Wednesday, October 22nd, 2014

What is Pahoehoe Lava?

lava types

Types of Lava Overview…

When lava is underground in its molten state it is called magma. As it reaches the ground, and air, it is called lava. Once lava begins to harden it can turn into a variety of shapes and colors. The color of lava depends on the temperature of the flow as well as what impurities are in the liquid rock. Colors can include black, red, gray, brown and tan, metallic sliver, pink, and green. Some lava has peridot (olivine) in it which are beautiful olive colored semi-precious crystals that, once they weather and break apart, are responsible for our famous green sand beaches.

When lava cools it also forms a myriad of different shapes and types of lava. There are two main types of lava pahoehoe (pa-hoy-hoy) and a’a (ah ah). Pahoehoe lava comes out smooth and dense and can form large areas that resemble flat parking lots or smooth bumps. A’a, on the other hand, forms individual rocks anywhere from a few inches to many feet in size. The rocks are porous and very jagged. In general, pahoehoe is very easy to walk on and a’a is very difficult, if not nearly impossible, to walk on (at least without getting hurt).

A third type of lava, pillow lava, forms only underwater and is created by lava entering the ocean underwater where the pressure of the ocean pushes against the lava to form pillow-like shapes that cool very quickly due to the ocean water.

Pahoehoe Lava…

Pahoehoe Lava is one of the most interesting forms of lava. Since it tends to flow more as a thick liquid it can pour uphill as well as downhill and can create a huge variety of interesting shapes.

If pahoehoe lava flows over a fairly flat ground it will coat the ground much like a parking lot – with a thick, smooth, flat coating of lava. However, when pahoehoe encounters barriers in the way, such as rocks, hills, fissures and bumps, it flows over and around them and produces different shapes.

One of the more interesting types of pahoehoe lava is called ropey pahoehoe and looks like a pahoe rope series of twisted ropes spaced evenly along the ground. The twisted ropes may be fairly straight, or may loop and wind in and out much like a fingerprint. Many visitors express interest in what could create such an unusual shape, but once you see ropey pohoehoe lava being created it is instantly clear how the shapes occur. As the pahoehoe flows, it usually encounters some minor barrier that slows up the front of the flow. As the front of the flow is slowing down, the faster flow behind it pushes the front and forces it to create a small ridge, which it pushes up and over the barrier. That ridge begins to cool and creates the next barrier, which in turn creates the next. The result is a series of ridges interspaced with valleys – which looks like 4 inch thick ropes of lava laying side by side or looping side by side. To walk over ropey pahoehoe it is best to walk on the top of the ridges, perpendicular to the ridges.

pahoe 2 While ropey pahoehoe is an official name for that particular formation of lava there are a number of other different types of pahoehoe lava – of which we have given our own name but these are not official. One type of especially metallic lava we call quiet flow. Normally, pahoehoe lava sounds like styrofoam being walked on – but quiet flow is much denser and when you walk from normal pahoehoe lava up onto quiet flow your footsteps immediately become softer. The quiet flow pahoehoe also is much more metallic and whereas normal pahoehoe sounds like rock if you break a bit, the quiet flow pahoehoe sounds like Christmas tree ornaments breaking – a light tinkling of glass. Quiet flow pahoehoe also tends to produce very glassy hollow bubbles, which if broken also sound like glass hitting a floor. The quiet flow lava is far denser than normal pahoehoe and we have witnessed a regular pahoehoe flow suddenly producing a quiet flow, and then reverting back to a normal flow.

Another unofficial, but yet different type of pahoehoe lava we call black flow.   pahoe3Black flow lava is especially dark and has a rougher feel to it. Black flow tends to create large mounds and coat tumulus and is much harder to walk on than normal pahoehoe flows. You can usually tell a black flow from other types of flow because it is extremely dark and looks like it isn’t much fun to walk on. In general we try to skirt around black flows just because it is much more difficult to hike on.

All the differences in pahoehoe flows stems completely from the temperature of the lava as well as chemical composition. Since both can change during a flow – it is possible to get all types of pahoehoe flowing from a single flow, over time.

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