Volcanic eruptions produce three types of materials: volcanic gas, lava, and fragmented debris called tephra.

6.3.1 Volcanic Gas

As discussed earlier, magma contains gas. At high pressures, the gases are dissolved within magma. However, if the pressure decreases, the gas comes out of solution, forming bubbles. The main component of volcanic gas emissions is water vapor, followed by carbon dioxide (CO₂), sulfur dioxide (SO₂), and hydrogen sulfide (H₂S). Volcanoes release gases when erupt, and through openings called fumaroles (Figure 6.12). They can also release gas into soil and groundwater.

6.3.2 Lava

Lava is the molten (i.e., liquid) rock that erupts onto Earth’s surface from volcanos. Remember that magma is different from lava, as magma is the molten rock underground.

The structures that lava forms depends on its viscosity and gas content. Viscosity is a measure of a fluid’s resistance to flow. In other words, it describes how stiff or sticky a fluid is. Lava that flows easily has low viscosity, and lava that is sticky and stiff has high viscosity. Temperature and composition are factors that affect the viscosity of a lava (or magma). In general, hotter lavas tend to have lower viscosity (i.e., are runnier) while colder lavas have higher viscosity (i.e., are stickier). You may recall that, when we discuss the composition of lava (or magma), we use the words felsic and mafic. Felsic lavas have higher silica content, meaning they have a higher proportion of SiO₂. The higher silica content enables more polymerization (formation of long molecules) which stiffens the lava, making it more viscous. On the other hand, mafic lavas have lower silica content which means they experience less polymerization on the chemical level, making them less viscous. Overall, the trend is this: the higher the silica content, the higher the viscosity of the lava.  

The general relationship between the silica content and gas content of molten rock is shown in Figure 6.13. Although there are many exceptions to this trend, mafic magmas typically have 1% to 3% volatiles, intermediate magmas have 3% to 4% volatiles, and felsic magmas have 4% to 7% volatiles—so felsic magmas tend to have higher gas content.

Eruption Style Depends on Viscosity and Gas Content

Differences in viscosity and volatile levels have significant implications for the nature of volcanic eruptions. When magma is deep beneath the surface and under high pressure from the surrounding rocks, the gases remain dissolved. As magma approaches the surface, the pressure exerted on it decreases. Gas bubbles start to form, and the more gas there is in the magma, the more bubbles form. If the magma is runny (i.e., has a low viscosity), gases will easily rise up and escape to the surface, preventing pressure from becoming excessive in the magma chamber. Assuming that it can break through to the surface, this magma will flow out relatively gently a. An eruption that involves a steady, non-violent flow of magma is called effusive.

If the magma has a high viscosity—like felsic ones typically have—or if it has a particularly high gas content, gases will not be able to escape easily, causing pressure to build up as the magma rises toward the surface. Viscous magma doesn’t flow easily, so even if there is a conduit for it to move towards surface, it may not flow out. Under these circumstances, pressure will continue to build as more magma moves up from lower in the chamber and more gases continue to exsolve. When the gas exsolves into bubbles, viscosity increases further. Eventually some part of the volcano will break and then all of that pent-up pressure will lead to an explosive eruption.

Under pressure!

A good analogy for a magma chamber in the upper crust is a plastic bottle of pop on the supermarket shelf. Go to a supermarket and pick one up off the shelf (something not too dark). You’ll find that the bottle is hard because it was bottled under pressure, and you should be able to see that there are no gas bubbles inside.

Buy a couple small bottles of pop (you don’t have to drink it!). If you take care to not shake one bottle and slowly open it, you can pour out a thin stream of fluid without spillage. In this scenario, the gas bubbles that form when you remove the lid (and thus remove the pressure on the pop) had time to escape, so it replicates an effusive eruption. Now, if you shake the other bottle and open it quickly (best to do this outside!), you’ll enhance the processes of bubble formation, and when you open the lid, the pop will come gushing out in a thick, frothy flow, just like an explosive volcanic eruption.

