HMM Pele’s hair, formed during passive outgassing from a lava lake, are very long and thin. They show few, highly elongated vesicles, often extending along the entire hair, and almost no round vesicles. KI samples are related to a lava fountain and are shorter and coarser. They are more vesicular than HMM and show a higher VND. Frequently two populations of vesicles occur: a large population of elongated vesicles and a small population of mainly near-spherical vesicles (Fig. 5). Finally, LOE Pele’s hair, related to a littoral explosion, shows intermediate sizes between KI and HMM with abundant highly elongated and few smaller and rounded vesicles.

The ‘spurting’ velocity, that is the velocity of the magma droplet (Pele’s tear) when ejected into the atmosphere, is considered as the main factor controlling the formation of Pele’s hair11. No experimental study has been carried out on the factors governing the shape of the Pele’s hair, but it seems reasonable that higher velocities favor the formation of longer and thinner hair. Additional parameters can modulate the shape of the Pele’s hair: i) size of the original magma droplet, in turn influencing the quenching rate; ii) viscosity of the magma and iii) the residence time of the hair in the hot environment. For a lava fountain of Kilauea Iki 1959, Wilson30 calculated a spurting velocity of 297 m/s for the smaller pyroclasts (0.84 mm), while Mattox and Mangan10 calculated a minimum value of ejection velocities ranging between 1–15 m/s for a littoral explosion of Kilauea volcano. No published data about the Halemaumau lava lake is available, but for a similar lava lake at Villarrica initial velocities of 12 m/s have been calculated31.

The formation of Pele’s hair in viscous melts occurs when quenching time is shorter than the time of separation of the filament from the droplet to which they are attached (Pele’s tear) and the diameter of the thin strands is about 1/5 of that of the Pele’s tear12. According to this, we have roughly estimated the diameter of the original magma droplet considering its diameter as being 5 times the median value of the thickness distribution for each group of Pele’s hair (Fig. 2). Results show larger volumes for KI (about 1 mm3) than for LOE (0.2–0.3 mm3) and HMM (about 0.01 mm3). It is obvious that for the very thin HMM Pele’s hair the quench time (t quench ) is shorter than for the thicker KI Pele’s hair. Longer t quench allows the formation of a second population of vesicles not deformed by the stretching. Therefore, in Pele’s hair containing round vesicles the t quench must be longer than the time of the second vesiculation (t ves ). As shown in Fig. 4c, the rare rounded vesicles (elongation < 0.1) have a maximum diameter of the equivalent sphere of 8 µm in HMM; 20 µm in LOE and up to 100 µm in KI. With these values it is possible to calculate the maximum t ves considering a vesicles growth rate of 3.2 × 10–4 cm/s, as reported by Mangan and co-workers32 for the basalts of Kilauea volcano. The results are of about 1 s for HMM, 3 s for LOE, 15 s for KI. Maximum t quench are equal or higher than these values and the obtained results are consistent with the experimental finding of Mastin and co-workers13.

Viscosity strongly influences the capacity of a droplet to deform under the action of the gas expansion in the bursting bubbles or wind and is a function of several parameters, including the melt composition, temperature, vesicle and crystal contents33,34,35. Following Giordano and co-workers36 we have calculated the viscosity (η) of the three magmas, given the melt composition listed in Supplementary Table S2, assuming an average water content of 0.5 wt.%37 and the liquidus temperature at atmospheric pressure calculated by MELTS software38. For the three magmas results are η HMM = 1.67, η KI = 1.19 and η LOE = 1.62 log Pa s (Supplementary Table S3). The influence of the pre-eruptive crystals on bulk viscosity can be considered negligible: the few microlites of olivine in KI and of plagioclase in LOE probably formed after fragmentation, as witnessed by i) their small size, ii) the presence of skeletal crystals indicating a very rapid crystallization, and iii) the distortion of vesicles clearly due to successive crystal growth (Fig. 3f). In any case their abundance is very low and most of the hair is completely aphyric. As suggested by Manga and co-workers16 the viscosity is strongly influenced by the shape and orientation of the vesicles: highly elongated vesicles are aligned to the flow direction and contribute less shear stress to the magma when compared to spherical vesicles. The occurrence of more elongated vesicles in HMM and LOE than in KI should support higher shearing. However, as previously shown, the t quench is very fast in all three cases and the difference in the elongation of the vesicles has low influence on the viscosity.

