11.2: Materials Produced by Volcanic Eruptions

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

Volcanic Gas

Magma contains gas. Most of the gas emitted by volcanoes is water, but gas also contains carbon dioxide (CO₂), sulphur dioxide (SO₂), and hydrogen sulphide (H₂S), in order of decreasing abundance.

.At high pressures, the gases are dissolved in the magma, but if the pressure decreases, the gas comes out of solution, forming bubbles, similar to what happens when you open a bottle of pop. Pop is bottled under pressure, forcing carbon dioxide gas to dissolve into the fluid. As long as the bottle is closed and the pressure remains high, you’ll see few to no bubbles in the pop. But if you open the bottle, air rushes out, decreasing the pressure on the pop. The pop will begin to fizz as carbon dioxide gas comes out of solution and forms bubbles.

Volcanoes release gases when erupt, and through openings called fumaroles (Figure 11.7). They can also release gas into soil and groundwater.

Figure 11.7 A fumarole at Puʻu ʻŌʻō Crater, Hawaii. The yellow crust along the margin of the fumarole is made of sulphur crystals. The crystals form when sulphur vapour cools as it is released from the fumarole. Source: U. S. Geological Survey (2016), Public Domain. View source.

Lava

The ease with which lava flows, and the structures it forms depend on how much silica and gas are in the lava. The more silica, the more polymerization (formation of long molecules) happens, stiffening the lava. The stiffness of lava is described in terms of viscosity: lava that flows easily has low viscosity, and lava that is sticky and stiff has high viscosity.

In general, higher-silica (felsic) lava contains more gas than low-silica lava. When the gas forms into bubbles, viscosity increases further. Consider the pop analogy again: If you shook the bottle vigorously, then opened it, the pop would gush out in a thick, frothy flow. In contrast, if you took care to not shake the bottle before opening it, you could pour out a thin stream of fluid. The presence of gas not only makes high-silica lava more viscous, but can affect mafic lavas in that way as well.

Concept Check: Viscosity

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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 can go 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 11.8 shows a dome formed from rhyolitic lava in the crater of Mt. St. Helens.

Figure 11.8 Lava dome in the crater of Mt. St. Helens. Source: Terry Feuerborn (2011), CC BY-NC 2.0. View source.

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

Figure 11.9 Lava flows. Left: A geologist collects a sample from a basaltic lava flow in Hawaii. Right: an andesitic lava flow from Kanaga Volcano in the Aleutian Islands. Source: Left- U. S. Geological Survey (2014), Public Domain. View source.; Right- Michelle Combs, U. S. Geological Survey (2015), Public Domain. View source.

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 11.10 (top) shows a view into a lava tube through a hole in the overlying rock, called a skylight. Figure 11.10 (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.

Figure 11.10 Lava tubes. Top: An opening in the roof of a lava tube (called a skylight) permitting a view of lava flowing through the tube (Puʻu ʻŌʻō crater, Kīlauea). The opening is approximately 6 m across. Bottom: Inside a lava tube that channelled lava away from Mt. St. Helens in an eruption 1,895 years ago. Sources: Top: U. S. Geological Survey (2016), Public Domain. View source. Bottom: Thomas Shahan (2013), CC BY-NC 2.0. View source.

Lava Structures

Pahoehoe

Lava flowing on the surface can take on different shapes as it cools. Basaltic lava with an unfragmented surface (e.g., Figure 11.9, right), is called pahoehoe. (pronounced pa-hoy-hoy). Pahoehoe can be smooth and billowy. It can also develop a wrinkled texture, called ropy lava (Figure 11.11). Ropy lava forms when the outermost layer of the lava cools and develops a skin (visible as a dark layer in Figure 11.11, 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 11.11 (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.

Figure 11.11 Ropy lava from Hawaii. Left: Ropy texture forming as a thin surface layer of lava cools and is wrinkled by the motion of lava flowing beneath it. Right: Cross-section view of ropy lava. Sources: Left: Z. T. Jackson (2005), CC BY NC-ND 2.0. View source.; Right: Fiddledydee (2011), CC BY-NC 2.0. View source..

A’a and Blocky Lava

When the outer layer of the lava flow can’t 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 11.12 (left) shows a close-up view of the advancing front of an a’a lava flow (the flow is moving toward the viewer). Figure 11.12 (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.

Figure 11.12 Aa lava flows. Left: Close-up view of a’a forming during an eruption of Pacaya Volcano in Guatemala. Field of view approximately 1 m across. Right: Rubbly reddish-brown a’a lava flow viewed from Chain of Craters Road, Hawai’i Volcanoes National Park. Pahoehoe is visible in lighter grey in the foreground. Sources: Photo of Hawaiian aa and pahoehoe: Roy Luck (2009), CC BY 2.0. View source. Pacaya a’a: Greg Willis (2008), CC BY-SA 2.0 (labels added). View source..

