1. Volcano Anatomy and Explosivity

A volcano is a location where molten rock flows out, or erupts, onto Earth’s surface as lava. Magma is the name for molten rock under the Earth’s surface and lava is the name for molten rock on the Earth’s surface. Volcanic eruptions can happen on land or underwater. Some volcanic eruptions flow from mountains, such as the eruption of Mt. St. Helen’s in Washington State (Figure 1C.1.1, left) but others do not. Fissure eruptions (Figure 1C.1.1, right) are volcanic eruptions flowing from long cracks in the Earth such as the eruption along the Kamoamoa fissure on the flank of Kilauea in Hawaii.

Left: Photograph of Mt. St. Helen's erupting with a large ash cloud. Right: photograph of lava running over the ground surface in Hawaii erupting from a long crack in the ground.
Figure 1C.1.1 Left: July 22, 1980, eruption of Mount St. Helens in Washington sent pumice and ash 6 to 11 mi (10-18 km) into the air, and was visible in Seattle, Washington (100 mi/160 km north). Right: Kamoamoa fissure eruption on the flanks of the Hawai’ian Kilauea Volcano in March of 2011. Source: Left: USGS (1980) Public Domain. Found here. Right: USGS (2011) Public Domain. Found here.

1.1 The Parts of a Volcano

The main parts of a volcano are shown in Figure 1C.1.2. The magma chamber sits deep in the Earth and is the large repository of molten rock. When volcanoes erupt, magma moves upward from the magma chamber, through conduits (the plumbing network inside the volcano) and up to the vents. The vents are the openings at the Earth’s surface through which the magma and gases escape. There is a central vent, which is the main opening at the top of the volcano, but volcanoes can also have many smaller side vents from which flank eruptions occur. A crater is the bowl-shaped depression surrounding the vent.

With continued eruptions, volcanic material accumulates around the vent forming a volcanic mountain. The accumulated material might consist of layers of solidified lava flows, fragments of volcanic debris that were ejected explosively from the volcano (the general term for this ejected material is pyroclastic material), or a combination of both.

Labeled diagram illustrating the parts of a volcano. Triangular shaped hill with dark and light colored layers representing lava flows and pyroclastic material, respectively. There is a large red oval under the hill (magma chamber) and a central tube (conduit) connecting the magma chamber to the surface (main vent) and smaller tubes branching off this to side vents where flank eruptions occur. The bowl shaped depression at the main vent is the crater.
Figure 1C.1.2 The parts of a volcano (not to scale). Source: Lindsay Iredale (2024) CC BY 4.0 adapted from Karla Panchuk (2017), CC BY 4.0. Found here. Alteration and addition of labels.

Another bowl-shaped structure that can form around the vent of a volcano is a caldera. While craters are relatively small, with diameters on the scale of 10s to 100s of meters, and are built up over time as volcanoes deposit material around the vent, calderas are much larger (km in scale) and form when a volcano collapses on itself. This process is illustrated in Figure 1C.1.3. It begins when an eruption occurs and the magma chamber beneath the volcano is drained. If a significant part of a volcano’s mass is supported by magma within the chamber, then depleting the magma chamber also reduces the support for the volcano. The loss of support causes part of the volcano to collapse into the void created by an empty magma chamber, leaving behind a broad basin rimmed by the remnants of the volcano. Over time, the basin can fill with water. If there is still activity within the magma chamber, magma may force its way upward again causing the floor of the caldera to be lifted or erupting to form a new volcano within the caldera.

Formation of a caldera. Calderas are the result of a volcano collapsing into a drained magma chamber. Source: Karla Panchuk CC BY 4.0. Modified after U. S. Geological Survey (2002)
Figure 1C.1.3 Formation of a caldera. Calderas are the result of a volcano collapsing into a drained magma chamber. Source: Karla Panchuk (2017), CC BY 4.0. Found here. Modified after U. S. Geological Survey (2002), Public Domain. Image source.

The island of Santorini (Figure 1C.1.4) is an example of a caldera. The island itself is the rim of the caldera, and the bay is the flooded basin. The two small islands in the middle of the bay formed from magma refilling the chamber that feeds the volcano (Figure 1C.1.4 right). The caldera formed after an enormous eruption between 1627 and 1600 BCE. The eruption is thought to have contributed to the downfall of the Minoan civilization, and some speculate that it might also be the source of the myth of Atlantis, a story about a lost continent that sank beneath the sea after a natural disaster.

Left: Satellite photograph of Santorini which is a horseshoe shaped ring of land surrounded by ocean. Inside the horseshoe is a large bay with a small island in the center. Right: Photograph from the island across the bay where the small central island can be seen.
Figure 1C.1.4 The Greek Island of Santorini. Left: Aerial view of the island forming a ring around a flooded caldera. Right: A view from the rim of the caldera. The other side of the rim is visible in the distance. Source: Karla Panchuk (2017), CC BY-SA 4.0. Found here.

Check your understanding: Volcano Anatomy

1.2 Volcanic Explosivity

Whether a volcano erupts explosively or effusively (non-explosively) is determined by several interconnected factors:

  • viscosity of the magma
  • composition of the magma
  • temperature of the magma
  • amount of gas trapped in the magma.

