4.4: Convergent Plate Boundaries

At two of the three types of convergent plate boundaries, lithosphere is downwelled (subducted) at a subduction zone. Lithosphere (sediments, oceanic crust, and solid upper mantle layer) that enters a subduction zone was formed at an oceanic ridge, generally millions of years earlier (up to about 170 million years ago). Lithosphere slowly cools during the millions of years it travels horizontally at the top of the mantle away from where it was formed. As it cools, its density increases so that, by the time it enters a subduction zone, its density exceeds that of the mantle material beneath it and it can sink into (and through) the asthenosphere.

Why does the lithosphere sink through the asthenosphere only at subduction zones where two plates meet? It would seem that it should sink sooner, since, before it reaches the subduction zone, its density is already higher than that of the underlying asthenosphere. There are two reasons it does not sink sooner. First, the difference in density between lithosphere supporting old, cool oceanic crust and the warmer asthenosphere below is very small. Second, the resistance to flow is high because the lithosphere material is stretched out as a flat plate across the top of the asthenosphere (like a canoe paddle that meets strong water resistance when held flat for the thrust stroke but slices through the water easily when turned sideways). In a subduction zone, the edge of the subducting plate is bent downward (equivalent to turning the paddle blade sideways), thus allowing it to sink much more easily.

On today’s Earth, most subduction zones surround the Pacific Ocean (Fig. 4-11). At each of these subduction zones, it is the Pacific Plate that is being subducted. The Pacific Ocean floor is being destroyed by subduction faster than new seafloor is created at its ridges, so the Pacific Ocean is becoming smaller. In contrast, the Atlantic and Indian Oceans are becoming larger. The oldest remaining seafloor sediment found in the Pacific Ocean (170 million years old) was retrieved from a hole drilled south of Japan by the International Ocean Drilling Program. All Pacific Ocean oceanic crust that existed prior to 170 million years ago, and the associated sediment, are believed to have entered subduction zones where they were either destroyed or added to the edges of continents. Most of the ocean floor is younger than the Pacific Ocean so almost no oceanic crust older than 170 million years old still exists on Earth because almost all older crust has entered subduction zones and been destroyed. The oldest oceanic crust so far discovered is in a small area called the Herodotus Basin between Cyprus, Crete and Egypt in the Mediterranean Sea dating to about 340 million years old. This area is thought to be a fragment of the Tethys Sea that once bordered the supercontinent Pangea (Fig. 4-9a).

Subduction Zones Next to Continents

The subduction zones that are located along the coastlines of continents mark convergent plate boundaries where a plate that has oceanic crust at its edge converges with a plate that has continental crust at its edge (Fig. 4-12). As the two plates move toward each other, the lithosphere of the plate supporting oceanic crust is thrust beneath the plate that supports the less dense continental crust. As the oceanic crust sinks beneath the continental crust, a subduction zone is formed with a characteristic deep trench parallel to the coast.

Figure 4-12. Subduction zone at a continental margin. A plate with oceanic crust is subducted beneath a plate with continental crust. This process forms a trench, a chain of volcanoes near the edge of the continent, and a chain of coastal hills or mountains, which are formed by compaction of the continental crust edge and accretion of compacted sediments and sedimentary rock from the subducting plate.

The edge of a continent that is carried into the plate boundary at a subduction zone is squeezed and thickened, forming a chain of coastal mountains along the edge of the continent. Because ocean sediments and sedimentary rocks have lower densities, like the continental rocks from which much of the sediment originated (Chap. 8), they are relatively buoyant. As a result, these materials tend to be “scraped off” the oceanic plate as they are dragged downward into the subduction zone rather than subducted with it. The sedimentary materials are further compressed, folded, and lifted as more such material accumulates from continuing subduction of additional oceanic crust. Thus, some of the marine sediments from the subducting oceanic crust are collected in the subduction zone and contribute to the formation of the coastal mountains. This process explains why Darwin found marine fossils high in the Andes Mountains (Chap. 2).

