Source: USGS
Subduction zones occur where plates are moving towards each other and one plate plunges, or subducts beneath the other plate. Subduction zones often occur near edges of continents, where the denser, cooler oceanic plates subduct below the continental plate. Subduction zone boundaries occur off the coast of Washington and British Colombia in the Cascadia Subduction zone.
Subduction zones are characterized by very large magnitude earthquakes often accompanied by tsunamis which are series of waves generated in a body of water by a rapid disturbance that vertically displaces the water.
As the subducting plate warms and sinks it produces magma which rises to the surface in the form of a line of volcanoes parallel to the plate boundary. The range of volcanoes between Mt Lassen in Northern California and Mt Baker in Washington State are a result of this phenomenon.