4.5: Volcanic Structures

Volcano Features and Types

There are several different types of volcanoes based on their shape, eruption style, magmatic composition, and other aspects.

The figure shows the main features of a typical stratovolcano:

1. magma chamber
2. bedrock or upper layers of lithosphere
3. the conduit or narrow pipe through which the lava erupts
4. the base or edge of the volcano
5. a sill of magma between layers of the volcano
6. a branch pipe or feeder tube to the sill
7. layers of tephra (ash) from previous eruptions
8. flank (side) of the volcano
9. layers of lava erupted from the vent that flowed down the sides of the volcano
10. the throat of the volcano
11. a parasitic cone, a small volcano located on the flank of a larger volcano such as Shastina on Mount Shasta (Kilauea sitting on the flank of Mauna Loa is not considered a parasitic cone because it has its own separate magma chamber [13].)
12. layers of lava on the main summit and parasitic cone
13. the vents of the parasitic cone and the main volcano
14. the crater
15. clouds of ash blown into the sky by the eruption; this settles back onto the volcano and surrounding land.

Figure \(\PageIndex{1}\): Mt. Shasta in California with Shastina, its parasitic cone

The largest craters are called calderas, such as the Crater Lake Caldera in Oregon.

Figure \(\PageIndex{2}\): Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.

Many volcanic features are produced by viscosity, a basic property of lava. Viscosity is the resistance to flow by a fluid. Low viscosity magma flows easily more like syrup such as the basaltic volcanism that occurs in Hawaii on shield volcanoes. High viscosity magma is thick and sticky, typically felsic or intermediate, that flows slowly, similar to toothpaste.

Shield Volcano

The largest volcanoes are shield volcanoes. They are characterized by broad low-angle flanks, small vents at the top, and mafic magma chambers. The name comes from the side view, which resembles a medieval warrior’s shield. They are typically associated with hotspots, mid-ocean ridges, or continental rifts with rising upper mantle material. The low-angle flanks are built up slowly from numerous low-viscosity basaltic lava flows that spread out over long distances. The basaltic lava erupts effusively, meaning the eruptions are small, localized, and predictable.

Figure \(\PageIndex{3}\): Mauna Kea, a shield volcano in Hawaii. (By Nula666; CC BY-SA 3.0 via Wikimedia Commons.)

Typically, shield volcano eruptions are not much of a hazard to human life—although non-explosive eruptions of Kilauea (Hawaii) in 2018 produced uncharacteristically large lavas that damaged roads and structures. Mauna Loa (see USGS page) and Kilauea (see USGS page) in Hawaii are examples of shield volcanoes. Shield volcanoes are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift [14].

 

Figure \(\PageIndex{4}\): Eruption of Kilauea in 2018 produced relatively high viscosity a’a lava shown here crossing and destroying a road. This eruption caused much property damage in Hawaii.

The largest volcanic edifice in the solar system is Olympus Mons on Mars. This (possibly extinct) shield volcano covers an area the size of the state of Arizona. This may indicate the volcano erupted over a hotspot for millions of years, which means Mars had little, if any, plate tectonic activity. [15] [16].

Figure \(\PageIndex{5}\): Olympus Mons, an enormous shield volcano on Mars, the largest volcano in the solar system.

Basaltic lava forms special landforms based on magma temperature, composition, and content of dissolved gases and water vapor. The two main types of basaltic volcanic rock have Hawaiian names—pahoehoe and aa. Pahoehoe might come from low-viscosity lava that flows easily into ropey strands.

Figure \(\PageIndex{6}\): Ropey pahoehoe lava.

Aa (sometimes spelled a’a or ʻaʻā and pronounced “ah-ah”) is more viscous and has a crumbly blocky appearance [17]. The exact details of what forms the two types of flows are still up for debate. Felsic lavas have lower temperatures and more silica and thus are higher viscosity. These also form aa-style flows.

Figure \(\PageIndex{7}\): Blocky a’a lava. (By Librex; CC BY 2.0 via Wikimedia Commons.)

Low-viscosity, fast-flowing basaltic lava tends to harden on the outside into a tube and continue to flow internally. Once lava flow subsides, the empty outer shell may be left as a lava tube. Lava tubes, with or without collapsed roofs, make famous caves in Hawaii, Northern California, the Columbia River Basalt Plateau of Washington and Oregon, El Malpais National Monument in New Mexico, and Craters of the Moon National Monument in Idaho.

Fissures are cracks that commonly originate from shield-style eruptions. Lava emerging from fissures is typically mafic and very fluid. The 2018 Kilauea eruption included fissures associated with the lava flows. Some fissures are caused by volcanic seismic activity rather than lava flows. Some fissures are influenced by plate tectonics, such as the common fissures located parallel to the divergent boundary in Iceland.

Figure \(\PageIndex{8}\): Volcanic fissure and flow, which could eventually form a lava tube. (By M. Patrick; public domain via USGS.)

Cooling lava can contract into columns with semi-hexagonal cross sections called columnar jointing. This feature forms the famous Devils Tower in Wyoming, possibly an ancient volcanic vent from which the surrounding layers of lava and ash have been removed by erosion. Another well-known exposed example of columnar jointing is the Giant’s Causeway in Ireland.

