Post on 12-Mar-2020
Cascadia Subduction Zone (“Cascadia Megathrust”) 1
"The earth, instead of appearing as an inert statue, is a living, mobile thing.”, J. Tuzo Wilson, 1968
Tectonics 102 (fall 2017 supplement to Geology of NPs): Part 1, More on Tectonics
Gary Oberts, Nov. 1, 2017
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Mid-Atlantic Ridge
East Pacific RiseMarianna and Aleutian Trenches
Hawaiian Islands Archipelago
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Plate boundaries and movement speed
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Range of movement: 1.3 – 18.3 cm / Yr = 0.5 – 7.2 in. / Yr
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Structure and Processes of the Tectonic Earth
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Precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches
and submarine mountain ranges
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1) Divergent boundaries (spreading centers, ridges, rises)
In ocean along spreading ridges
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On land
Iceland
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Afar Region, Horn of Africa
(East Africa Rift Valley)
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Ocean Depth
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Isotopic age-dating
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Repeated magnetic reversals of the Earth magnetic field in the geologic past
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2) Convergent boundaries (subduction zones)
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a) Oceanic-Continental – most major subduction zones like Juan de Fuca (below), Alaska, Chile, Indonesia
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Alaska
Cascadia Subd. Zone
Aleautian Chain Subd. Zone
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Nazca Plate subducting under So. American Plate (~8 cm or 3” per year)
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Farallon Plate (and Juan de Fuca
remnant)
< Likely the current situation
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Farallon subduction.mpg
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b) Oceanic-Oceanic – ocean-
located subduction zones like the Mariana
Trench (~11,000 m deep), usually with volcanic
island arc behind subduction zone
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“Accretion” (add-on, append, paste in variable shapes)
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Formation of an Accretionary Wedge
Graphics from Nat. Geog. DVD #734
c) Continental-Continental –Major mountain building
often results, like Himalayans/Hindu Kush
Ranges and Tibetan Plateau
Moving at a rate of about
*29.5’/100 yrs.😱😱😱
* 61 mm/yr 35
3) Transform boundaries (lateral, slip)
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~3”/Yr
~1”/Yr
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“Triple Junctions” (3 types)
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Spreading
Colliding
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Gorda Plate >
Complex (spreading and colliding)
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Mantle “Hot Spots”
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Continental Environments
Super-volcano
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Oceanic Environments
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New seamount (submerged island) of Lo’ihi• 400,000 years old
• 3,000’ below sea level• 10,000’ above sea floor
• Will emerge from sea in 10,000 – 100,000 years
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