Modern Theory of Plate Tectonics


The Lithosphere is divided into plates

There are three types of plate boundaries

  • Divergent--plates move away from each other
  • Convergent -- plates move toward each other
  • Transform -- plates slide past each other

Plates move through the process of convection in the mantle.

Almost all volcanism and Earthquake activity can be explained by the distribution and motion of the plates


A quick little movie showing continental outlines and some history of plate motion
A walk through of plate tectonics by Volcano World --Highly recommended
A lecture on plate tectonics At Green Bay--Also very nice

 

Divergent Plate Boundaries

Features of the mid-oceanic ridge systems

Geographic Features

located in the middle of oceans

Elevated in comparison with surrounding ocean

contain a central down-dropped "valley".

Volcanic Features

Magma arises from partial melting of upwelling mantle

Magma composition is that of Basalt--low silica content.

Large lava flows from fissures.

Seismic Features

Earthquakes are very shallow

Caused by more or less vertical (gravitational) movement of large blocks of ocean crust in the central valley as the ridge is lifted by upwelling magma. The Stresses acting at a divergent margin are largely tensional.

Hydrothermal Features

At the mid oceanic ridges, water soaks into the oceanic crust, is heated by the underlying magma, and rises back to the sea floor as superheated "geysers" or hydrothermal vents. These hydrothermal vents are associated with vast amounts of mineral deposits--copper, gold among them.  See these web sites:

British Museum Natural History -- Things living near the deep sea vents--a long time ago
American Museum of Natural History Expedition to the black Smokers
NOVA Online covering mission to see Black Smokers
Another PBS Site showing Vent Critters
More critters by Volcano World
Australian site studying mining potential of vents off of Papua new Guinea
British site on Deep ocean mineral deposits
Texas A&M Hydrothermal Vents site
NOAA Video Clips of Ocean Vents
 

The Force that Splits Continents: How a Mid-Oceanic Ridge System Starts.

Continental rifting

Oceans start out as rifts in continents.

Upwelling begins under continental crust, causes uplift, and fracturing at a Continental Rift

Continental Rift looks like a large valley in a generally uplifted area. A large crack.
Examples:

  • Basin and Range of the Western United States (yep, right here)
  • East African Rift

Typically three rifts join at Triple Junctions

Continental rifts are areas of uplift and of divergent movement, and divergence continues along two arms of a given triple junction--leaving one Failed Arm.

Small Oceans

As divergent motion continues, the rift fills with basaltic lava and widens out, forming the first Oceanic Ridge.  Of course, the rift eventually fills with sea water as well.

As the oceanic crust continues to form the two parts of the continent continue to move away from each other.

Since the new plate boundary is in the center of the mid oceanic ridge, the joint between the continental crust and the new oceanic crust is called the Passive Margin of the continent.

Examples of small incipient ocean basins:

  • Red Sea
  • Gulf of California

Old Oceans

As the ocean matures the Passive Margins begin to accumulate large amounts of sediment from the erosion of material on the continent.  Examples include:

  • East coast of north and South America
  • South eastern and southwestern coast of India
  • Most of the coast of Africa

Eventually, as oceanic crust gets older, it moves far from the mid-oceanic ridge.
Subduction may begin at the edge of an old ocean as old oceanic crust begins to sink back into the mantle.

 

Convergent Boundaries

What Happens at a convergent plate boundary?

Types of Convergent Plate Boundaries

There are three types of convergent plate boundaries:

  • Ocean to Ocean
    • Aleutian Islands
  • Ocean to Continent
    • West coast of South America
    • East coast of Asia

Continent to Continent

  • India and Asia
  • Italy and Europe
  • Africa and Europe (still a tiny bit of ocean left: the Mediterranean)

Subduction: the defining Process of Convergent Plate Boundaries

Subduction is a term that describes how, in a convergent plate boundary, one plate moves underneath another plate and is recycled into the mantle. The plate that goes under is the subducted plate, while the plate that remains on the surface is the overriding plate. Because subduction is so closely associated with the process of convergence, convergent plate boundaries are also called subduction zones, and the terms are loosely equivalent.