Figure \(\PageIndex{9}\): Left: Devils Tower in Wyoming has columnar jointing. Right: Columnar jointing on Giant’s Causeway in Ireland.

Stratovolcano

A stratovolcano, also called a composite cone volcano, has steep flanks, a symmetrical cone shape, a distinct crater, and rises prominently above the surrounding landscape. The term composite refers to the alternating layers of pyroclastic fragments like ash and bombs, and solidified lava flows of varying composition. Examples include Mount Rainier in Washington state and Mount Fuji in Japan.

Figure \(\PageIndex{10}\): Left: Mount Rainier towers over Tacoma, Washington. Right: Mt. Fuji in Japan, a typical stratovolcano: symmetrical, increasing slope, and a visible crater at the top.

Stratovolcanoes usually have felsic to intermediate magma chambers, but can even produce mafic lavas. Stratovolcanoes have viscous lava flows and domes, punctuated by explosive eruptions. This produces volcanoes with steep flanks [14].

Lava Domes

Lava domes are accumulations of silica-rich volcanic rock, such as rhyolite and obsidian. Too viscous to flow easily, the felsic lava tends to pile up near the vent in blocky masses. Lava domes often form in a vent within the collapsed crater of a stratovolcano and grow by internal expansion. As the dome expands, the outer surface cools, hardens, and shatters, and spills loose fragments down the sides. Mount St. Helens has a good example of a lava dome inside of a collapsed stratovolcano crater. Examples of stand-alone lava domes are Chaiten in Chile and Mammoth Mountain in California [18][14].

Figure \(\PageIndex{11}\): Lava domes have started the rebuilding process at Mount St. Helens, Washington.

Caldera

Calderas are steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice into an empty magma chamber. Calderas are generally very large, with diameters of up to 25 km (15.5 mi). The term caldera specifically refers to a volcanic vent but it is frequently used to describe a volcano type. Caldera volcanoes are typically formed by eruptions of high-viscosity felsic lava having high content of volatiles.

Crater Lake, Yellowstone, and the Long Valley Caldera are good examples of this type of volcanism. The caldera at Crater Lake National Park in Oregon was created about 6800 years ago when Mount Mazama, a composite volcano, erupted in a huge explosive blast. The volcano ejected large amounts of volcanic ash and rapidly drained the magma chamber, causing the top to collapse into a large depression that later filled with water. Wizard Island in the middle of the lake is a later resurgent lava dome that formed within the caldera basin [14].

Figure \(\PageIndex{12}\): Timeline of events at Mount Mazama. 1. Eruptions of ash and pumice. The cataclysmic eruption started from a vent on the northeast side of the volcano as a towering column of ash, with pyroclastic flows spreading to the northeast. 2. Caldera collapse. As more magma was erupted, cracks opened up around the summit, which began to collapse. Fountains of pumice and ash surrounded the collapsing summit, and pyroclastic flows raced down all sides of the volcano. 3. Steam explosions. When the dust had settled, the new caldera was 5 miles (8 km) in diameter and 1 mile (1.6 km) deep. Ground water interacted with hot deposits causing explosions of steam and ash. 4. Today. In the first few hundred years after the cataclysmic eruption, renewed eruptions built Wizard Island, Merriam Cone, and the central platform. Water filled the new caldera to form the deepest lake in the United States. (By USGS and NPS; public domain via Wikimedia Commons.)

Figure \(\PageIndex{13}\): Wizard Island sits in the caldera at Crater Lake.

The Yellowstone volcanic system erupted three times in the recent geologic past—2.1, 1.3, and 0.64 million years ago—leaving behind three caldera basins. Each eruption created large rhyolite lava flows as well as pyroclastic flows that solidified into tuff formations. These extra-large eruptions rapidly emptied the magma chamber, causing the roof to collapse and form a caldera. The youngest of the three calderas contains most of Yellowstone National Park, as well as two resurgent lava domes. The calderas are difficult to see today due to the amount of time since their eruptions and subsequent erosion and glaciation.

Figure \(\PageIndex{14}\): Map of calderas and related rocks around Yellowstone.

Yellowstone volcanism started about 17 million years ago as a hotspot under the North American lithospheric plate near the Oregon/Nevada border. As the plate moved to the southwest over the stationary hotspot, it left behind a track of past volcanic activities. Idaho’s Snake River Plain was created from volcanism that produced a series of calderas and lava flows. The plate eventually arrived at its current location in northwestern Wyoming, where hotspot volcanism formed the Yellowstone calderas [19].

Figure \(\PageIndex{15}\): Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bishop Tuff ash bed (brown dashed line), and the modern May 18, 1980 ash fall from Mt. St. Helens (yellow).

The Long Valley Caldera near Mammoth, California, is the result of a large volcanic eruption that occurred 760,000 years ago. The explosive eruption dumped enormous amounts of ash across the United States, in a manner similar to the Yellowstone eruptions. The Bishop Tuff deposit near Bishop, California, is made of ash from this eruption. The current caldera basin is 17 km by 32 km (10 mi by 20 mi), large enough to contain the town of Mammoth Lakes, major ski resort, airport, major highway, resurgent dome, and several hot springs [20].

