The Products of Volcanic Eruptions

Products of Volcanic Eruptions

The drama of a volcanic eruption transfers materials from inside the Earth to our planet’s surface. Products of an eruption come in three forms lava flows, pyroclastic debris, and gas. Note that we use the name flow for both a molten, moving layer of lava and for the solid layer of rock that forms when the lava freezes.

Lava Flows 

The character of a lava flow depends on its viscosity.

Sometimes it races down the side of a volcano like a fast moving, incandescent stream, sometimes it builds into a rubble-covered mound at a volcano’s summit, and sometimes it oozes like a sticky but scalding paste. Clearly, not all lava behaves in the same way when it rises out of a volcano. Therefore, not all lava flows look the same. Why? The character of a lava primarily reflects its viscosity (resistance to flow), and not all lavas have the same viscosity. Differences in viscosity depend, in turn, on chemical composition, temperature, gas content, and crystal content. Silica content plays a particularly key role in controlling viscosity. Silica poor (basaltic) lava is less viscous, and thus flows farther than does silica-rich (rhyolitic) lava (figure above). To illustrate the different ways in which lava behaves, we now examine flows of different compositions.

Basaltic lava flows 

Features of basaltic lava flows. They have low viscosity thus can flow for long distances. Their surface and interior can be complex.

Basaltic (mafic) lava has very low viscosity when it first emerges from a volcano because it contains relatively little silica and is very hot. Thus, on the steep slopes near the summit of a volcano, it can flow very quickly, sometimes at speeds of over 30 km per hour (figure above a). The lava slows down to less-than-walking pace after it starts to cool (figure above b). Most flows measure less than a few km long, but some flows reach as far as 600 km from the source. How can lava travel such distances? Although all the lava in a flow moves when it first emerges, rapid cooling causes the surface of the flow to crust over after the flow has moved a short distance from the source. The solid crust serves as insulation, allowing the hot interior of the flow to remain liquid and continue to move. As time progresses, part of the flow’s interior solidifies, so eventually, molten lava moves only through a tunnel-like passageway, or lava tube, within the flow the largest of these may be tens of meters in diameter. In some cases, lava tubes drain and eventually become empty tunnels.

The surface texture of a basaltic lava flow when it finally freezes reflects the timing of freezing relative to its movement. Basalt flows with warm, pasty surfaces wrinkle into smooth, glassy, rope-like ridges; geologists have adopted the Hawaiian word pahoehoe (pronounced “pa-hoy-hoy”) for such flows (figure above c). If the surface layer of the lava freezes and then breaks up due to the continued movement of lava underneath, it becomes a jumble of sharp, angular fragments, creating a rubbly flow also called by its Hawaiian name, a’a’ (pronounced “ah-ah”) (figure above d). Footpaths made by people living in basaltic volcanic regions follow the smooth surface of pahoehoe rather than the foot-slashing surface of a’a’. 

During the final stages of cooling, lava flows contract, because rock shrinks as it loses heat, and may fracture into polygonal columns. This type of fracturing is called columnar jointing (figure above e). 

Basaltic flows that erupt underwater look different from those that erupt on land because the lava cools so much more quickly in water. Because of rapid cooling, submarine basaltic lava can travel only a short distance before its surface freezes, producing a glass-encrusted blob, or “pillow” (figure above f). The rind of a pillow momentarily stops the flow’s advance, but within minutes the pressure of the lava squeezing into the pillow breaks the rind, and a new blob of lava squirts out, freezes, and produces another pillow. In some cases, successive pillows add to the end of previous ones, forming worm-like chains.

Andesitic and rhyolitic lava flows

This rhyolite dome formed about 650 years ago, in Panum Crater, California. Tephra (cinders) accumulated around the vent.

Because of its higher silica content and thus its greater viscosity, andesitic lava cannot flow as easily as basaltic lava. When erupted, andesitic lava first forms a large mound above the vent. This mound then advances slowly down the volcano’s flank at only about 1 to 5 m a day, in a lumpy flow with a bulbous snout. Typically, andesitic flows are less than a few km long. Because the lava moves so slowly, the outside of the flow has time to solidify; so as it moves, the surface breaks up into angular blocks, and the whole flow looks like a jumble of rubble called blocky lava.
Rhyolitic lava is the most viscous of all lavas because it is the most silicic and the coolest. Therefore, it tends to accumulate either above the vent in a lava dome (figure above), or in short and bulbous flows rarely more than 1 to 2 km long. Sometimes rhyolitic lava freezes while still in the vent and then pushes upward as a column-like spire up to 100 m above the vent. Rhyolitic flows, where they do form, have broken and blocky surfaces.

Volcaniclastic Deposits 

On a mild day in February 1943, as Dionisio Pulido prepared to sow the fertile soil of his field 330 km (200 miles) west of Mexico City, an earthquake jolted the ground, as it had dozens of times in the previous days. But this time, to Dionisio’s amazement, the surface of his field visibly bulged upward by a few meters and then cracked. Ash and sulfurous fumes filled the air, and Dionisio fled. When he returned the following morning, his field lay buried beneath a 40-m-high mound of gray cinders Dionisio had witnessed the birth of Paricutín, a new volcano. During the next several months, Paricutín erupted continuously, at times blasting clots of lava into the sky like fireworks. By the following year, it had become a steep-sided cone 330 m high. Nine years later, when the volcano ceased erupting, its lava and debris covered 25 square km. 

