7.3: The Variety of Volcanoes in the Cascades and the Modoc Plateau

Magma Viscosity, Eruptive Style and Volcano Type

Volcanoes come in many different shapes and sizes. The shape of a volcano and the way it will behave when it erupts are largely due to a property of magma called viscosity. Viscosity is the resistance to flow. Highly viscous fluids flow slowly, whereas fluids that flow easily are said to have lower viscosity. For example, honey has higher viscosity than water. When low-viscosity magmas erupt at the surface of the earth as lava, they quickly flow away from the vent, creating a broad, flat volcano. More viscous lava, on the other hand, travels less far and thus accumulates near the vent, often building a relatively steep cone.

Viscosity is also a major player in determining the explosivity of an eruption. When magma is deep in the earth, it has gasses dissolved within it. The gasses remain dissolved due to the high pressure at great depth. However, as the magma rises upwards and the pressure decreases, the gasses come out of solution, forming bubbles. A similar process can be seen in a bottle of soda. Soda is bottled under pressure, with gas dissolved in the liquid. Opening the cap releases the pressure, causing the gasses to come out of solution. In low-viscosity lavas, the lava flows easily out of the way of the expanding bubbles, and the gasses easily escape the lava. In high-viscosity lavas, the lava can’t flow as easily around the expanding bubbles; instead, the expanding gasses rip the lava apart into fragments, creating violent, explosive eruptions.

Felsic lavas tend to be more viscous than mafic lavas and thus tend to form steeper volcanoes that exhibit more explosive eruptions. Mafic lavas tend to form broad, flat volcanoes with effusive eruptions. An effusive eruption is where lava pours from a vent in the form of a lava flow, with little explosion of material into the air. Viscosity is not the factor that determines explosivity. The temperature of the lava, dissolved gas content of the lava, and the rate of eruption also contribute to eruption style. Even highly viscous felsic lava can erupt in an effusive lava flow if the eruption is slow enough.

Because subduction zones produce a variety of magma compositions, they also produce a variety of volcano shapes and sizes. Subduction zones are known for their iconic composite volcanoes (sometimes called composite cones or stratovolcanoes), and indeed composite volcanoes are known to form only at subduction zones, but subduction zones also produce a variety of other volcano types.

Classifying Cascade Volcanoes

Before you learn about the specific types of volcanoes, complete the activity, “anatomy of a volcano” to learn the names for some basic features present in most volcanoes. Place the terms in the correct box. Start with the terms that you are most confident about, then for the rest, make your best educated guess. Attempt the activity as many times as you need to learn the terminology.

Query \(\PageIndex{1}\)

Accessibility guidance for this activity

Volcanoes generally are classified by their size and shape. The two types of large, mountain-forming volcanoes found in the Cascades are shield volcanoes and composite volcanoes. Shield volcanoes are broad and flat, in contrast to composite volcanoes, which have a distinctive steep triangular profile. Two types of smaller volcanoes found in the Cascades and Modoc Plateau are cinder cones and lava domes.

Composite Volcanoes

Composite volcanoes can be the most picturesque of all volcanoes. A classic composite volcano is conical with a concave shape that is steeper near the top. These mountains commonly have snow-covered peaks standing high above the surrounding mountainous terrain. Composite volcanoes are also sometimes called stratovolcanoes or composite cones. Many of the iconic volcanoes of the Cascade Range are composite volcanoes, including Mount Shasta in California (see 7.5: Mount Shasta) as well as numerous Pacific Northwest volcanoes, such as Mount Rainier, Mount St. Helens and Mount Hood.

Composite volcanoes are large volcanoes (many thousands of feet or meters tall) and are probably the most complex type of volcanic edifice as they experience multiple eruptions, erupt lavas with a range of compositions, and experience many different types of eruptions. This variety of compositions and eruption styles leads to a variety of materials. Composite volcanoes are composed of a mixture of volcanic materials, including lava flows, pyroclastic deposits, and lava domes. Although composite volcanoes can erupt a range of compositions from basalt to rhyolite, intermediate (andesitic) and dacitic magmas are most common.

Composite volcanoes may have multiple vents, but most have a main vent at the summit. Accumulation of materials around this vent during successive eruptions gives these volcanoes their distinctive shape, but eruptions can also destroy large areas of their summits, such as the May 1980 explosion and landslide at Mount St. Helens. Composite volcanoes are active over long periods (tens to hundreds of thousands of years) and can experience multiple stages of growth and destruction over many eruptions in that time. Figure \(\PageIndex{1}\) shows the many different hazards associated with eruptions of composite volcanoes, which build and shape the volcanic edifice.

