A boundary between tectonic plates, called as plate margin. There are four major types of plate boundary.
- Where two plates are moving apart from each other, as at mid-oceanic-ridges, new crust is formed and the boundary is said to be Constructive, Accreting or Diverging plate boundary.
- Where two plates are moving towards each other, or converge along a line and collide wherein leading edge of one plate overrides the other plate and the overridden plate is subducted, called as Destructive boundary.
- Collision boundaries are also sites of convergence, but at these neither plate is consumed. Example- A collision boundary is the line of Himalayan chain s formed where the Indian plate meets the Asian plate.
- A conservative boundary may form along a transform fault where the relative plate is parallel to the boundary. Ex- San Andreans fault of California.
Points at which three plates meet are called Triple Junctions. At such junctions, subduction, transform motion, or divergence may occur in any combination.
Constructive Margin or Divergent Plate Boundary:
Constructive margins are rift zone or spreading zone. This is the zone of tension, where lithosphere splits, separates and moving apart, and thus, hot magma comes out through the cracks and solidifies leading to the formation of new crust.
Ocean ridge formation:
When the sea – floor spreading theory was first proposed, it was commonly accepted that the oceanic ridge is a portion of the upwelling mantle driven by convection currents and then cooled. It appears from result of seismic studies, indicating that shallow earthquakes take place along the ridges due to faulting or fracturing.
Plate divergence takes place when the lithosphere under tension splits along a rift and the fragmented portion moves apart in two opposite directions, ascending magma from the asthenosphere wells up below the cracking and widening ridges. As the magma approaches the ocean floor the pressure decreases and partial melting is increased. Some of the magma cools below the crust, where coarsely crystalline Gabbro is formed and some is forced into the fractures in the crust as wall like intrusions, called Dykes or flow out as basalt lava – especially Pillow lava. As the newest sea floor is the hottest and therefore the weakest part of the crust, the crests of ridges continue to be sites of spreading.
Here we also have to mention that the sediment formed of the skeletons of marine animals, of wind – blown dust from the continents and more rarely of material carried away from the continents by turbidity currents is collected in the basins between fractured blocks of oceanic crust.
Destructive Margin or Convergent Plate Boundary:
Much of the evidence for the nature of destructive plate boundaries come from seismic studies. According to depth, three classes of earthquake are recognized.
- Shallow, with a focus within the upper 50km of crust.
- Intermediate, with a focus from 50-250km depth.
- Deep, with a focus deeper than 250km and a maximum depth of about 650-700km.
When one plate subducted into the another plate, this zone is called as Benioff Zone. This zone is characterized by intense earthquake. Whereas, shallow earthquake is being occurred in all marginal oceanic trench and intermediate earthquake occur closer to the continent.
The second evidence had been discovered earlier by the Dutch Geophysicist, Venning, Meinez, who found that the trenches around the east and west Indies are belt of negative gravity anomalies.
The third evidence is that trenches are areas of low heat flow in the crust, compared with the high heat flow zones of the axial rifts of the oceanic ridges.
What happens at the upper surface of the crust during a collision depends upon the nature of the crust.
Ocean – Ocean collision:
Where oceanic crust collides with oceanic crust an Island Arc may be formed above a subduction zone, as in the Aleutians or the Marianas.
Most of the Island Arc of the world occur around the Western and northern edges of the Pacific ocean, where the Pacific plate is being subducted below the oceanic edges of the Eurasian and American plates. As the down bending lithospheric plate descends into the mantle it is heated, and it is thought that the crustal basalt melts and forms new magma bodies. Some of the magma differentiates as the crystallization temperatures of the constituent minerals are passed, so, that those minerals which melt first become concentrated in the rising magma bodies. As a result of differentiation the mafic materials from the basalt stay behind but the felsic components rise to that the newly created magma is of intermediate composition which produces andesite volcanoes and lava flow.
Andesite and basalt outpouring first forms a chain of submarine volcanoes and then, as they grow above sea-level, a chain of volcanic Islands. Eventually larger Islands may form with many eruptive centers. All of these stages can be recognized in the Island arcs of the western pacific. So, common are andesitic volcanoes around the margin of the Pacific Ocean that the line of the Island arc and the continental volcanoes of the Pacific coasts of the Americas, is called the Andesite line.
