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Plate Tectonics II

Boundary characteristics and Driving Forces

How and why do the plates move?

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 trench toward overriding plate

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

Plate #1 Plate #2

X = prone to earthquakes

Fracture Zone(inactive seismicity)

Fracture Zone(inactive seismicity)

Central Rift Valley (new oceanic crust is formed here)

Transform Fault(active seismicity)

Plate #1 Plate #2

Plate #1

X X X X X X X X X

X X X X

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.

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Faults at subduction zones� Reverse faulting in

subduction zones at deep-sea trench� 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

� Accretionary wedge at trench� Composed of sediment

scraped off down-going plate� Can be uplifted eventually if

subduction leads to continent-continent collision

Subduction zone examplesOceanic-continental Convergent Boundary

broad coastal plain& continental shelf

(“passive” continentalmargin)

narrow shelf dropping offquickly to the trench(“active” cont. margin)

Peru-Chile Trench

Andes Mtns.(volcanic mtn. range)

ocean-continent

Subduction zone examples

notice the island arcs andvolcanic mountain ranges(e.g., Japan) landward of

the trenches

Mariana Islands(volcanic island arc)

Mariana Trench

1991 eruption of Mt. Pinatubo

ocean-ocean

Earthquakes at subduction zones

� 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

x

active magmatic arc (volcanic island arc)

partial melting of subducting plate near base of lithosphere

sea level

rising bodies of magma

trench

oceanic lithosphere

oceanic crust

x

xx

xx

x

x

xx

x

xx

xx

x

xxx

x x = earthquake focix

x

x

f ore-arc basinbackarc basin

accretionary prism

oceanic lithosphere Benioff Zone

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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 earth-quakes, shallow to deep foci

� India-Asia is classic example

deformed magmatic arc & suture zone

(massive mountain chain)

oceanic crust

x

xx

xx

x

x

xx

x

xx

xx

x

xxx

xx x = earthquake focix

x

x

x

xxx

xx

oceanic lithosphere

continental lithosphere

continental lithosphere

continental crust

continental crust

xx

x x

old magmatic

arc

deep-sea sediments & oceanic crust

squeezed between the two continents

Himalaya &Tibetan Plateau

continent-continentcollision

Faults 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

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

extension of San Andreas Fault)

Mt. Rainer

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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.

� 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 - subductioneliminates much of sea floor and oceanic ridge.

� Terminal - last of the sea floor is eliminated and continents collide forming a continental mountain chain.

Hotspots – a whole ‘nother story� 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

What drives this plate motion?oceanic

crustoceanic

crustcontinental

crustmagmatic arc(volc. mtn. chain)

landward of trench

Asthenosphereupper Mantle (ductile)

200

400

600

800

1000

km

Mantle (rigid)

spreading center

spreading center

Lithospheresea level

subductionzone

trench

Pushing force

Pulling force

Convection in the asthenosphereand/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 subductedinto the mantle).

Convection connections� There are competing hypotheses as to how and where

mantle convection occurs.� 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.

� 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.

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Plate Tectonics Puzzler

oceanic crust

oceanic crust

continental crust

Asthenosphere (ductile)

200

400

600

800

1000

km

Mantle

Lithosphere

continental margin

continental margin

ocean-continent collision:

magmatic arc (volcanic mtn.

chain)

ocean-ocean collision:

magmatic arc (island arc)

oceanic crust

sea level

continental crust

trenchtrench

1200

1400

1600

1800

2000

2200

2400

2600

2800

3000

source of hot spot?

hot spot islandand linear chain of seamounts(see facing page)

Liquid Outer Core

spreading center

spreading center

Quick question: How manyplates are shown here?

Hint: Look for boundaries.

There are 5 plates.

4 51 2 3

Plate Tectonics summary fig.

� 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!