Re: What are the three types of plate boundaries?

Dear Krystal,

The three types of plate boundaries are:

Constructive plate boundaries.  These are boundaries where the Earth’s 
tectonic plates are moving apart.  For the most part, these boundaries are 
located on the ocean floors, where they form a continuous chain of 
volcanic mountains and rifts called mid-ocean ridges that extend 
throughout the Earth’s oceans.  A good example is the Mid-Atlantic Ridge 
that runs down the middle of the Atlantic Ocean; the tectonic plates of 
Europe and Africa on the east side of the Mid-Atlantic Ridge are moving 
away from the North American and South American plates to the west.  As 
the plates move apart magma wells up to fill the space between them, and 
this is why constructive plate boundaries are the sites of volcanic 
activity.  The upwelling magma is formed by the partial melting of the 
rocks of the Earth’s mantle deep below, and the magma is basaltic in 
composition.  As it cools and solidifies it creates rocks called gabbro 
and, where it erupts onto the sea floor, basalt.  The gabbro and basalt 
form new oceanic crust, so constructive plate boundaries are sites where 
the Earth’s crust is growing.

Destructive plate boundaries.  If new crust is being created at 
constructive plate boundaries, then this growth must be balanced by the 
destruction of old crust somewhere else – if this were not the case, then 
the Earth would expand in diameter, and there is no evidence that it 
expands.  The sites where crust is being destroyed are destructive plate 
boundaries, where two tectonic plates are in collision.  At every 
destructive boundary the crust forming one of the plates is diving back 
down into the mantle beneath the other, overriding plate, a process called 
subduction.  Destructive boundaries can occur where the two plates are 
both composed of oceanic crust, where one is composed of oceanic and the 
other of continental crust, or where both are composed of continental 
crust.

Destructive plate boundaries are often the sites of major volcanoes.  
These volcanoes are the familiar stratocone volcanoes such as Mount Fuji 
in Japan.  This volcanic activity is driven by the heating up of the plate 
that is descending back into the mantle, causing it and part of the 
overriding plate to melt.  This melting typically occurs about 100 
kilometers behind the actual plate boundary, so the line of volcanoes 
produced is also behind the actual boundary, located on the overriding 
plate.  In collision between two pieces of oceanic crust the result is a 
chain of volcanic islands, of which Indonesia is a prime example.  Where 
oceanic crust collides with a plate carrying a continent, the result is a 
chain of volcanoes on the continent, such as the Cascade volcanic chain in 
the Pacific Northwest of the USA and the Andes Mountains of South 
America.  Where two plates carrying continents collide the result is a 
range of (usually non-volcanic) mountains where the continental crust is 
squeezed by the powerful forces driving the two plates together; the 
Himalayan Mountains are the prime modern example, and the Appalachian 
Mountains are an old chain created by a continent-continent collision 
about 300 million years.

Transcurrent boundaries.  The third type of plate boundary is called a 
transcurrent boundary.  A transcurrent boundary is where two tectonic 
plates slide past one another, without either colliding or pulling apart.  
The most famous example active today is in Mexico and California, where 
the Pacific Plate is moving north past the North American plate.  In this 
example the San Andreas Fault is the actual plate boundary.  The San 
Andreas is a transcurrent or strike-slip fault, and earthquakes occur 
along it very frequently as the two plates move past one another.

I hope this answers your question.

Best wishes,

David Scarboro