Pahoehoe lava

Pahoehoe is a smooth and continuous lava crust. Other main subaerial lava flow types are aa lava and blocky lava.

Pahoehoe forms when the effusion rate is low and consequently the velocity of lava flow is slow1. Pahoehoe lava flow is usually at least 10 times slower than typical aa lava flow4. Higher effusion rate results in lava flow being shattered which is how the rubbly and clinkery aa lava surface forms. Pahoehoe and aa lava are strikingly different in appearance but their composition may be identical or very similar. Lava flow that was originally pahoehoe may transform into aa lava but the reverse is impossible — once lava crust is broken it can not return back to smooth and continuous form2.

Only low-viscosity (usually basaltic) lava can form pahoehoe. Aa lava is much more common and is not as picky about the composition of lava flow. Aa lava can be basaltic, andesitic, tephritic, etc. Blocky lava needs more felsic compositions (silica content generally over 55%). Blocky lava is composed of larger blocks than aa lava and these blocks have much smoother surface.

Best known examples of pahoehoe lava flows are from the Big Island of Hawaii and the term ‘pahoehoe’ itself (just as ‘aa’) originates from the Hawaiian language. Pahoehoe is also known as ropy lava and it has several more varieties named entrail, festooned, filamented, sharkskin, shelly, etc3.

Pahoehoe lava is not common on La Palma. Most fresh lava flows there have highly irregular surfaces typical to aa lava. I found these nice examples of pahoehoe while trying to walk on a lava flow formed in 1949. I said trying because aa lava makes very difficult and even dangerous walking. Its surface is irregular and uppermost clinkery part of it is usually loose. One has to be very careful not to lose balance because fresh lava surface is sharp and can easily cut deep wounds on bare hands.

Hot basaltic pahoehoe lava in Hawaii.

Hawaiian lava from the Kilauea volcano (Pu’u O’o vent).

Fresh pahoehoe lava in Hawai’i (Kilauea volcano, Pu’u O’o vent).

This pahoehoe formed on La Palma, Canary Islands during the eruption of Cumbre Vieja rift in 1949 (Hoyo del Banco vent). These examples of pahoehoe can be also described as ropy lava.

This lava is probably not basaltic. The Canaries are a fine example of alkaline volcanism and historic eruptions on La Palma are generally basanitic/tephritic in composition. I believe it is probably the case here as well.

A hand sample of pahoehoe lava. Width of sample is 11 cm.

Larger view of the same lava flow of 1949 on La Palma in the foreground. It is almost completely aa lava. Pahoehoe forms only small part of it. In the background is a green volcanic cone of the Bejenado volcano which is mostly phonolitic in composition. It is covered with pine trees. Behind Bejenado is an edge of the Taburiente volcano and between them is the Caldera de Taburiente (the original caldera which ironically is not caldera at all in geological sense).

Trachybasaltic pahoehoe lava flow in Tenerife.

Pahoehoe lava flow and the peak of Mount Teide in Tenerife.

References

1. Walker, George P. L. (1999). Basaltic Volcanoes and Volcanic Systems. In: Encyclopedia of Volcanoes (Ed. Sigurdsson, H.). Academic Press. 283-289.
2. Francis, P. & Oppenheimer, C. (2003). Volcanoes, 2nd Edition. Oxford University Press.
3. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.
4. Kilburn, Christopher R. J. (1999). Lava Flows and Flow Fields. In: Encyclopedia of Volcanoes (Ed. Sigurdsson, H.). Academic Press. 291-305.


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