Seafloor Spreading Theory

Explains how ocean crust is formed at ocean ridges and destroyed at deep- sea trenches

•Supports Wagener’s continental drift theory •Unlike Wagner’s theory: continents are not push through ocean crust, in fact riding along while ocean crust moves from ocean ridges

•Think of a Conveyor Belt

Seafloor Spreading Theory

• The evidence for Seafloor Spreading suggest continental and ocean crust move as enormous slabs labeled Tectonic Plates

Tectonic Plates

• Tectonic Plates: huge piece of crust & upper mantle that fit together at their edges to cover Earth's surface

• There are about 12 major plates and several smaller ones

• Move a few centimeters a year similar to the rate at which fingernails grow

Theory of Plate Tectonics

• Theory that describes how tectonic plates move and shape Earth's surface

• Move in different directions @ different rates• Edges of the plates are called boundaries

Plate Boundaries

• Plate Interaction - The plates interact with each other along three types of plate boundaries:

• Divergent Boundaries• Convergent Plate Boundaries• Transform Boundaries• Because of the motion of the plates they can

collide, pull apart, or scrap against each other

Divergent Boundaries

• Regions where plates are breaking apart to move away from each other

• Mostly found along seafloor in rift valleys• Example east Africa Rift Zone

Convergent Boundaries • Region where two plates run into each other just

like two cars hitting head on in an accident• When two plates collide the denser plate descends

below the other, less dense plate, process called subduction

• There are three major types of convergence zones– Ocean-Ocean Convergence – Ocean-Continent Convergence – Continent-Continent Convergence

Ocean-Ocean Convergence

• Oceanic plates are dense and can easily sink into the Earth's mantle. When two oceanic plates collide, one runs over the other which causes the latter to sink into the mantle along a zone called a subduction zone.

• The process of subduction forms a very deep depression in the ocean floor called a trench.

• Magma floats back to surface, often erupts and forms an arc of volcanic islands.

• For example the Aleutian Islands Alaska

Ocean-Ocean Convergence

Ocean-Continent Convergence

• Continental plates are less dense than oceanic plates so the latter always subducts beneath the former. Andesitic magma rises from the subduction zone to form a magmatic arc on the continental crust.

• For example, the Andes mountains is a magmatic arc on the South American continental crust which formed as the Nazca plate is subducted below South America.

Ocean-Continent Convergence

Continent-Continent Convergence

• Continental plates do not easily subduct so that when two continents collide the two continents weld together.

• The result is a mountain range in the interior of a continent.

• For Example, The Himalaya Mountains

Continent-Continent Convergence

Transform Boundaries

• Region where plates are sliding past each other along one fault or a group of faults

• The motion of the two plates is parallel but in opposite directions so that the boundary is a shear zone but neither plate is gaining or loosing area.

• Example The San Andreas fault in California is a transform boundary between the Pacific and North American Plates.

Questions1. What type of plate boundaries are

found along the edges of the N American Plate?

Transform & Convergent along the western edge and Divergent

boundary is found along the Eastern Edge

QuestionsWhat is the deepest know Ocean trench?

ANY QUESTIONS?