12.4 Hot Spots, Folding and Faulting, and Mountain Building

Describe landforms that result from tectonic processes away from plate boundaries.

There are two tectonic processes that do not fit into the three types of plate boundary interactions discussed in the previous section, but are nevertheless important in building landforms: hot spots and folding and faulting. In this section we describe those processes, and we summarize the tectonic processes that result in mountain building.

Hot Spots

So far, the types of volcanoes that we have discussed all occur at plate margins where divergence or subduction is occurring. But not all volcanoes occur on plate boundaries. There are some 30,000 islands in the Pacific Ocean, and most of them are volcanic islands that formed far from plate boundaries. Hawai‘i, Easter Island, Tahiti, and the Galápagos Islands are a few examples of non-plate boundary volcanic islands. In the Atlantic Ocean, the Canary Islands and the Cape Verde Islands are also volcanic in origin but are not found directly on plate boundaries.

Volcanic islands such as these are the result of geologic hot spots. A hot spot is a location at the base of the lithosphere where high temperatures cause the overlying crust to melt. A hot spot results from a mantle plume, a mostly stationary column of hot solid rock that extends from deep in the mantle up to the base of the lithosphere (Figure 12.25).

hot spot

A location at the base of the lithosphere where high temperatures cause the overlying crust to melt.

mantle plume

A mostly stationary column of hot rock that extends from deep in the mantle up to the base of the lithosphere.

Figure 12.25

A hot spot. A plume of hot solid rock anchored deep in the mantle slowly rises and melts the base of the lithosphere. The resulting magma then rises up through the crust. In oceanic crust, it creates submarine lava flows that build oceanic islands once they rise above sea level.

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Mantle plumes are incompletely understood. They are thought to be rooted in Earth’s outer core, to remain mostly stationary, and to rise at a rate of several centimeters per year. The plumes are not liquid, but rather slowly deforming rock. As a hot spot approaches the lower lithosphere, a decrease in pressure lowers the rock’s melting point and allows it to melt into magma. The buoyant magma melts its way through the overlying crust, where it forms a volcano. As a lithospheric plate moves over a hot spot, a line of volcanoes may be formed (Figure 12.26).

Figure 12.26

Island formation at a hot spot. (A) As the overlying plate moves over a stationary hot spot, it creates a chain of volcanoes. Old volcanoes that have moved off the hot spot become extinct and are eroded and diminished in size. Eventually, the inactive volcanoes are moved into deeper water, where they become flat-topped seamounts. (B) The Hawaiian Islands were formed by a stationary hot spot. As the Pacific plate moves over the hot spot, new islands are formed and old islands are moved into deeper water. The maximum ages of the islands are given in millions of years (Ma). A new island, named Lo‘ihi, is forming and will rise above the sea in about 10,000 years.

Animation

Hawai’i formation

http://qrs.ly/a83w0pv

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If you look carefully at a bathymetric map of the world’s ocean basins, you will find many linear ridges that look as if a giant knife had made long incisions in the crust. Volcanic islands and submerged seamounts are often associated with these linear ridges. Hot spots melt through the crust as the plates move over them, creating these lines of inactive volcanoes, called hot spot tracks (Figure 12.27).

Figure 12.27

Hot spot tracks. Hot spot tracks are found throughout the world’s ocean basins. Most of their volcanic mountains are no longer active and are submerged beneath the surface of the oceans. On this map hot spot tracks are shown in red. Red dots show the location of volcanoes that sit atop hot spots.

An active continental hot spot persists beneath the North American plate at the location of Yellowstone National Park. This hot spot has produced a number of volcanic eruptions, including one of Earth‘s largest known eruptions 640,000 years ago (see Section 14.2). It also produced a string of volcanic landforms called calderas (collapsed volcanoes) (Figure 12.28). Some scientists think that the magma body that resides just below Yellowstone National Park today could erupt again.

Figure 12.28

Yellowstone hot spot. (A) Some 16 million years ago, the oldest of the extinct volcanoes on the Yellowstone hot spot track, McDermitt Caldera, was located over the hot spot. The southwestern direction of plate movement has transported McDermitt Caldera, and the other now-extinct volcanoes produced by the Yellowstone hot spot, to the southwest. The ages of the calderas are given in millions of years (Ma). Yellowstone’s hot springs and geysers, such as the Grand Prismatic Spring (B) and Old Faithful (C), are the active result of the active magma body that resides beneath the park.
(B. © Justin Reznick/E+/Getty Images; C. U.S. Geological Survey)

Bending and Breaking: Folding and Faulting

When hit with a hammer, rocks break into smaller fragments because they are brittle. When they are slowly compressed and heated, however, rocks in many cases are able to deform and fold (bend) before they fault (break). A fold is a wrinkle in the crust resulting from deformation caused by geologic stress, and a fault is a fracture in the crust where movement and earthquakes occur.

fold

A wrinkle in the crust resulting from deformation caused by geologic stress.

fault

A fracture in the crust where movement and earthquakes occur.

