Pacific Northwest Geography & Tectonics: Flash Open

Novice

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 How does geography reveal tectonic features?

Scroll over the bathymetric relief map to learn about the geographic provinces of the Pacific Northwest, including the subducting plate, the subduction zone, coast mountains, central valley, and cascades.

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Keypoints:

All these geographic features are related to the subduction zone:

  • Offshore plate boundary (often accompanied by a trench when older cold oceanic plate dives beneath another oceanic plate or continent. The Juan de Fuca Plate is young and warm, so doesn't result in such a trench as it doesn't dive as steeply)
  • Coast Range due to compression of the plate
  • Central valley, which is caught between the Coast Range and the
  • Arc Volcanoes, which grow above the point where the subducting plate reaches about 100 km depth. Melting of the rock requires a particular pressure/temperature regime.

Related Animations

Plate Boundary: Convergent margin

The subduction zone iswhere two tectonic (lithospheric) plates come together, one subducting (diving) beneath the other. The plates are locked together and periodically overcome the friction causing the leading edge of the overlying plate to surge back, lifting a wall of water producting a tsunami.

Animation Novice
Subduction Zone: Simplified model of elastic rebound

Oblique view of a highly generalized animation of a subduction zone where an oceanic plate is subducting beneath a continental plate. (See sketch below for parts.) This scenario can happen repeatedly on a 100-500 year cycle. The process which produces a mega-thrust earthquake would generate a tsunami, not depicted here.

Animation Novice
GPS Records Variable Deformation across a Subduction Zone

Subduction zones show that there are 3 distinct areas of movement in the overlying plate:

  1. constant movement above the locked leading edge,
  2. see-saw pattern of back-&-forth movement above a zone that alternately locks then slips, and
  3. no movement far inland above the deeper part of the diving oceanic plate.
Animation Novice
Pacific Northwest vs Japan: Similar tectonic settings

This UNAVCO animation compares Japan's subduction zone at the location of the 2011 earthquake with a mirror-image subduction zone in the Pacific Northwest. There are many similarities.

Animation Novice

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