4.3: Types of Volcanism

There are numerous types of volcanism; some of the more common ones are summarized in Table \(\PageIndex{1}\). You don't need to memorize these; this is just here to give context to our learning.

Table \(\PageIndex{1}\): Summary of Common Types of Volcanism

Type	Tectonic Setting	Size & Shape	Magma & Eruption Characteristics	Example
Cinder cone	Various. Some form on the flanks of other volcanoes	Small (10s to 100s of m) and steep (>30°)	Most are mafic and form from the gas-rich early stages of a shield- or rift-associated eruption	Eve Cone, northern BC
Composite volcano	Almost all are at subduction zones	Medium size (1000s of m) and moderate steepness (10 to 30°)	Magma composition varies from felsic to mafic, and from explosive to effusive	Mt. St. Helens and other Cascade Peaks Andes Mountains
Shield volcano	Most are at mantle plumes, some on spreading ridges	Large (up to several 1000 m high and 200 km across), not steep (typically 2 to 10°)	Magma is almost always mafic, and eruptions are typically effusive, although cinder cones are common on the flanks of shield volcanoes	Kilauea, Hawaii
Large igneous provinces	Associated with “super” mantle plumes	Enormous (up to millions of km2) and 100s of m thick	Magma is always mafic. Individual flows can be 10s of meters thick	Columbia River basalts
Sea-floor volcanism	Generally associated with spreading ridges but also mantle plumes	Most of the oceanic crust formed at spreading ridges	At normal eruption rates pillows form. At faster rates, lava flows develop.	Juan de Fuca ridge Mid Atlantic ridge
Kimberlite	Older parts of continents	The remnants are typically 10s to 100s of m across	Most appear to have had explosive eruptions forming cinder cones. The youngest one is over 10 ka, and all others are over 30 Ma.	Lac de Gras kimberlite field, NWT

The sizes and shapes of typical shield, composite and cinder-cone volcanoes are compared on Figure \(\PageIndex{1}\), although, to be fair, Mauna Loa is the largest shield volcano on Earth. Mauna Loa rises from the surrounding flat sea floor, and its full diameter is in the order of 200 km, with a diameter of about 100 km above sea level. Its elevation is 4169 m above sea level. Mt. St. Helens, a composite volcano, rises above the surrounding hills of the Cascade Range. It is about 6 km across at the base, and its height is 2550 m above sea level. Cinder cones are much smaller. On this drawing even a large cinder cone is just a dot.

Figure \(\PageIndex{1}\): Profiles of a Shield Volcano (Mauna Loa and Kilauea), a Composite Volcano (Mt. St. Helens), and a Large Cinder Cone

Cinder Cones

Cinder cones, like Eve Cone in northern BC (Figure \(\PageIndex{2}\)), are typically only a few hundred meters in diameter and few are more than 200 m high. Most are comprised of fragments of mafic volcanic rock that were blasted out during a high-gas-pressure early phase of an eruption that may have subsequently become gentle (lava flows). Most cinder cones were created during a single eruptive phase that might have lasted weeks or months. Because cinder cones are made up almost exclusively of loose fragments, they have very little strength and can be easily, and relatively quickly, eroded away. Since these are mafic in composition, they produce relatively gentle eruptions.

Figure \(\PageIndex{2}\): Eve Cone, Which Rises About 170 m Above the Surrounding Plateau, Formed Approximately 700 Years Ago.

Composite Volcanoes

Composite volcanoes, like Mt. St. Helens in Washington State (Figure \(\PageIndex{3}\)), are almost all associated with subduction at convergent plate boundaries—either ocean-continent or ocean-ocean boundaries. At many such volcanoes magma is stored in a magma chamber in the upper part of the crust. For example, at Mt. St. Helens, there is evidence of a magma chamber that is approximately 1 kilometer in width and extends from about 6 to 14 km depth below surface (Figure \(\PageIndex{4}\)). Since these are felsic or intermediate in composition, they produce relatively violent eruptions.

Figure \(\PageIndex{3}\): The North Side of Mt. St. Helens in South-Western Washington State, 2003. The large 1980 eruption reduced the height of the volcano by 400 m, and a sector collapse removed a large part of the northern flank. Between 1980 and 1986 the slow eruption of more lava led to construction of a dome inside the crater.

Figure \(\PageIndex{4}\): A Cross-Section Through the Upper Part of the Crust at Mt. St. Helens Showing the Zoned Magma Chamber.

The rock that makes up Mt. St. Helens ranges in composition from rhyolite (fine-grained and felsic) to basalt (fine-grained and mafic), and that implies that the types of past eruptions have varied widely in their character. As already noted, felsic magma doesn’t flow easily and doesn’t allow gases to escape easily. Under these circumstances pressure builds up until some part of the volcano gives way, and then an explosive eruption results. This type of eruption can also lead to rapid melting of ice and snow on a volcano, and that typically triggers large mudflows known as lahars. Hot, fast moving pyroclastic flows (fast-moving, high-density current of hot volcanic ash, gas, pumice, and rock fragments) and lahars are the two main causes of casualties in volcanic eruptions. Pyroclastic flows killed approximately 30,000 during the 1902 eruption of Mt. Pelée on the Caribbean Island of Martinique. Most were incinerated in their homes.

In contrast, mafic eruptions (and some intermediate eruptions), produce lava flows. If they are thick enough, they can cool in a columnar jointing pattern (Figure \(\PageIndex{5}\)). Lava flows serve to both flatten the profile of the volcano (because the lava typically flows farther than the pyroclastic debris falls) and to protect it from erosion.

