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Beauty and the Beast Plate Tectonics of the Pacific Northwest bd Su NORTH AMERICAN Va nc ou PLATE ve Mt. Garibaldi Rid ge Glacier Peak de Fu ca GORDA PLATE Plateau Mt. Hood Three Sisters Cascade Volcanoes Mt. Mazama Mt McLoughlin Mt. Mountains Shasta Basin Range Province Ca Ne lif PACIFIC PLATE JUAN de FUCA PLATE NORTH AMERICAN PLATE Crustal Earthquake Volcanic Earthquake Magma Rifting Earthquake va or Earthquake Mid-Ocean Locked Zone Ridge Earthquake Ju Su an bd “THE BIG ONE” de uct Fu ing ca Pla te Plate Sweats Hot Water da s ni a Plate Boundaries: Hotspot Divergent Convergent Basin and Range Province Slab and da va Ne ey ra l er Val Si eat ge Gr an Lassen Peak R st a Co ea dr An lt n au Sa F PLATE Offshore Earthquake Mt. Jefferson Go rd a Ri dg e Klamath Juan de Fuca Ridge tte ame Will lley Va Willamette Valley Coast Range Mt. St. Helens Cascade Volcanoes t ge d Pu un So Washington Oregon Mt. Adams Boundary PACIFIC Mt. Rainier Columbia Plate Jua n Mt. Baker Olympic Mtns. JUAN de FUCA PLATE Olympic Mountains and Coast Range Canada get Pu und So g tin s. Idaho uc rI Cascadia Subduction Zone Transform Binder Foldout & CEETEP Poster Beauty and the Beast “The same geological processes that threaten our lives with earthquakes, tsunamis, and volcanic eruptions also nourish our spirits by creating the spectacular mountains, valleys, and coastlines of the Pacific Northwest.” Mt. St. Helens National Olympic National Park Volcanic Monument Plate Tectonics The Earth’s interior is convecting. Plates are driven by cooling of Earth. Gravity provides the “ridge push” and “trench pull” forces to move plates. About a dozen plates; some plates have continents; some don’t. Plates move a few centimeters per year (the rate of fingernail growth). Volcanoes and earthquakes are concentrated at plate boundaries. Lithosphere in Earth Context oceanic crust continental crust Thickness of lithosphere (= tectonic plates) ~ 100 km. Lithosphere is crust on top and mantle on bottom. Oceanic crust is basalt, thickness = 7 km. Continental crust is granite, average thickness = 40 km. Granite is less dense than basalt. Earth - vs Egg Earth –vs- Egg Earth radius = 6370 km Lithosphere thickness = 100 km What % of Earth radius is lithosphere? ~1.6% 2% Egg radius = 0.75 inch Egg shell thickness = 0.015 inch 2% What % of egg radius is shell? How do these compare? Lithosphere is to Earth as shell is to egg. Mechanical Behaviors Rock Tongs Elastic: bends and recovers. Brittle: elastic UP TO A LIMIT, then BREAKS. COLD rocks are brittle. Ductile: permanent deformation. WARM rocks are ductile. Lithosphere (= Tectonic Plate) Granite Basalt 40 km thick, variable 7 km thick Lithosphere: outer ~100 km. Contains crust + upper mantle. Cold, rigid, and brittle; can fracture. Asthenosphere: deeper, hotter mantle. Can flow like silly putty. A VISCOELASTIC SOLID. NOT LIQUID!! “Oceanic Crust” “Continental Crust” “Earth’s Mantle” Parks and Plates ©2005 Robert J. Lillie Three Kinds of Plate Boundaries Transform Divergent Convergent Plate Boundaries of the Western US Transform Plate Boundary Strike-slip faulting San Andreas Fault North American Plate Pacific Plate Strike-Slip Fault Produced by shearing forces. Transform Plate Boundaries Transform Plate Boundaries Earthquakes on Transform Boundaries Strike-slip faulting with horizontal motion parallel to fault. Shallow depths, almost never deeper than 20 km. Why? Many small EQs (M<6). Some strong (M6) and major (M7) EQs. Divergent Plate Boundaries Spreading ocean ridges Continental rift zones Divergent Boundaries in Oceans are Seafloor Spreading Centers Example: Mid-Atlantic Ridge. Divergent Plate Boundaries Divergent Plate Boundaries Normal Fault Foam Faults Produced by extensional forces. Age of Juan de Fuca and Gorda Plates New oceanic plate formed at the spreading ridge. Plate is young at the ridge and gets older with