Plate Boundary Animations
header with G7S emblem
Left End RoundButton Fill - Center Slice 25x25Buttom Right Normal - Left CropHomeScience LinksClassworkScience TopicsScience SimulationClass ClipsButton Fill - Right Normal - Left CropButton Fill - Center Slice 25x25Button - Right Round End
Divergent Plate Boundaries
Almost all the Earth’s new crust forms at divergent boundaries, but most are not well known because they lie deep beneath the oceans. These are zones where two plates move away from each other, allowing magma from the mantle to rise up and solidify as new crust.
 

Convergent Plate Boundaries
Crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised.  Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) under another.  The location where sinking of a plate occurs is called a subduction zone.
The type of convergence – called by some a very slow "collision" – that takes place between plates depends on the kind of lithosphere involved. Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates.
Oceanic-Continental place convergence occurs when an oceanic plate moves under (subducts) a continental plate. Oceanic-continental convergence creates many of the Earth's active volcanoes, such as those in the Andes and the Cascade Range in the Pacific Northwest.
 
As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed. Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.