The process of Pele’s hair formation is summarized in Fig. 6. The very elongated shape of the HMM Pele’s hair can be attributed to a combined effect of very small size of droplets formed during the rupture of bubbles enclosed within a very thin magma envelope on the surface of the lava lake, and high spurting velocity. The wind transporting the Pele’s hair away from the lava lake can also act as a strain factor. The highly elongated shape of the vesicles suggests that they were already present in the magma forming the wall of the bubbles and deformed during bursting, detachment and transport. t quench is very short due not only to the small size of the magma droplet, but also to the brief residence of the fragments in a hot environment and to wind chill. Further vesiculation and crystallization is therefore inhibited and only rarely few round vesicles form after the deformation. This suggests that deformation time (t def ) is shorter than t quench , allowing the pronounced stretching of the hair and vesicles. In most of the cases t quench is shorter than t ves and crystallization time.

Figure 6 Schematic process of formation of Pele’s hair in the three studied eruptions. t def : time of deformation; t ves : time of second vesiculation; t quench : time of quenching. Relevant parameters discussed in the text are also reported: sp. vel: spurting velocity; th: thickness of the Pele’s hair, corresponding to the median of the thickness distribution (Fig. 2 and Supplementary Table S1). VPT: volume of the initial Pele’s tear, estimated considering th equal to about 1/5 the diameter of the Pele’s tear; max t ves : maximum time of second vesiculation, considering a growth rate of 3.2 × 10-4 cm/s and the maximum size of rounded vesicles for each sample; η: viscosity calculated from the chemical composition (see the text for details). For HMM the stretching of a small sized, vesiculated Pele’s tear creates a very thin Pele’s hair with long and thin vesicles. The maximum t ves is in the order of 1 second; the almost complete absence of second, round vesicles indicates that the t quench is shorter than this value. In LOE the coarser size of the magma droplet and the lower spurting velocity form thicker Pele’s hair, allowing, after the deformation, the nucleation of few, rounded bubbles in a maximum t ves of 3 s. Their occurrence only in some fragments suggests that t quench is similar to this value. In KI the coarse dimension of the original droplet is probably the main factor controlling its limited stretching, in spite of the lower viscosity of the magma and the higher spurting velocity. The relatively high thickness of the resulting Pele’s hair and the longer permanence in the hot lava fountain allow the nucleation of a second population of sub-spherical bubbles during or after the deformation. The maximum t ves is in the order of 15 s and the hair quenches after this second vesiculation event. Full size image

The thickness of the Pele’s hair formed after the littoral explosion (LOE) is higher than the HMM case, as a consequence of low spurting velocity and of the larger size of the droplets. This is probably due to the different style of fragmentation, related to the rapid vaporization of trapped water in contact with hot magma. In these fragments the vesicles are strongly elongated in the same direction of the hair, indicating that the fragmentation occurred in an already vesiculated magma, whose vesicles rapidly deformed as the magma droplet was stretched. t quench is higher than the previous case, as a consequence of the coarser diameter of the magma droplets and longer residence in the hot steam column (335–550 °C10). It is noteworthy that a second population of few and rounded vesicles only occurs in the thicker filaments which cooled sufficiently slowly to allow ongoing gas exsolution. Their formation and the crystallization of the few plagioclase microlites can be explained with a further, short episode of vesicle and crystal nucleation after the deformation of the droplet and before quenching.

