Transcript 17.3 Plate Boundaries

17.3 Plate Boundaries
Objectives
 Describe how Earth’s tectonic plates result in many geologic
features
 Compare and contrast the 3 types of plate boundaries and the
features associated with each
 Generalize the processes associated with subduction zones
Main Idea: Volcanoes, mountains, and deep-sea trenches form
at the boundaries between the plates
Theory of Plate Tectonics
Evidence from seafloor spreading suggested that
continental and oceanic crust moves as enormous
slabs which geologists call tectonic plates
 Huge pieces of crust and rigid upper mantle that fit together
at their edges to cover Earth’s surface
 Theory describes how plates move and shape Earth’s surface
 Attributes earthquakes, volcanoes, mountains, etc. to
movement and interaction of the rigid plates
 Move in different directions and at different rates
 Interact with each other at their boundaries
Tectonic Plates
12 major plates & several smaller ones
 Move slowly (few cm/yr; similar to fingernails)
 Can carry both continents & oceans
Continental Plates: composed mostly of granite
Oceanic Plates: most basalt, which is considerably heavier
 Continents are lighter and more buoyant; therefore they float higher on Earth’s
mantle than the ocean’s crust
Plate Boundaries
Divergent Boundaries
 When 2 plates are moving apart from each other
 Can occur both on continents and in oceans
Source comes from rising convection currents in mantle
• Rising current pushes up on bottom of
lithosphere, lifting it and flowing laterally
beneath it
• Lateral flow causes plate material above
to be dragged along in direction of flow
Oceanic Divergent Boundaries
Rising convection current below lifts lithosphere producing a
mid-ocean ridge
 Mid-Atlantic ridge is classic example
Effects found at divergent boundary between oceanic plates:
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Submarine mountain range
Volcanic activity
Shallow earthquake activity
Creation of new seafloor and a widening ocean basin
Continental Divergent Boundaries
Pull-apart not vigorous enough to create a clean,
single break through thick plate material
 Continental plate is arched upwards from convection currents below and
pulled thin and fractured into a rift-shaped structure
East Africa Rift Valley is prime
example of the effects of
continental divergent boundaries
Magma flowing laterally
pulling apart plates and
creating the rift valley
Convergent Boundaries
2 plates moving toward one another
 When 2 plates collide, the denser plate eventually descends below the
other, less dense plate in a process called subduction
3 types of convergent boundaries
 Oceanic-oceanic
 Oceanic-continental
 Continental-continental
Oceanic-oceanic convergent
boundaries
Subduction zone is formed when a denser
oceanic plate descends below another oceanic
• Subducted plate descends
plate
 Creates an ocean trench
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into mantle, recycling
oceanic crust formed at
ridge
Water carried into Earth
by subducted plate lowers
MP of plate causing it to
melt at shallower depths
Once turned to magma, it
is less dense so it rises to
surface where it often
erupts and forms an arc of
volcanic islands
Oceanic-Continental convergent
boundaries
Again, the denser plate (oceanic plate) is
subducted
 Also produces a trench & volcanic arc
 Results in mountain range with many volcanoes along edge of
continental plate
Continental-Continental convergent
boundaries
Form when 2 continental plates collide, long
after an oceanic plate has converged with a
continental plate
 Forms vast mountain range, such as the Himalayas
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Recall that oceanic continents are
often carried along attached to
oceanic crust
Over time, oceanic plate can be
completely subducted, dragging an
attached continent behind toward
subduction zone
Continental crust that pulls behind
cannot descend because it’s less
dense so the edges of both
continental plates collide and
become crumpled, folded, and
uplifted
Transform Boundaries
Region where 2 plates slide horizontally past each other
 Instead of new crust being formed or destroyed, the crust is only deformed or
fractured somewhat along these boundaries
San Andreas fault is an
example of a transform
boundary