Pahoehoe basalts

ropy lava flows on volcanic islands: Hawaii and Iceland

large slab [307 kb] detail [339 kb]

Figs. 1-2: This magnificent sample is a slab of a fresh pahoehoe flow, from Hawaii. Pahoehoe is a word in the Hawaiian language, describing the characteristic ropy texture of basalt flows whose physical properties and setting allow them to flow from the parent vent or fissure, the surface developing the characteristic texture. The slab is some 76 x 65 x 10 cm in size, and seems absolutely pristine. The interior is highly vesicular (full of gas bubbles, innumerable mm-size cavities in the glassy, congealed lava). Royal Ontario Museum specimen 38173. This marvelous slab was on display in the museum foyer in December 2019. It is from a 1998 eruption in Hawaii, and was donated by the USGS Hawaiian Volcano Observatory.

"Rock of the Month # 258, posted for December 2022" ---

Basaltic lavas, emerging at the Earth’s surface, are relatively hot and fluid. They have lower viscosity than more siliceous magmas, and as such tend to be emitted as flows from vents and especially fissures, or from the ends of lava tubes, and their activity is less explosive than the andesites, dacites and rhyolites that are the more chemically “evolved” products of stratovolcanoes. The behavior of the basaltic lava depends on its temperature, viscosity and rate of cooling, and the topography on which it flows. Lava emitted in pulses may flow below the congealing surface of the earliest pulses, moving in lava tubes towards the toe of the flow. There it is emitted and congeals, one lobe after another, in this way building up what may be great lengths and thicknesses. Somewhat cooler melt, more viscous, generally moves more slowly, and the surface, instead of the smooth, ropy consistency of pahoehoe, is cindery clinker, forming rough and ragged surfaces, known as aa (or a’a), another Hawaiian term.

One practical aspect of the different morphologies of the lava flows is the relative ease of travel over them. The pahoehoe flow is quite easy to walk over, but it is best to watch for pits, on a scale of roughly 1-10 m2, that may be evidence of pre-existing lava tubes or other voids. Thus the surface may be quite thin! This (roof of a lava tube or other cavity) may be the origin of the Hawaiian slab (Figs. 1-4). The aa flows, being very rough and jagged, are more of a challenge, whether or not (as in Iceland) they may be colonized by layers of pale moss that make them look deceptively civilized!

close-up surface [371 kb] interior [345 kb]

Figs. 3-4: A close-up of the surface texture of the Hawaiian sample, and another of the vesicular interior of the flow (upper, outer surface, seen in Figs. 1-3, below). Royal Ontario Museum specimen 38173.


For an excellent guide to the Icelandic geology, starting with the nature of lavas, see Thordarson and Hoskuldsson (2014, pp.14-21). Such elemental aspects of geology are of course covered in physical geology and geography texts, such as Holmes (1965, pp.294-302), or Plummer et al. (2016). For more popular texts, and lots of illustration, see works such as Weisel and Johnson (1994), and, for Hawaii and Iceland respectively, Holland and Lanting (2004) and Sigurdsson (2016).

The flow featured in Figs. 5-8 is on the lava field from the Fagradalsfjall volcano, on the Reykjanes peninsula, the southwestern extremity of Iceland. The area lies near three population centres: it is southwest of the capital, Reykjavik, southeast of the main airport at Keflavik, and near the south-coast port of Grindavik. The rift that generated the Geldingadalur flow hosted eruptive activity for six months, from 19 March to 18 September 2021 (see Global Volcanism Program, 2022).

An elegant first-hand description of a visit to the tourist-attractive volcanic display is provided by Heidi Julavits (2021), who braved the windy hillsides to observe the eruption in the last week of May. She notes that the 2021 lava, at the time the latest scenic feature on the island, was named Fagradalshraun, the Lava of the Beautiful Valley.

The first volcanism recorded on the Reykjanes peninsula in more than eight centuries, the 2021 event is estimated to have generated some 151 million m3 of lava, covering an area of 4.8 km2. This implies an average thickness of flows of some 31 m, obviously much greater in some valleys and other topographic depressions, thinning to zero on the margins. In one valley, the average depth of new lava was estimated at about 200 feet (61 metres: Julavits, 2021).

On Wednesday, 03 August 2022, across a ridge from Geldingadalur, about 1 km to the northeast, the rift reopened, with lava fountaining and flows emerging from a linear fissure, generating the Meradalir flow. The last significant eruption in this area was some 870 years earlier, in the middle of the 12th century (Pratt, 2022). The recent eruption, though spectacular, was short, ending on its 19th day, 21 August 2022.

