Lava

• 1. Introduction

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  Lava is molten rock expelled from beneath a planet's surface, and it is one of the most ancient and powerful forces shaping worlds. On the night of the 10th of January, 1977, a crater wall at Mount Nyiragongo in what was then Zaire simply failed. A fluid lava lake drained out in under an hour. The resulting flow tore down steep slopes at up to 100 kilometres per hour and overwhelmed several villages while their residents slept. The mountain was later designated a Decade Volcano in 1991, a recognition of how dangerous it remained.

  That event raises a question most of us never think to ask: why was that lava so fast when most lava barely moves at walking pace? The answer lies in chemistry. Lava is not a single substance. It is a family of materials, each with a radically different personality depending on what it is made of, how hot it runs, and where it flows. Understanding those differences means understanding how volcanoes destroy towns, build islands, and carve landscapes across the solar system.

• 2. The Chemistry of Flow

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  Silicon is the key ingredient that separates slow lava from fast, explosive from gentle. Silicon ions in molten rock bind tightly to four oxygen ions each in a tetrahedral arrangement, and when those clusters link together they form long chains that make the melt thick and resistant to movement. The more silica a lava contains, the more viscous it becomes, and viscosity shapes almost everything else about how an eruption behaves.

  At the felsic end of the spectrum, lavas with silica content above 63% include rhyolite and dacite. These are extraordinarily stiff, with a viscosity ranging from roughly 100 million centipoise for hot rhyolite at 1200 degrees Celsius to 100 billion centipoise for cool rhyolite at 800 degrees. Water, for comparison, sits at about 1 centipoise. That extreme resistance usually means felsic lavas explode rather than flow. When they do manage to pour out, they tend to form short, thick coulees, lava domes, or lava spines. Unusually hot rhyolite lavas, those above 950 degrees, can travel for many tens of kilometres, as they have in the Snake River Plain of the northwestern United States.

  Intermediate or andesitic lavas sit in the range of 52 to 63 percent silica. They erupt at temperatures between 850 and 1100 degrees Celsius, and their typical viscosity of around 3.5 million centipoise is roughly that of smooth peanut butter. They often build the steep composite volcanoes associated with the Andes. At the fluid end sit the mafic, or basaltic, lavas, with silica between 45 and 52 percent, erupting at around 10 to 100 Pa dot s. Their low viscosity lets them travel great distances, spreading into the broad, gentle shield volcanoes seen across Hawaii.

• 3. Extreme Compositions

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  At the margins of the known lava world sit compositions that challenge expectations. Komatiite, an ultramafic lava with silica content below 45 percent and magnesium oxide above 18 percent, is thought to have erupted at temperatures of 1600 degrees Celsius. At that heat, polymerization of mineral compounds essentially vanishes. The estimated viscosity of komatiite magma runs as low as 100 to 1000 centipoise, similar to light motor oil. Most ultramafic lavas are no younger than the Proterozoic. No modern komatiite is known anywhere on Earth, because the mantle has cooled too much to generate such magnesium-rich melts.

  On the opposite extreme for temperature, natrocarbonatite lavas at Ol Doinyo Lengai in Tanzania erupt at measured temperatures of only 491 to 544 degrees Celsius. Ol Doinyo Lengai is the sole known active carbonatite volcano on Earth. These lavas are composed mostly of sodium carbonate, with roughly half as much calcium carbonate and half again as much potassium carbonate. Their viscosity is barely above that of water, making them the most fluid of all known lavas despite their cool temperatures.

  Iron oxide lavas represent yet another curiosity. At Kiruna in Sweden, these are thought to have produced the iron ore deposits that formed during the Proterozoic. Iron oxide lavas of Pliocene age also appear at the El Laco volcanic complex on the Chile-Argentina border, though some geologists interpret those rocks as silicate flows altered by later hydrothermal activity. Sulfur lava flows at Lastarria volcano in Chile extend up to 250 metres long and 10 metres wide, formed by the melting of sulfur deposits at temperatures as low as 113 degrees Celsius.

• 4. How Lava Moves

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  Most lava does not behave like water pouring from a tap. The crystals that form as lava cools give it thixotropic and shear-thinning properties. A typical lava is what physicists call a Bingham fluid: it resists flowing at all until a stress threshold called the yield stress is crossed. Once moving, it behaves somewhat like toothpaste being squeezed from a tube, with shear concentrated in a thin layer next to the channel walls while the interior moves as a semisolid plug. When the crystal content reaches about 60 percent, the lava stops behaving like a fluid altogether and becomes a crystalline mush.

