NOVEMBER 2005
Tectonic Plates grinding into new strata have caused this year's appalling earthquake tragedies. Is it over?
The world's most powerful earthquake in more than 40 years struck deep under the Indian Ocean off the west coast of Sumatra, the epicenter of Sunday's 9.0 magnitude, A violent movement of the Earth's tectonic plates displaced an enormous amount of water, sending powerful shock waves in every direction. quake triggering massive tsunamis that obliterated villages and seaside resorts in six countries across southern and Southeast Asia.
Plate Tectonics
Plate
tectonics refers to the process of plate
formation, movement, and destruction. It finds its
foundations in two theories, continental drift and
sea-floor spreading. Continental
drift describes the movements of
continents over the Earth's surface. Sea-floor
spreading refers to the creation new oceanic
plate material and movement away from the midocean ridge.
It was Alfred Wegener in the early 1900's who
brought forth the concept that the "shell" of
the Earth's surface was fractured, and these
"pieces" drifted about. Blasphemy in the minds
of scientists of Wegener's day, some 50 years later his
ideas were finally accepted. Wegener was able to piece
together (pardon the pun) several bits of information
which led to his conclusion that the present
configuration of the continents is not the same as it was
in the past. In fact, the continents were one
"super-continent"called Pangea
in his texts.
Modern Theory of Plate Movement
and Continental Drift
The movement of lithospheric plates referred to as continental drift, is believed to be caused by the radioactive decay of elements in the core and mantle that produces heat. The heat in turn creates convection currents in the mantle which "drive" the plates along their path of movement. When plates collide, heavier, more dense plates dive beneath lighter, less dense crustal plates along subduction zones. As the heavier plates move downward they melt and molten rock material makes its way to the surface, finally being extruded onto the surface as lava (Figure TL.12). Some plates slip past one another, creating earthquakes,
Figure TL.11 Mid-ocean Ridge
(Courtesy USGS)
In the late 1950's and early 1960's oceanographic research was opening the final frontier on Earth, the mysteries of the ocean floor. During the 1950's seismologists showed that earthquake activity was concentrated along the longest continuous mountain system on Earth, the mid-ocean ridge. Known for over a century, the mid-ocean ridge system in the Atlantic Ocean rises some 6,500 feet above the surrounding ocean floor and extends for more that 37,500 miles (60,000 km) in all the world's oceans. At the crest of the ridge system lies a trough or rift. These rifts are about 20 miles wide (30 km) and 6,500 feet (2,900 m) deep and are a site where lava is expelled on to the ocean floor. The youngest material is found near the ridge with rocks of increasing age further away. It appears that the mid-ocean ridge is the site of sea floor spreading leading to plate movement. The key to sea floor spreading was found in the magnetic properties of rock lying on the sea floor.
And finally this is possibly the most threatening picture
below of these boundaries. As we might be persuaded to
speculate that such great shifts as have already
happened, stemming from an original shift in Antartica,
could now lead to further shifts along these paths.
from www.uwsp.edu/.../tectonics_
introduction.html