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DISTRIBUTION OF VOLCANIC ACTIVITY

DISTRIBUTION OF VOLCANIC ACTIVITY

Most volcanoes are located at active plate boundaries called interplate volcanism. The prefix “inter-“ means “between.” In contrast, some volcanoes are not associated with plate boundaries, but rather are located within the plate far from plate boundaries. These are called intraplate volcanoes, and many are formed by hotspots and fissure eruptions. The prefix “intra-“ means “within.” The following discusses the location of volcanism in more detail with mid-ocean ridges, subduction zones, and continental rifts representing interplate volcanism, and hot spots representing intraplate volcanism.

VOLCANOES ALONG MID-OCEANIC RIDGES

Although most volcanism occurs on the ocean floor along the mid-ocean ridge (a type of divergent plate boundary), they are also the least observed since most are under 10,000 to 15,000 feet of ocean, an exception being Iceland. As the oceanic plates diverge and thin, hot mantle rock is allowed to rise, pressure from depth is released which causes the ultramafic mantle rock (peridotite) to melt partially. The resulting magma is basaltic in composition based on the concept of partial melting discussed earlier. Because most volcanoes on the ocean floor are basaltic, most of the oceanic lithosphere is also basaltic near the surface with phaneritic gabbro and ultramafic peridotite forming underneath. Icelandic volcanism is an example of this, but lying above sea level.

An underwater volcanic eruption occurs when basaltic magma erupts underwater forming pillow basalts and/or in small explosive eruptions. Lava erupting into seawater forms pillow-shaped structures (see figure) hence the name. In association with these seafloor eruptions, an entire underwater ecosystem thrives in parts of the mid-ocean ridge. This ecosystem exists around tall vents emitting black, hot mineral-rich water called deep-sea hydrothermal vents (also known as black smokers).

This hot water, up to 380 °C (716 °F), is heated by the magma and dissolves many elements, which support the ecosystem. Deep underwater where the sun cannot reach, this ecosystem of organisms depends on the heat of the vent for energy and vent chemicals as its foundation of life called chemosynthesis. The foundation of the ecosystem is hydrogen sulfide-oxidizing bacteria that live symbiotically with the larger organisms. Hydrogen sulfide (H2S, the gas that smells like rotten eggs) needed by these bacteria is contained in the volcanic gases emitted from the hydrothermal vents. The source of most of this sulfur and other elements is the Earth’s interior. Below are three short videos regarding a deep-sea submersible submarine and deep-sea hydrothermal vents.

VOLCANOES ALONG CONVERGENT BOUNDARIES

Volcanoes are a vibrant manifestation of plate tectonics processes. Volcanoes are common along convergent and divergent plate boundaries, but are also found within lithospheric plates away from plate boundaries. Wherever mantle can melt, volcanoes may be the result.
Volcanoes erupt because mantle rock melts. The first stage in creating a volcano is when mantle rock begins to melt because of extremely high temperatures, lithospheric pressure lowers, or water is added.

SUBDUCTION ZONES

During the process of subduction, water is expelled from the hydrated minerals causing partial melting by flux melting in the overlying mantle rock. This creates a mafic magma that rises through the lithosphere and can change composition by interacting with surrounding continental crust as well as by magma differentiation. These changes then evolve basaltic magma into more silica-rich rock in volcanoes and plutons. These silica-rich rocks are felsic to intermediate rocks such as andesite, rhyolite, pumice, and tuff. The “Ring of Fire” surrounding the Pacific Ocean is dominated by subduction and contains volcanoes with silica-rich magma. These volcanoes are discussed in more detail in the stratovolcano section.

Large earthquakes are extremely common along convergent plate boundaries. Since the Pacific Ocean is rimmed by convergent and transform boundaries, roughly 80 percent of all earthquakes occur around the Pacific Ocean basin, called the Ring of Fire. A description of the Pacific Ring of Fire along western North America is below:

  • Subduction at the Middle American Trench creates volcanoes in Central America.
  • The San Andreas Fault is a transform boundary.
  • Subduction of the Juan de Fuca plate beneath the North American plate creates the Cascade volcanoes like Mount St. Helens, Mount Rainer, Mount Hood and more.
  • Subduction of the Pacific plate beneath the North American plate in the north creates the long chain of the Aleutian Islands volcanoes near Alaska.

CONTINENTAL RIFTS

In addition to volcanoes at the mid-ocean ridge and subduction zones, some volcanoes are at continental rifts where the lithosphere is diverging and thinning such as in the Basin and Range Province in North America and the East African Rift Basin in Africa. The thinning allows for some of the lower crustal rocks or upper mantle rocks to rise releasing some pressure and causing partial melting. The magma generated is less dense than the surrounding rock and rises through the crust to the surface erupting as basalt. These basaltic eruptions are usually in the form of flood basalts, cinder cones, and basaltic lava flows. For example, relatively young cinder cones are located in south-central Utah, the Black Rock Desert Volcanic Field, which is part of the Basin and Range crustal extension. The 1-minute video (below) illustrates volcanism in the Basin and Range Province. These Utah cinder cones and lava flows started erupting 6 million years ago with the last volcanic eruption 720 years ago.

HOTSPOTS

The primary source of intraplate volcanism is hotspots. Hotspots occur when lithospheric plates glide over a hot mantle plume, which is an ascending column of hot rock (solid, not magma) originating from deep within the mantle. A chain of ancient volcanoes formerly active but now inactive for millions of years can be seen on the seafloor, or on continents, which leads to an active intraplate volcano, indicating hotspot volcanism. The Pacific oceanic plate overrode a hotspot mantle plume producing a long volcanic island chain beginning with the Emperor Seamounts in the northwest Pacific and terminating at the Hawaiian Islands with currently active volcanoes. When the North American continental plate overrode a mantle plume hotspot, a chain of ancient volcanic calderas formed extending from Southwestern Idaho to the Yellowstone caldera.

Once the ascending magma reaches the lithosphere, it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across. Think of the Bowen’s Reaction Series and the temperatures of the magmas that contain the respective minerals. If hot mafic magma rises beneath felsic continental crust spreads into a head below the felsic boundary, the higher heat of the mafic magma may cause the felsic rock above it to melt. There may be mixing of the mafic material from below with the felsic above to form intermediate magmas, or the felsic magma may melt and rise higher forming granitic batholiths or even emerging as a felsic volcano. Such felsic (granitic) batholiths lie at the core of the Sierra Nevada Mountains and comprise the dramatic features of Yosemite. Since most mantle plumes are beneath the oceanic lithosphere, the early stages of volcanism typically take place on the seafloor. Over time, basaltic volcanoes may form islands like those in Hawaii. If the hotspot is under continental lithosphere, then magma of more felsic to intermediate (silica-rich) composition rises into an explosive volcano like Mt. St. Helens or the Yellowstone caldera. Two three-minute videos (below) illustrates hotspot volcanoes.

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Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Kategoria: Moje artykuły | Dodał: kolo (2019-04-04)
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