Plate tectonics theory describes how the lithosphere (crust) of the Earth is broken into various plates. These plates drift on the asthenosphere (between the Earth’s crust and upper mantle) slowly. As plates move away from each other, the lithosphere thins and tears.
At these divergent plate boundaries, new oceanic lithosphere is created in the gaps from upwelling magma from the mantle. This upwelling magma forms mid-ocean ridges, long mountain chains that mark the boundaries between diverging plates.
Plate tectonics provides the mechanism to recycle the Earth’s crust. The fixed size of the Earth implies that the crust must be destroyed at about the same rate it is being created. Destruction (recycling) of crust occurs along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) beneath another. The location where the sinking of a plate occurs is called a subduction zone.
A transform boundary is where plates slide past each other without creating or destroying the lithosphere.
Divergent Plate Boundaries
Divergent boundaries occur where plates are moving apart. Hot mantle rock rises, and partial melting occurs. New crust is created by the magma pushing up from the mantle, creating elevated regions. These elevated regions can be considered continental rift zones or mid-ocean ridges, depending on whether the lithosphere is capped by continental or oceanic crust. Fissures, normal faults, rift valleys, and mountain ranges (due to volcanoes) can occur at these boundaries.
Continental Rift Zones such as the East African Rifts occur when the continent pulls apart, stretching and thinning the crust. The underlying asthenosphere is hot and buoyant, causing the area to rise to high elevations. As it is cold and brittle, the upper part of the crust deforms. This deformation causes earthquakes, elevated ridges (horsts), and down-dropped valleys (grabbens). Grabbens can fill up with sediments due to erosion of horsts and volcanic activity, forming basins.
Mid-Ocean Ridges such as the Mid-Atlantic Ridge occurs when continents completely rift apart. As continents continue to pull apart, the buoyant asthenosphere elevates a ridge on the seafloor that may be 100’s to up to 4000 kilometers wide. Although the ridge is quite hot, the upper part of the oceanic crust is cold and brittle. This causes earthquakes and normal faulting. Chains of mid-ocean ridges, including the Mid-Atlantic Ridge, East Pacific Rise, and the Indian Ocean Ridge, extend in a near-continuous fashion for more than 50,000 kilometers, making it the longest mountain range on Earth, even though it is mainly covered by water.
Transform Plate Boundaries
At a transform plate boundary, two plates are sliding past one another laterally. At shallow levels (generally above 50 kilometers), this occurs across Trinidad, with the South American Plate and the Caribbean Plate sliding past one another. In Trinidad, this strike-slip motion is compensated with a vast network of strike-slip, normal fault, and a few thrust or reverse faults across the island and surrounding seas. However, there is no clear transform boundary similar to the San Andreas Fault Zone, another transform boundary in the Western United States.
Fracture Zones
Along mid-ocean ridges, such as the Mid-Atlantic Ridge, a network of divergent and transform boundaries exist, where volcanism and earthquakes occur. However, further away from the ridge, inactive extensions of the transform boundaries exist, called fracture zones.
Oceanic crust age differences and ridge-ridge transform faulting associated with offset mid-ocean ridge segments lead to the formation of fracture zones. Credit: Bob Stern, UT, Dallas/Wikipedia
Convergent Plate Boundaries
When lithospheric pates converge, the place with a thinner, less buoyant crust usually descends (subducts) beneath the other plate. The region where a plate descends deeply within the mantle is called a subduction zone.
The general premise of convergent plate margins is that at boundaries where plates move toward one another, one plate (the denser or heavier plate) moves under, or subducts, under the more buoyant or “lighter” plate. There are three sub-types of convergent plate boundaries: oceanic-oceanic, oceanic-continental, and continental-continental.
Oceanic-Oceanic Convergence
When two oceanic plates converge, one plate subducts beneath the other. Generally, two features are formed: a trench, where the oceanic plates at the bottom of the ocean meet, and a volcanic island arc. This is because water released from the subducting slab facilitates the melting of the overlying mantle. The melt rises to form volcanoes.
In areas with significant sediment accumulation on the seabed base, this sediment is incrementally scraped off and forms accretionary wedges that can form islands, such as Barbados. A depression between an accretionary wedge and an island arc, which tends to accumulate sediment, is called a forearc basin.
In the Lesser Antilles, all of these features exist. To the east and north of the Caribbean Islands, the Puerto Rico Trench exists. Though there is a subtle trench to the East of the Lesser Antilles, the significant accumulation of sediment from the Orinoco River makes this trench and the depression between the accretionary wedge and island arc less pronounced. Nearly all Eastern Caribbean islands are volcanic in origin, with most having active volcanism presently. Lastly, the accretionary wedge is responsible for forming islands such as Barbados, which rises approximately 25 millimeters every 1000 years.
Oceanic-Continental Convergence
The denser oceanic plate subducts beneath, the more buoyant continental plate at this boundary. Like oceanic-oceanic convergence, a deep-sea trench, accretionary wedge, a forearc basin, and volcanism (albeit continental volcanism) are common. Off the coast of South America along the Peru-Chile trench, the oceanic Nazca Plate is subducted beneath the South American Plate. The overriding South American Plate is being lifted, creating the towering Andes mountains.
Continental-Continental Convergence
At a continental-continental convergent zone, the thinner, oceanic crustal part of the downgoing plate is consumed through subduction initially. Then, the thicker, more buoyant crusts meet and are too light to be subducted. Instead, the thick crusts are deformed, buckling, and pushed upward or sideways. If convergence continues, once thick continental crust block may thrust underneath the other, resulting in a broad region of high elevation. An example of this is the Himalayan mountain range. It dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics. The Himalayas, towering as high as 8,854 m above sea level, form the highest continental mountains in the world.
Hotspots
A “hotspot” is a region in the mantle where magma forms due to a “plume” of heat from deep within the mantle. These plumes are relatively narrow and are thought to be fixed relative to the deep mantle.
These plumes of intense heat melt the lithosphere at the surface, producing volcanism, sometimes far away from plate boundaries, where volcanoes are expected and common. As the hotspot is thought to be fixed and plates are constantly moving, volcanoes get progressively older away from the largest and most active volcanoes. An excellent example of this is the Hawaiian Island Chain. This can be used to track plate movements over millions of years.
