Samiha Sehgal delves into the fiery depths of volcanoes to explore what geophysicists can learn from the strange fibreglass-like slivers of volcanic glass that are blown out after some eruptions
A sensible crew cut, a chic bob, an outrageous mullet. You can infer a lot about a person by how they choose to style their hair. But it might surprise you to know that it is possible to learn more about some objects in the natural world from their “hair” – be it the “quantum hair” that can reveal the deepest darkest secrets of what happens within a black hole, or glassy hair that emerges from the depths of our planet, via a volcano.
In December 2017 University of Oxford volcanologist Tamsin Mather travelled to Nicaragua to visit an “old friend”: the Masaya volcano, some 20 km south of the country’s capital of Managua. Recent activity had created a small, churning lava lake in the centre of the volcano’s active crater, one whose “mesmerising” glow at night attracted a stream of enchanted tourists.
For those who could draw their eyes away from the roiling lava, however, another treat awaited: a gossamer carpet of yellow fibres strung across the downwind crater’s edge. Known to geologists as “Pele’s hair”, Mather describes these beautiful deposits as like “glistening spiders’ webs”, shiny and glass-like, looking like “fresh cut grass after some dew”.
These glassy strands, often blown along by the wind, have been found in the vicinity of volcanoes across the globe – not only Masaya, but also Mount Etna in Italy, Erta Ale in Ethiopia, and across Iceland, where they are instead dubbed nornahár, or “witches’ hair”. They have even been found produced by underwater volcanoes at depths of up to 4.5 km below sea level. However, Pele’s hair is arguably most associated with Hawaii, from whose religion (not the footballer) the deposits take their name (see box “The legend of Pele”).
Lava fountains and candy floss
Although you might hardly guess it from its fine nature, Pele’s hair has quite the violent birth. It forms when droplets of molten rock are flung into the air from lava fountains, cascades, particularly vigorous flows or even bursting gas bubbles. This material is then stretched out into long threads as the air (or, in some cases, water) quenches them into a volcanic glass. Pele’s hair can be both thicker and finer than its human counterpart, ranging from around 1 to 300 µm thick (Jour. Research US Geol. Survey 5 93). While the strands are typically around 5–15 cm in length, some have been recorded to reach a whopping 2 m long.
Katryn Wiese – an earth scientist at the College of San Mateo in California – explains that the hairs form in the same way that glass blowers craft their wares. “Melt a silica-rich material like beach sand and as it cools down, blow air through it to elongate it and stretch it out,” she says. Key to the formation of Pele’s hair, Wiese notes, is that the molten lava does not have time to crystallize as it cools. “Pele’s hair is really no different than ash. Ash is basically small beads of microscopic glass, whereas Pele’s hair is a strung-out thin line of glass.”
Go to a funfair and you’ll see this same process at play at the candy floss stall. “Sugar is melted by a heat coil in the centre of a cotton candy machine and then the liquid melted sugar is blown outwards while the device spins,” Wiese explains, to produce “thin threads of liquid that freeze into non-crystalline sugar or glass”.
Just as there is a fine art to spinning cotton candy, so too does the formation of Pele’s hair require very specific conditions to be met. First, the lava has to cool slowly enough so it can stretch out into thin strands. Second, the lava must be sufficiently fluid, rather than being more viscous. That’s why Pele’s hair is only formed by so-called basaltic eruptions, where the magma has a relatively low silica content of around 45–52%.
The composition of the initial lava is also a factor in the colour of the hairs, which can range from a golden yellow to a dark brown. “Hawaiian glasses are classically amber coloured,” notes Wiese. She explains that basalts from Hawaii are primarily made up of silica and aluminium oxides (a mix of iron, magnesium and calcium oxides), as well as trace amounts of other elements and gases. “The gases often contribute to oxidation of the elements and can also lead to different colours in the glass – the same process as blown glass in the art world.”
The legend of Pele
Both Pele’s hair and Pele’s tears take their name from the Hawaiian goddess of volcanoes and fire: Pelehonuamea, “She who shapes the sacred land”, who is believed to reside beneath the summit of the volcano Kīlauea on the Big Island – the current eruptive centre of the Hawaiian hotspot.
Many ancient legends of Pele depict the deity as having a fiery personality. According to one account, it was this temperament that brought her to Hawaii in the first place, having been born on the island of Tahiti. As the story goes, Pele seduced the husband of her sister Nāmaka, the water goddess. This led to a fight between the siblings that proved the final straw for their father, who sent Pele into exile.
Accepting a great canoe from her brother, the king of the sharks, Pele voyaged across the seas – trying to light her fires on every island she reached – pursued by the vengeful Nāmaka. Mirroring how the Hawaiian islands were erupted in sequence as the Earth’s crust moved relative to the underlying hotspot, Pele moved along the chain repeatedly trying to dig a fiery crater in which to live, only for each to be extinguished by Nāmaka.
The pair had their final confrontation on Maui, with Nāmaka defeating Pele and tearing her apart at the hill known today as Ka Iwi o Pele – “the bones of Pele”. Her spirit, meanwhile, flew to Kīlauea, finding its eternal home in the Halema‘uma‘u pit crater.
Tears and hairs – volcanic insights
Another important factor in the formation of Pele’s hair is the velocity at which magma is “spurted” out during an eruption, according to Japanese volcanologist Daisuke Shimozuru, who was studying Pele’s hair and tears in the 1990s.
Based on experiments involving jets of ink released from a nozzle at different speeds, Shimozuru concluded that thread-like expulsions like Pele’s hair are only formed when the eruption velocity is sufficiently high (Bulletin of Volcanology 56 217). At lower speeds, the molten material is instead quenched without being stretched, forming glassy droplets, referred to as Pele’s tears, sometimes with a hair or two attached.
