Plate Boundaries

 

There are 3 types of plate movements.

 

• Constructive - plates separating

• Destructive - plates colliding

• Conservative - plates sliding past each other.

 

Constructive plate movement - This happens when tectonic plates move apart, magma rises up to bridge the gap and produces new crust made of igneous rock, basalt.  sometimes magma comes out with great force producing undersea volcanoes.  This is quite evident in the middle of Atlantic Ocean.  The so called Mid-Atlantic ridge runs the whole length of the Atlantic and actually cuts through the middle of Iceland, which is why they have hot underground water.

 

 

 

As the magma rises up through the gap it forms ridges, and underwater mountains, resulting in symmetrical pattern either side of the ridge, thus providing strong evidence for the theory of continental drift (see animation above).  The most convincing evidence, however, comes from the magnetic direction of the rocks.  As the liquid magma erupts out of the gaps, the iron particles in the rocks tend to align themselves with the Earth's magnetic field and as it cools they set in position.  Every 500,000 years or so the earth's magnetic field tend to switch direction.  This means the rock on either side of the ridge has bands of alternate magnetic polarity.  This pattern is found to be Symmetrical either side of the ridge (see animation below).

 

Mid-Atlantic Ridge

 

Destructive plate movement - This occurs when plates move towards each other.  Different types of collision may take place at these boundaries.  Oceanic and Continental plates or two continental plates can collide with each other.

Oceanic and Continental plates colliding:  Since the oceanic plate is more dense, it is always forced underneath the continental plate.  This is called subduction (see animation below).  As the oceanic plate is pushed down a deep trench is formed. The plate melts and creates pressure in the surrounding area due to all the melting rock.  The resulting molten rock finds its way to the surface and volcanoes form.  Earthquakes also happen as the two plates slowly grind past each other.  The continental crust is not destroyed.  It is simply compressed, folded into anticlines and synclines and thickened to form a fold mountain range similar to the Andes in South America.

 

 

Two continental plates colliding:  When two continental plates collide head on, neither of them is subducted.  Instead the sediment layers laying between the two continent land masses get squeezed.  The effect is to form fold mountains similar to the Himalayas.  This type of fold mountain range has no volcanoes or deep focus earthquakes.  India, in fact detached it self from Africa and piled into the bottom of Asia.  It is still doing so, pushing the Himalayas up and up.  This means Mount Everest is getting taller by a few centimeters every year as India continues to push up into the continent of Asia.

 

 

Conservative plate movement: 

 

When plates slide past each other this type of movement results.  Here, material is neither created or destroyed.  The best known example of this is the San Andres Fault in California (see animation below).  It marks the boundary between the Pacific plate and the North American plate.  These plate of rock don't glide smoothly past each other.  They catch on each other and as the forces build up they suddenly jerk.  This sudden jerking only lasts a few seconds, but brings devastation in a built and heavily populated area.  Building come tumbling down and many people get killed.  The city of San Francisco sits along side this fault line.  It was completely destroyed in 1906 and was again hit by a powerful tremor in 1991.  This could happen again any time.

 

These days in earthquake zones, developers try to build earthquake-proof buildings which are designed to withstand small amount of shaking.  In poorer countries, earthquakes usually cause much devastation where they have badly constructed properties, overcrowding and inadequate rescue services.

 

SITES
Title	URL
VolcanoWorld	http://volcano.und.nodak.edu
MTU's Volcanoes Page	http://www.geo.mtu.edu/volcanoes
USGS Cascades Volcanoes Observatory	http://vulcan.wr.usgs.gov
Global Volcanism Network	http://www.volcano.si.edu/gvp/
USGS Volcanoes Site	http://volcanoes.usgs.gov