Iceland pahoehoe [407 kb] pahoehoe with sublimates 596 kb]

Figs. 5-6: Two photographs of the Geldingadalur lava flows erupted in 2021, showing details of the pahoehoe structures preserved on surface. Left: metre-scale development of the ropy texture. Tom Erickson for scale! Right: colourful area of a local small-scale gas vent, the surface colours due to fumarolic sublimates, deposited from sulphur-rich volcanic gases escaping to the atmosphere.

Pahoehoe worldwide (and beyond)

Focusing on pahoehoe, our main topic this month, it will be found that such flows are of worldwide extent (on continents as well as islands), and extend back in the geologic record, far into the Precambrian. Picking out just a few occurrences, across space and time, youngest on back ….

  • Hawaii (Jones, 1993; Coombs and Rowland, 1994; Kauahikaua et al., 1998; Byrnes and Crown, 2001). Kilauea and Mauna Loa volcanoes are very active, and in fact Mauna Loa, which last erupted in 1984, is active as I write this, with activity beginning on 27 November 2022.
  • Iceland (Thordarson and Hoskuldsson, 2014).
  • The Galapagos archipelago (Bruno et al., 1992; Standish et al., 1998).
  • Xitle, the youngest and only basaltic volcano in the Chichinautzin volcanic field, located on the south edge of Mexico City (Canon-Tapia et al., 1995). The upper third of the flows are highly vesicular, the lower two-thirds lacking in vesicles. There are other occurrences in Mexico, including the iconic 20th century Paricutin.
  • Lastarria in Chile, with pahoehoe and remelted flows of fumarolic native sulphur (also reported in Hawaii, Japan, and the Galapagos islands: Naranjo, 1985).
  • Columbia River basalts, Washington, U.S.A. (Self et al., 2005).
  • The late-Cretaceous Deccan traps of western India, notably in the state of Maharashtra. This great outpouring of continental flood basalts includes well-documented multitudes of compound pahoehoe flows (Keszelyi et al., 1999; Duraiswami, 2003; Bondre et al., 2004; Sen and Sabale, 2011).
  • The Jurassic continental tholeiitic basalts of the North Mountain basalt of Nova Scotia (Kontak, 2008).
  • Jurassic Orange Mountain basalt, New Jersey, U.S.A. (Laskowich and Puffer, 2016).
  • The Neoproterozoic Midcontinent Rift of North America (Green, 1987; Hart and Pace, 2006; Cundari et al., 2010).
  • It is likely that pahoehoe flows also developed elsewhere, such as on Mars! The basaltic meteorites known as shergottites (see, e.g., First and Hammer, 2016) are rich in olivine, pyroxene and glass. However, given the violence of the transport of meteorites from Mars to Earth, delicate surface features seen in this Rock of the Month would not survive!

top [232 kb] base [204 kb]

Figs. 7-8: Two sides of a pair of small samples from the Geldingadalur flow. These pieces are from the upper portion of the flows, as seen in Figs. 5-6, and so are likely from a later stage of the 6-month eruptive cycle. Vapour (apparently mostly steam) was rising from cavities and fractures in the flows. The flow and samples were scarcely one year old, when the field photographs were taken and the samples collected, on 26 July 2022.

Left: the top surface, with pahoehoe structure and, right: the underside, displaying the vesicular interior of the lava (compare with the Hawaiian sample, Fig. 4). Close-up, the top surface of each piece displays a submetallic lustre, varying in hue from bronze to bluish-grey. Scattered through the bubbly, glassy black fabric of the rock are sparse 1-3-mm phenocrysts of glassy-lustred plagioclase feldspar. The colourful patina is very delicate, more of a lustre than a thick surface layer! However, blue glassy pahoehoe is a lava found in small amounts (<5% of total pahoehoe) in the active Kalapana flow field of Kilauea (Oze and Winter, 1997). It has a thick outer rind, large pockets of vesicles in the flow centre and distinctive blue-silver surface colouration.

Notes on the two samples: a) larger, 386 grams, 13x9x4 cm, magnetic susceptibility (mean of 3 readings) 0.896x10-3 SI units (not appreciably magnetic), and b) smaller, 155 grams, 9x8x3 cm, magnetic susceptibility (mean of 3 readings) 0.207x10-3 SI units (not appreciably magnetic). Both samples have very low "heft", due to their high porosity. Some basalts are highly magnetic, but these evidently carry next to no magnetite.

eruption - view 1 [118 kb]

eruption - view 2 [168 kb]

Figs. 9-10: Two dramatic views of the 2022 Meradalir eruption at night. In each case, the source fissure is obvious. In Fig. 9, the photo records a good sense of the flow of the lava, and one can imagine that a pahoehoe sheet will soon be the result (in an extended sequence of eruptions, of course, one flow can be overlain by the next, and in some cases the final surface flow may be blocky aa, not more pahoehoe). . These were taken in the early hours of Sunday 07, which was to be day 5 of the event, which stretched over 19 days in August 2022. Photographs (c) Tom Hart.