This description of Paricutín’s eruption, and that of  Vesuvius at the beginning of this chapter, emphasizes that volcanoes can erupt large quantities of fragmental igneous material. Geologists use the general term volcaniclastic deposits for accumulations of this material. Volcaniclastic deposits include pyroclastic debris (from the Greek pyro, meaning fire), which forms from lava that flies into the air and freezes. They also include the debris formed when an eruption blasts apart pre-existing volcanic rock that surrounds the volcano’s vent, the debris that accumulates after tumbling down the volcano in landslides or after being transported in water-rich slurries, and the debris formed as lava flows break up or shatter. 

Pyroclastic debris from basaltic eruptions

Pyroclastic debris from basaltic eruptions.

Basaltic magma rising in a volcano may contain dissolved volatiles (such as water). As such magma approaches the surface, the volatiles form bubbles. When the bubbles reach the surface, they burst and eject clots and drops of molten magma upward to form dramatic fountains (figure above a). To picture this process, think of the droplets that spray from a newly opened bottle of soda. Solidification of the pea-sized fragments of glassy lava and scoria produces a type of lapilli (from the Latin word for little stones). Pieces of this type of lapilli are informally known as cinders. Rarely, flying droplets may trail thin strands of lava, which freeze into filaments of glass known as Pelé’s hair, after the Hawaiian goddess of volcanoes, and the droplets themselves freeze into tiny streamlined glassy beads known as Pelé’s tears. Apple- to refrigerator-sized fragments called blocks (figure above b) may consist of already-solid volcanic rock, broken up during the eruption such blocks tend to be angular and chunky. In some cases, however, blocks form when soft lava squirts out of the vent and then solidifies such blocks, also known as bombs, have streaked, polished surfaces.

Pyroclastic debris from andesitic or rhyolitic eruptions

The components of an explosive eruption.

Andesitic or rhyolitic lava is more viscous than basalt, and may be more gas-rich. The lava flows tend to be blocky to start with, and blocks of flows may tumble down the volcano. Eruptions of these lavas also tend to be explosive. Debris ejected from explosive eruptions includes fragments of pumice and ash. Ash consists of particles less than 2 mm in diameter, made from both glass shards formed when frothy lava explosively breaks up during an eruption, and from pulverized pre-existing volcanic rock (figure above a). Two types of lapilli are produced by explosive eruptions: pumice lapilli consists of angular pumice fragments formed from frothy lava (figure above b); accretionary lapilli consists of snowball-like lumps of ash formed when ash mixes with water in the air and then sticks together (figure above c). 

Much of the pyroclastic debris erupted from an exploding volcano billows upward in a turbulent cloud that can reach stratospheric heights (figure above d). Some, however, rushes down the flank of the volcano in an avalanche-like current known as a pyroclastic flow (figure above e). Pyroclastic flows were once known as nuées ardentes (French for glowing cloud), because the debris they contain can be quite hot 200C to 450C. 

Unconsolidated deposits of pyroclastic grains, regardless of size, constitute tephra. Ash, or ash mixed with lapilli, becomes tuff when buried and transformed into coherent rock. Tuff that formed from ash and/or pumice lapilli that fell like snow from the sky is called air-fall tuff, whereas a sheet of tuff that formed from a pyroclastic flow is an ignimbrite. Ash and pumice lapilli in an ignimbrite is sometimes so hot that it welds together to form a hard mass.

Other volcaniclastic deposits

In cases where volcanoes are covered with snow and ice, or are drenched with rain, water mixes with debris to form a volcanic debris flow that moves downslope like wet concrete. Very wet, ash-rich debris flows become a slurry called a lahar, which can reach speeds of 50 km per hour and may travel for tens of kilometers. When debris flows and lahars stop moving, they yield a layer consisting of volcanic debris suspended in ashy mud.

Volcanic Gas 

The gas component of volcanic eruptions.

Most magma contains dissolved gases, including water, carbon dioxide, sulphur dioxide, and hydrogen sulphide (H2O, CO2, SO2, and H2S). In fact, up to 9% of a magma may consist of gaseous components, and generally, lavas with more silica contain a greater proportion of gas. Volcanic gases come out of solution when the magma approaches the Earth’s surface and pressure decreases, just as bubbles come out of solution in a soda when you pop the bottle top off. 

In low-viscosity magma, gas bubbles can rise faster than the magma moves, and thus most reach the surface of the magma and enter the atmosphere before the lava does. Thus some volcanoes may, for a while, produce large quantities of steam, without much lava (figure above a). The last bubbles to form, however, freeze into the lava and become holes called vesicles (figure above b). In high-viscosity magmas, the gas has trouble escaping because bubbles can’t push through the sticky lava. When this happens, explosive pressures build inside or beneath the volcano.

Credits: Stephen Marshak (Essentials of Geology)

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