A pop bottle is a better analogue for a volcano than the old baking soda and vinegar experiment that you did in elementary school, because pop bottles—like volcanoes—come pre-charged with gas pressure. All we need to do is release the confining pressure and the gases come bubbling out, forcing the pop with them.

Chemical Composition Affects the Thickness and Shape of Lava Flows

The thickness and shape of a lava flow depends on its viscosity. The greater the viscosity, the thicker the flow, and the shorter the distance it travels before solidifying. Highly viscous lava might not flow very far at all, and simply accumulate as a bulge, called a lava dome, in a volcano’s crater. Figure 6.15 shows a dome formed from rhyolitic lava in the crater of Mt. St. Helens.

Less viscous rhyolitic lava can travel further, as with the thick flow in Figure 11.9 (right). The left of Figure 6.16 shows thin streams of freely-flowing, low-silica, low-viscosity basaltic lava.

Low-viscosity basaltic lava flows may travel extended distances if they move through conduits called lava tubes. These are tunnels within older solidified lava flows. Figure 6.17 (top) shows a view into a lava tube through a hole in the overlying rock, called a skylight. Figure 6.17 (bottom) shows the interior of a lava tube, with a person for scale. Lava tubes form naturally and readily because flowing mafic lava preferentially cools near its margins, forming solid lava levées that eventually close over the top of the flow. Lava within tubes can flow for 10s of km because the tubes insulate the lava from the atmosphere and slow the rate at which the lava cools. The Hawai’ian volcanoes are riddled with thousands of old, drained lava tubes, some as long as 50 km.

6.3.3 Lava Structures

Pahoehoe

Lava flowing on the surface can take on different shapes as it cools. Basaltic lava with an unfragmented surface, like that in Figure 6.16 (right), is called pahoehoe. (pronounced pa-hoy-hoy). Pahoehoe can be smooth and billowy. It can also develop a wrinkled texture, called ropy lava, as shown in Figure 6.18. Ropy lava forms when the outermost layer of the lava cools and develops a skin (visible as a dark layer in Figure 6.18, left), but the skin is still hot and thin enough to be flexible. The skin is stiffer than the lava beneath it, and is dragged by flowing lava and folded up into wrinkles. Figure 6.18 (right) is a close-up view after a cut has been made to show the internal structure of a wrinkled lava flow. Notice the many holes, or vesicles, within the lava, formed when the lava solidified around gas bubbles.

A’a and Blocky Lava

If the outer layer of the lava flow cannot accommodate the motion of lava beneath by deforming smoothly, the outer layer will break into fragments as lava moves beneath it. This could happen if the lava flow develops a thicker, more brittle outer layer, or if it moves faster. The result is a sharp and splintery rubble-like lava flow called a’a (pronounced like “lava” but without the l and v). Figure 6.19 (left) shows a close-up view of the advancing front of an a’a lava flow (the flow is moving toward the viewer). Figure 6.19 (right) shows an a’a lava flow viewed from the side. Compare the texture of the a’a flow with the texture of the lighter-grey pahoehoe lava in the foreground of the picture.

Higher viscosity andesitic lava flows also develop a fragmented surface, called blocky lava. This is visible in the toe of the andesitic lava flow from Figure 6.16 (right). The difference between a’a and the andesitic blocky lava is that the blocky lava has fragments with smoother surfaces and fewer vesicles.

Lava Pillows

When lava flows into water, the outside of the lava cools quickly, making a tube (Figure 6.20 (top left)). Blobs of lava develop at the end of the tube (Figure 6.20 (top right)), forming pillows. The bottom left of Figure 6.20 shows pillows covering the sea floor, and the bottom right shows the distinctive rounded shape of pillows in outcrop. Because pillows always form underwater, finding them in the rock record gives us information that the environment was underwater.

Columnar Joints

When lava flows cool and solidify, they shrink. Long vertical cracks, or joints, form within the brittle rock to allow for the shrinkage. Viewed from above, the joints form polygons with 5, 6, or 7- sides, and angles of approximately 120º between sides (Figure 6.21).

