giant breaking waves (like normal surf waves
that curl over as they approach shore).
Rather, they come in much like very strong tides.
The force of an oceanic earthquake also sends
out tsunami waves in all directions
away from the epicenter.
Tsunamis can spread across an ocean,
as seen in the next diagrams-
a tsunami from its origin off the coast of South America
reached Hawaii in about ten hours.
The most powerful earthquake in 40 years
erupted under the Indian Ocean near Sumatra
on Dec. 26, 2004.
It caused giant, deadly waves to crash ashore in nearly a
dozen countries, killing over 150,000 people.
Measured at 8.9, it was the strongest
earthquake worldwidesince a 9.2 quake hit
Alaska in 1964.
Billions of dollars of damage was caused by the Asian tsunamis caused by the
earthquake.
It will be the costliest disaster in world history.
Up on the northern tip of the island of Sumatra,
Indonesia,close to the epicenter
of the quake, the regional capital
Banda Aceh was devastated.
The first tsunamis hit early in the morning…
Locals described a wall of water, slamming into
the coastline.
Some felt the quake first, others saw the sea sucked away from beaches only to
return minutes later as a wall of water up to 30 feet high.
Tsunami moves toward land Kalutara, Sri Lanka…
Tsunami receding,Kalutara, Sri Lanka.
A surge of water swept through beachfront hotels.
There were loud crashes as buildings collapsed.
Eyewitnesses say two tidal waves struck early
in the morning.
The force of the second wave was so strong it threw
people and vehicles in its path.
Among the dead were tourists from many nations.
It was a record year for the tourist industry and hotels along the coasts were fully
booked, filled with foreign vacationers.
Banda Aceh, Indonesia prior to the tsunami…
Satellite image after the tsunami…
Banda Aceh harbor before…
…after
Sickness and infection remain a major concern.
The countries are ravaged- not just homes but crops and means of earning a
livelihood, such as fishing boats, have been destroyed.
Nations around the world responded in sending aid,
manpower, food and water.
The United States government donated
$350 million and independent charity organizations have sent additional aid, totaling
over $1 billion.
But recovery will be slow…
TSUNAMI
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