Cinder Cone

Cinder cones are small volcanoes with steep sides and made of pyroclastic fragments that have been ejected from a pronounced central vent. The small fragments are called cinders and the largest are volcanic bombs. The eruptions are usually short-lived events, typically consisting of mafic lavas with a high content of volatiles. Hot lava is ejected into the air, cooling and solidifying into fragments that accumulate on the flank of the volcano. Cinder cones are found throughout western North America [14].

Figure \(\PageIndex{16}\): Sunset Crater, Arizona is a cinder cone.

A recent and striking example of a cinder cone is the eruption near the village of Parícutin, Mexico that started in 1943 [21]. The cinder cone started explosively shooting cinders out of the vent in the middle of a farmer’s field. The volcanism quickly built up the cone to a height of over 90 m (300 ft) within a week and 365 m (1200 ft) within the first 8 months. After the initial explosive eruption of gases and cinders, basaltic lava poured out from the base of the cone. This is a common order of events for cinder cones: violent eruption, cone, and crater formation, low-viscosity lava flow from the base. The cinder cone is not strong enough to support a column of lava rising to the top of the crater, so the lava breaks through and emerges near the bottom of the volcano. During nine years of eruption activity, the ashfall covered about 260 km² (100 mi²) and destroyed the nearby town of San Juan [14].

Figure \(\PageIndex{17}\): Left: Soon after the birth of Parícutin in 1943. Right: Lava from Parícutin covered the local church and destroyed the town of San Juan, Mexico.

Flood Basalts

A rare volcanic eruption type, unobserved in modern times, is the flood basalt. Flood basalts are some of the largest and lowest viscosity types of eruptions known. They are not known from any eruption in human history, so the exact mechanisms of eruption are still mysterious. Some famous examples include the Columbia River Flood Basalts in Washington, Oregon, and Idaho, the Deccan Traps, which cover about 1/3 of the country of India, and the Siberian Traps, which may have been involved in the Earth’s largest mass extinction.

Figure \(\PageIndex{18}\): World map of flood basalts. Note the largest is the Siberian Traps.

Carbonatites

Arguably the most unusual volcanic activity is a carbonatite eruption. Only one actively erupting carbonatite volcano exists on Earth today, Ol Doinyo Lengai, in the East African Rift Zone of Tanzania. While all other volcanism on Earth originates from silicate-based magma, carbonatites are a product of carbonate-based magma and produce volcanic rocks containing greater than 50% carbonate minerals. Carbonatite lavas are very low viscosity and relatively cold for lava. The erupting lava is black and solidifies to brown/grey rock that eventually turns white. These rocks are occasionally found in the geologic record and require special study to distinguish them from metamorphic marbles. They are mostly associated with continental rifting [22].

Figure \(\PageIndex{19}\): Crater of Ol Doinyo Lengai in 2011. Note the white carbonatite in the walls of the crater.

A summary of types of volcanoes with associated rock types and other characteristics is shown in the figure below.

\(\PageIndex{20}\): Igneous rock types and related volcano types. Mid-ocean ridges and shield volcanoes represent more mafic compositions, and stratovolcanoes (composite volcanoes) generally represent a more intermediate or felsic composition and a convergent plate tectonic boundary. Note that there are exceptions to this generalized layout of volcano types and igneous rock composition.

References

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).

14. USGS. Volcanoes: Principal Types of Volcanoes. (2011). Available at: http://pubs.usgs.gov/gip/volc/types.html. (Accessed: 30th July 2016)

15. Carr, M. H. Geologic map of the Tharsis Quadrangle of Mars. IMAP (1975).

16. Frankel, C. Worlds on Fire: Volcanoes on the Earth, the Moon, Mars, Venus and Io. (Cambridge University Press, 2005).

17. Peterson, D. W. & Tilling, R. I. Transition of basaltic lava from pahoehoe to aa, Kilauea Volcano, Hawaii: Field observations and key factors - ScienceDirect. J. Volcanol. Geotherm. Res. 7, 271–293 (1980).

18. USGS. Volcanoes General - What are the different types of volcanoes? (2016). Available at: https://www2.usgs.gov/faq/categories/9819/2730. (Accessed: 14th March 2017)

19. USGS. Yellowstone Volcano Observatory. (2012). Available at: https://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_geo_hist_52.html. (Accessed: 30th July 2016)

20. United States Geological Survey. USGS: Volcano Hazards Program CalVO Long Valley Caldera. Geologic history of the Long Valley, Mono Basin region (2016). Available at: https://volcanoes.usgs.gov/volcanoes/long_valley/geo_hist_summary.html. (Accessed: 25th May 2017)

21. Luhr, J. F. & Cuasay, M. Parícutin : The volcano born in a Mexican cornfield. (U.S. Geoscience Press, 1993).

22. Bell, K. & Keller, J. Carbonatite volcanism: Oldoinyo Lengai and the petrogenesis of natrocarbonatites. (Springer Science & Business Media, 2012).