Viscosity is the resistance of a material to flow. So, materials that are highly viscous do not flow easily. They can be though of as “sticky”, like honey. Materials that have low viscosity flow easily and are fluid and runny, like water. Magma that is viscous (sticky) is far more explosive than runny magma.

Check your understanding: Viscosity

The viscosity of magma is a function of its composition and temperature. Figure 1C.1.5 below illustrates this relationship. This is the same figure showing igneous rock classification as seen back in Chapter 1A.1 (Figure 1A.1.2) on Earth’s composition and shown again when explaining partial melting in Chapter 1A.3 but with temperature and viscosity information added.

Terminology note: Magmas get named in two ways. One is based on the igneous rock type they solidify to make (basaltic magma solidifies to make basalt and andesitic magma solidifies to make andesite, etc.). The other is based on silica content: mafic, intermediate, felsic. These two naming conventions often get used interchangeably, so a basaltic magma is the same as a mafic magma and an andesitic magma is an intermediate magma, etc.

Remember that minerals do not all melt at the same temperature (this was the chocolate chip ice cream analogy), which means magmas of different mineral compositions will have different temperatures (newly melted ice cream is still pretty cold, but melted chocolate chips will be quite warm!). Three magmas important in building volcanoes are rhyolitic, andesitic, and basaltic magmas. Rhyolitic magmas are cooler than andesitic magmas, which are cooler than basaltic magmas. To connect magma temperatures to viscosity is intuitive: which flows more easily, warm honey just heated up in the microwave or cold honey just taken out of a fridge? The warm honey flows easier; it is less “sticky”/less viscous.

The relationship between viscosity and magma composition is less intuitive, although for any who like to cook, a good analogy might be that of using a roux or cornstarch to thicken a soup (or sauce, or gravy…mmm…gravy). The more roux/cornstarch added, the thicker (more viscous) the soup will be. For magma, this “thickening” agent is silica. On the igneous rock chart, working from left to right through the compositions, a felsic magma composition (rhyolite) has the most silica, and is therefore the most viscous magma, an intermediate magma composition (andesite) has slightly less silica, and a mafic magma composition (basalt) has less silica still and is therefore the least viscous (see top and bottom arrows on Figure 1C.1.5).

From left to right fine grained igneous rocks are rhyolite, andesite, basalt. Arrows indicate that magma silica content is highest on the left and decreases to the right, magma temperature is lowest on the left and increases to the right, and magma viscosity is highest on the left and decreases to the right. Andesitic and Rhyolitic magmas (felsic and intermediate compositions) are high viscosity, high silica, cold magmas; "sticky magma". Basaltic magma (mafic composition) is low viscosity, low silica, hotter magma; "runny magma"
Figure 1C.1.5 Igneous rock classification chart with information about how composition (silica content) relates to magma temperature and magma viscosity as indicated by arrows. The general relationship is magma compositions on the left-hand side of the diagram (rhyolite and andesite) are higher viscosity with lower temperatures and higher silica; these magmas are “sticky”. Magma compositions towards the right-hand side of the diagram (basalt) have lower viscosity with higher temperatures and lower silica; these magmas are “runny”. Source: Lindsay Iredale (2024). CC BY 4.0.

The final factor affecting explosivity of an eruption, the amount of gas trapped in a magma, is also related to viscosity. A thicker, stickier, more viscous magma is more easily able to trap gas inside it. Magmas that have more trapped gas are more explosive than magmas where the gas can easily escape. For the final food analogy of this section, think of two bottles of pop. One was accidentally left uncapped all night and went mostly flat before it was capped again and has since been sitting on the counter – this will be the “runny” magma. The other has been tightly capped, never opened, and bouncing around in a backpack all day – this will be the “sticky” magma. (It is not a perfect analogy because in this case the pops are the same viscosity, but it works because one has more gas trapped inside than the other – which is what is needed for the analogy). Which of those two pop bottles is more likely to explode and spray your nice white shirt with pop when you try to open it? The one that is fresh and has been bouncing around all day! All of that trapped gas in the pop is going to come rushing out when the cap is removed, and the pressure gets released causing an explosion of pop. The flat pop does not have much gas left to lose so it does not explode as much. The same thing happens with volcanic eruptions; the more gas that is trapped within a magma the more likely those gases will explode out (along with the magma) once the pressure is released, which for an eruption is once the magma starts to move to the surface.

This means basaltic magmas tend to create effusive eruptions while rhyolitic and andesitic magmas make explosive eruptions.

Check your understanding: magma compositions

 

Check your understanding: Explosivity

Drag and drop the correct labels to complete the chart below. This incorporates all of the magma elements controlling volcanic explosivity.

References

Friedrich, W. L., Kromer, B., Friedrich, M., Heinemeier, J., Pfeiffer, T., & Talamo, S. (2006). Santorini Eruption Radiocarbon Dated 1627-1600 B.C. Science (312)5773, 548. doi: 10.1126/science.1125087

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