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(With help from Clague et al, 2006, At Risk: Earthquakes and Tsunamis on the West Coast.)
Continental crust – The crust under continents (up to 60 km. thick). Rock is rich in silicon and aluminum (granitic) and is less dense than oceanic crust.
Continental shelf – Submerged shallow (<200m) fringe around continent which is part of continental plate.
Continental slope – The slope leading from the shelf to the abyss.
Crust – The outermost layer of the Earth.
Divergent boundary (oceanic) – Site of spreading found at sub-sea ridge where two plates are created from magma and move in opposite directions, for example Juan de Fuca and Pacific move east and west from Juan de Fuca Ridge.
Earthquakes – A sudden release of energy from strain built up in the Earth’s crust, usually located near plate boundaries.
Juan de Fuca plate – The oceanic plate in the deep ocean off of southwestern British Columbia, Washington and Oregon states and the northern part of California.
Juan de Fuca Ridge – The active volcanic section of the mid-ocean ridge that separates the Juan de Fuca and Pacific plates
North American plate – One of the large continental plates, that forms the crust of the Earth.
Oceanic crust – Crust on ocean floor (up to 8km. thick). Rock rich in silicon and magnesium and much more dense than continental crust.
Pacific Plate – One of the large oceanic plates that forms the crust of the Earth.
Plate tectonics – Generally accepted theory that the Earth’s crust is made up of crustal plates of different sizes that “float” on a plastic upper mantle and move more or less independently of one another. Dynamic earthquake activity is associated with the boundaries of these plates. Oceanic plates are pushed from the rear by seafloor spreading and pulled by subduction under the continent.
Subduction zone –The over-lapping part of the crust where one plate (such as a dense oceanic plate) descends at an angle under another plate (such as a lighter continental plate).
Tsunamis – Series of waves made by a major disturbance on the ocean floor such as a submarine earthquake, landslide, volcanic eruption or by impact from outer space. Trench – A long narrow deep area of the ocean floor where flexing oceanic plates slide down and under a lighter, less dense plate.
Sub-sea ridge – An underwater divergence boundary where new plate material is pushed out as magma and cools in long fractures on the sea floor. See Juan de Fuca Ridge. |
