Pahoehoe is a smooth and continuous lava crust. Pahoehoe forms when the effusion rate is low and consequently the velocity of lava flow is slow2. Pahoehoe lava flow is usually at least 10 times slower than typical aa lava flow5. Higher effusion rate results in lava flow being shattered which is how the rubbly and clinkery aa lava surface forms.
Pahoehoe and aa lava are strikingly different in appearance, but their composition may be identical or very similar. Lava flow that was originally pahoehoe may transform into aa lava but the reverse is impossible - once lava crust is broken it can not return back to smooth and continuous form3.
Only low-viscosity (usually basaltic) lava can form pahoehoe. Aa lava is much more common and is not as picky about the composition of lava flow. Aa lava can be basaltic, andesitic, tephritic, etc. Blocky lava needs more felsic compositions (silica content generally over 55%). Blocky lava is composed of larger blocks than aa lava and these blocks have much smoother surface.
Best known examples of pahoehoe lava flows are from the Big Island of Hawaii and the term ‘pahoehoe’ itself (just as ‘aa’) originates from the Hawaiian language. Pahoehoe is also known as ropy lava and it has several more varieties named entrail, festooned, filamented, sharkskin, shelly, etc.
Forming basaltic pahoehoe lava flow in Hawaii. Such lava flows move slowly and are not overly dangerous when compared with some other volcanic phenomena.
However, their destructive power is practically unstoppable. You can easily walk away from a lava flow like that in most cases, but you will not be able to save your house if it happens to be on the way of such a slow-moving disaster. Here is all that is left of a house engulfed by a pahoehoe lava flow.
Although it is possible to predict which regions are in danger of being covered by lava flows, people, if allowed, stubbornly continue to build their houses in these districts. Destruction that inevitably follows keeps the attention of public focused on these flows and provides money for scientists studying them.
Hawaiian lava from the Kilauea volcano (Pu’u O’o vent).
Lava flows are often imagined as fiery rivers of molten rock. That is indeed often the case, but it is also very common that there is no easily defined boundary of an active lava flow. The lava often flows largely underground. When I was hiking in Hawaii with a clear target in my mind to find an active lava flow I was perplexed and amazed when I suddenly discovered that I am actually standing right in the middle of such a flow. There were no river of lava. The red glowing material was just seeping out of the ground here and there, some in front of me and some already behind.
This is potentially very dangerous situation not so much because of lava itself because it moves slowly and contrary to popular belief it is impossible to sink into it. But because of toxic volcanic gases that are involved with all types of lava flows. I tried to be very careful, trying to observe the flows so that the wind was blowing the gas away from me. Here is a picture of volcanic gas seeping out from the ground. If you are also trying to find flowing lava, always try to keep a safe distance with such a thick cocktail of hazardous gases and pay attention to the wind direction.
Pahoehoe lava flows form a beautiful and otherworldly barren landscape in southern part of Hawai’i.
Walking on a pahoehoe lava flow is relatively easy, especially when compared with walking on an aa-type lava flow which is practically impossible undertaking. However, hiking on a smooth lava surface is not without some nasty surprises. As mentioned earlier, lava crust solidifies quickly, but the interior keeps moving which often leaves empty space beneath the thin surface. It can easily break when you step on it which may result in badly scratched legs because the edges of a broken lava flow are razor sharp.
Or even worse, you might have a broken leg which understandably is a major problem if you are miles away from nearest roads. Through practice hikers will learn relatively quickly where it is safer to step and where it pays to be extra careful. Generally speaking, smaller lava lobes are safer and larger gently sloping, but otherwise relatively smooth parts are the most dangerous ones.
This image shows also that voids in lava flows are a common feature. Such a long tunnel-like caves are called lava tunnels. Large amount of lava can move through these tunnels which makes it easier for a volcano to grow itself laterally larger. This is a Thurston lava tunnel in Hawaii. Lava tunnels are especially common within silica-poor basaltic lavas.
This lava flow formed on La Palma, Canary Islands during the eruption of Cumbre Vieja rift in 1949 (Hoyo del Banco vent). Examples like that can be also described as ropy lava which is a subtype of pahoehoe. The wrinkled surface of ropy lava is a result of the interior of the lava flow moving more quickly than the exterior.
Coulée is a volcanic landform which is an intermadiate stage between lava dome and lava flow. The lava that forms coulée is too thick to flow like a normal lava flow. However, it may resemble huge pahoehoe flow (this coulée is more than 50 meters wide) when gravity forces it to flow downhill just a little and the slightly more easily flowing interior warps the surface just like it does in a ropy pahoehoe flow. Light-colored mound in the foreground is composed of pumiceous lapilli. Picture taken in Tenerife, Canary Islands.
No comments:
Post a Comment