page_title: Geology of the United States
section_title: Cascades Volcanic Province
hierarchical_section_title: Geology of the United States / Pacific Province / Cascades Volcanic Province
caption_attribution_description: English: Detailed geological map of the Cascade Range and environs. Yellow rings indicate recorded earthquakes. Black lines show faults active within past 10,000 years.
context_page_description: The richly textured landscape of the United States is a product of the dueling forces of plate tectonics, weathering and erosion. Over the 4.5 billion-year history of our Earth, tectonic upheavals and colliding plates have raised great mountain ranges while the forces of erosion and weathering worked to tear them down. Even after many millions of years, records of Earth's great upheavals remain imprinted as textural variations and surface patterns that define distinctive landscapes or provinces.
  The diversity of the landscapes of the United States can be easily seen on the shaded relief image to the right. The stark contrast between the ‘rough' texture of the western US and the ‘smooth' central and eastern regions is immediately apparent. Differences in roughness result from a variety of processes acting on the underlying rock. The plate tectonic history of a region strongly influences the rock type and structure exposed at the surface, but differing rates of erosion that accompany changing climates can also have profound impacts on the land.
context_section_description: Where the Sierra Nevada ends a chain of explosive volcanic centers, the Cascade volcanoes, begins. The Cascades Province forms an arc-shaped band extending from British Columbia to Northern California, roughly parallel to the Pacific coastline. Within this region, 13 major volcanic centers lie in sequence like a string of explosive pearls.
  Although the largest volcanoes like Mount St. Helens get the most attention, the Cascades is really made up of a band of thousands of very small, short-lived volcanoes that have built a platform of lava and volcanic debris. Rising above this volcanic platform are a few strikingly large volcanoes that dominate the landscape.
  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.
  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.
  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.
  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.
  The small Juan de Fuca Plate and two platelets, the Explorer Plate and Gorda Plate are the meager remnants of the much larger Farallon oceanic plate. The Explorer Plate broke away from the Juan de Fuca about 4 million years ago and shows no evidence that it is still being subducted. The Gorda platelet split away between 18 and 5 million years ago and continues to sink beneath North America.
  The Cascade Range made its first appearance 36 million years ago, but the major peaks that rise up from today's volcanic centers were born within the last 1.6 million years (during the Pleistocene). More than 3000 vents erupted during the most recent volcanic episode that began 5 million years ago. As long as subduction continues, new Cascade volcanoes will continue to rise.
caption: Geological map of the Cascade Range. Yellow indicates earthquakes, black lines indicate faults
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