  Speeds vary enormously. Hawaiian basaltic flows typically move at about 0.25 miles per hour, with maximum speeds on steep slopes reaching 6 to 30 miles per hour. The Nyiragongo event, where a lava lake drained catastrophically, produced the record speed of 20 to 60 miles per hour. A key scaling principle governs how fast a flow can go: the average speed scales as the square of the flow's thickness divided by its viscosity. That means a rhyolite flow would need to be about a thousand times thicker than a basalt flow to achieve a comparable speed.

  Lava keeps itself surprisingly warm over long distances by developing an insulating crust. Geologists of the United States Geological Survey drilled repeatedly into the Kilauea Iki lava lake, which formed in a 1959 eruption. After three years, the solid surface crust was still only 14 metres thick, even though the lake was about 100 metres deep, and its base sat at 1065 degrees Celsius. Residual liquid was still present at depths of around 80 metres nineteen years after the eruption.

• 5. Surface Forms and Landforms

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  Aā and pāhoehoe are the two names most associated with Hawaiian lava, and both were introduced as formal geological terms by Clarence Dutton. Aā, which in Hawaiian means stony rough lava and also to burn or blaze, is basaltic lava characterized by a rubbly surface of broken fragments called clinker. The sharp, angled texture makes aā a strong radar reflector, visible from orbit in bright tones on Magellan imagery. Accretionary lava balls as large as 3 metres across are common on aā flows. Pāhoehoe, meaning smooth, unbroken lava, advances as a series of small lobes and toes that continually break out from a cooled crust. Its rounded texture makes it a poor radar reflector and dark on Magellan pictures. Pāhoehoe can transition into aā if it becomes turbulent, but the reverse does not happen.

  Underwater, lava forms pillow structures. As lava emerges from a submerged vent, water chills its surface instantly into a solid crust, which then cracks and oozes additional large blobs as more lava pushes through. Because water covers most of Earth's surface and most volcanoes are near or under water, pillow lava is actually the most common lava type on Earth.

  Lava tubes represent one of lava's most remarkable long-distance delivery systems. A crust forms over a flowing channel, and beneath it, insulated by rock, lava can travel many kilometres without losing significant heat. Extensive lava tubes of Tertiary age in North Queensland, Australia, include some extending for 15 kilometres. The highest lava fountain ever recorded occurred on the 23rd of November, 2013, at Mount Etna in Italy, reaching a stable height of around 2500 metres for 18 minutes and briefly peaking at 3400 metres.

• 6. Towns in the Path

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  The human toll of lava flows reads like a catalogue of hard luck and occasional resilience. Garachico on the island of Tenerife was destroyed by the eruption of Trevejo in 1706, then rebuilt. San Sebastiano al Vesuvio in Italy was destroyed in 1944 by the most recent eruption of Mount Vesuvius, during the Allies' occupation of southern Italy, and was later rebuilt. Kalapana, Hawaii, was overwhelmed by Kīlauea in 1990 and abandoned. Kapoho, Hawaii, was largely inundated in June 2018, with its subdivision Vacationland Hawaii completely destroyed.

  The Nisga'a villages of Lax Ksiluux and Wii Lax K'abit in northwestern British Columbia were destroyed during the eruption of Tseax Cone in the 1700s. Parícutin village, the settlement that gave its name to the volcano, was buried along with nearby San Juan Parangaricutiro between 1943 and 1952. Goma in the Democratic Republic of Congo was damaged during the 2002 eruption of Nyiragongo. La Palma's Todoque neighbourhood and El Paraíso neighbourhood in El Paso were affected by the 2021 Cumbre Vieja eruption.

  The word lava comes from Italian and is probably derived from the Latin labes, meaning a fall or slide. Its first documented connection to volcanic extrusion appears in a short account by Francesco Serao of the 1737 eruption of Vesuvius, where he used the phrase "a flow of fiery lava" to compare the movement of molten rock to water and mud running down the volcano's flanks after heavy rain. That same volcano has since destroyed and rebuilt the towns around it repeatedly, and Plymouth, Montserrat, which was destroyed by pyroclastic tephra in 1995 and remains the de jure capital of the island even though it is abandoned, shows that the threat reaches far beyond lava flows alone.