Pahoehoe Basalt
Pahoehoe is basaltic lava with a smooth, billowy or ropy surface. The word is Hawaiian, and it describes fluid lava that advances in small lobes beneath a thin, flexible skin.
Pahoehoe is basaltic lava with a smooth, billowy or ropy surface. The word is Hawaiian, and it describes fluid lava that advances in small lobes beneath a thin, flexible skin.
๐ชจ Geology Guide Specimen: Open Pahoehoe Basalt in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.
Skin and Flow: What Makes the Ropes?
A thin skin forms on a lobe of runny lava as it meets air. As molten lava keeps moving underneath, it drags and wrinkles the skin into twisted, rope-like folds. The surface is often glassy and iridescent, and the flow's interior is full of gas bubbles.
Pahoehoe to a'a: Why Does Lava Change Style?
The same lava can change from pahoehoe to the rough, clinkery lava called สปaสปฤ as it travels. Cooling, loss of gas, crystallization and faster flow raise its viscosity until the skin tears instead of wrinkling. The change goes one way: สปaสปฤ does not turn back into pahoehoe.
Lava Tubes: Where Is It Seen?
Pahoehoe flows at Kฤซlauea and other Hawaiian volcanoes build insulating crusts and form lava tubes that carry molten rock many kilometers with little cooling. Similar flows are found in Iceland and elsewhere, and empty tubes remain as caves. Compare the submarine form, pillow basalt.
Pahoehoe Basalt at a Glance
| Property | Value |
|---|---|
| Type / variety | Ropy Effusive Basaltic Lava |
| Composition | 48โ52% SiOโ โข Low-Viscosity Fluid Tholeiitic Melt |
| Geology Guide group | Igneous Rocks (Plutonic & Volcanic) |
| Rock Classification | Mafic Volcanic (Basaltoids) |
| Mohs hardness | Mohs 5.5โ6 |
| Texture / habit | Smooth, billowing, or twisted ropy surface crust |
| Density | 2.80โ2.95 g/cmยณ |
| Lustre and cleavage | Iridescent glassy surface skin over vesicular interior |
| Geological setting | Hawaiian-type shield volcano lava tubes and sheet flows |
| Diagnostic feature (Geology Guide) | Dark, fluid lava flow whose thin, viscoelastic surface skin was dragged and wrinkled by hot molten rock moving underneath into smooth, twisted coils resembling coiled rope. |
| Name origin | Hawaiian pฤhoehoe |
| Related lava type | สปAสปฤ |
See Also
- Basalt โ the rock type
- Pillow Basalt โ basalt erupted underwater
- Scoria โ basaltic cinders from fire-fountaining