Figure \(\PageIndex{1}\): Composite volcanoes erupt a variety of volcanic materials and produce a variety of volcanic hazards, which build and alter the shape of the volcano. The features pictured in the diagram do not all occur simultaneously. For example, lava flows are a product of an effusive eruption, whereas pyroclastic flows usually require an explosive eruption. This diagram by the USGS is in the public domain. Access a detailed description.

Shield Volcanoes

Although shield volcanoes are the largest volcanoes on Earth, they do not form soaring mountains with conical peaks like composite volcanoes. Instead, they are broad volcanoes with gentle slopes and are shaped somewhat like a warrior’s shield lying flat on the Earth. In contrast to the concave shape typical of composite volcanoes, shield volcanoes have a convex shape as they are flatter near the summit. While the most famous shield volcanoes are found in Hawaii, there are also shield volcanoes in California (see 7.6: Lassen Volcanic National Park and 7.7: Medicine Lake Volcano).

Like composite volcanoes, shield volcanoes are built by repeated eruptions that occur intermittently over vast periods of time (up to a million years or longer). However, shield volcanoes are more consistent in their style of eruption. They are usually constructed almost entirely of basaltic and/or andesitic lava flows which are very fluid when erupted. When erupted at high rates, this low-viscosity lava travels fast and can cover large areas, giving a shield volcano its broad, flat shape. Figure \(\PageIndex{2}\) shows the features of a typical shield volcano.

Figure \(\PageIndex{2}\): Shield volcanoes erupt mostly lava flows. In some cases these lava flows are fed by fire fountains. Other volcanic hazards include vog (volcanic smog) and laze, which is formed when hot lava interacts with ocean water. This diagram by the USGS is in the public domain. Access a detailed description.

Cinder Cones

Cinder cones are the most common type of volcano in the world. They may look like an idealized depiction of a volcano as they are steep, conical hills that usually have a prominent crater at the top. There are many cinder cones in both the California Cascades and the Modoc Plateau, including one creatively named Cinder Cone, in Lassen Volcanic National Park (see 7.6: Lassen Volcanic National Park).

Cinder cones are more technically known as scoria cones. Scoria forms when blobs of gas charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rock containing cavities created by trapped gas bubbles. Cinder is a more colloquial term that is widely used in the United States for pieces of scoria that are roughly nut- to fist-size.

Most cinder cones are basaltic to basaltic andesite in composition, but they may be andesitic (intermediate). This low-viscosity mafic lava results in eruptions that are usually mildly to moderately explosive. A fountain of scoria erupts from a central vent and accumulates in a ring around the vent. Unlike shield and composite volcanoes, cinder cones usually form in a single eruptive episode. Cinder cone eruptions usually have a short duration. Half of all historic cinder cone eruptions have lasted less than 30 days; and 95% have lasted less than 1 year. Most cinder cones are a few hundred feet tall (100 to 150 m), and rarely are larger than 600 to 900 feet (200 to 300 m) in height.

While sometimes they are nearly perfect cones, cinder cones frequently have an asymmetric shape. Cinder cones that form over a linear fissure vent are elongated, and ones that form in areas with strong prevailing winds may be much taller on the downwind side. Elongated cinder cones may also form when the location of the vent shifts during the eruption. They may also be breached when lava flows erupting from near the base of the cone carry away a part of the flank. Because cinder cones are usually mafic in composition, they are also often associated with lava flows. Much like an uncapped bottle of shaken soda, volcanic eruptions can often begin as mildly explosive eruptions, then transition to effusive eruptions as the gas content decreases.

Cinder cones commonly occur in association with other volcanoes. They may occur within calderas, near volcanic domes, and as satellite cones on the flanks of composite and shield volcanoes. Cinder cones frequently occur in volcanic fields with as many as hundreds of other cinder cones. Figure \(\PageIndex{3}\) shows a schematic diagram of a cinder cone.

Cinder cones can also be relatively ephemeral. Sometimes a cinder cone starts an eruptive sequence and what begins as a cinder cone may eventually develop into a larger shield volcano or composite volcano.

Figure \(\PageIndex{3}\): Cinder cones are formed from the eruption of tephra from a single vent. They are also often associated with lava flows that emerge from vents near the base of the cone. This diagram by Trista Thornberry-Ehrlich, Colorado State University for the National Park Service is in the public domain. Access a detailed description.

Lava Domes

Lava domes form from the slow extrusion of highly viscous felsic lava. The low gas content and slow rate of eruption keep these eruptions effusive despite the high viscosity of the lava. However, these lavas are too thick to spread out into a lava flow. Most lava domes are small and many do not have a crater. Some dome-forming eruptions start with highly explosive eruptions that wane into dome-building ones as the gas content in the magma decreases. Other dome-building eruptions begin effusively until the growing dome explodes or collapses to produce pyroclastic flows. Lava domes can collapse because they may become over-pressured from gasses trapped inside or become over-steepened.

