![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
pahoehoe & aa
Dec. 17th, 2025 06:56 ampahoehoe (pah-HOH-ay-hoh-ay, puh-HOH-ee-hoh-ee) - n., basaltic (i.e. mafic) lava with a smooth or billowy surface.
aa or a'a (AH-ah) - n. basaltic (i.e. mafic) lava with a jagged, clinkery surface.
Fresh aa flowing over cool pahoehoe:
Thanks, WikiMedia!
So a bit of volcanology. I ran mafic and felsic as a pair a while ago, but in sum, lava with a lot of silica, called felsic, is viscous and traps gas, so is associated with explosive eruptions, while lava with very little silica, called mafic or basaltic, is runny and lets gas escape, and so it associated with lava flows and shield volcanoes such as the entire Hawaii archipelago. If the surface of a lava flow cools rapidly, the skin solidifies then gets broken up as the lava beneath it flows on, becoming aa -- but if it cools slowly, it flows smoothly and becomes pahoehoe. The Anglicized forms of the Hawaiian words for these two types of lava flow were popularized by American geologist Clarence Dutton starting in the 1880s. The Hawaiian words themselves are pāhoehoe, from nominalizing prefix pā- meaning "having the qualities of" + hoe-hoe, reduplication of hoe, to paddle (so essentially, "like paddle ripples"), and ʻaʻā, to burn/glow/fury.
---L.
aa or a'a (AH-ah) - n. basaltic (i.e. mafic) lava with a jagged, clinkery surface.
Fresh aa flowing over cool pahoehoe:
Thanks, WikiMedia!
So a bit of volcanology. I ran mafic and felsic as a pair a while ago, but in sum, lava with a lot of silica, called felsic, is viscous and traps gas, so is associated with explosive eruptions, while lava with very little silica, called mafic or basaltic, is runny and lets gas escape, and so it associated with lava flows and shield volcanoes such as the entire Hawaii archipelago. If the surface of a lava flow cools rapidly, the skin solidifies then gets broken up as the lava beneath it flows on, becoming aa -- but if it cools slowly, it flows smoothly and becomes pahoehoe. The Anglicized forms of the Hawaiian words for these two types of lava flow were popularized by American geologist Clarence Dutton starting in the 1880s. The Hawaiian words themselves are pāhoehoe, from nominalizing prefix pā- meaning "having the qualities of" + hoe-hoe, reduplication of hoe, to paddle (so essentially, "like paddle ripples"), and ʻaʻā, to burn/glow/fury.
---L.
