Most major mountain belts are generated when one or more buoyant crustal fragments collide with a continental margin as a result of subduction. Whereas oceanic lithosphere, which is relatively dense, readily subducts, continental lithosphere contains significant amounts of low-density crustal rocks and is, therefore, too buoyant to be subducted deeply or permanently. Consequently, the arrival of a crustal fragment at a trench results in a collision between the two continental blocks.

Cordilleran-Type Mountain Building

A Cordilleran-type orogeny (episode of mountain building), named after the North American Cordillera, is associated with a Pacific-like ocean—in that, unlike the Atlantic, the Pacific may never close. The rapid rate of seafloor spreading in the Pacific basin is balanced by a high rate of subduction. In this setting, island arcs and small crustal fragments are often carried along until they collide with an active continental margin and accrete (join) onto it. This process of collision and accretion has generated many of the mountainous regions that rim the Pacific. These accreted blocks of crust are called terranes. Geologists use this term to describe any crustal fragment that consists of a distinct and recognizable series of rock formations and has been transported and accreted by plate tectonic processes. Notice that terrane is a different word from terrain; the two are pronounced the same, but terrain refers to the shape of the surface topography, or “lay of the land.”

The Nature of Terranes 

What is the nature of the crustal fragments that have become terranes? Some may have been microcontinents similar to the modern-day island of Madagascar, located east of Africa in the Indian Ocean. Many others were island arcs similar to Japan, the Philippines, and the Aleutian Islands. Still others may have been submerged oceanic plateaus created by massive submarine outpourings of basaltic lavas (Figure 1). More than  of these relatively small crustal fragments exist in the modern world.

Accretion and Orogenesis 

Small geologic structures, such as seamounts, are generally subducted along with the descending oceanic slab. However, thick sections of oceanic crust, such as the Ontong Java Plateau (which is almost as big as Alaska) or an island arc dominated by low-density andesitic igneous rocks, are too buoyant to subduct. In these situations, a collision between the crustal fragment and the continental margin occurs.

The sequence of orogenic events that occur when small crustal fragments reach a Cordilleran-type margin is shown in Figure 2. The upper crustal layers are “peeled” from the descending plate and thrust in relatively thin sheets onto the adjacent continental block (Figure 2A,B). Convergence does not generally end with the accretion of a crustal fragment. Rather, new subduction zones typically form seaward of the accreted terrane, and they can carry other island arcs or microcontinents toward a collision with the continental margin (Figure 2C,D). Each collision displaces earlier accreted terranes further inland, adding to the zone of deformation as well as to the thickness and lateral extent of the continental margin.

The North American Cordillera 

The correlation between mountain building and the accretion of crustal fragments was first developed in studies of the North American Cordillera (Figure 3). Researchers determined that some of the rocks in the orogenic belts of Alaska and British Columbia contained fossil and paleomagnetic evidence indicating that these strata previously lay much closer to the equator.

It is now known that many of the terranes that make up the North American Cordillera were scattered throughout the Pacific, like the island arcs and oceanic plateaus currently distributed in the western Pacific. During the breakup of Pangaea, the eastern portion of the Pacific basin (the Farallon plate) began to subduct under the western margin of North America. This activity resulted in many additions of crustal fragments along the entire Pacific margin of the continent—from Mexico’s Baja Peninsula to northern Alaska (refer to Figure 2). Geologists expect that many modern microcontinents will likewise be accreted to active continental margins surrounding the Pacific, producing new orogenic belts.

Alpine-Type Mountain Building:
Continental Collisions

Alpine-type orogenies are episodes of mountain building that occur where two continental masses collide. They are named after the Alps, which have been intensively studied for more than 200 years. Mountain belts formed by the closure of major ocean basins include the Himalayas, Appalachians, Urals, and Alps. Continental collisions result in the development of mountains characterized by laterally shortened and vertically thickened crust, achieved through deformation, such as folding and large-scale thrust faulting. Prior to the collision of the two large landmasses, this type of orogeny may also involve the accretion of smaller continental fragments or island arcs that occupied the ocean basin that once separated the two continental blocks.

