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Volcanoes at Plate Boundries Volcanoes are common around subduction zones. (Subduction Zone's are places where one plate is forced under another one).  Magma is made above the subducted plate, and rises toward the surface because it is less dense than the surrounding rock.  Eventually the magma turns to lava and and a volcano is formed. Hot Spots Hot Spots originate at the boundry between the mantle and the outer core.  They are narrow plumes of hot mantle material that rises up and melts the rock at the base of the lithosphere.  This creates pools of magma that rise to the surface and form hot spot volcanoes.  When the plates move, the hot spot doesn't.  The result ?...Hawaii.  Island chains are created by hot spots.

Earthquakes at Plate Boundries At transverse boundries, sometimes rocks get jammed between the plates and then slip suddenly after awhile.  This causes the Earthquake.  Earthquakes are most dangerous on land like in California.  Earthquakes at subduction zones and continent-continent plate collisions are more dangerous to humans. Growth of Continents at Subduction Zones If  there is a lot of excess sediment on one of the subducted plate in a subduction zone,  it is turned to rock by heat and pressure.  This rock becomes part of the other plate.  This causes the plate to grow larger at it's edges.  This process is called Continental Accretion.