13.4: Mountain Building

While some mountains are volcanoes and form when molten rock erupts on Earth’s surface, faulting and folding on a large scale also build mountains. In fact, the majority of mountain building on Earth is from folding and faulting rather than from volcanic eruptions.

The type of stress acting on rocks will determine whether folding will happen and what kind of faults will form. Plate tectonic processes are what determine the overall stresses acting in a region, and different processes occur in different tectonic settings. What this means in the big picture is that plate tectonic setting determines what kind of mountains will form.

Mountain Building Along Convergent Margins

Ocean-Ocean Collision

Volcanic island arcs are mountain chains made of volcanoes that form along subduction zones where two oceanic plate margins collide. The Kuril Islands (Figure 13.32; view in Google Earth) are an example. The Kuril-Kamchatka Trench marks a subduction zone where the Pacific Plate is being subducted beneath the Okhotsk Plate.

Figure 13.32 Kuril Islands, a volcanic island arc formed along a subduction zone. The Kuril-Kamchatka Trench marks where the Pacific Plate is subducting beneath the Okhotsk Plate. Source: Karla Panchuk (2018), CC BY-NC-SA. Map © 2016 Google, Data: SIO, NOAA, U.S. Navy, NGA, GEBCO, US Dept of State Geographer. Image Landsat/ Copernicus.

Ocean-Continent Collision

Continental volcanic arcs form along subduction zones where oceanic plate boundaries collide with continental plate boundaries and subduct beneath the continental boundaries. The Cascade Range (Figure 13.33) is an example of a continental volcanic arc. It has formed because of the subduction of the Juan de Fuca plate beneath the North American plate.

Figure 13.33 Cascade volcanic arc, a continental volcanic arc formed where the Juan de Fuca plate is subducting beneath the North American plate. Source: Karla Panchuk (2018), CC BY-NC-SA. Map © 2016 Google, Data: SIO, NOAA, U.S. Navy, NGA, GEBCO, US Dept of State Geographer. Image Landsat/ Copernicus.

While volcanoes are an important feature of these mountain chains, folding and faulting of continental rocks are also important. The process of producing long mountain chains when tectonic plates collide and deform is called orogeny. Mountain chains formed in this fashion are called orogens.

The Sierra Nevada mountain range in Utah and Nevada (view in Google Earth) was formed by an orogeny that began around 140 million years ago. The mountain range was built up by igneous intrusions and volcanic eruptions along a continental volcanic arc (Figure 13.34). The terrain was altered further inland as well. Sheets of rock were thrust on top of each other, and pushed inland along a detachment fault, similar to the example of the McConnell Thrust in Figure 13.29.

Figure 13.34 Orogeny in an ocean-continent collision zone. Mountains form from subduction zone volcanism, and from large sheets of rock that are thrust inland and folded. Materials accumulating on the leading edge of the continent in an accretionary wedge are eventually smashed onto the continent, adding to continental crust. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Ron Blakey, NAU Geology (n.d.). View source. Click for terms of use.

The continental crust flexed downward because of the weight of the mountains, and this formed a fore arc basin seaward of the new mountain range. Sediments accumulated within that basin. The leading edge of the continent also collected sediments and igneous rock scraped off the subducting plate, forming an accretionary wedge. Over time, the force of the collision would smash the basin sediments and the accretionary wedge against the continent, turning it into new continental crust.

Continent-Continent Collision

When two plate margins of continental crust collide, the mountain chain that forms is a result only of folding and faulting of rocks, not volcanism. The Himalayas (Figure 13.35) are an example of a mountain chain formed by continent-continent collision. The collision also raised the plateau to the north of the Himalaya mountain chain.

Figure 13.35 Himalayas, a mountain range formed by the collision of two continents. Source: Karla Panchuk (2018), CC BY-NC-SA. Satellite Photo © 2016 Google, Data: SIO, NOAA, U.S. Navy, NGA, GEBCO. Image Landsat/ Copernicus.

The Alleghenian Orogeny, which brought together North America and Africa in the process of forming Pangea, is an example of mountain building in a continent-continent collision zone. Before the continents came into contact with each other, mountain building on the eastern coast of North America would have involved deformation from an ocean-continent collision, as with Figure 13.34. But as subduction proceeded, the subducting plate drew Africa closer and closer to North America. The gap between the two continents began to close, and fill with sediments (Figure 13.36, top).

Figure 13.36 Orogeny by continent-continent collision. The formation of Pangea included the merging of Africa and North America. This closed an ocean basin and stopped subduction along the coast of North America. Volcanism ended with the closure of the ocean basin, but mountains continued to grow through folding and faulting. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Ron Blakey, NAU Geology (n.d.). View source. Click for terms of use.

