The Earth’s crust is not continuous, in one piece — it’s cracked into larger pieces called tectonic plates. Tectonic plates are basically the large sections of Earth’s lithosphere that cover the planet’s surface. These plates seem stable but they are in perpetual motion, driven by the heat from the planet’s interior.
Their interactions at the boundaries lead to several geological phenomena, and this article focuses on one such interaction: convergence.
What’s a convergent boundary?
In some parts of the planet, plates are pushing toward one another. In others, they are pushing away from each other. Some tectonic plates are also sliding past one another, as is the case in San Andreas.
A convergent boundary is where two tectonic plates move toward each other, often causing one plate to slide under the other — but not always. When a plate slides under another, the process is called subduction and typically leads to plates bending down in the seafloor trench.
Sometimes, convergent plates can also lead to significant geological activity and changes, such as the formation of mountain ranges and the generation of earthquakes. In fact, the Earth’s largest mountains formed as a result of tectonic collision.
Convergent boundaries, therefore, play an important role in shaping the Earth’s landscape and contribute to its dynamic geology. But that’s just the short version.
Introduction to Tectonic Plates
Plate tectonics is a relatively new theory — it’s not even a century old. Heck, it’s half a century newer than general relativity.
The theory that revolutionized our understanding of Earth’s geology, came to prominence in the mid-20th century. Building upon the earlier concept of continental drift proposed by Alfred Wegener in 1912, the theory of plate tectonics offers a comprehensive explanation for many of Earth’s geological phenomena.
The theory says that the Earth’s outermost layer, the lithosphere, is not a single, unbroken shell. Rather, it is divided into numerous large slabs called tectonic plates. These plates are in constant motion, gliding over the semi-fluid layer of the mantle beneath them. It is at the boundaries of these plates, where they interact with each other, that the most dramatic geological activities occur.
The understanding and acceptance of the plate tectonics theory was a process that evolved over decades. Early 20th-century geologists noted how the eastern coast of South America and the western coast of Africa seemed almost to fit together like puzzle pieces. This observation led to Wegener’s hypothesis of continental drift, which suggested that the continents were once a supercontinent named Pangaea that had drifted apart over time.
But the mechanism behind plate tectonics wasn’t well understood, and many geologists were skeptical.
It wasn’t until the mid-20th century, with advancements in seafloor exploration and the discovery of symmetrical patterns of magnetic reversals on either side of mid-ocean ridges, that Wegener’s hypothesis morphed into the theory of plate tectonics. These findings showed that new ocean floor was being created at mid-ocean ridges and being consumed in subduction zones, providing the mechanism for continental drift that Wegener’s theory lacked.
Convergent Boundaries: A Meeting of Plates
There are three main types of tectonic plate boundaries, each characterized by the relative motion of the plates involved: convergent boundaries, divergent boundaries, and transform boundaries.
Convergent Boundaries
This occurs when two plates move towards each other. If both are of similar density, as with two continental plates, they typically push up against each other, forming mountains. An example of this is the Himalayas, resulting from the convergence of the Indian and Eurasian plates.
If one plate is denser than the other, as is the case when an oceanic plate meets a continental plate, the denser plate subducts, or slides under the less dense plate. This process can form deep-sea trenches and volcanic mountain ranges, such as the Andes in South America.
This movement is not a gentle approach but a forceful collision that occurs over millions of years. The results of this slow, yet immensely powerful process, manifest in different ways depending on the type of plates involved.
Divergent Boundaries
These boundaries are characterized by two plates moving away from each other. When this occurs on the ocean floor, it results in seafloor spreading, a process that creates new oceanic crust and mid-ocean ridges. The Mid-Atlantic Ridge is a classic example of this type of boundary.
Divergent boundaries can also occur on continents, leading to the formation of rift valleys like the East African Rift Valley.
Transform Boundaries
At these boundaries, two plates slide horizontally past each other. The movement at these boundaries is usually fairly steady, but it’s not always smooth and can sometimes lock up, causing stress to build over time. When this stress is released, it results in earthquakes.
The most famous example of a transform boundary is the San Andreas Fault in California.
Continental Convergence: Formation of Mountains
Of course, geology is rarely straightforward, and there are multiple types of convergent boundaries. This largely depends on the type of crust involved. Specifically, oceanic crust is typically denser than continental crust, and this difference in density affects how different plates interact.
Continental convergence occurs when two tectonic plates carrying continental crust move toward each other. Unlike oceanic crust, which is denser and can sink into the mantle in a process called subduction, continental crust is less dense and does not subduct. Instead, when two continental plates converge, they push against each other, causing the crust to deform and crumple.
Think of it like pushing two pieces of carpet together. They won’t sink or go beneath one another, but crumple up. This is precisely what happens during continental convergence, but on a much larger and slower scale. Now imagine what happens if instead of two pieces of carpet you have two continents.
The result of this collision is the creation of mountain ranges. The rock at the boundary is forced upwards, sometimes reaching kilometers and kilometers high as tectonic forces continue to push.