Volcanoes

Another process that occurs at subduction zones is the formation of a line of volcanoes on the continental crust located a few tens or hundreds of kilometers inland from the plate boundary. The volcanoes are formed when oceanic crust and some associated sedimentary material are subducted beneath the edge of the continental plate. As this oceanic crust and sediment are subducted deeper into the Earth, they are heated by the friction of their movement, by the hotter mantle material below, and by increasing pressure. This causes water and other volatile constituents to be released by the subducting plate and its associated sedimentary material as fluids, which migrate upwards and combine with the mantle material above the plate. This causes physical and chemical changes in the mantle material, lowering its melting point, and it melts to form magma with a high concentration of water and other volatile constituents. The resulting magma rises through the overlying continental crust and toward Earth’s surface, where it erupts to form volcanoes near the edge of the continental plate (Fig. 4-13a,c,d). The erupting magma is rich in silica, which makes it more viscous (resistant to flow). As the magma erupts, pressure decreases, and water and other volatile constituents become gaseous and expand rapidly. The result is that these eruptions are often explosive, ejecting large amounts of ash and other volcanic material. 

Figure 4-13. (a) Mt. St. Helens, in Washington State, is a subduction zone volcano that erupted violently in 1980. The top of the mountain was blown away during the eruption, leaving behind a large crater. This explosive eruption released about 24 megatons of thermal energy and ejected about 0.1 km³ of ash. (b) The eruption of Mt. Pinatubo in the Philippines in 1991 was the largest volcanic eruption worldwide between 1912 and 2022 when the underwater Hunga Tonga eruption, shown here, was of similar magnitude. The Mt. Toba eruption 73,000 years ago was very much larger: Mt. Pinatubo erupted about 4 km³ of ash, Toba produced at least 800 km³. (c) Mt. St. Helens, May 1980. (d) Mt. Saint Helens September 10th, 1980.

Most active subduction zones in which the two plates have continental and oceanic crust at their respective edges are in the Pacific Ocean, and therefore, they are often called “Pacific-type margins.” Such margins include the west coast of Central and South America, the west coast of North America from northern California to southern Canada, and the coast of South-central Alaska. These areas have a well-developed ocean trench, coastal mountain range, and inland chain of volcanoes that characterize Pacific-type plate margins (Fig. 4-14).

Figure 4-14. Stretches of the west coasts of Mexico, California, Oregon, Washington, Canada, and Alaska are characterized by trenches and continental subduction zone volcanoes. The active volcanoes associated with the subduction of the Juan de Fuca and Gorda Plates under the North American Plate do not extend farther south than Mount Lassen in northern California. To the south, the San Andreas Fault, which can be traced from Cape Mendocino through southern California and into northern Mexico, is a transform fault and not a subduction zone. Continuing southward, active volcanoes are again found in central Mexico, where the transform faults of California end and the subduction zone between the Cocos Plate and the North American Plate begins.

Exotic Terranes

Throughout the oceans are small areas where the seafloor is raised a kilometer or more above the surrounding oceanic crust. Called oceanic plateaus, these areas constitute about 3% of the ocean floor (Fig. 4-11). Some oceanic plateaus are extinct seafloor volcanoes, others are old volcanic ridges, and still others are fragments of continental crust called “microcontinents.” Small extinct undersea volcanoes can be broken up and subducted with oceanic crust. However, the larger oceanic plateaus are too thick to be subducted. Therefore, when sections of oceanic plateau enter a subduction zone, their rocks become welded onto the edge of the continent to form exotic terranes (Fig. 4-15). Exotic terranes may also be formed when islands, including those originally formed at magmatic arc subduction zones (see the next section), are carried into a subduction zone where they impact the edge of a continent. Much of the Pacific coast of North America consists of exotic terranes.

Figure 4-15. Formation of exotic terranes. Oceanic plateaus, inactive oceanic ridges, and volcanoes are scraped off the oceanic crust as it enters a subduction zone. The scraped-off material forms new continental crust that is welded onto the edge of the continent and is called an exotic terrain.

