4.2 Volcano Shape

Charlene Estrada

Types of Volcanoes

There are all types of volcanoes on our planet: some are huge, some are no bigger than hills, some are explosive, and some are less so. There are several broad types of volcanoes based on their shape, eruption style, magma composition, and other aspects. However, all volcanoes are potentially deadly!

Figure 4.2.1. What image comes to mind when you think of a volcano? Does it look like the one in this figure? If so, you thought of a strato volcano, a.k.a composite volcano.”Volcano Structure” By W. Crochot. CC BY SA 4.0.

Volcanoes have a cone-shaped structure that has been built over long periods of geologic time after multiple eruptions. At the very top of this cone is a crater. All active volcanoes sit atop a plume of magma called a magma chamber. The composition of this magma can vary from felsic to mafic depending on factors such as region, overlying crust composition, and tectonic setting.

When the magma chamber experiences too much pressure, it will erupt. The molten rock and volatile gases explode upward through a pipe-like column called a chimney. When the magma reaches the surface, it is called lava, even though the composition does not change.

Magma chambers, cones, craters, chimneys, and lava are features that are diagnostic of volcanoes. However, that is where the similarities end. Below, we will explore the different types of volcanoes on our planet.

Cinder Cones

Cross section of a cinder-cone with a central vent in the middle, fragments of rock along the rim, and a crater at the top.
Figure 4.2.2. Schematic of a typical cinder cone volcano.

Cinder cones are small volcanoes with steep sides, made of tephra and volcanic bombs ejected from a clear central vent. Cinders themselves are composed primarily of mafic lava with more volatile gases than average. Cinders are smaller pieces of tephra, or molten rock, that will erupt with lava from the volcano and rapidly cool and solidify in the air. Larger tephra rocks (over 2.5 inches) are called volcanic bombs, which are potentially deadly to anyone within range.

Cinder cone volcanoes do not last relatively long, but they are common in the United States and Mexico. Because they are usually mafic in composition, they produce extrusive igneous rock deposits such as scoria.

Lava Domes

A rounded dome of lava above a central volcanic crater
Figure 4.2.3. A rounded dome of lava above a central volcanic crater of Volcán Chaitén, Chile.

Lava Domes are fairly small structures made of felsic rocks that form within the collapsed craters of stratovolcanoes. These domes are made of extrusive rocks such as rhyolite, pumice, and obsidian that are piled around the vent. The dome-like structure is the result of the high-viscosity of the felsic to intermediate lava, which is too sticky to move long distances.

Lava domes have appeared in Mount St. Helens, Mammoth Mountain in California, and Chaiten in Chile (see Fig. 4.2.3).


Conical and steep volcano with bright orange lava flows
Figure 4.2.4. The Mayon volcano in the Philippines.

Mount St. Helens, Mount Vesuvius, Mount Fuji, Mount Pinatubo, Krakatoa— these infamous volcanoes belong to a frightening class of volcanoes that are historically known for their destruction and hazards. Stratovolcanoes (also called composite volcanoes) have steep sides and a symmetrical cone shape with an easily identifiable crater on top. These are called “composite” or “strato” because of the different layers of volcanic materials (such as ash) and lava that build up the volcano [1].

Stratovolcanoes can have magma that is anywhere from felsic to mafic in composition, although most of these volcanoes tend to be intermediate in composition. Stratovolcanoes usually form along subduction zones between oceanic-oceanic or continental-oceanic lithosphere. A good example of these volcanoes can be found along the Pacific Northwest. There, an ancient subduction zone used to exist between the North American plate and Farallon plate which formed the Cascade Mountain range and deadly stratovolcanoes including Mount Rainier and Mount St Helens. Check out the interactive model of Mt St Helens by clicking on Fig. 4.2.5.


Interactive Model of Mt St. Helens Landscape
Figure 4.2.5. Mount St Helens landscape. Click this image to go to an interactive model of the volcano and surrounding landscape by Sara Carena, CC BY-N-SA.

Shield Volcanoes

A large, shallow sloped volcano in the New Mexico desert.
Figure 4.2.6. The Sierra Grande shield volcano at Capulin Volcano National Monument, New Mexico.

The largest type of volcano is a shield volcano. These are characterized by very broad, shallow slopes, and small vents. The word “shield” refers to the shield-like shape of the volcano when it is viewed from the side. Shield volcanoes are sourced from low-viscosity mafic magma, and they typically have basaltic lava that has reached far distances along the volcanic slope. We typically observe shield volcanoes in areas where the upper mantle rises to meet the crust. These areas include hot spots, mid-ocean ridges, and continental rift zones.

Figure 4.2.7. “Hawaiian Islands” by NASA’s Earth Observatory, Public Domain.