The zone where two continents collide and are “welded” together is a suture. The same term can be used to describe the boundary between two adjacent accreted terranes. This portion of a mountain belt often preserves slivers of oceanic lithosphere that were trapped between the colliding plates. The unique structure of these pieces of oceanic lithosphere, called ophiolites, helps identify the collision boundary.

Next, we will take a closer look at two examples of collisional mountains: the Himalayas and the Appalachians. The Himalayas, Earth’s youngest collisional mountains, are still rising. By contrast, the Appalachians are a much older mountain belt, in which active mountain building ceased about 250 million years ago.

The Himalayas

The mountain-building episode that created the Himalayas began between 50 and 30 million years ago, when India began to collide with Asia. Prior to the breakup of Pangaea, India was located between Africa and Antarctica in the Southern Hemisphere. As Pangaea fragmented, India moved rapidly, geologically speaking, a few thousand kilometers in a northward direction.

The subduction zone that facilitated India’s northward migration was near the southern margin of Asia (Figure 4A). Continued subduction along Asia’s margin created an Andean-type plate margin that contained a well-developed continental volcanic arc and an accretionary wedge. India’s northern margin, on the other hand, was a passive continental margin consisting of a thick platform of shallow-water sediments and sedimentary rocks.

Geologists have determined that two or perhaps more small crustal fragments were positioned on the subducting plate somewhere between India and Asia. During the closing of the intervening ocean basin, a small crustal fragment, which now forms southern Tibet, reached the trench and was accreted to Asia. This event was followed by the docking of India itself.

As the intervening ocean basin was closing up, the more deformable materials on the continental margins of these landmasses became highly folded and faulted (Figure 4B). Two major thrust faults and many smaller ones sliced through the Indian crust. Subsequent motion along these thrust faults caused slices of the Indian crust to be stacked one upon the other. Today, these slices make up the bulk of the highest peaks in the Himalayas—many of which are capped by tropical marine limestones that formed along what was once the continental shelf.

The formation of the Himalayas was followed by a period of uplift that raised the Tibetan Plateau. Seismic evidence suggests that a portion of the Indian subcontinent was thrust beneath Tibet—a distance of perhaps 400 kilometers (about 250 miles). If this occurred, the added crustal thickness would account for the lofty landscape of southern Tibet, which has an average elevation of more than 4500 meters (about 14,800 feet), higher than the tallest mountain in the contiguous United States.

The collision with Asia slowed but did not stop the northward movement of India, which has since penetrated at least 2000 kilometers (about 1200 miles) into the mainland of Asia. Crustal shortening and thickening accommodated some of this motion. Much of the remaining penetration into Asia caused lateral displacement of large blocks of the Asian crust by a mechanism described as escape tectonics. As shown in Figure 5, when India continued its northward trek, parts of Asia were “squeezed” eastward, out of the collision zone, thus “escaping” the impacting India block. These displaced crustal blocks include much of Southeast Asia (the region between India and China) and sections of China.

Why was the interior of Asia deformed to such a large extent, while India has remained essentially intact? The answer lies in the nature of these diverse crustal blocks. Much of India is a continental shield composed mainly of old Precambrian rocks (Figure 6). This thick, cold slab of crustal material has been intact for more than  billion years and is mechanically strong as a result. By contrast, Southeast Asia was assembled more recently, from the collision of several smaller crustal fragments. Consequently, it is still relatively “warm and weak” from recent periods of mountain building (refer to Figure 5).

The Appalachians

The Appalachian Mountains provide great scenic beauty near the eastern margin of North America, from Alabama to Newfoundland. Mountain belts of similar origin that formed during the same period and were once contiguous are found in the British Isles, Scandinavia, northwestern Africa, and Greenland (Figure 7). The orogenies that generated this extensive mountain system lasted a few hundred million years and resulted in the assembly of the supercontinent Pangaea. Detailed studies of the Appalachians indicate that this mountain belt was the result of three distinct episodes of mountain building.

