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Pahoehoe refers to a type of lava flow; these are smooth-topped lava flows, caused by hot fluid lava flowing easily and smoothly. When the surface of the lava cools it forms a crust, but underneath the crust the red hot lava keeps flowing. As the lava flow expands the crust folds, wrinkles, cracks, and rips, allowing more lava to spill thru; this lava crusts over in turn. Depending on the heat, speed, and composition of the lava, pahoehoe can cool into all kinds of odd forms; pillows and ropy folds, toes, blisters and coils. Pahoehoe flows are sometimes referred to as lava sculpture.

Although pahoehoe is referred to as 'smooth lava', it is actually a rough-grained, basaltic rock. There is an outer crust about 1 cm thick that is very glassy; next is a transition zone of a few centimeters, before finding the core of the rock, which is also called pahoehoe. The crust may be silvery, a glassy dark-blue, or black.

Pahoehoe is one of the two main types of lava flows. The other is aa (or a'a). Aa is formed when the lava picks up broken bits of lava, rubble, and clinkers, rolling into broken clumps and balls, and generally making a mess of things. Pahoehoe flows may turn into a'a flows as they cool and reach virgin landscape. Pahoehoe flows often cover cooling a'a flows, as the hot a'a is warm enough to keep the new lava from cooling and breaking.

As the lava flow drains out from under the hard pahoehoe crust, lava tubes may form, forming long tunnels, some big enough for a human to walk through. Shelly pahoehoe refers to lumps of rolls of pahoehoe that have been drained from the inside, to form a complex of smaller fragile 'lava tubes'; these can be dangerous to walk over, as the shells may break, causing you to fall or the sharp fragments of the crust to cut your lower legs.

Some other forms of pahoehoe:

Ropy pahoehoe: A very common type of pahoehoe. This forms when a partially cooled crust is bunched up and wrinkled, due to the turbulence of the underlying lava flow. The resulting folds look something like coiled ropes, but perhaps even more like some fabric bunched and wrinkled into fine folds. Ropy pahoehoe may appear in the form of lava coils, where the lava piled up gently into a spiral, looking like an oversized Ammonite.
Picture: http://volcanoes.usgs.gov/Products/Pglossary/pahoehoe_ropy.html

Pahoehoe toe: Another very common formation. A toe is a budding out of the lava flow, a smaller protrusion from the main mass of lava. Lava toes may have ropy or smooth (or other) surfaces, but usually they are smaller, smooth flows occurring at the edges of the flow. As toes fill and grow with new lava, they may bud new toes of their own.
Picture: http://volcanoes.usgs.gov/Products/Pglossary/pahoehoe_toe.html

Pahoehoe entrails: Pahoehoe that was formed by fast moving lava flowing down a steep slope. Long flat tongues of lava that look something like an animal's entrails spilling over and around each other in a giant tangle. Not having butchered many animals myself, I would have compared it to melted wax, but to each their own.
Picture: http://volcanoes.usgs.gov/Products/Pglossary/pahoehoe_entrail.html

Pahoehoe blister: Small glass bubbles on the surface of pahoehoe formed when gasses try to escape the flow, but are captured in the forming glassy crust. Somewhat akin to Limu o Pele, but much stronger and thicker due to the less explosive nature of the gasses and slower cooling time.
Picture: http://volcanoes.usgs.gov/Products/Pglossary/pahoehoe_blister.html

Spiny pahoehoe: Pahoehoe without the glassy crust. Formed when viscous, cooler lava cools at a rate slow enough that there is relatively complete crystallization of the rock, and glass does not form. The surface is rough, or spiny. It is also called toothpaste pahoehoe or sharkskin pahoehoe.

Rubbly pahoehoe: Usually we think of lava as either 'smooth' pahoehoe and 'crumbly' a'a. This works pretty well with the Hawaiian volcanoes, but in other parts of the world it isn't uncommon to see a cross between the two. Rubbly pahoehoe is a pahoehoe flow in which the flow top is composed of broken bits of small pahoehoe lobes (whereas in aa the top of the flow is composed of clinkers). It's debatable whether this should actually be considered a type of pahoehoe flow, but it is a lava formation that has to do with pahoehoe.

Pahoehoe is a Hawaiian word, now used regularly in English as the proper name for this sort of lava. In Hawaiian it should be written pāhoehoe. It may be pronounced either 'pahoyhoy', 'paHO-ehHO-eh', or 'puh-hoh-ee-hoh-ee'.


Tem42 has covered pahoehoe very well above, but please excuse me an anecdote.

Flowing pahoehoe is possibly one of the most beautiful things in the world.

Let me back up. A while back, I was in Hawaii with my wife. We went to Volcano National Park, and the only thing going on beside the usual sulfur vents, etc. was a steam plume from where lava was running into the ocean. We decided to walk out to the closest safe place and timed it so that the hike would get us there near dusk. We watched as the plume turned from steam-white in the sun to bottom-lit by staggeringly hot rock red in the dark, and then walked back out at night. This was a popular thing to do, so there was a stream of people with flashlights headed in either direction on a path marked out in advance. From above, I'm sure it would look like some sort of thrill-seeking ant trail.

On our way back in the dark, we looked across the expanse of volcanic rock and saw glowing red on the slope in the distance. Why, there was perfectly serviceable lava over there, why would they just point everyone to go and watch well-lit water vapor?

A few days later, we returned to the park with the idea of hiking up and seeing the lava. We went into the main visitor center, staffed with helpful rangers, and inquired about it. We were told, flat out, there was no way at all that we would be allowed to. Absolutely not. Would you like to buy a map or a book about the park?

We drove down to the impromptu parking lot near the active area, which was created after the road further down was covered in lava a while back. We went to the impromptu ranger station and approached someone. We told them what we wanted to do. Rather than blowing us off, they grabbed another ranger and the interrogation began.

"Let's see your shoes." We showed that we were wearing sturdy boots which protected and supported the ankle.

"You know how far it is?" We didn't, but they helpfully told us.

"You've hiked on lava before?" We told them we'd walked out to the sea vent and knew how hard hiking on hilly broken black glass was.

"Let's see your lights." We showed them the trio of fairly powerful flashlights.

"Let's see your water." We opened our backpacks and showed them that we had 4 liters of fluid each, 2 of water and 2 of sugary lemonade.

"Let's see your food." We showed them the various granola bars, nuts, etc. that we had.

There were a couple of glances and shrugs between rangers, which appeared to settle it. They gave us detailed instructions with visual landmarks of how to get up there, how to approach the flow from the side, how fast a'a lava could go downhill, and every other safety precaution they could think of. They wrapped it up with something along the lines of "Most of us have been up to that flow, but that doesn't mean that it won't kill you."

With some variation of "We never spoke, we were never here" we were on our way.

We got up to the altitude of the flow with only humorous problems (Seriously? Who thinks a bee hive is going to be in the only thousand square feet of non-rock on many, many square miles of volcanic glass?) When we got to that level and looked across, there was nothing but the wavering distortion of air caused by the lava's heat.

We began picking our way in that direction. Based on the heat, we kept picking our way uphill. We wanted to find the leading edge where there wasn't potentially lethal fresh lava uphill from us. With a little bit of effort, we found it. A massive field, obscuring the horizon with its heat, nipping at our shins while we tried to photograph the tiniest rivulet on the leading edge.

It was gorgeous.

Pa*ho"e*ho`e (?), n. Min.

A name given in the Sandwich Islands to lava having a relatively smooth surface, in distinction from the rough-surfaced lava, called a-a.

<-- Sandwich islands = Hawaii -->


© Webster 1913.

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