Acknowledgements: The 2022 ILSG (Institute on Lake Superior Geology) Iceland field trip was led by Phil Larson and Allan MacTavish, ably assisted by Peter Hinz and Tom Hart. Thanks to Tom Hart for the use of the eruption photographs, Figs. 9-10 - and to Tom Erickson for providing the platform for sorting images after the field trip. Katherine Dunnell searched the catalogues at the museum for the wonderful pahoehoe sample.


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Bruno,BC, Taylor,GK, Rowland,SK, Lucey,PG and Self,S (1992) Fractal analysis: a new remote sensing tool for lava flows. Lunar and Planetary Science 23, 171-172.

Byrnes,JM and Crown,DA (2001) Relationships between pahoehoe surface units, topography, and lava tubes at Mauna Ulu, Kilauea volcano, Hawaii. J.Geophys.Res. 106 no.B2, 2139-2151.

Canon Tapia,E, Walker,GPL and Herrero Bervera,E (1995) Magnetic fabric and flow direction in basaltic Pahoehoe lava of Xitle volcano, Mexico. J.Volc.Geotherm.Res. 65, 249-263.

Coombs,CR and Rowland,SK (1994) Thermal erosion in a lava tube: Honoapo, Mauna Loa volcano, Hawaii. GSA Abs.w.Progs. 26 no.7, 533pp., 118 -119, Seattle.

Cundari,R, Hollings,P and Smyk,M (2010) Geology and geochemistry of the Rove basalts, a possible Mid Continent Rift related extrusive unit south of Thunder Bay, Ontario. Abs. 56th Annual Meeting, Institute on Lake Superior Geology, vol.56 part 1, 76pp., 12- 13, International Falls, MN.

Duraiswami,RA (2003) Morphology, structure and emplacement of pahoehoe flows from the western Deccan volcanic province. J.Geol.Soc.India 61, 236.

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Holmes,A (1965) Principles of Physical Geology. Thomas Nelson and Sons Ltd, London, 2nd edition, 1288pp.

Jones,AC (1993) The cooling rates of pahoehoe flows: the importance of lava porosity. Lunar and Planetary Science 24, 731- 732.

Julavits,H (2021) The fire geyser. New Yorker 97 no.25, 42-51, 23 August.

Kauahikaua,J, Cashman,KV, Mattox,TN, Heliker,C, Hon,KA, Mangan,MT and Thornber,CR (1998) Observations on basaltic lava streams in tubes from Kilauea volcano, island of Hawai`i. J.Geophys.Res. 103 no.B11, 27303-27323.

Keszthelyi,L, Self,S and Thordarson,T (1999) Application of recent studies on the emplacement of basaltic lava flows to the Deccan Traps. In `Deccan Volcanic Province' (Subbarao,KV editor), Geol.Soc.India Memoir 43(1), 485- 520.

Kontak,DJ (2008) On the edge of CAMP: geology and volcanology of the Jurassic North Mountain basalt, Nova Scotia. Lithos 101, 74- 101.

Laskowich,C and Puffer,JH (2016) Prehnite and zeolite distribution in the Orange Mountain basalt, Paterson, New Jersey. Mineral.Record 47 no.4, 479- 490.

Naranjo,JA (1985) Sulphur flows at Lastarria volcano in the North Chilean Andes. Nature 313, 778-780.

Oze,CJ and Winter,JD (1997) A petrographical study of blue glassy pahoehoe from Kilauea volcano, Hawai'i. GSA Abs.w.Progs. 29 no.6, 137, Salt Lake City.

Plummer,CC, Carlson,DH and Hammersley,L (2016) Physical Geology. McGraw Hill Higher Education, 15th edition, (xx+595+57=) 672pp.

Pratt,SE (2022) Eruption in Fagradalsfjall, Iceland, NASA Earth Observatory, 07 August.

Self,S, Thordarson,T and Widdowson,M (2005) Gas fluxes from flood basalt eruptions. Elements 1 no.5, 283- 287.

Sen,B and Sabale,AB (2011) Flow types and lava emplacement history of Rajahmundry traps, west of River Godavari, Andhra Pradesh. J.Geol.Soc.India 78, 457- 467.

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Graham Wilson, posted 30 November-05,07 December 2022, note added 30 January 2023.

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