Our simplified overview begins roughly 750 million years ago, with the breakup of a supercontinent called Rodinia that predates Pangaea. Much like the breakup of Pangaea, this episode of continental rifting and seafloor spreading generated a new ocean between the rifted continental blocks. Located within this widening ocean basin was a microcontinent near the edge of ancestral Africa.

About 600 million years ago, for reasons geologists do not completely understand, plate motion changed dramatically, and this ancient ocean basin began to close. This led to the development of multiple subduction zones, and the stage was set for the three orogenic events that would lead to the collision of North America and Africa (Figure 8A).

Taconic Orogeny 

Around 450 million years ago, the marginal sea between the volcanic island arc and ancestral North America began to close. The collision that ensued, called the Taconic Orogeny, caused the volcanic arc along with ocean sediments located on the upper plate to be accreted to the edge of the larger continental block. The remnants of this volcanic arc and oceanic sediments are recognized today as the metamorphic rocks through much of the Appalachian mountain belt (Figure 8B). For example, schists beneath New York City and Washington, DC, formed at this time. In addition to this pervasive regional metamorphism, numerous magma bodies intruded the crustal rocks along the entire continental margin.

Acadian Orogeny 

A second episode of mountain building, called the Acadian Orogeny, occurred about 350 million years ago. The continued closing of the ancient ocean basin resulted in the collision of a microcontinent with North America (Figure 8C). This orogeny involved thrust faulting, metamorphism, and the intrusion of many large granite bodies. This event also added substantially to the width of North America, particularly in eastern New England.

Alleghanian Orogeny 

The final orogeny, called the Alleghanian Orogeny, occurred between  and  million years ago, when Africa collided with North America. This collision displaced material that was accreted earlier by as much as 250 kilometers (155 miles) toward the interior of North America. This event also displaced and further deformed the continental shelf sediments and sedimentary rocks that had once flanked the eastern margin of North America (Figure 8D). Today these folded and thrust-faulted sandstones, limestones, and shales make up the largely unmetamorphosed rocks of the Valley and Ridge Province (Figure 9). This structural signature of mountain building can be found as far inland as central Pennsylvania and West Virginia.

With the collision of Africa and North America, the young Appalachians, perhaps as majestic as the Himalayas, lay along the suture, in the interior of Pangaea. The tectonic forces that built the mountains ceased to drive them upward. Then, about 180 million years ago, the new supercontinent began to break into smaller fragments, a process that ultimately created the modern Atlantic Ocean. Because this new zone of rifting occurred east of the suture that formed when Africa and North America collided, remnants of Africa remain stuck to the North American plate (Figure 8E). The crust underlying Florida is an example of one of these remnants.

Other mountain ranges built from continental collisions include the Alps and the Urals. The Alps formed as Africa and several smaller crustal fragments collided with Europe during the closing of the Tethys Sea. Similarly, the Urals were deformed and uplifted during the assembly of Pangaea, when northern Europe and northern Asia collided, forming a major portion of Eurasia. Unlike the Appalachian belt, however, the Urals did not break apart again after their orogenesis.

• A terrane is a relatively small crustal fragment (microcontinent, volcanic island arc, or oceanic plateau) that has been carried by an oceanic plate to a continental subduction zone and then accreted onto the continental margin. The North American Cordillera formed by the accretion of many successive terranes.

• The Himalayas and Appalachians were formed by collisions between continents when the intervening ocean basin subducted completely. The Appalachians were caused by the collision of ancestral North America with ancestral Africa more than 250 million years ago. The Himalayas were formed by the collision of India and Eurasia starting around  million years ago, and they are still rising.

• microcontinents: Relatively small fragments of continental crust that may lie above sea level or may be submerged.

• suture: A zone along which two crustal fragments are joined together. For example, following a continental collision, the two continental blocks are sutured together.

• terranes: Crustal blocks bounded by faults, whose geologic history is distinct from the histories of adjoining crustal blocks.