One of the most spectacular examples of this process is the Himalayan Mountain Range, formed by the ongoing collision between the Indian Plate and the Eurasian Plate. This convergence is still taking place today, meaning the Himalayas are still rising, albeit at a rate of just a few millimeters per year. However, they are also being eroded, at a rather similar rate — meaning the height of the Himalayas remains rather stable.
Examples of continental convergence:
1. The Himalayas: As mentioned before, the Himalayas are the result of a collision between the Indian Plate and the Eurasian Plate. This ongoing collision started about 50 million years ago and continues to this day, resulting in the world’s highest mountain range, which includes Mount Everest, the highest peak on Earth above sea level.
2. The Alps: The Alps, stretching across eight European countries, were formed as a result of the collision between the African and Eurasian Plates. This process started around 30-40 million years ago.
3. The Appalachians in North America: While the Appalachian Mountains are now eroding, they were once similar in height to the Himalayas. These mountains were formed around 300 million years ago due to the collision of North America with Africa during the formation of the supercontinent Pangaea.
4. The Urals: The Ural Mountain range, which primarily runs through western Russia, is the result of a collision between the Siberian and Baltica plates about 300-250 million years ago. The Urals are considered the boundary between Europe and Asia.
Oceanic Convergence: The Birth of Trenches
Oceanic convergence occurs when two tectonic plates, at least one of which carries oceanic crust, move towards each other. This interaction involves the process of subduction, in which one plate, typically the denser oceanic one, descends beneath the other into the mantle, the layer beneath the Earth’s crust.
If both converging plates are oceanic, the older, denser plate usually subducts beneath the younger, less dense one. An example is the boundary where the Pacific Plate is subducting beneath the Philippine Sea Plate, creating the Mariana Trench, the deepest part of the world’s oceans.
When an oceanic plate converges with a continental plate, the denser oceanic plate is the one that subducts. As it descends, it carries water into the mantle, which lowers the melting point of the mantle rock, causing it to melt and form magma. This magma can rise to the surface, leading to volcanic activity in the overlying continental crust. This process forms volcanic arcs, such as the Andes in South America, where the Nazca Plate is subducting beneath the South American Plate.
Therefore, oceanic convergence results in some of the most distinctive features of Earth’s geology, including deep-sea trenches, volcanic arcs, and related seismic activity. This dynamic process plays a crucial role in the recycling of Earth’s crust, as the subducted oceanic crust eventually melts and may resurface as volcanic material.
Examples of oceanic convergence
1. The Mariana Trench: As mentioned previously, the Mariana Trench in the western Pacific Ocean is the deepest point in the world’s oceans. It’s formed by the convergence of the Pacific Plate and the Philippine Sea Plate, both of which are oceanic plates.
2. The Andes Mountain Range: The Andes, stretching along the western edge of South America, are an example of oceanic-continental convergence. Here, the Nazca Plate (an oceanic plate) is subducting beneath the South American Plate (a continental plate). The subduction of the Nazca Plate has led to the uplift of the Andes and numerous volcanic eruptions.
3. The Aleutian Islands: Located off the coast of Alaska, the Aleutian Islands are formed due to the subduction of the Pacific Plate under the North American Plate. This has led to the creation of a volcanic island arc, a common feature in oceanic-oceanic convergence.
4. The Japan Trench: Another example of oceanic-oceanic convergence is the Japan Trench in the Pacific Ocean. Here, the Pacific Plate is subducting beneath the Okhotsk Plate. This subduction zone is associated with frequent earthquakes and the formation of the Japanese island arc.
Seismic Activity: Earthquakes and Convergent Boundaries
Seismic activity, which includes earthquakes and volcanic eruptions, is a common feature at convergent boundaries due to the immense forces and movements involved.
When two plates converge, the process isn’t always smooth. The plates may become locked together, unable to easily slide past or beneath each other due to friction. During this locking period, stress gradually accumulates within the rock. Over time, the stress may exceed the strength of the rock, causing it to fracture and release the stored energy as seismic waves. These waves travel through the Earth’s crust, causing the ground to shake in an event we perceive as an earthquake.
The depth of these earthquakes can vary significantly at convergent boundaries, depending on the exact location of stress release along the subducting plate. Near the surface, shallow-focus earthquakes can occur. However, as the subducting plate descends deeper into the mantle, it can also generate intermediate and deep-focus earthquakes. This wide range of earthquake depths is a distinguishing feature of convergent boundaries.
Volcanic activity at convergent boundaries is usually associated with subduction. When an oceanic plate subducts beneath a continental plate, it carries water and other volatiles into the mantle. The addition of these substances lowers the melting point of the mantle rock, leading to the formation of magma. This magma is less dense than the surrounding rock, causing it to rise toward the surface and potentially lead to volcanic eruptions.
Examples of seismic activity at convergent boundaries are plentiful, including the frequent earthquakes along the Pacific Ring of Fire and the volcanic activity in the Andes. Thus, while these processes can be destructive, they are also an integral part of Earth’s dynamic geology.
The Transformative Power of Convergent Boundaries
Convergent tectonic boundaries are a primary force in shaping Earth’s landscape. Their influence is visible in the majestic mountain ranges, deep ocean trenches, and seismic activity we observe. Understanding these geological processes provides valuable insights into the Earth’s past, present, and future geological transformations.
