Plate Boundaries The action in plate tectonics is almost always at the plate boundaries. Two plates moving towards each other form a convergent boundary 1.Ocean crust meets ocean crust 2.Ocean crust meets continental crust 3.Continental Crust meets continental crust Two plates moving away from each other form a divergent boundary
Divergent Plate Boundaries in the oceans Extensional Stresses stretching ocean crust Creates New Seafloor Volcanoes Eruptions of basalt Low in Si, high in Fe Earthquakes Shallow foci Low to modest magnitudes
Partial melting of peridotite in the mantle produces basalt. Why doesn’t this process just produce more peridotite? Different mineral components in the peridotite melt at different T’s What will happen if only a small fraction of the rock melts? Will the melted part have the same composition as the original rock?
The mid ocean ridges
Iceland Mid Atlantic Ridge above the water
The mid ocean ridges have a rich variety of life and geologic structure Tube Worms Spider Crabs
Formations on the ridges Black Smokers Pillow Basalt
Divergent Plate Boundaries under continents Extensional Stresses stretching Eventually creates New Seafloor Volcanoes Basalt and granite (Bimodal composition) Earthquakes Shallow foci Modest magnitudes -- larger than oceanic rifts.
The rift valley in Africa is one of the most clear examples of continental rifting Oldoinyo Lengai Erta Ale Killimanjaro
The rift occurring under the Red Sea is more advanced
Why do we get both high & low Si rocks at continental rifts? Basalt is dense and ponds at base of crust Heat from basalt melts some of the Si-rich crust producing granite/rhyolite magma Both types of magma may eventually erupt at the surface
Convergent Plate Boundaries Compressional Stresses Old Seafloor Destroyed (Subduction) Volcanoes Explosive and Very Dangerous (lots of gas) Earthquakes Shallow to Deep foci Large magnitudes Mountain Belts/High Deformation Accretion – Building of the Continents
Island arcs: Indonesia, Japan, Taiwan, Philippines, Kamchatka Peninsula, Aleutian Islands.
The Andes mountains are the result of a ocean-continent collision
Deep, severe earthquakes and explosive volcanoes occur as oceanic crust subducts
Continent-Continent Collision Why are there no volcanoes when continents collide? No plate is sinking into the mantle and melting!
The Indian subcontinent slammed into Asia several million years ago
India is now a part of Asia. The Asian continent has grown.
Appalachian mountains were similar to the Himalaya mountains MYA (collision between North America and Africa)
Build-up of North America through time Pieces added to western North America in the last 200 million years
Transform Plate Boundaries Shear Stresses No destruction or creation of crust Volcanoes are rare Earthquakes Shallow foci Large magnitudes Occur both in ocean basins and on continents
Transform faults are evident in the ocean ridges
The San Andreas fault is a transform boundary
Island Chains Created by plates drifting over hot spots.
Other “Hot Spots” around the world
Major plate boundaries today
Lets play “Name that boundary”! Island Arc Transform Boundary Subduction (Trench) Divergent plate boundary Continental Rift Zone Converging Continental
The interior structure of Earth has been determined mostly from a)Drilling b)Exploration c)X-Rays d)Earthquake waves e)Chemical analysis