The stubby shape of KI Pele’s hair is apparently in contrast with the viscosity calculated for this poorly evolved magma, lower than the HMM and LOE cases, and with the much higher spurting velocities. The limited elongation of the KI Pele’s hair can be explained with a less efficient stretching, probably related to the dimensions of fragments in the fountain which were coarser than at HMM or LOE. Although the spurting velocities of magma fragments in the lava fountain were very high, the higher diameter of droplets led to the formation of thicker and less elongated filaments which cool down relatively slow also because of the very long residence in the hot fountain. The occurrence of different size populations of vesicles is generally related to distinct pulses of nucleation and growth17,27,39. Deformation of vesicles is expressed by the Capillary number (Ca) which is directly proportional to radius16. Therefore, small vesicles, formed during a second pulse of nucleation, are less deformed both due to their radius and to their formation during the last stages or after the stretching, but before the quenching (t def < t ves < t quench ). Coarser elongated vesicles formed before or during the stretching. This is also supported by the distribution of smaller and round vesicles across the hair cross section, while the coarser and elongated vesicles are mainly concentrated in the axial portion (Fig. 5b, c). A similar distribution of vesicles of different size and a comparable VND is observed in coarse scoria fragments of the same eruption40 and is interpreted as an evidence of no post-fragmentation gas expansion. However, the coarser scoria clasts are dominated by round, relaxed vesicles. In Pele’s hair the t quench is short enough to prevent the elongated vesicles from relaxing to spheres13. Only in Pele’s hair showing evident thickening (e.g. KI3 and KI6), also the coarser vesicles show relatively higher sphericity (Fig. 5a), implying some relaxation. As in the previous case, the olivine microlites formed during or after the eruption and before the complete quenching of the hair. SR-µCT analyses revealed that KI Pele’s hair is more vesicular than the other samples. This can be related to secondary vesiculation, but is probably mainly due to the original gas content of the magma at the moment of Pele’s hair formation.

The different morphologies and vesicularities of Pele’s hair from the three eruptive styles suggest that the eruptive mechanism is a main factor determining their internal and external morphological features. However, Pele’s hair sampled during persistent degassing of Masaya volcano (Nicaragua), has coarser diameters (over 200 μm) and has both elongated vesicles extending discontinuously along the length of the hair and smaller and rounded vesicles5. Masaya Pele’s hair is therefore more similar to LOE or KI samples than to HMM, although the HMM samples come from a more similar eruptive style. The differences in morphology and vesiculation of Pele’s hair formed by similar fragmentation mechanisms are therefore probably related to the higher viscosity of Masaya magma. Higher viscosity melts would lead to the formation of coarser droplets stretched in thicker filaments by the gas expansion and/or by the wind. These would quench less rapidly than in the HMM case. A second episode of vesiculation could therefore occur before the complete solidification of the fragment, forming the small and rounded vesicles very rare in the HMM samples. Pele’s hair produced during littoral explosions were commonly found during the eruption of Kilauea, in the period 1992–199410, and submarine hydrovolcanic explosions on Loihi seamount8 and Gorda Ridge10. In the latter two a few photographs of Pele’s hair closely resemble LOE samples, even if in10 fragments seem to be more irregular and thick. Anyway no description on morphologies and vesicle textures is presented and therefore it is not possible to compare them. Pele’s hair of KI (Kilauea Iki 1959) was compared with that of the 1969–72 eruption3, and while numerical data on the length and thickness of the Pele’s hair or vesicle dimensions are not available, the images show long and thin strands, very different from the stubby shape of KI. In the 1969–72 eruption high fountaining was restricted to episodes in 1969 and in the later years magma was frequently ponded and/or drained back into the eruptive craters suggesting a scenario more closely resembling the current Halemaumau eruption. Pele’s hair erupted during lava fountain at Piton de La Fournaise41 displays the occurrence of highly stretched vesicles and a second population of rounded vesicles very similar to both KI and LOE, but a much higher microlite content, probably determining a higher magma viscosity.

This study of Pele’s hair from different eruptive styles at Kilauea and qualitative comparison with hair formed from similar eruptions at other volcanoes shows that, although the fragmentation mechanism (and therefore the eruptive style) is a key parameter in controlling the shape and vesicularity of Pele’s hair, other factors are also important. Caution should be used when trying to infer the eruptive mechanism from the texture and shape of Pele’s hair. The following parameters are also important:

1. dimensions of the magma droplet originating the Pele’s hair, in its turn influencing the quenching rate; 2. spurting velocity; 3. viscosity of the magma; 4. vesicle content in the initial magma droplet; 5. residence time in the hot environment; 6. wind.

It was not possible to quantify the last three parameters. However, our data allowed to determine that dimensions of the original Pele’s tear increases from HMM to LOE and KI. Spurting velocity is much higher for KI than for HMM and LOE, while viscosity is comparable for HMM and LOE, but lower for KI. Therefore, we suggest that spurting velocity and viscosity are secondary controls, and the size of the original magma droplet is the key factor controlling the shape and texture of Pele’s hair.

The results of this study prove the capability of SR-μCT for the investigation of small and delicate volcanic products such as Pele’s hair and that the analytical protocol described in this paper can be applied to other magmas to better constrain the Pele’s hair formation process for a wider range of magma compositions and viscosities.