<P> The Cascades volcanoes define the Pacific Northwest section of the Ring of Fire, an array of volcanoes that rim the Pacific Ocean . As if volcanic hazards were not enough, the Ring of Fire is also infamous for its frequent earthquakes . In order to understand the origins of this concentrated band of Earth hazards, we have to slice deep into the Earth . </P> <P> A slice of the Earth from the Pacific Ocean through the Pacific Northwest might look something like the adjacent image . Beneath the Cascades, a dense oceanic plate plunges beneath the North American Plate; a process known as subduction . As the oceanic slab sinks deep into the Earth's interior beneath the continental plate, high temperatures and pressures allow water molecules locked in the minerals of solid rock to escape . The water vapor rises into the pliable mantle above the subducting plate, causing some of the mantle to melt . This newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Range) above the subduction zone . </P> <P> A close - up look at the Cascades reveals a more complicated picture than the simple subduction zone shown in the image on theleft . Not far off the coast of the North Pacific lies a spreading ridge; a divergent plate boundary made up of a series of breaks in the oceanic crust where new ocean crust is created . On one side of the spreading ridge new Pacific Plate crust is made, then moves away from the ridge . On the other side of the spreading ridge the Juan de Fuca Plate and Gorda Plate move eastward . </P> <P> There are some unusual features at the Cascade subduction zone . Where the Juan de Fuca Plate sinks beneath the North American Plate there is no deep trench, seismicity (earthquakes) are fewer than expected, and there is evidence of a decline in volcanic activity over the past few million years . The probable explanation lies in the rate of convergence between the Juan de Fuca and North American Plates . These two plates converge at 3--4 centimeters per year at present . This is only about half the rate of convergence of 7 million years ago . </P>

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