What is a Convergent Boundary?

Background Knowledge

To answer the question “What Is A Convergent Boundary?”, we first need to understand the meaning of tectonic plates. The lithosphere of the Earth that includes the crust and the uppermost mantle is not continuous in nature but is subdivided into several pieces called tectonic plates. The tectonic plates are of two types: the oceanic plates, and the continental plates. The tectonic plates of the lithosphere are not static in nature but are constantly moving towards and away from each other. It was these tectonic movements that gave birth to the continents we see today after the continental plates broke away from the supercontinent of Pangaea.

What Is A Convergent Boundary? Where Are Convergent Boundaries Located?

Now that we know what tectonic plates mean, we can easily answer the question of what a convergent boundary is.

Convergent Boundary Definition:

Convergent boundaries, also referred to as destructive plate boundaries, are locations on the lithosphere where two or more tectonic plates move towards each other leading to high levels of tectonic activities.

What Happens At A Convergent Boundary?

Convergent boundaries are highly unstable areas of the lithosphere of the Earth. Some or all of these activities happen at the convergent boundaries: subduction of the denser plate underneath the less dense one, melting of parts of the subducted plates, plate collision, faulting and folding, crustal deformation, magma generation, volcanic eruptions, and earthquakes.

Three Types Of Convergent Boundaries

There are three types of convergent boundaries depending on the nature of the tectonic plates converging with each other. They are as follows:

Continental-Oceanic

When a continental and an oceanic plate collide at the convergent boundary of the two plates, subduction zones often develop. The thinner and denser oceanic plate usually subducts below the thicker and less dense continental plate. Deep oceanic trenches are produced in the process and the subduction activity might lead to earthquakes or volcanic eruptions. The formation of volcanoes at such sites is explained below as an answer to the question: How Do Volcanoes Form At Convergent Boundaries?

An example of this type of convergent boundary is the Washington-Oregon coastline of the US. Here the oceanic plate of Juan de Fuca is subducting beneath the North American continental plate that is moving in a westward direction. The tectonic activity at this convergent boundary has led to the formation of the volcanic Cascade Mountain Range.

Oceanic-Oceanic

A convergent boundary might also feature two oceanic plates. When two such plates approach each other, the older and hence the denser plate usually subducts beneath the other. This results in the creation of magma chambers at the subduction zones which might ultimately lead to volcanic eruptions and the formation of volcanic island chains in the ocean.

An example of such a boundary has been found between the Pacific and the Philippine oceanic plates that led to the formation of the Japanese islands.

Continental-Continental

At convergent boundaries that involve two continental plates, the plate tectonics is a bit more complex than in the other two types of convergent boundaries. Since two continental plates are colliding, subduction becomes questionable as the difference in density between the plates is usually quite low. Instead, subduction might happen to some extent if the heavier lithosphere below the crust might break free from it due to the forces of friction and pressure created at the convergent boundary. Such types of convergent boundaries are also subjected to extensive faulting and folding of the rocks within the two plates that are colliding with each other.

The Himalayas were born as a result of the collision between two continental plates (the Indo-Australian Plate and the Eurasian plate) at the convergent boundary between these plates. The process of folding at such convergent boundaries is explained below under the sub-heading: What Type Of Mountain Ranges Form At Convergent Plate Boundaries?

How Is Magma Generated Along Convergent Plate Boundaries?

At the subduction zone of a convergent boundary, the denser tectonic plate slides underneath the relatively less dense plate. As the plate slides to greater depths of about 100 km below the surface of the Earth, it comes into contact with the relatively hotter environment of the mantle. As the fluids are released from the subducting plate into the hot mantle below, the process of partial melting of the parts of the subducted plate and the sediments carried by it begins to take place. Thus, a viscous and hot liquid called magma is generated which then moves up through the vent between the two sliding tectonic plates.

How Do Volcanoes Form At Convergent Boundaries?

A volcano is basically a rupture in the crust of the Earth that allows the eruption of lava, volcanic ash, and gases from the magma chamber below the volcano. Volcanoes are formed at three locations on the crust of the Earth: at convergent boundaries, divergent boundaries, and hot spots.

When two tectonic plates approach each other, subduction zones might be created at the convergent boundary between these two tectonic plates. Usually the denser plate subducts under the other plate creating a deep oceanic trench. As the denser plate descends, it might enter depths where the higher temperature environment cause the materials making it up to partially melt. This partial melting leads to the formation of magma chambers (as mentioned above) where the magma produced is less dense than the surrounding mantle. The magma thus ascends through the crack between the two plates, melting and fracturing the plates as it moves upwards. If the magma chamber does not solidify before it reaches the surface, it might lead to a volcanic eruption at the surface, leading to the formation of a volcano.

The volcanoes located in the Pacific Ring of Fire and the Mount Etna volcano on the east coast of Sicily were formed in this manner.

What Type Of Mountain Ranges Form At Convergent Plate Boundaries?

Fold mountain ranges form at convergent plate boundaries. When tectonic movements cause two tectonic plates to approach each other at the convergent plate boundary, deposits of sedimentary and metamorphic rocks at such boundaries often crumple and fold to form mountains called fold mountains. The presence of mechanically vulnerable layers like a layer of salt in such deposits speeds up the process of folding.

The Himalayan range is a classic example of fold mountains formed in this manner. These mountains were formed as a result of the collision between the Eurasian Plate and the Indo-Australian Plate at the convergent boundary. The Andaman and Nicobar Islands located in the Indian Ocean and part of the territory of India were also formed in a similar manner.