4.4: Volcanism

When magma emerges onto the Earth’s surface, the molten rock is called lava. A volcano is a type of land formation created when lava solidifies into rock. Volcanoes have been an important part of human society for centuries, and studying them has greatly increased as our understanding of plate tectonics and has made them less mysterious. The next sections describe volcano location, type, hazards, and monitoring.

Distribution and Tectonics

Most volcanoes are interplate volcanoes. Interplate volcanoes are located at active plate boundaries created by volcanism at mid-ocean ridges, subduction zones, and continental rifts. The prefix “inter-“ means between, such as between tectonic plates. Some volcanoes are intraplate volcanoes. The prefix “intra-“ means within, and intraplate volcanoes are located within tectonic plates, far removed from plate boundaries. Many intraplate volcanoes are formed by hotspots.

Figure \(\PageIndex{1}\): Association of volcanoes with plate boundaries

Volcanoes at Mid-Ocean Ridges

Most volcanism on Earth occurs on the ocean floor along mid-ocean ridges, a type of divergent plate boundary. These interplate volcanoes are also the least observed and famous since most of them are located under 3000-4500 m (10,000-15,000 ft) of the ocean and the eruptions are slow, gentle, and oozing. One exception is the interplate volcanoes of Iceland. The diverging and thinning oceanic plates allow hot mantle rock to rise, releasing pressure and causing decompression melting. Ultramafic mantle rock, consisting largely of peridotite, partially melts and generates magma that is basaltic. Because of this, almost all volcanoes on the ocean floor are basaltic. In fact, most oceanic lithosphere is basaltic near the surface, with phaneritic gabbro and ultramafic peridotite underneath [10].

Figure \(\PageIndex{2}\): Map of mid-ocean ridges throughout the world.

When basaltic lava erupts underwater it emerges in small explosions and/or forms pillow-shaped structures called pillow basalts. These seafloor eruptions enable entire underwater ecosystems to thrive in the deep ocean around mid-ocean ridges. This ecosystem exists around tall vents emitting black, hot mineral-rich water called deep-sea hydrothermal vents, also known as black smokers.

Figure \(\PageIndex{3}\): Left: Pillow basalt on the seafloor near Hawaii. Right: Black smoker hydrothermal vent with a colony of giant (6 feet) tube worms.

Without sunlight to support photosynthesis, these organisms instead utilize a process called chemosynthesis. Certain bacteria are able to turn hydrogen sulfide (H₂S), a gas that smells like rotten eggs, into life-supporting nutrients and water. Larger organisms may eat these bacteria or absorb nutrients and water produced by bacteria living symbiotically inside their bodies [11].

Figure \(\PageIndex{4}\): Distribution of hydrothermal vent fields.

The video shows some of the ecosystems found around deep-sea hydrothermal vents.

Volcanoes at Subduction Zones

The second most commonly found location for volcanism is adjacent to subduction zones, a type of convergent plate boundary. The process of subduction expels water from hydrated minerals in the descending slab, which causes flux melting in the overlying mantle rock. Because subduction volcanism occurs in a volcanic arc, the thickened crust promotes partial melting and magma differentiation. These evolve the mafic magma from the mantle into more silica-rich magma. The Ring of Fire surrounding the Pacific Ocean, for example, is dominated by subduction-generated eruptions of mostly silica-rich lava; the volcanoes and plutons consist largely of intermediate-to-felsic rock such as andesite, rhyolite, pumice, and tuff.

Figure \(\PageIndex{5}\): Distribution of volcanoes on the planet. Click here for an interactive map of volcano distributions.

Volcanoes at Continental Rifts

Some volcanoes are created at continental rifts, where crustal thinning is caused by diverging lithospheric plates, such as the East African Rift Basin in Africa. Volcanism caused by crustal thinning without continental rifting is found in the Basin and Range Province in North America. In this location, volcanic activity is produced by rising magma that stretches the overlying crust. Lower crust or upper mantle material rises through the thinned crust, releases pressure, and undergoes decompression-induced partial melting. This magma is less dense than the surrounding rock and continues to rise through the crust to the surface, erupting as basaltic lava. These eruptions usually result in flood basalts, cinder cones, and basaltic lava flows (see video). Relatively young cinder cones of basaltic lava can be found in south-central Utah, in the Black Rock Desert Volcanic Field, which is part of the zone of Basin and Range crustal extension. These Utah cinder cones and lava flows started erupting around 6 million years ago, with the last eruption occurring 720 years ago [12].

Figure \(\PageIndex{6}\): Basaltic cinder cones of the Black Rock Desert near Beaver, Utah.

The video shows the Basin and Range volcanic processes.

Hotspots

Hotspots are the main source of intraplate volcanism. Hotspots occur when lithospheric plates glide over a hot mantle plume, an ascending column of solid heated rock originating from deep within the mantle. The mantle plume generates melts as material rises, with the magma rising even more. When the ascending magma reaches the lithospheric crust, it spreads out into a mushroom-shaped head that is tens to hundreds of kilometers across.

Figure \(\PageIndex{7}\): Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate.

Since most mantle plumes are located beneath the oceanic lithosphere, the early stages of intraplate volcanism typically take place underwater. Over time, basaltic volcanoes may build up from the seafloor into islands, such as the Hawaiian Islands [13]. Where a hotspot is found under a continental plate, contact with the hot mafic magma may cause the overlying felsic rock to melt and mix with the mafic material below, forming intermediate magma. Or the felsic magma may continue to rise, and cool into a granitic batholith or erupt as a felsic volcano. The Yellowstone caldera is an example of hotspot volcanism that resulted in an explosive eruption.

Figure \(\PageIndex{8}\): The track of the Yellowstone hotspot, which shows the age of different eruptions millions of years ago.

A zone of actively erupting volcanism connected to a chain of extinct volcanoes indicates intraplate volcanism located over a hotspot. These volcanic chains are created by the overriding oceanic plate slowly moving over a hotspot mantle plume. These chains are seen on the seafloor and continents and include volcanoes that have been inactive for millions of years. The Hawaiian Islands on the Pacific oceanic plate are the active end of a long volcanic chain that extends from the northwest Pacific Ocean to the Emperor Seamounts, all the way to the subduction zone beneath the Kamchatka Peninsula. The adjacent overriding North American continental plate moved across a mantle plume hotspot for several million years, creating a chain of volcanic calderas that extends from Southwestern Idaho to the presently active Yellowstone caldera in Wyoming.

Figure \(\PageIndex{9}\): The Hawaii-Emperor seamount and island chain.

The three-minute video illustrates hotspot volcanoes.

References

10. Thorarinsson, S. The Lakagigar eruption of 1783. Bull. Volcanol. 33, 910–929 (1969).

11. United States Geological Survey. Exploring the deep ocean floor. (1999). Available at: http://pubs.usgs.gov/gip/dynamic/exploring.html. (Accessed: 25th July 2016)

12. United States Geological Survey. Black Rock Desert Volcanic Field. (2012). Available at: http://volcanoes.usgs.gov/volcanoes/black_rock_desert/. (Accessed: 25th July 2016)

13. Rhodes, J. M. & Lockwood, J. P. Mauna Loa Revealed: Structure, Composition, History, and Hazards. Washington DC American Geophysical Union Geophysical Monograph Series 92, (1995).