Lava domes can form volcanic edifices in their own right but also occur in clusters. Lava domes may be extruded in the summit craters of composite volcanoes or within calderas as part of a post-caldera eruptive phase. Like many terms for volcano types, there are several synonym variations of ‘lava dome’. These include ‘volcanic dome’, ‘plug dome’, or sometimes, just ‘dome’.

​​Because they are made of viscous lava, domes are usually steep-sided. They typically have rough brecciated surfaces, and sometimes they have glassy rinds due to quenching of the hot lava once it has erupted onto the surface. Most domes are relatively small volcanoes with limited volume. Like cinder cones, lava domes usually grow in a single eruptive episode, which frequently last days to months. The features of a lava dome are shown in figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Lava domes form close to the eruption vent, often within a crater formed by an earlier eruptive phase. This diagram by Trista Thornberry-Ehrlich, Colorado State University for the National Park Service is in the public domain. Access a detailed description.

Practice Classifying Volcanoes

Video \(\PageIndex{1}\): How to Classify Volcanoes

For further review of volcano classification and to practice visually identifying volcano types, please watch the video, “How to Classify Volcanoes.”

Accessibility guidance for this video

Apply your knowledge of volcano classification to the California Cascades in the activity “Classifying California Cascade Volcanoes”.

Query \(\PageIndex{2}\)

Accessibility guidance for this activity

Volcanoes of the California Cascades

Most of the major Cascade volcanoes are located in Oregon and Washington, but most lists of major Cascade volcanoes will include three in California: Mount Shasta, Lassen Peak, and Medicine Lake Volcano. Mount Shasta, an iconic composite volcano, is the second-highest peak in the whole Cascade range and although it is the fifth-highest peak in California, it is the only peak in the top five that is a volcano. Lassen Peak, a lava dome, is known for its 1915 eruption and for being the site of a National Park. For most of the 20th Century, until the 1980 eruption of Mount Saint Helens, the Lassen Peak eruption was the most well-known volcanic eruption in the United States. Medicine Lake Volcano is one of the largest volcanoes in the Cascade Range by volume, at around 600 km³ (143 mi³). It is usually described as a shield volcano, but studies of older rocks at the core of the volcano suggest that its classification may not be that straightforward.

Though most maps of Cascade volcanoes show only the large, young composite and shield volcanoes, there are countless other smaller cinder cones, domes, and eroded volcano remnants throughout the Cascades, including the California Cascades. In the eastern part of the range, the Cascades transition to the Modoc Plateau, which is characterized by smaller volcanoes and lava flows rather than large, prominent composite volcanoes.

In the Cascades and Modoc Plateau, smaller volcanoes tend to occur in clusters. Some of these clusters form lines, with a north-northwest trend. This trend is consistent with the strike of faults in the area. Faults in this area are normal faults associated with Basin and Range extension. The linear clusters of cinder cones and other small volcanoes are thought to have resulted from the interaction of Cascadia and Basin and Range tectonics. Cascadia subduction is mainly responsible for the generation of magma, but as the magma moves toward the surface, it exploits pre existing weaknesses in the crust created by Basin and Range faulting.

Acknowledgments

The descriptions of volcano types are slightly modified from text provided by the National Park Service, which is in the public domain. Links to the original text can be found in the reference section on this page.

References

1. Earle, S. (2019). Chapter 4 Volcanism. In Physical Geology (2nd ed.). BCcampus. https://opentextbc.ca/physicalgeology2ed/part/chapter-4-volcanism/
2. Johnson, C., Affolter, M. D., Inkenbrandt, P., & Mosher, C. (2017). Chapter 4: Igneous Processes and Volcanoes. In An Introduction to Geology. https://slcc.pressbooks.pub/introgeology/chapter/4-igneous-processes-and-volcanoes/
3. U.S. National Park Service. (n.d.-a). Cinder Cones. Retrieved March 13, 2024, from https://www.nps.gov/articles/000/cinder-cones.htm
4. U.S. National Park Service. (n.d.-b). Composite Volcanoes (Stratovolcanoes). Retrieved May 24, 2024, from https://www.nps.gov/articles/000/composite-volcanoes.htm
5. U.S. National Park Service. (n.d.-c). Shield Volcanoes. Retrieved May 24, 2024, from https://www.nps.gov/articles/000/shield-volcanoes.htm
6. U.S. National Park Service. (n.d.-d). Volcanic Domes. Retrieved May 24, 2024, from https://www.nps.gov/articles/000/domes.htm