Subduction Zones at Magmatic Arcs

At oceanic convergent plate boundaries where both plates have oceanic crust at their edges, a chain of volcanoes erupts magma to form an island chain parallel to the subduction zone. Because the most prominent examples of this type of plate boundary on the present-day Earth are curved (arced), these boundaries are called magmatic arcs (or “island arcs”). Processes that occur at such plate boundaries are similar to processes that occur at subduction zones at a continent’s edge. However, because no continental crust is present, the oceanic crust of one plate (which has higher density) is subducted beneath the oceanic crust of the other plate (which has lower density). Because oceanic crust cools with age, and density increases with decreasing temperatures, older oceanic crust has a higher density and is subducted beneath younger crust. For example, at the Aleutian Islands plate boundary, the older Pacific Plate is being subducted below the younger lithosphere of the Bering Sea portion of the North American Plate.

The subduction of oceanic crust at magmatic arcs forms a trench system that parallels the plate boundary (Fig. 4-16). Sedimentary materials from the subducting plate accumulate on the edge of the nonsubducting plate, just as they do at subduction zones at a continent’s edge. They may form a chain of low sedimentary arc islands joined by an underwater ridge called an “outer arc ridge.” Behind the ridge is an outer arc basin, an area of the non-subducting plate where the crust is affected little by the subduction processes.

Figure 4-16. At magmatic arc subduction zones, as the plate with the higher-density oceanic crust is subducted, a chain of volcanic islands (magmatic or island arc) is formed along the edge of the nonsubducting plate. Sometimes low sedimentary islands also form between the island arc and the trench. At some magmatic arcs, such as shown here, the oceanic plate subducts fast enough that the adjacent plate (shown with a continental crust edge in this example) does not move toward the plate boundary as fast as the subduction occurs. In this situation, a trench and island arc form outside a back-arc basin, which is created by stretching and thinning of the non-subducting plate edge (the continent edge in this example). The directions of plate motion shown are relative to each other. In some cases, the trench may migrate seaward, while the non-subducting plate migrates more slowly in the same direction so that a back-arc basin is still formed.

Most present-day magmatic arc subduction zones are around the Pacific Ocean. They include Indonesia (Fig. 4-17), the Mariana Islands, the Aleutian Islands, and Japan (Fig. 4-6). Each of these areas has the characteristic trench and magmatic arc, some have the low outer arc islands, and some have shallow, sediment-filled back-island basins, as described in the next section.

Figure 4-17. The outer arc ridge, volcanoes, and back-arc basins of Indonesia and the Philippines. The numerous active volcanoes on the major islands of Indonesia, including Sumatra and Java, are evidence of the very active nature of this oceanic convergence. The eruptions of Krakatau in 1883 and Tambora in 1815 were two of the largest eruptions of the past several centuries.

Magmatic Arc Volcanoes

On the nonsubducting plate of a magmatic arc subduction zone, a line of volcanic islands (the magmatic arc) forms parallel to the plate boundary (Fig. 4-16). The islands are constructed by rising magma, which is produced by the sinking, heating, and subsequent melting of subducted oceanic crust and mantle material. These volcanic islands are equivalent to the chain of volcanoes formed on the continental crust of subduction zones at the edges of continents. Like their continental counterparts, magmatic arc volcanoes often erupt explosively because subducted sediments and their associated water are also heated and erupted with the magma. One of the best known of these explosive eruptions is the 1883 eruption of Krakatau (Krakatoa) in Indonesia (Fig. 4-17). This eruption altered the Earth’s climate for several years afterward, and the eruption and resulting tsunami killed an estimated 36,000 people. More recently, in 2022, Hunga Tonga, a large volcano on the Tonga subduction zone whose peak was about 150 m underwater at the time, erupted explosively. The force of the eruption was hundreds of times more powerful than the atomic bomb dropped on Hiroshima in World War 2. The volcano ejected an estimated 10 km3 of rock ash and sediment and created a massive tsunami that caused damage thousands of kilometers away. Hunga Tonga also caused the largest explosive shock wave that has ever been recorded. This shock wave travelled around the world several times. The eruption also ejected a huge amount of water vapor to the atmosphere, equivalent to about 10% of all the water vapor in Earth’s atmosphere at any one time, or enough to fill 58,000 swimming pools. Fortunately, the explosion occurred in a sparsely populated area of the Pacific and caused only 6 known deaths. 

