def. Pahoehoe:

A type of lava with a smooth or undulating surface. Pahoehoe lava flows often resemble coils of rope and are most commonly found in basalt. Pahoehoe is Hawaiian word that is pronounced “pah-hoy-hoy”.

 

Pahoehoe is a term used to describe a lava flow that has a smooth or undulating surface. Many pahoehoe flows resemble coils of rope or, in some cases, messy piles of rope. Pahoehoe is mostly commonly found in basalt.

Pahoehoe forms when lava flows slowly and a thin crust of cooled material can form on top of the hot lava, preventing heat from escaping and allowing the lava to retain its fluidity (or to retain its low viscosity, to use a more formal geologic term). Lava that flows slowly and cools slowly will form volcanic rocks with smooth or undulating morphologies and, often, will form volcanic rocks that resemble coils or piles of rope.

When lava flows more quickly, the surface crust tends to break and the lava loses heat more quickly. The lava becomes less fluid (or more viscous, since cooler lava is more viscous), which makes the volcanic rock have a blocky, rough morphology. A lava flow with a blocky, rough morphology is known as a’a (pronounced “ah-ah”), another Hawaiian word.

Both pahoehoe and a’a lava flows can be produced at the same volcano. For example, pahoehoe lava flows may form when the lava flows slowly across a gentle slope. However, if the slope becomes steeper and lava starts to flow more quickly the morphology of the lava flow can change from pahoehoe to a’a.

There are many great places to see pahoehoe and a’a lava flows. For example, you can see these types of lava flows in basaltic rocks on the volcanic islands of Hawaii and Iceland. A few years ago, my husband and I went on vacation to the volcanic island of Reunion in the Indian Ocean. We had and incredible trip and saw many amazing lava flows. The pictures of pahoehoe and a’a lava flows in this post were taken during that vacation.

Here are some more pictures of pahoehoe lava flows:

 

 

 

 

And here are some pictures of a’a lava flows:

 

 

 

That’s all for this week’s word – stay tuned for next week!