Download - 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 Plates12 major plates & several smaller ones Move slowly (few cm/yr; similar to fingernails) Can carry both continents & oceans
Continental Plates: composed mostly of graniteOceanic 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 BoundariesRising convection current below lifts lithosphere producing a mid-ocean ridge Mid-Atlantic ridge is classic example
Effects found at divergent boundary between oceanic plates: Submarine mountain range Volcanic activity Shallow earthquake activity Creation of new seafloor and a widening ocean basin
Continental Divergent BoundariesPull-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 plate Creates an ocean trench
• Subducted plate descends 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
• 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 BoundariesRegion 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