1. 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

2. 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

3. 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

4. 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

5. 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:
Submarine mountain range
Volcanic activity
Shallow earthquake activity
Creation of new seafloor and a widening ocean basin

6. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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