Modern Theory of Plate Tectonics
The Lithosphere is divided into plates
There are three types of plate boundaries
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
Divergent Plate Boundaries
Features of the mid-oceanic ridge systems
located in the middle of oceans
Elevated in comparison with surrounding ocean
contain a central down-dropped "valley".
Magma arises from partial melting of upwelling mantle
Magma composition is that of Basalt--low silica content.
Large lava flows from fissures.
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.
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:
Museum Natural History -- Things living near the deep sea vents--a long
The Force that Splits Continents: How a Mid-Oceanic Ridge System Starts.
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.
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.
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:
As the ocean matures the Passive Margins begin to accumulate large amounts of sediment from the erosion of material on the continent. Examples include:
Eventually, as oceanic crust gets older, it moves far from the mid-oceanic
What Happens at a convergent plate boundary?
Types of Convergent Plate Boundaries
There are three types of convergent plate boundaries:
Continent to Continent
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 crust,
ocean 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:
Features of Convergent Plate Boundaries
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.
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.
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.
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.
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
The main feature of a transform boundary is a transform fault, simply the fracture separating the two plates along which they slide.
Transform faults are a conspicuous feature of the mid-oceanic ridge where they link "offsets" in the ridge.
Plates slide past each other very quickly, and major earthquakes are a frequent phenomenon.