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An In-depth Look at Earthquakes at divergent boundaries – shallow only, usually weak at translational boundaries – shallow only, often strong at convergent boundaries – often strong Plate Tectonics II continent-continent – shallow and intermediate subduction zones – shallow, intermediate, and deep, in that order, moving away from trench toward overriding plate Boundary characteristics and Driving Forces How and why do the plates move? Faults at divergent boundaries normal faulting in central rift valley and between blocks Tension pulls apart basalt blocks Blocks in rift valley fall down relative to others Volcanism in rift valley creates seamounts transform faulting on both sides of rift valley Rift valley not continuous but is punctuated by transform faults Two plates slide past each other at transform faults Fracture zones are inactive extensions of transform faults Earthquakes at divergent bdry’s Shallow focus, low magnitude quakes most occur at transform faults some occur in central rift valley Central Rift Valley (new oceanic crust is formed here) Plate #1 Transform Faults have strike-slip motion created by two blocks sliding past one another; these faults are prone to earthquake activity due to the frictional stresses that build up along the faults. Plate #2 Transform Fault (active seismicity) X X X X X X X X X Plate #1 Fracture Zone (inactive seismicity) Plate #2 Fracture Zone (inactive seismicity) X X = prone to earthquakes X X X Plate #1 1 Faults at subduction zones Reverse faulting in subduction zones at deep-sea trench Subduction zone examples Oceanic-continental Convergent Boundary ocean-continent ocean-ocean: more dense plate subducts under less dense plate; get island arc ocean-continent: ocean crust subducts under continental crust; get volcanic mountain chain Andes Mtns. (volcanic mtn. range) Accretionary wedge at trench Composed of sediment scraped off down-going plate Can be uplifted eventually if subduction leads to continentcontinent collision Peru-Chile Trench narrow shelf dropping off quickly to the trench (“active” cont. margin) broad coastal plain & continental shelf (“passive” continental margin) Earthquakes at subduction zones Subduction zone examples ocean-ocean Earthquake focus can be shallow, intermediate, or deep Often very high magnitude Can actually see downgoing plate by location of earthquake foci Benioff Zone – intermediate and deep earthquakes occur in this inclined zone, tilted away from trench toward volcanic arc, extends downward up to 700 km active magmatic arc (volcanic island arc) accretionary prism backarc basin notice the island arcs and volcanic mountain ranges (e.g., Japan) landward of the trenches 1991 eruption of Mt. Pinatubo rising bodies of magma oceanic lithosphere x x x x x partial melting of subducting plate near base of lithosphere Mariana Islands (volcanic island arc) trench f ore-arc basin x x xx x x x sea level x x oceanic crust oceanic lithosphere Benioff Zone x x x Mariana Trench x xx x x x x = earthquake foci 2 Continent-continent collisions thrust faulting at continent-continent collisions a sort of extreme form of reverse faulting major mountain building regions another site of strong earthquakes, shallow to deep foci India-Asia is classic example deep-sea sediments & oceanic crust squeezed between the two continents deformed magmatic arc & suture zone (massive mountain chain) x continental crust old magmatic arc x x x x x x x x x xx x x x xx x x x x x x continental crust x continental lithosphere oceanic lithosphere x x oceanic crust x = earthquak e foci Earthquakes at translational bdry’s transform faults, or “strike-slip” faults exist where one plate slides past another ocean crust neither created nor destroyed at these boundaries San Andreas Fault through California is the classic example other long transform faults on northern and southern edges of the Scotia Plate and the Caribbean Plate x x x x x x x continental lithosphere Himalaya & Tibetan Plateau continent-continent collision Faults at translational bdry’s Shallow focus but can be strong … Pacific Northwest – 3-in-1 The Pacific Northwest combines all 3 plate boundary types in one region … Divergent: Juan de Fuca Ridge Convergent: Cascadia Subduction Zone Translational: Mendocino Fault (northwest extension of San Andreas Fault) Mt. Rainer 3 The Wilson Cycle (J. Tuzo Wilson) Wilson Cycle refers to the sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins. Hotspots – a whole ‘nother story Stages in basin history are: Embryonic - rift valley forms as continent begins to split. Juvenile - sea floor basalts begin forming as continental fragments diverge. Mature - broad ocean basin widens, trenches eventually develop and subduction begins. Declining - subduction eliminates much of sea floor and oceanic ridge. Terminal - last of the sea floor is eliminated and continents collide forming a continental mountain chain. What drives this plate motion? oceanic crust spreading center km oceanic crust spreading center trench Lithosphere Pushing force Pulling force 800 ne zo 600 n io 400 continental crust sea level t uc bd su 200 magmatic arc (volc. mtn. chain) landward of trench Asthenosphere upper Mantle (ductile) intraplate volcanism (within a plate) like Hawaii, or located on a spreading ridge like Iceland linear chains of islands, seamounts, or ridges form due to plate moving over stationary hot spot hot spots are surface expressions of magma plumes rooted deep in the mantle over time, plate continues to move over hot spot, resulting in linear chain of volcanoes as plate moves, volcanoes move off hotspot, become seamounts Convection connections There are competing hypotheses as to how and where mantle convection occurs. The whole mantle model has convective flow throughout the entire mantle. According to this model, subducted slabs of oceanic lithosphere sink through the 660-km boundary between the asthenosphere and lower mantle, all the way down to the core-mantle boundary, where they melt. The layered mantle model has two separate zones of convection, one in the asthenosphere and the other in the lower mantle. According to this model, there is very little mixing between the two layers, and slabs of lithosphere either melt or pile up at the bottom of the asthenosphere. Mantle (rigid) 1000 Convection in the asthenosphere and/or lower in the mantle partly drives movement of the plates. In addition, the leading edges of suducting plates are pulled down by gravity, while plates at spreading ridges are pushed apart. Convection can basically be defined as a process in which hot, less dense material rises (such as magma that feeds a spreading ridge), and cold, more dense material sinks (such as old oceanic lithosphere that is being subducted into the mantle). 4 Plate Tectonics Puzzler ocean-ocean collision: oceanic magmatic arc crust (island arc) km 200 400 sea level hot spot island and linear chain of seamounts (see facing page) oceanic crust spreading center ocean-continent collision: magmatic arc (volcanic mtn. chain) trench 1 continental crust continental margin Lithosphere trench 2 3 Plate Tectonics summary fig. 4 Asthenosphere (ductile) oceanic crust spreading center continental crust The figure below is a theoretical schematic of the different types of plate boundaries, rather than a representation of an actual, present-day location on Earth. Enjoy! continental margin 5 600 800 1000 1200 1400 1600 Mantle 1800 2000 Quick question: How many plates are shown here? Hint: Look for boundaries. 2200 There are 5 plates. 2400 2600 2800 3000 source of hot spot? Liquid Outer Core 5