Figure 6.22 shows a side view of columnar joints in a basaltic lava flow in Iceland.

6.3.4 Pyroclastic Materials

The pop bottle analogy illustrates another key point about gas bubbles in fluid, which is that the bubbles can propel fluid. In the same way that shaking a pop bottle to make more bubbles will cause pop to gush out when the bottle is opened, gas bubbles can violently propel lava and other materials from a volcano, creating an explosive eruption.

Collectively, loose material thrown from a volcano is referred to as tephra. Individual fragments are referred to in general terms as pyroclasts, so sometimes tephra is also referred to as pyroclastic debris. Pyroclasts are classified according to size.

Volcanic Ash

Particles less than 2 mm in diameter are called volcanic ash. Volcanic ash consists of small mineral grains and glass. Figure 6.23 shows volcanic ash on three scales: in the upper left is ash from the 2010 eruption of Eyjafjallajökull in Iceland. The image was taken with a scanning electron microscope at approximately 1000 times magnification. In the upper right is ash from the 1980 eruption of Mt. St. Helens, collected in Yakima, Washington, about 137 km northeast of Mt. St. Helens. Individual particles are under 1 mm in size. Figure 11.16 (bottom) shows a village near Mt. Merapi in Indonesia dusted in ash after an eruption 2010.

Lapilli

Fragments with dimensions between 2 mm and 64 mm are classified as lapilli. Figure 6.24 (upper left) shows lapilli at the ancient city of Pompeii, which was buried when Mt. Vesuvius erupted in 79 C.E. Figure 6.24 (lower left) is a form of lapilli called Pele’s tears, named after the Hawai’ian diety Pele. Pele’s tears form when droplets of lava cool quickly as they are flung through the air. Rapidly moving through the air may draw the Pele’s tears out into long threads called Pele’s hair (Figure 6.24, right). The dark masses in Figure 6.24 (right) within the Pele’s hair are Pele’s tears.

Blocks and Bombs

Fragments larger than 64 mm are classified as blocks or bombs, depending on their origin. Blocks are solid fragments of the volcano that form when an explosive eruption shatters the pre-existing rocks. Figure 6.25 shows one of many blocks from an explosive eruption at the Halema‘uma‘u crater at Kīlauea Volcano in May of 1924. The block has a mass of approximately 7 tonnes and landed 1 km from the crater.

Bombs form when lava is thrown from the volcano and cools as it travels through the air. Traveling through the air may cause the lava to take on a streamlined shape, as with the example in Figure 6.26.

Effects of Gas on Lapilli and Bombs

The presence of gas in erupting lava can cause lapilli and bombs to take on distinctive forms as the lava freezes around the gas bubbles, giving the rocks a vesicular (hole-filled) texture. Pumice (Figure 6.27) forms from gas-filled felsic lava. Figure 6.27 (right), shows a magnified view of the sample on the left. The dark patches in the photograph are mineral crystals that formed in the magma chamber before the lava erupted. Pumice floats on water because some of the holes are completely enclosed, and air-filled.

The mafic counterpart to pumice is scoria (Figure 6.28, left). Mafic lava can also form reticulite (Figure 6.28, right), a rare and fragile rock in which the walls surrounding the bubbles have all burst, leaving behind a delicate network of glass.

Text Attributions and References

This text of this chapter is adapted from:

• Section 11.2 of Physical Geology, First University of Saskatchewan Edition (2019) by Karla Panchuk. Licensed under CC BY-NC-SA 4.0, except where otherwise noted.
• Sections 4.2 of Physical Geology – 2nd Edition (2019) by Steven Earle. Licensed under CC BY 4.0, except where otherwise noted.

Other references used to compose text include:

• U. S. Geological Survey (2013) Mt. St. Helens National Volcanic Monument. Retrieved on 11 June 2017. Visit website

Media Attributions

1. Figure 6.13: Original image by Schminke, 2004. (Schminke, H-U., 2004, Volcanism, Springer-Verlag, Heidelberg). Modified by Steven Earle.