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 11.9 (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 11.13 (top left)). Blobs of lava develop at the end of the tube (Figure 11.13 (top right)), forming pillows. The bottom left of Figure 10.13 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.

Figure 11.13 Pillow lavas. Top left: A tube of lava extruding underwater. Hot lava can be seen through cracks in the wall of the tube. The image is approximately 1 m across. (Pacific Ocean, near Fiji). Top right: The rounded end of a tube with cracks showing the lava within. (Pacific Ocean, near Fiji). Bottom left: sea floor near the Galápagos Islands covered with pillow lavas. Bottom right: A boulder made of 2.7 billion year old pillows derived from the Ely Greenstone in north-eastern Minnesota. Sources: Top left- NSF and NOAA (2010), CC BY 2.0. View source.; Top right- NSF and NOAA (2010), CC BY 2.0. View source.; Bottom left- NOAA Okeanos Explorer Program, Galápagos Rift Expedition 2011 (2011), CC BY 2.0. View source.; Bottom right- James St. John (2015), CC BY 2.0. View source..

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 11.14).

Figure 11.14 Columnar joints viewed from above, Giant’s Causeway, Northern Ireland. Source: Meg Stewart (2012), CC BY-SA 2.0. View source.

Figure 11.15 shows a side view of columnar joints in a basaltic lava flow in Iceland.

Figure 11.15 Columnar joints in a basaltic lava flow, Svartifoss (Black Fall) Vatnajökull National Park, Iceland. Source: Ron Kroetz (2015), CC BY-ND 2.0. View source.

Practice with Structures Made by Lava

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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 11.16 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.

Figure 11.16 Volcanic ash. Upper left: Ash from 2010 eruption of Eyjafjallajökull in Iceland, magnified approximately 1000x. Upper right- Ash from the 1980 eruption of Mt. St. Helens, collected at Yakima, Washington. Bottom: Indonesian village after the eruption of Mt. Merapi in 2010. Sources: Upper left: Birgit Hartinger, AEC (2010), CC BY-NC-ND 2.0. View source. Upper right: James St. John (2014), CC BY 2.0 (scale added). View source. Bottom: AusAID/Jeong Park (2010), CC BY 2.0. View source.

Lapilli

Fragments with dimensions between 2 mm and 64 mm are classified as lapilli. Figure 11.17 (upper left) shows lapilli at the ancient city of Pompeii, which was buried when Mt. Vesuvius erupted in 79 C.E. Figure 11.17 (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 11.17, right). The dark masses in Figure 11.17 (right) within the Pele’s hair are Pele’s tears.

Figure 11.17 Lapilli are pyroclasts ranging between 2 mm and 64 mm in size. Upper left: lapilli from the site of the ancient city of Pompeii. Lower left: Pele’s tears, a type of lapilli that forms when droplets of lava fly through the air. Right: Pele’s hair, which form when Pele’s tears are drawn out into thin threads as they fly. Sources: Upper left: Pauline (2009), CC BY-NC-ND 2.0. View source.; Lower left: James St. John (2014), CC BY 2.0 (scale added) View source.; Right: James St. John (2009), CC BY 2.0 (scale added) View source.

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 11.18 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.

Figure 11.18 Volcanic block weighing approximately 7 tonnes thrown 1 km from the Halema‘uma‘u crater at Kīlauea Volcano on May 18, 1924. Source: U. S. Geological Survey (1924), Public Domain. View source.

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 11.19.

Figure 11.19 Volcanic bomb with a streamlined shape. Source: James St. John (2016), CC BY 2.0 (scale added). View source.

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 11.20) forms from gas-filled felsic lava. Figure 11.20 (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.

Figure 11.20 Lapilli-sized pumice collected from the shores of Lake Atitlán in Guatemala by H. Herrmann. The lake is a flooded caldera, and is surrounded by active volcanoes. Right: Magnified view showing vesicular structure and amphibole crystals (dark patches). Source: Karla Panchuk (2017), CC BY 4.0.

The mafic counterpart to pumice is scoria (Figure 11.21, left). Mafic lava can also form reticulite (Figure 11.21, right), a rare and fragile rock in which the walls surrounding the bubbles have all burst, leaving behind a delicate network of glass.

Figure 11.21 Mafic lapilli with vesicular textures. Left: Scoria from Mount Fuji, Japan. Scoria is the denser mafic counterpart to pumice. Right: Reticulite from Kīlauea Volcano. Reticulite is a delicate network of volcanic glass that forms when the walls separating gas bubbles pop. Sources: Left- James St. John (2014), CC BY 2.0 (scale added). View source. Right- James St. John (2014), CC BY 4.0 (scale added). View source..

Practice with Types of Pyroclastic Material

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References

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