Folding and faulting can occur anywhere on Earth’s surface. Generally, folding occurs most often where two plates are converging, particularly in regions of subduction and collision. Faulting, too, occurs most often near plate boundaries, but many regions far away from plate boundaries are also faulted. Folding and faulting are particularly important elements of lithospheric plate movement because each can produce prominent surface landforms.

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Folded Landforms

Many mountain ranges are the result of folding, which can occur anywhere compressional forces are pushing the crust together. Folds may also occur at just about any spatial scale, from centimeters to kilometers, and in nearly any rock type, as shown in Figure 12.29.

Figure 12.29

Folding. (A) These folded volcanic rocks are located in Hamersley Gorge, Karijini National Park, Australia. The distance across this photo is only a few tens of meters. (B) A satellite image of kilometers-long folds in sedimentary rocks in the Béchar Basin, northwestern Algeria. These rocks were folded in a continental collision between the African and Eurasian plates beginning about 65 million years ago. The ground distance of this satellite image is approximately 100 km (60 mi) across.
(A. © Ignacio Palacios/Lonely Planet Images/Getty Images; B. Image courtesy of the Image Science & Analysis Laboratory, NASA Johnson Space Center)

The two major types of folds are anticlines and synclines. An anticline is a fold in the crust with an archlike ridge, and a syncline is a fold in the crust with a U-shaped dip. Generally, when synclines and anticlines occur across a wide region, ridges form on the anticlines and valleys form on the synclines (Figure 12.30).

Figure 12.30

Anticlinal ridges and synclinal valleys. (A) Like a rug left rumpled on the floor when its two ends have been pushed toward each other, Earth’s crust deforms into synclinal and anticlinal folds when it is compressed. (B) An anticlinal ridge and a synclinal valley are visible in this satellite image of the Zagros Mountains of western Iran. These folds are a result of collision between the Arabian and Eurasian plates.
(B. EROS Center, U.S. Geological Survey)

anticline

A fold in the crust with an archlike ridge.

syncline

A fold in the crust with a U-shaped dip.

When a folded surface is eroded by streams, softer rock is often removed more quickly than harder rock, leaving the more resistant rock to form a ridge. This process can result in an inverted topography, with synclinal ridges and anticlinal valleys (Figure 12.31).

Figure 12.31

GEO-GRAPHIC: Inverted topography. (A) Erosion of anticlinal ridges can result in anticlinal valleys. Similarly, layers of sedimentary rock that are resistant to erosion may form synclinal ridges. (B) This roadcut exposes a synclinal ridge near Hancock, Maryland.
(B. © Mark Burnett/Photo Researchers/Getty Images)

Block Landforms

Faulting results when rocks can deform no further by folding, and they break. When this happens, the energy stored in the rocks is released and travels through the crust as seismic waves that shake the ground in an earthquake. (Faulting and earthquakes are covered in greater detail in Section 14.3.)

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Through time, continued faulting can create blocks of crust that move vertically relative to each other. These blocks are called fault blocks. Like folds, fault blocks occur across many spatial scales and in a variety of tectonic settings. There are many fault-block mountain ranges in the world, some of which have dramatic relief (Figure 12.32).

Figure 12.32

The Teton Range, Wyoming. (A) This diagram illustrates the fault block system that formed the Teton Range. The range formed when a 65 km (40 mi) long fault block was lowered, beginning about 13 million years ago. Erosion by streams and glaciers subsequently cut into the higher block, removing most of the overlying rocks and creating the jagged mountain topography seen today. (B) Grand Teton, standing at 4,197 m (13,770 ft) and visible here, is the highest peak in the Teton Range.
(B. © John Wang/Photodisc/Getty Images)

Orogenesis: Tectonic Settings of Mountains

How do mountains form?

All of Earth’s mountain ranges were formed by different types of plate movement, although some do not lie at plate boundaries.

The building of mountain ranges by any tectonic process is called orogenesis (from the Greek oros, “mountain,” and genesis, “origin”). Most mountains are grouped together to form linear ranges. These ranges, called orogenic belts, form most commonly along plate boundaries, particularly in areas of collision and subduction. Away from plate boundaries, isolated mountains and orogenic belts form in areas of rifting, hot spot tracks, and folding and faulting. Figure 12.33 summarizes the tectonic settings and processes covered in this chapter and their roles in orogenesis.

orogenesis

Mountain building.

orogenic belt

A linear mountain range.

Figure 12.33

Mountain ranges. (A) This table summarizes the tectonic settings of mountain ranges and specifies whether the setting produces volcanically active mountains. (B) Earth’s 10 longest terrestrial mountain ranges are labeled here. Red and orange areas show the highest surface elevations. The accompanying table orders the ranges by their length and indicates their tectonic setting.

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