Oceanic crust can be subducted, but continental crust cannot be subducted because it is too light, and "floats" on the asthenosphere. Thus, in an ocean to ocean convergent boundary, either oceanic plate can be subducted; generally the older plate will be subducted. In an ocean to continent convergent boundary, the oceanic plate will always be subducted, never the continental plate.  Because any plate may contain both continental and oceanic crustocean to continent convergent boundaries can change to continent to continent convergent boundaries; as the oceanic crust is consumed, the continent on the subducting plate is brought closer to the subduction zone and eventually rams into the overriding continent. The continent on the subducting plate may begin to follow the oceanic crust, but subduction will stop shortly. Examples of continent to continent convergence:
 

Fast and Slow Oceans

Oceanic crust is more likely to subduct with increasing age.  This may be related to the increasing density of the crust as it cools, or with the increasing distance from the mid-oceanic ridge and the movement of currents in the upper mantle.

"Slow Ocean-Fast Continent" subduction occurs when an ocean to continent convergence forces relatively young oceanic crust to be subducted; In such a case the continent is overtaking the oceanic crust and the subduction occurs right at the edge of the continent. Examples:

  • West coast of South America
  • Northwestern coast of United States.


"Fast Ocean-Slow Continent" subduction occurs when old oceanic crust is subducted under continental crust. In this case, convergent plate motion may not be able to keep up with the speed of the descending oceanic crust. As the crust descends, the subduction zone may progress further and further in the direction of the oceanic plate, pulling with it a piece of the continent which cannot "keep up". An pull-apart ocean basin forms behind the detached continental fragment.  Examples:

  • Japan Pulled away from Asia

Features of Convergent Plate Boundaries

Geographic Features

Trenches form wherever oceanic plates are subducted. These are the deepest parts of the ocean

Island Arcs form On the leading edge of the overriding oceanic plates as magma, derived from the subducted plate, erupts and forms volcanoes.

  • Aleutian Islands

Volcanic Mountain Ranges  form on the leading edge of the overriding continental plates as magma, derived from the subducted plate, erupts and forms volcanoes. Also, some folding of Sedimentary rocks (derived from the thick sediments of the passive margins) and other rocks occurs as a result of compressional stresses between the colliding plates.

  • Cascade mountains of the American northwest
  • Andes Mountains of western South America

Giant Collisional Mountain Ranges form as a result of continent to continent collision. Partial subduction of subducted continent creates an area of double thickness of continental crust. Remnants of oceanic crust are pushed up between the continents, and the terrain is characterized by intricate folding, a direct result of massive compressional stresses.

  • Himalayan Mountains
  • Alps

Volcanic Features

Magma is derived from the partial melting of the subducting oceanic plate, and partial mixing with materials from the overriding plate. This magma typically has a higher silica content than the magma on the mid oceanic ridge. Instead of great flows, this magma erupts into tall volcanic peaks like mount Fuji, Mount St Helens, Etc.

Seismic Features

Earthquakes are generated by the friction between the subducting plate and the overriding plate. Because of this there are both deep and shallow earthquakes distributed along a plane that is shallowest near the oceanic trench where the plate is subducted and is deepest far away from the trench, under the overriding plate.  This is called the Wadati-Benioff Zone. Earthquakes tend to be frequent and violent.

 

What is a Transform Boundary?

A Transform Boundary is where two plates slide past each other.

Features of a Transform Boundary

Geographic

The main feature of a transform boundary is a transform fault, simply the fracture separating the two plates along which they slide.

  • San Andreas Fault

Transform faults are a conspicuous feature of the mid-oceanic ridge where they link "offsets" in the ridge.

Seismic

Plates slide past each other very quickly, and major earthquakes are a frequent phenomenon.

(No Volcanism)