While a subduction zone existed, the addition of water to the mantle permitted partial melting of mantle rocks, and thus volcanic activity. However, when the two continents collided, the subduction zone closed off and volcanism was no longer possible. As the continents smashed together, deep faults formed and stacked blocks of crust on top of each other. Old faults were reactivated. Rocks also began to shift along the boundary between an earlier orogen—the Taconic Orogen—and North America (Figure 13.36, bottom). When the continents had finally merged, Africa met North America along a suture zone with remnants of a continental volcanic arc on one side, and folded and faulted sedimentary rocks on the other.

Orogens Are Everywhere!

Orogens can be found around the world (Figure 13.37, in cyan). Orogens along the edges of continents or in oceans indicate places where mountain belts are forming along subduction zones. Orange zones are shields: regions where the ancient stable basement rock (called a craton), is exposed. Pink zones are platforms, where the craton is covered with sediments and sedimentary rocks. Where orogens are present in the midst of shields and platforms, this indicates a place where two continental plate margins have collided, and the plates merged to form a single large continent.

Figure 13.37 Global distribution of orogens, stable crust, and extended crust. Extended crust comprises regions where plates once began to thin and stretch. Basins are areas where tectonic forces have warped plates downward. Large igneous provinces are locations where sustained volcanic eruptions covered broad regions of the crust with lava. Source: U. S. Geological Survey (2005), Public Domain. View source.

What is classified as a craton depends on the timeframe under consideration. The regions in Figure 13.37 identified as shields and platforms are in fact comprised of crust stitched together by ancient orogens. The shading in Figure 13.38 shows what part of North America consists of cratons (stable crust) older than 2.5 billion years. The gaps between the cratons are filled by orogens that have stitched the cratons together. Even these ancient cratons consist of older pieces of crust sutured by orogens.

Figure 13.38 Fragments of North America made of crust older than 2.5 billion years. Source: Karla Panchuk (2018), CC BY-NC-SA. Based on Whitmeyer and Karlstrom (2007).

Mountain Building Along Divergent Margins

Where plates are thinning and separating, systems (groups) of normal faults form. Mountain ranges can form when faulted blocks tilt backward or are lifted relative to the surrounding rock. Blocks that are elevated compared to adjacent blocks can form another type of mountain, called a fault-block mountain. Fault block mountains formed in eastern North America when Pangea began to split up, and Africa pulled away from North America (Figure 13.39).

Figure 13.39 Fault-block mountains formed in a rift zone. Magma can move up along normal faults, resulting in igneous intrusions, or volcanic eruptions. Over time, valleys between elevated blocks will fill with sediment as the blocks erode. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Ron Blakey, NAU Geology (n.d.). View source. Click for terms of use.

Over time, elevated blocks erode, filling up valleys with sediment. The thinning of continental crust that occurs with rifting can decrease the pressure on mantle rocks enough to trigger partial melting. Magma can move up along the normal faults, forming igneous intrusions, or feeding volcanoes. The Palisades Sill in New York and New Jersey is a result of rift-zone magmatism. It is a cliff-like feature resulting from erosion that exposed the tip of a structure like the sills in Figure 13.39. Mountain building that combines faults and volcanic activity from decompression melting is currently happening in the East African Rift Zone.

Another location you can see fault-block mountains is in the Basin and Range Province in Nevada (Figure 13.40). Stretching of Earth’s crust has resulted in a series of normal faults in a horst and graben arrangement. (Horsts are fault blocks raised higher, and graben are blocks that fall lower). In the region shown in Figure 13.40, the dark green linear features are vegetation-covered horst mountains, and the tan colored features are graben valleys filled with sediment.

Figure 13.40 Fault block mountains in Nevada formed by a series of normal faults in a horst and graben configuration. In the photograph, horst mountains are dark green with vegetation, and graben valleys are tan colored from the accumulation of eroded sediments. Source: Karla Panchuk (2018), CC BY-NC-SA. Satellite Photo © 2016 Google, Data: LDEO-Columbia, NSF, NOAA. Image Landsat/ Copernicus. Normal Faults: U. S. Geological Survey, Public Domain. View source.

Intraplate Mountain Chains

Geologic features forming away from plate boundaries are referred to as intraplate (within-plate) features. Intraplate chains of volcanic mountains such as the Hawaiian-Emperor seamount chain are formed by hotspots, where a plume of hot mantle rock rises up and melts the overlying lithosphere (Figure 13.41). The plume remains in the same location, but the plate does not. A chain of volcanoes forms because as the plate moves, a new part of the plate is exposed to the plume.

Figure 13.41 Creation of the Hawai’ian islands by the Pacific Plate moving over a hotspot. A hotspot is a plume of hot mantle material that melts the lithosphere. Islands are older to the northwest as they get further from the hotspot. Source: Karla Panchuk (2021), CC BY-NC-SA. Adapted from U. S. Geological Survey (2006), Public Domain. View source.

References

Whitmeyer, S., & Karlstrom, K. (2007). Tectonic model for the Proterozoic growth of North America. Geosphere, 3(4), 220-259. DOI: 10.1130/GES00055.