distance away from the ridge. Juan de Fuca Plate is “only” 10 million years old when it subducts. Earthquakes on Divergent Boundaries Normal faulting due to extension. Shallow depths, rarely deeper than 10 km. Why? Many small EQs (M<6). Some strong (M6) and major (M7) EQs. World Map of Plates Explore Spreading Ocean Ridges and Transform Plate Boundaries Convergent Plate Boundaries Ocean - Ocean subduction zone, Marianas Ocean – Continent subduction zone, Cascadia Continent – Continent collision zone, Himalayas Why does an oceanic plate subduct beneath a continental plate? Swimming Pool “Earth’s Mantle” Parks and Plates ©2005 Robert J. Lillie “Oceanic Crust” “Continental Crust” “Earth’s Mantle” Parks and Plates ©2005 Robert J. Lillie Easy to Subduct. Parks and Plates ©2005 Robert J. Lillie Hard to Subduct! Parks and Plates ©2005 Robert J. Lillie Continental Crust is THICKER, and therefore MORE BOUYANT than Oceanic Crust Parks and Plates ©2005 Robert J. Lillie Plate capped with thin Oceanic Crust subducts beneath plate capped with thick Continental Crust Parks and Plates ©2005 Robert J. Lillie Convergent Plate Boundary Entire region is the “Cascadia Subduction Zone” Cascadia Subduction Zone Reverse (Thrust) Fault Foam Faults Produced by compressional forces. Earthquakes on Convergent Boundaries Thrust faulting due to compression. EQs to 100 km depth on plate boundary megathrust AND within the converging plates. EQs to greater depths occur within subducting oceanic plate. Wide range of magnitudes. Most M8 & M9 great EQs are shallow megathrust events in subduction zones. Cascadia Animation World Map of Plates Explore Convergent Plate Boundaries Robert J. Lillie Bernard Garcia National Parks in two mountain ranges. Parks and Plates ©2005 Robert J. Lillie Terrane Accretion A TERRANE consists of crust that is too thick and buoyant to subduct. The continent grows outward as terranes are added to the margin. Olympic National Park Coastal Ranges Marine sedimentary layers deposited on the seafloor then scraped off onto the continental margin. Olympic National Park Pillow Basalt Formation of Pillow Lava Lava Erupts into Cold Ocean Water Oceanic Crust Video of Pillow Lava Terrane Accretion Beauty and the Beast Plate Tectonics of the Pacific Northwest bd Su NORTH AMERICAN Va nc ou PLATE ve Mt. Garibaldi Rid ge de Fu ca e dg Ri Cascade Volcanoes Mt. Mazama Basin Offshore Earthquake and Range a rr Province y le da va Ne ge an al tV ea Gr e Si R st oa Lassen Peak Ca Ne lif PACIFIC PLATE JUAN de FUCA PLATE Crustal Earthquake Basin and Range Province Volcanic Earthquake Magma Rifting Earthquake Slab Go rd a tte ame Will lley Va Three Sisters Mt McLoughlin Mt. Mountains Shasta s C ea dr An lt n au Sa F PLATE NORTH AMERICAN PLATE Mt. Jefferson Klamath GORDA PLATE Juan de Fuca Ridge Plateau Mt. Hood Cascade Volcanoes t ge d Pu un So Washington Oregon Mt. St. Helens Willamette Valley Boundary PACIFIC Mt. Rainier Mt. Adams Coast Range n Glacier Peak Columbia Plate Jua Mt. Baker Olympic Mtns. JUAN de FUCA PLATE Olympic Mountains and Coast Range Canada get Pu und So g tin s. Idaho uc rI Cascadia Subduction Zone or va Earthquake Mid-Ocean Locked Zone Ridge Earthquake Ju Su an bd “THE BIG ONE” de uct i n ca g Pla Fu Plate Sweats Hot Water te ni da a Plate Boundaries: Hotspot Divergent Convergent Transform Geology of Olympic Na0onal Park h"p://geomaps.wr.usgs.gov/parks/olym/index.html Robert J. Lillie Yachats Forma=on Oceanic Basalt Lava Flows ~ 35 Million Years Old (Late Eocene -‐ Oligocene) Heceta Head Lighthouse State Scenic Viewpoint, Oregon Beauty and the Beast Mt. St. Helens National Volcanic Monument, Washington Robert J. Lillie U. S. Geological Survey “The same geological processes that threaten our lives with earthquakes, tsunamis, and volcanic eruptions also nourish our spirits by creating the spectacular mountains, valleys, and coastlines of the Pacific Northwest.” Crater Lake National Park