At all convergent plate boundaries, the distance between the plate boundary and the line of volcanoes is shorter where the subducting plate’s angle of descent is greater. Because the lithosphere cools and its density increases as it ages, older lithosphere tends to sink more steeply (faster) into the asthenosphere than younger lithosphere. Thus, the distance between the trench and the volcano chain is less where the subducting lithosphere is older.

Back-Arc Basins

Sometimes a subducted plate’s rate of destruction can exceed the rate at which the two plates are moving toward each other, particularly where old lithosphere is sinking steeply into the asthenosphere. Under these circumstances, the edge of the nonsubducted plate is stretched, which causes a thinning of the lithosphere at its edge. The thinning may create a back-arc basin (sometimes called a “back-island basin”) behind a magmatic arc (Fig. 4-16). In extreme cases, the lithosphere is stretched and thinned so much that magma rises from below to create new oceanic crust in the basin.

Back-arc basins are generally shallow seas with a floor consisting of large quantities of sediment eroded by wind and water from the newly formed mountains of the magmatic arc and from nearby continents. The Mariana Trench subduction system, where the Pacific Plate is being subducted beneath the Philippine Plate, provides an example of a back-arc basin. The back-arc basin lies between the Mariana Trench–Island subduction zone and the Philippine Trench. About 200 km to the west of the Mariana subduction zone is a now-inactive oceanic ridge that is the former location of back-arc spreading (Fig. 3-4).

Collisions of Continents

Continental collision plate boundaries form at the convergence of two plates that both have continental crust at their edges. When the lithosphere of one plate meets the edge of the other plate, neither is sufficiently dense to be dragged into the asthenosphere and subducted. Therefore, the continental crusts of the two plates are thrust up against each other. As the collision continues, more continental crust is thrust toward the plate boundary, and the two continents become compressed. One continent is generally thrust beneath the other, lifting it up. The forces created and the energy released by such a collision are truly immense, raising a high mountain chain along the plate boundary. The effect of such collisions is not unlike the effect of a head on collision of two cars, in which each car’s hood is compressed and crumpled upward, and one car may ride partially under the other.

Collision plate boundaries are relatively rare on the Earth today, but the geological record indicates that they were more common in the past, such as when Pangaea was formed from preexisting continents. The most prominent continental collision plate boundaries today are the ones between India and Asia and between Africa and Eurasia (Fig. 4-11). Older examples, formed before the present spreading cycle, include the Ural Mountains of Russia and the Appalachian Mountains of North America.

The ongoing India–Asia collision is a particularly vigorous collision. It began about 40 million years ago when the edges of the continental shelves of the two continents first collided (Fig. 4-18). The movement of India toward Asia before the collision was very fast in relation to the speed at which other plates are known to move. The high speed explains the extreme violence of the India–Asia collision, during which India has been thrust under the Asian continent, creating the Himalaya Mountains and compressing and lifting the vast high steppes of Asia. The Himalaya Mountains continue to be uplifted, and the many powerful earthquakes felt throughout the interior of China and Afghanistan are caused by earth movements that release the compressional stress continuously built up by the India–Asia collision. This collision may also be compressing and fracturing the Indo-Australian Plate across its middle, as its northern section is slowed by the collision and its southern section continues northward.

Figure 4-18. The continental collision boundary between India and Asia. About 50 million years ago, the northern margin of the Indian continental crust began a collision with the southern margin of the Asian continental crust. India is being thrust under the Asian continent as the collision continues. The Himalayas and the high Tibetan Plateau were both created by the collision. The tops of the Himalaya Mountains are formed from sedimentary rocks scraped off the oceanic crust of the ocean floor destroyed as the continents first came together.

A similar continental collision began about 200 million years ago between Africa and Eurasia. The convergence is slower than the India–Asia convergence, so the collision is less violent. Nevertheless, the Africa–Eurasia collision is responsible for building the Alps and for the numerous earthquakes that occur on the Balkan Peninsula (parts of Turkey, Greece, Albania, Bosnia and Herzegovina, Bulgaria, Croatia, Macedonia, Romania, Slovenia, Serbia, and Montenegro) and the adjacent region of Asia (parts of Turkey, Armenia, Georgia, Azerbaijan, and Iran).