Plate Tectonics II. An In-depth Look at Earthquakes. Faults at divergent boundaries. Earthquakes at divergent bdry s

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1 Plate Tectonics II 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 continent-continent shallow and intermediate subduction zones shallow, intermediate, and deep, in that order, moving away from 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 etensions 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 is formed here) Plate #1 Plate #2 Fracture Zone (inactive seismicity) Transform Fault (active seismicity) X X X X X X X X X Plate #1 Plate #2 X = prone to earthquakes 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. X X X X Plate #1 Fracture Zone (inactive seismicity) 1

2 Faults at subduction zones Reverse faulting in subduction zones at deep-sea Subduction zone eamples Oceanic- Convergent Boundary ocean-continent ocean-ocean: more dense plate subducts under less dense plate; get island arc ocean-continent: ocean subducts under ; get volcanic mountain chain Andes Mtns. (volcanic mtn. range) Accretionary wedge at 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 ( active cont. margin) broad coastal plain & shelf ( passive margin) Earthquakes at subduction zones Subduction zone eamples 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 toward volcanic arc, etends downward up to 700 km active (volcanic island arc) accretionary prism backarc basin notice the island arcs and volcanic mountain ranges (e.g., Japan) landward of the es 1991 eruption of Mt. Pinatubo rising bodies of magma partial melting of subducting plate near base of Mariana Islands (volcanic island arc) f ore-arc basin Benioff Zone Mariana Trench = earthquake foci 2

3 Continent-continent collisions thrust faulting at continent-continent collisions a sort of etreme form of reverse faulting major mountain building regions another site of strong earthquakes, shallow to deep foci India-Asia is classic eample Himalaya & Tibetan Plateau deep-sea sediments deformed magmatic continent-continent & arc & suture zone squeezed between (massive mountain chain) collision the two continents Faults at translational bdry s transform faults, or strike-slip faults eist where one plate slides past another ocean neither created nor destroyed at these boundaries San Andreas Fault through California is the classic eample other long transform faults on northern and southern edges of the Scotia Plate and the Caribbean Plate old magmatic arc = earthquake foci Earthquakes 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 etension of San Andreas Fault) Mt. Rainer 3

4 The Wilson Cycle (J. Tuzo Wilson) Wilson Cycle refers to the sequence of events leading to the formation, epansion, contracting and eventual elimination of ocean basins. Stages in basin history are: Embryonic - rift valley forms as continent begins to split. Juvenile - sea floor basalts begin forming as fragments diverge. Mature - broad ocean basin widens, es eventually develop and subduction begins. Declining - subduction eliminates much of sea floor and ridge. Terminal - last of the sea floor is eliminated and continents collide forming a mountain chain. Hotspots a whole nother story intraplate volcanism (within a plate) like Hawaii, or located on a ridge like Iceland linear chains of islands, seamounts, or ridges form due to plate moving over stationary hot spot hot spots are surface epressions 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 km What drives this plate motion? Pushing force (volc. mtn. chain) landward of Pulling force 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 ridges are pushed apart. subduction zone Lithosphere Asthenosphere upper Mantle (ductile) Mantle (rigid) Convection can basically be defined as a process in which hot, less dense material rises (such as magma that feeds a ridge), and cold, more dense material sinks (such as old that is being subducted into the mantle). 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 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 miing between the two layers, and slabs of either melt or pile up at the bottom of the asthenosphere. 4

5 Plate Tectonics Puzzler Plate Tectonics summary fig. km ocean-ocean collision: (island arc) hot spot island and linear chain of seamounts (see facing page) ocean-continent collision: (volcanic mtn. chain) margin Lithosphere Asthenosphere (ductile) 4 5 margin 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! source of hot spot? Mantle Liquid Outer Core Quick question: How many plates are shown here? Hint: Look for boundaries. There are 5 plates. 5