According to Kenna Rubin – a volcanologist at the University of Rhode Island – studying the shape of these black globules can shine a light on the properties of the lava that formed them. They can provide information not only about the ejection speed, but also related parameters such as the temperature, viscosity and the distance they travelled in the atmosphere before solidifying.
Furthermore, the tears can preserve tiny bubbles of volcanic gases within themselves, trapped in cavities known as “vesicles”. Analysing these gases can reveal many details of the chemical composition of the magma that released them. These can be a useful tool to shine a light on the exact nature of the hazard posed by such eruptions.
In a similar fashion, Pele’s hair can also offer valuable insights to volcanologists about the nature of the eruptions that formed them – thereby helping to inform models of the hazards that future volcanoes may pose to nearby life and property.
Window within, and to the past
“Pele’s hair and tears are a subset of the pantheon of particles ejected by a volcano when they erupt,” notes Rubin. By examining the particles that come out over time, as well as studying the geophysical activity at a volcano, such as seismicity and gas ejection, researchers “can then make inferences about the conditions that were extant in past eruptions”. In turn, she adds, “This allows us to look at old eruption deposits that we didn’t witness erupting, and infer the same kinds of conditions.”
While Pele’s hair and tears are both relatively rare volcanic products, when they do exist they can help to constrain the eruption conditions – offering a window into not only recent but also past eruptions when so-called “fossil” samples have been preserved.
Alongside the composition of the glasses (and any trapped gases within such), the shape of hairs and tears can shine a light on the various forces that affected them as they were flying through the air cooling. In fact, the presence of the hair around a volcano is itself a sign that the lava is of the least viscous type, and is undergoing some form of fountaining or bubbling.
There are, of course, many other types of material or fragments of rock that get ejected into the air when volcanoes erupt. But the great thing about Pele’s hair is that, having cooled from lava to a glass, it represents the lava’s bulk composition. As Wiese notes, “We can quickly determine the composition of the lavas that are erupting from just a single sample.”
For example, Mather collected samples of Pele’s hair from Masaya during a 2001 return visit to her cherished Nicaraguan haunt, enabling Mather and her colleagues to determine the composition of the lava erupting from Masaya’s vent in terms of both major elements and lead isotopes (Journal of Atmospheric Chemistry 46 207; Atmospheric Environment 37 4453). As Mather says, “With other measurements we can think about how this composition changes with time and also compare it with the gas and particles that are dispersed in the plume.”
Pele’s curse
There is an urban legend on the islands that anything native to Hawaii – whether it be sand, rock or even volcanic glass – cannot be removed without being cursed by Pele herself. Despite invoking Hawaii’s ancient volcano goddess, the myth is believed to actually be quite recent in origin. According to one account, it was dreamt up by a frustrated park ranger who were frustrated by tourists taking rocks from the island as souvenirs. Another attributes it to tour drivers, who tired of tourists bringing said rocks onto their buses, and leaving dirt behind.
Either way, the story has taken hold as if it were an ancient Hawaiian taboo, one that some take extremely seriously. Volcanologist Kenna Rubin, for one, often receives returned rocks at her office at the University of Hawaii. “Tourists and visitors find my contact details online and return the lava rocks, or Pele’s hair,” she explains. “They apologise for taking the items as they feel they have been cursed by the goddess.”
The legend of Pele’s curse may be fictitious, but the hazards presented by Pele’s hair are very real, both to the unwitting visitor to Hawaii, and also the state’s permanent residents. Like fibreglass – which the hairs closely resemble – broken slivers of the hair can gain sharp ends that easily puncture the skin (or, worse, the eye) and break into smaller pieces as people try to remove them.
Not only can an active lava lake produce enough of the hair to carpet the surrounding area, but strands are easily picked up by the wind. From Kīlauea Volcano, for example, the US Geological Survey notes that prevailing winds tend to blow much of the Pele’s hair that is produced south to the Ka‘ū Desert, where it builds up in drifts against gully walls (see photo). In fact, hairs have been known to be carried up to tens of kilometres from the originating volcanic vent – and it is not uncommon on Hawaii to find Pele’s hair snagged on trees, utility poles and the like.
Hair in the catchment
Wind-blown Pele’s hair also poses a threat to the many locals who collect rainwater for drinking. “As ash, laze [“lava haze” – a mix of glass shards and acid released when basaltic lava enters the ocean] and Pele’s hair have been found to contain various metals and are hazardous to ingest, catchment users should avoid accumulating it in their water tanks,” the Hawaii State Department of Health advises in the event of volcanic activity.
However, even though Pele’s hair has the potential to harm humans, there are some residents of Hawaii who do benefit from it – birds. Collecting the strands like the bits of straw they resemble, our avian friends have been known to use the volcanic deposits to feather their nests; in fact, one made entirely from Pele’s hair has been preserved for posterity in the collections of the Hawaii Volcanoes National Park.
Pele’s tears can also serve as a proxy for the severity of eruptions. In a study published this March, geologist Scott Moyer and environmental scientist Dork Sahagian showed that the diameter of vesicles preserved in Pele’s tears from Hawaii is related to the height of the lava fountains that formed them (Frontiers in Earth Science 12 10.3389/feart.2024.1379985). Fountain height, in turn, is constrained by the separated gas content of the source magma, which controls eruption intensity.
It’s clear that Pele’s hair and tears are far more than a beautiful natural curiosity. Thanks to the tools and techniques of geoscience, we can use them to unravel the mysteries of Earth’s hidden interior.