The mafic magma below shield volcanoes does not contain too many volatile gases; therefore, when shield volcanoes erupt, they are not very explosive. Instead, these volcanic eruptions are fairly small and predictable, which makes them less of a potential hazard than others. We can find a perfect example of shield volcanoes with the Hawaiian Islands at Mauna Loa and Kilauea (right). Click here to view an interactive model of Mauna Loa, the world’s largest shield volcano!

Kilauea is the most active volcano in the world, although it does not cause many human fatalities. The eruption of Kilauea from fissures in Hawaii in 2018, however, produced lavas that did considerable damage to roads and structures [1].


Sedona, AZ is not just a resort town with rust-red rocks! About 15 million years ago, this region was volcanically active due to the migrating continental hot spot, which is responsible for the San Francisco Volcanic Field. This hot spot produced both mafic magmas and lavas and near Sedona, a large shield volcano formed that we now call “House Mountain”. House mountain is pretty huge! This large shield volcano covers the area around 84 smaller volcano vents in an area of 180 square miles. While that is still much smaller than Mauna Loa (2,035 square miles!), that is still a pretty huge volcano [2]!

House Mountain shield volcano looms over desert Verde Valley in the distance.
Figure 4.2.8. View of House Mountain shield volcano near Sedona, AZ in the Verde Valley.


Figure 4.2.9.Crater Lake” CC BY SA 3.0

Calderas are large (up to 15 miles in diameter!), crater-like depressions that form after a volcano has collapsed after it has emptied much of its magma chamber. It takes a very explosive eruption to form a caldera, so it should come as no surprise that most calderas are found at volcanoes with highly viscous felsic and intermediate magma.

Because the caldera is a basin or depression, it often is filled in by water to become a crater-lake. The Yellowstone Caldera and Crater Lake, Oregon (above) is a notable example of this type of volcano. In the figure below, you can see a diagram of how a caldera forms at Crater Lake from the Mount Mazama stratovolcano [1]. This volcano has an explosive eruption that drains the magma chamber, and causes a collapse of the vent. That collapsed feature then fills with water.

Stages of caldera lake formation: 1. A felsic/intermedia eruption occurs. 2. The volcanic dome collapses under the violence of the eruption. 3. Steam exits the crater/depression over time and water precipitates. 4. A lake forms in the crater/depression of the volcano.
Figure 4.2.10. “Mount Mazama Eruption” by the United States Geologic Survey, Public Domain.

Flood Basalts

Map of Northern Russia showing the extent of the Siberian flood basalts, which influenced the Permian-Triassic Extinction in 252 Ma.
Figure 4.2.11. Map of Northern Russia showing the extent of the Siberian flood basalts (blue line), which influenced the Permian-Triassic Extinction in 252 Ma.

Flood basalts are an uncommon type of eruption, but they are by far the largest and longest. As the name suggests, Flood Basalts are large-scale eruptions of basaltic lava. We have not seen flood basalts throughout human history, but the evidence of flood basalt activity has been found in the geologic record. We currently estimate that once volcanism begins, flood basalts will erupt for up to 1-3 million years!

Some notable examples of flood basalts include the Deccan Traps that cover about one-third of India and the Siberian Traps, which can be found in Russia. We now think that flood basalt volcanism can be a key contributor in causing mass extinctions in our planet’s history. For instance, the Siberian Traps, which were active about 252 million years ago, may have expelled greenhouse gases into the atmosphere in such large amounts that the entire planet’s temperature could have rapidly increased by 5°C!

This process of rapid warming, in combination with other factors, may have caused the largest mass extinction the world ever experienced, the Permian-Triassic Mass Extinction.

Super Volcanoes

“Super-volcanic” eruptions can impact the entire planet, and the life inhabiting it, for years. The eruption can exceed 100,000 atomic bombs! For those lucky enough to survive the initial blast, a massive amount of ash is also ejected into the atmosphere and will blanket land hundreds of kilometers away. The combination of toxic gases and ash will furthermore block out sunlight in the atmosphere and cause “volcanic winters” that last for years. Such a devastating eruption has not yet occurred in modern human society…yet.

Schematic of the Yellowstone Caldera, fed by a hotspot.
Figure 4.2.12. Schematic cross section of the Yellowstone Caldera.

The Yellowstone Hot Spot is an active caldera-type volcano that is capable of a super-volcanic eruption. Although this is a hot spot volcano, it differs greatly from the shield volcanoes of Hawaii! This is because Yellowstone is located on the continental plate of North America. This very thick plate produces felsic to intermediate magma, which will erupt violently.

Figure 4.2.13.Yellowstone Relief Map” by the United States Geologic Survey is licensed under Public Domain.

The Yellowstone caldera already erupted three times in the recent past: at 2.1, 1.3, and 0.64 million years ago [1]. Each eruption created large rhyolite lava flows and pyroclastic clouds of ash that solidified into tuff. These extra-large eruptions rapidly emptied the magma chamber, causing the roof to collapse and form a caldera. These eruptions left three calderas, and most of the roads and hotels of Yellowstone National Park are within the caldera [1].



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