In a geological context, composite volcanoes tend to form relatively quickly and do not last very long. Mt. St. Helens, for example, is made up of rock that is all younger than 40,000 years; most of it is younger than 3,000 years. If its volcanic activity ceases, then it might erode within a few tens of thousands of years. This is largely because of the presence of pyroclastic eruptive material, which is quite weak because it is made up of fragments that are not well stuck together.

Figure \(\PageIndex{5}\)The Development of Columnar Jointing in Basalt, Here Seen from the Top Looking Down. As the rock cools it shrinks, and because it is very homogenous it shrinks in a systematic way. When the rock breaks it does so with approximately 120˚ angles between the fracture planes. The resulting columns tend to be 6-sided but 5- and 7-sided columns also form.

Shield Volcanoes

Most shield volcanoes are associated with mantle plumes, although some form at divergent boundaries, either on land or on the sea floor. Since these are mafic in composition, they produce relatively gentle eruptions. The best-known shield volcanoes are those that make up the Hawaiian Islands, and of these the only active ones are on the big island of Hawaii. Mauna Loa, the world’s largest volcano and the world’s largest mountain (by volume) last erupted in 1984. Kilauea, arguably the world’s most active volcano, erupted almost continuously, from 1983 to 2018, and then started up again in late 2020. Loihi is an underwater volcano on the southeastern side of Hawaii. It is last known to have erupted in 1996 but may have erupted since then without being detected.

All the Hawaiian volcanoes are related to the mantle plume that currently lies beneath Mauna Loa, Kilauea and Loihi (Figure \(\PageIndex{6}\)). In this area the Pacific Plate is moving northwest at a rate of about 7 cm/year, and this means that the earlier formed—and now extinct—volcanoes have now moved well away from the mantle plume. As shown on Figure 7.3.8, there is evidence of crustal magma chambers beneath all three active Hawaiian volcanoes.

Figure \(\PageIndex{6}\) A Cross-Section Through the Crust and Upper Mantle in the Area of the Hawaii Mantle Plume

Large Igneous Provinces

While the Hawaii mantle plume has produced magma at a relatively slow rate for a very long time (at least 85 million years), other mantle plumes are less consistent, and some generate massive volumes of magma over relatively short time periods. Although their origin is still controversial, it is thought that the volcanism leading to large igneous provinces (LIP) is related to very high volume but relatively short duration bursts of magma from mantle plumes^[1]. An example of an LIP is the Columbia River Basalt Group (CRGB), which extends across Washington, Oregon and Idaho (Figure \(\PageIndex{7}\)). This volcanism, which covered an area of about 160,000 km² with basaltic rock up to several hundred meters thick, took place between 17 and 14 Ma.

Figure \(\PageIndex{7}\): A Part of the Columbia River Basalt Group at Frenchman Coulee, Eastern Washington. All the flows visible here have formed large (up to 2 meters in diameter) columnar basalts. The inset map shows the extent of the 17 to 14 Ma Columbia River Basalts, with the approximate location of the photo shown as a star.

Some other LIP eruptions have been much bigger. The eruption of the Siberian Traps (also basalt), which happened at the end of the Permian period, at 250 Ma, is estimated to have been 40 times the volume of the CRBG, and is thought to have been responsible for the greatest extinction of all time.

The mantle plume that is assumed to be responsible for the CRBG is now situated beneath the Yellowstone area in Wyoming, where it is associated with felsic volcanism. Over the past 2 million years three very large explosive eruptions at Yellowstone have yielded approximately 900 km³ of felsic magma, about 900 times the volume of the 1980 eruption of Mt. St. Helens, but only 5% of the volume of mafic magma in the CRBG.

Sea Floor Volcanism

Some LIP eruptions occur on the sea floor, the largest being the one that created the Ontong Java plateau in the western Pacific Ocean at around 122 Ma. But most seafloor volcanism originates at divergent boundaries and involves relatively low volume eruptions. Under these conditions, hot lava that oozes out in the cold seawater quickly cools on the outside and then behaves a little like toothpaste. The resulting blobs of lava are known as pillows, and they tend to form piles around a sea-floor lava vent (Figure \(\PageIndex{8}\)). In terms of area, there is very likely more pillow basalt on the sea floor than any other type of rock on Earth.

Figure \(\PageIndex{8}\): Modern Sea-Floor Pillows in the South Pacific. (Left: public domain image by NOAA, 1988, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F...alt_crop_l.jpg; and Right: 40 to 50 Ma pillows on the shore of Vancouver Island, near to Sooke. The pillows are 30 to 40 cm in diameter.

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{2}\): Eve Cone by FortGirl, 2007, via Flickr, CC BY SA NC 2.0, https://www.flickr.com/photos/fortgi...n/photostream/
• Figure \(\PageIndex{3}\): Photo by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{4}\): Steven Earle, CC BY 4.0, after Pringle, P. T., & Washington (State). Division of Geology and Earth Resources. (1993). Roadside geology of Mount St. Helens National Volcanic Monument and vicinity. Information circular/Washington Department of Natural Resources, Division of Geology and Earth Resources, 88. https://www.dnr.wa.gov/Publications/...helens_pt1.pdf
• Figure \(\PageIndex{5}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{6}\): Hotspot Cross-Sectional Diagram by J. E. Robinson, (2006). US Geological Survey public domain image via Wikimedia Commons, https://commons.wikimedia.org/wiki/F...al_diagram.jpg)
• Figure \(\PageIndex{7}\): Photo and inset drawing by Steven Earle, CC BY 4.0
• Figure \(\PageIndex{8}\): Photo by Steven Earle, CC BY 4.0

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1. Bryan, S. & Ernst, R. (2007). Revised definition of large igneous provinces (LIPs). Earth-Science Reviews, 86(1-4), 175-202. https://doi.org/10.1016/j.earscirev.2007.08.008 ↵