Lithospheric Plate System: Convergent and Divergent Plate Boundaries

The Earth’s surface is composed of six major lithospheric plates’ viz. Pacific, American, Eurasian, African, Austral-Indian, and Antarctic. Apart from those, there are some lesser plates and sub plates also. The below graphics shows these Lithospheric Plates.

Some important notable observations about these plates are as follows:

• American plate includes most of the continental lithosphere of North and South America.
• Most part of the Eurasian plate is continental lithosphere, but it is fringed on the west and north by a belt of oceanic lithosphere.
• African Plate is also known as the Nubia Plate. It is a mix of continental and oceanic lithosphere.
• The great Pacific plate occupies much of the Pacific Ocean basin and consists almost entirely of oceanic lithosphere.
• The Antarctic plate is almost completely enclosed by a spreading plate boundary. This means that the other plates are moving away from the pole. The continent of Antarctica forms a central core of continental lithosphere completely surrounded by oceanic lithosphere.
• The Austral-Indian plate is mostly oceanic lithosphere but contains two cores of continental lithosphere– Australia and peninsular India. The recent studies show that they may be different parts of two different plates.

Plate Boundaries

The above discussed Lithospheric Plates are composed of lithosphere, about 100 km thick, that “float” on the plastic asthenosphere. While the continents do indeed appear to drift, they do so only because they are part of larger plates that float and move horizontally on the upper mantle asthenosphere. The plate boundaries can be identified because they are zones along which maximum earthquakes occur.  Plate interiors have much fewer earthquakes.

There are three types of plate boundaries:

• Convergent Plate Boundaries: where plates move toward each other.
• Divergent Plate boundaries: where plates move away from each other.
• Transform Plate Boundaries: where plates slide past one another.

Convergent Plate Boundaries

The convergent plate boundaries are also responsible for nearly 75% of Earth’s volcanoes.  There are following types of Convergent Boundaries:

Ocean-Ocean Convergent Plate Boundary

When two oceanic plates meet and collide against each other, the denser of the two plates is pulled under the other and is subducted. It descends into the asthenosphere, or upper mantle, where it will lead to the generation of new magma. Such boundary would be called an Ocean-ocean convergent plate boundary.

Please note that when one oceanic plate is subducted under the other, the resulting new magma is less dense than the surrounding rock. Therefore it easily rises and erupts on the seafloor, ultimately building a volcano or a volcanic island in the sea. Areas of ocean-ocean convergence are characterized by ocean trenches, seafloor volcanoes, and volcanic islands.

Island Volcanic Arc

At ocean-ocean convergent boundaries, the resulting body of many volcanoes is called an island volcanic arc. An island volcanic arc may include islands that develop in the sea from the build-up of volcanic rocks. Thus, Island volcanic arcs are a chain of islands and mountains that form on the overriding or non-subducting oceanic plate. Examples of such arcs are Japan, the Philippines, the Tonga Islands, the Aleutian Islands, and the West Indies Islands etc. All of them have developed parallel to the direction of subduction.

Ocean-Continental Convergent Boundary

Convergence of an oceanic plate with a continental plate is similar to ocean-ocean convergence and often results in the volcanic. When an oceanic plate collides with a continental plate, the oceanic plate is always pulled under and subducted because it is denser than the continental plate. When the oceanic plate is subducted under the continental plate, it leads to the generation of new magma, which upwells and forms volcanoes on the non-subducting plate, or the continental plate. Thus Volcanoes are common on Ocean-Continent Boundary also. At ocean-continent boundaries, the resulting body of volcanoes is called a continental volcanic arc. Continental volcanic arcs are chains of volcanoes found on the margin of the continent above a subduction zone at ocean-continent boundaries. The most visible example is Andes Mountains off the west coast of the U.S.

Here we should also note that Pacific Ring of Fire, where subduction is taking place at numerous trenches that border the continental shores, has  450 volcanoes, more than 75% of all the volcanoes on Earth. This makes plate convergence responsible for nearly all volcanic activity on Earth.

Continent-Continent Boundary

When the continent and continent converge, the crust at both the sides is too light and buoyant to be subducted, so neither plate is subducted in continent-continent convergent boundary. Both continental masses press against the other, and both become compressed and ultimately fused into a single block with a folded mountain belt forming between them.

• This is the type of activity is responsible for forming the Himalayas, and is still going on. The Himalayas are still growing, as we all know.
• Please note that due to intense pressure between the colliding plates, metamorphic rocks formation is common at such boundaries.
• Please also note that Volcanoes are not common at Continent-continent convergent boundaries because there is no subduction of plates. Subduction is prerequisite for formation of the new magma.

Divergent Plate Boundaries

The Continental Drift Theory says that all the continents were once joined together in one giant supercontinent called Pangaea. Because of plate tectonics, Pangaea broke apart and the continents began their slow migration to their present locations. The Atlantic Ocean opened up in between North America and the west coasts of Europe and Africa. The agent for causing this is the Mid-Atlantic Ridge, a divergent plate boundary, where two plates are rifting and moving away from each other.  Thus, divergent plate boundaries are places of extension stress, where the crust is being extended, thinned, and rifted.

In the convergent plate boundaries are the destructive plate boundaries where the crustal material is consumed at the subduction zones. However, the divergent plate boundaries are constructive boundaries because it leads to formation of new Lithosphere. The creation of the new crustal material takes place at mid-ocean ridges, where the oceanic crust is rifted open and magma wells up to fill the opening. The magma then hardens to form the igneous rocks that make up the oceanic crust. This is the mechanism which forms maximum amount of rock material on earth.

Comparison: Divergent and Convergent Plate Boundaries

Kindly note & remember the following points:

Continental Rift Zones

Please note that the divergent plate boundaries can also develop on the continents, and here, we name them as Continental Rift Zones. Most of the features of Oceanic Divergent boundaries are valid for them also such as thinned crust; normal faults; shallow earthquakes; basaltic volcanoes etc.

While the Continental Rift Zones develop, the earth is stretched and thinned, leading to development of a  small body of water. When the rifting keeps continuing, the body of water grows bigger to form a juvenile ocean. After millions of years of rifting, the body of water becomes a mature ocean with two separate continents on each side. Red Sea and Gulf of Aden is the best example of this phenomenon.

Transform Plate Boundary

Transform plate boundaries are places where two plates are sliding past each other. At these boundaries, the plates are neither compression nor extension stress, but are under shear stress. Then there is neither creation nor consumption of the lithospheric material. So, the transform plate boundaries are basically faults and nothing else.

The transform plate boundaries can cause horizontal displacement of hundreds of kilometers of land on the continents which results in several types of landscapes such as ridges and troughs. In oceans, transform plate boundaries are part of fracture zones. Earthquakes are most common at transform plate boundaries. Volcanoes rarely develop at transform plate boundaries because transform boundaries do not allow for the upwelling or new creation of magma.