The products of volcanism that build volcanoes and leave lasting marks on the landscape include lava flows that vary in viscosity and gas content, and tephra ranging in size from less than a mm to blocks with masses of many tonnes. Individual volcanoes vary in the volcanic materials they produce, and this affects the size, shape, and structure of the volcano.

There are three types of volcanoes: massive caldera complexes, composite volcanoes (also called stratovolcanoes), and shield volcanoes. Figure 8.22 illustrates the size and shape differences amongst these volcanoes.

Shield volcanoes, which get their name from their broad rounded shape, are the largest. Figure 8.22 shows the largest of all shield volcanoes- in fact, the largest of all volcanoes on Earth- Mauna Loa, which makes up a substantial part of the Island of Hawai‘i and has a diameter of nearly 200 km. The summit of Mauna Loa is presently 4,169 m above sea level, but this represents only a small part of the volcano. It rises up from the ocean floor at a depth of approximately 5,000 m. Furthermore, the great mass of the volcano has caused it to sag downward into the mantle by an additional 8,000 m. In total, Mauna Loa is a 17,170 m thick accumulation of rock.

Kīlauea Volcano is also a shield volcano, albeit a much flatter one. Kīlauea Volcano rises only 18 m about the surrounding terrain, and is almost not visible in the scale of the diagram, however it still stretches over a distance of 125 km along the eastern side of the Island of Hawai‘i.

Composite volcanoes are the next largest. Mt. St. Helens is shown on the left of Figure 8.22. It rises 1,356 m above the surrounding terrain in the Cascade Range of the western United States, and has a diameter of approximately 6 km. Composite volcanoes tend to be no more than 10 km in diameter. Unlike shield volcanoes, composite volcanoes have a distinctly conical shape, with sides that steepen toward the summit.

Caldera complexes are the largest type of volcano and the most dangerous. They are formed by multiple caldera forming eruptions. Calderas are large depressions in the ground, formed from the collapse of the roof of the magma reservoir into the magma chamber. Yellowstone is a large caldera complex, made from three caldera forming eruptions, located in Wyoming, America. Yellowstone Caldera is a 50 by 70 km depression that is still active today.

Volcano Structure

Shield Volcanoes

Shield volcanoes, like the Sierra Negra volcano in the Galápagos Islands (Figure 8.23, top), get their gentle hill-like shape because they are built of successive flows of low-viscosity basaltic lava (Figure 8.23, bottom). The low viscosity of the lava means that it can flow for long distances, resulting in the greater size of shield volcanoes compared to composite volcanoes.

Composite Volcanoes (Stratovolcanoes)

Composite volcanoes, like Cotopaxi in Figure 8.24 (top), consist of layers of lava alternating with layers of tephra (blocks, bombs, lapilli, and ash; Figure 8.24, bottom). The layers (strata) is where the alternative name, stratovolcano comes from. Cotopaxi displays the characteristic shape of composite volcanoes, which have slopes that get steeper near the top of the volcano. The change in the slope reflects the accumulation of tephra fragments near the volcano’s vent. Composite volcanoes typically erupt higher viscosity andesitic and rhyolitic lavas, which do not travel as far from the vent as basaltic lavas do. This results in volcanoes of smaller diameter than shield volcanoes. A notable exception is Mt. Fuji in Japan, which erupts basaltic lava.

Figure 8.24 Composite volcano. Top: Cotopaxi in Ecuador exhibits the upward-steepening cone characteristic of composite volcanoes. Bottom: Diagram of a composite volcano showing alternating layers of lava and tephra. Sources: Karla Panchuk (2017) CC BY 4.0; Top photo by Simon Matzinger (2014) CC BY 2.0 view source. Click the image for more attributions. 

From a geological perspective, composite volcanoes tend to form relatively quickly and do not last very long. If volcanic activity ceases, it might erode away within a few tens of thousands of years. This is largely because of the presence of pyroclastic eruptive material, which is not strong.

Caldera Complexes (Supervolcanoes)

In certain areas of the world, huge calderas have been found to be the remains of volcanic eruptions of enormous scale (Figure 8.25). These calderas are volcanic features that are formed by the collapse of a huge amount of land due to the powerful eruptions. Caldera comes from Latin word, meaning cauldron. Calderas are generally circular shaped geographic formations like the picture in figure 8.25. These are not singular mountains but entire geographical areas. Yellowstone National Park in Wyoming is another caldera that has blown about a hundred times in the last 16 million years.

Supervolcanoes represent the most dangerous type of volcano. An eruption from a supervolcano could change life on Earth as we know it for many years. Supervolcanoes were not even accepted in volcanology until this millennium. Many supervolcano eruptions are thought to have occurred, the most recent in New Zealand less than 2000 years ago. That explosion was thought to have ejected about 100 cubic kilometers of material. A supervolcano eruption near what is now Colorado was thought to have let loose over 5,000 cubic kilometers of material millions of years ago. In comparison, the Mt. Saint Helens eruption ejected about 1 cubic kilometer of material.

The eruptions from supervolcanoes can be so large that the ash ejected into the air blocks the Sun and lowers the temperature on the entire planet. The lowered temperatures caused by these eruptions is called a volcanic winter. A supervolcano eruption at Lake Toba in northern Sumatra may have annihilated about 60% of the world’s human population about 75,000 years ago. One can only imagine how such a huge eruption would change the world in modern times.

The largest supervolcano in North America is the Yellowstone caldera, which had three super eruptions at 2.1 million, 1.3 million and 640,000 years ago, and much more recent smaller (but still enormous) eruptions. Long Valley caldera, south of Mono Lake in California, is the second largest supervolcano in North America, erupting extremely hot and explosive rhyolite around 700,000 years ago. An earthquake swarm in 1980 alerted geologists to the possibility of another eruption in the future, but the timing of such an event is unknown.

Supervolcanoes are a fairly new idea so the exact cause of supervolcano eruptions is still debated. However, scientists believe that an entire and very large magma chamber erupts in a catastrophic explosion. This enormous eruption creates a huge hole or caldera where the surface area collapses.

Video: Geoscience Videos – How to Classify Volcanos

References

Rubin, K. (n.d.) Mauna Loa Volcano. Retrieved 23 August 2017. Visit website