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