Chain of volcanic islands like Aleutians and Marianas form a limited source of sediments for the deep trenches along their margin. Such trenches are regarded as barren and the backarc basin between the islands and the continent may also be an area of thin oceanic crust and of limited sedimentation of its side nearest the island arc.
Two hypothesis have been offered to explain how oceanic crust come to be located in backarcbasins.
- The entrapment hypothesis suggests that a subduction boundary was formed within the oceanic crust and thus isolated the backarc crust behind younger arc volcanoes. This explains the situation of the Mariana, Aleutian and Bering sea.
- In the case of Japan arc, the sea of Japan crust is younger that the older rock of Japan, which are Paleozoic. In this case an hypothesis of lateral drift appears to be appropriate: it is proposed that subduction occurs beneath a continental margin and that a slice of continental crust opening up a rift which will progressively widen and be filled with new oceanic crust and become a back arc basin. The curvature of such arcs away from the Eurasian plate, the high heat flow beneath the sea of japan, and the age of the crust , all support this hypothesis.
Where oceanic crust collides with continental crust the oceanic crust may be subducted below the continent and a mountain chain formed above the subduction zone, as along the Andrean coast of South America. Ocean- continental collision characterized by mountain building, rock deformation metamorphism, earthquake and volcanic activity.
The Andes of Chile and Peru, where the oceanic plate subducted under continental plate, subjected to high pressure and severe scrapping between two plates and hence intense earthquakes. This type of subduction has been called Chilean type, is associated with relatively thin, warm, low – density oceanic crust sinks into the mantle less rapidly.
Along continental margins and to a lesser extent island volcanic arcs which stand high above sea – level, large quantities of sediments are carried from the land and swept across the continental shelves by currents. At the outer edges of shelves turbidity currents carry the sediments down to continental slope and along submarine canyons into the trenches. At a site of active subduction some of this sediment is trapped between the upper and lower plates and carried into the mantle, but most is scrapped into crumpled, under thrust wedges which pile up, one against another. The wedges progressively steepen in dip and are pushed upwards into the newly deposited sediments overlying them, which in their turn form folds dropped over the wedge crests. In many places groups of wedges may be pushed up to form structural highs, which trap younger sediments in Fore-arc Basins.
As the oceanic plate plunges deeper, it gets melted, the partial melting of mixture of basaltic rocks and sediments generates Andesitic magma and occasionally granite. Most of the rising magma will be emplaced in the overlying continental crust when it will cool and crystalise at a depth of several hundred km forming Batholiths. Some times magma also come out from numerous and occasionally explosive volcanic eruptions. Example where Nazca plate subducts, the molten material thrown up by Cotopaxi.
Continent – Continent Collision:
Where two continental blocks collide the sediments between them may be pilled up into a mountain, as in the Himalayas.
Continental collision during the Cenozoic Era has occurred along the great tectonic line forming the southern edge of the Eurasian plate. This line of collision extends from the Himalayan Ranges in the east, through the Zagros mountains of Iran in the center, through Turkey, the European alps and the Atlas mountain in the west.
As the ocean basin between converging continental blocks narrows, the sediments of marginal Geocynclines and deformed. As the oceanic basaltic crust is broken into thrust faulted blocks which are pushed up into the sediments. The thrust blocks ride up one over another is an imbricate pattern. The upper part of each thrust sheet of sediment bends over, under the force of gravity, into a horizontal position and the strate from very elongated flat lying folds called Nappes.
Erosion of the rising thrust sheets produces fine – grained sediment which may accumulated in narrow ocean troughs, during the early stages of collision, as Flysch. Flysch consists of shale and fine – grained sandstone which may become incorporated with the late stages of over – thrusting and into the younger nappes. At the late stages of formation of the orogeny the high mountains release coarse sands and gravel which are spread across the marginal continental foreland or are deposited in shallow lakes as sheet of thick Molasse.
The alpine – Himalayan chain was formed in this manner. The Advancing Indian plate enclosed the Tethys sea and crumpled its sediment into a huge mountain mass.
Conservative Plate Boundary:
In this margin, the plates slide past each other without the formation of new crust. This boundary is, thus, also called Transform fault boundary.