Just as Eskimo's are said to have many names for kinds of snow (not actually true, but reasonable), volcanologists have many more names for various lava surfaces than the standard a'a and pahoehoe. Still, these two categories cover most flow types you will encounter on basaltic shield, and the other can be thought of as refinements on these two basic classifications. A'a is known for its clinkery, rough, loosely consolidated surface. I have always thought that the name possibly came from the sound people make when they are walking over this material in bare feet. In any case, a'a is very prevalent on the surfaces of Mauna Loa and Kilauea, especially in proximal (near vent) regions. A'a is also common at the distal (distant) parts of surface flows when the flow has cooled enought to become more viscous.
The clinkery surface of an a'a flow belies its interior of thick, pasty lava. The diggers of wells and cess pools on the big island know the cooled interiors of a'a flows as blue rock. It is dense, hard, and unvesiculated. Sometimes this lava is exposed in the interior of a'a flows, should you be so foolish as to venture there. Three factors influence the formation of a'a. These are temperature, volatile content, and flow induce strain rate. The first of these are pretty straightforward. Cooler lava tends to form a'a. Also, loss of volatiles, principally water, also tend to favor the formation of a'a. Both of these effects are realized during high effusion eruptions, such as the early stages of a fissure eruption or high fountaining. Lava is fragmented into small parcels that more easily lose heat and gas. The reason loss of water favors a'a formation has to do with the formation of strings of silica-oxygen tetrahedra during early stages of cooling before descrete mineral crystals become apparent. These string tend to make the lava thicker, and the presence of water in the melt inhibits their formation. The third factor, strain rate, has to do with how rapidly the lava is flowing. There are numerous examples along the Pali's on the South side of the Big Island where a pahoehoe flow pouring over the cliff is converted to a'a because of the increase in "stirring". Walking on a moving a'a flow is extremely unpleasant, and not recommended!
The movement of a'a is something like a caterpillar (D9 type) tread. Clinkers on the top are carried forward and dumped in front of the flow to form a rubbly base layer as the flow moves ahead. This results in a series of triplicate structures that can be easily seen in the road cut at the hairpin turn where the Chain of Craters road descends the Pali. A thick, massive blue rock layer is sandwiched between two rubbly layers of cinder. As an a'a flow moves forward, it makes a silly tinkling sound like broken glass -- not at all what you might expect.
Pahoehoe is a much nicer form a basaltic lava. The more time you spend on a'a, the more you come to appreciate pahoehoe. Pahoehoe has a smooth often ropey surface that reflects the swirls and pools that attend its implacement. It forms from lava that is much more fluid than that forming a'a. This image shows a massive pahoehoe flow coming over a small cliff. Common features on such flows are bocas shown here, which is latin for mouth. Also one can find toes, which are small buds of surface pahoehoe. Originally it was thought that pahoehoe was emplaced by stacking these toes one on top of another. During the current eruption it has become increasingly clear that much of the lava emplaced in massive flow fields is emplaced beneath the surface raising it up. This is evidenced by fence lines plucked from the ground and large flexural cracks resulting from the uplift of adjacent slabs.
The surface of a pahoehoe flow often has a ropey texture as seen here. This flow is covering an earlier a'a flow and shows nicely the dramatic difference between these two types of lava. During a sustained eruption, flow sometimes pauses because of blockage of lava tubes (discussed below) or upslope pooling of lava. When lava flows return to lower reaches of the volcano, the first flows tend to be a'a as lava cools as it passes over cool ground. Subsequent flows override this early emplaced a'a, which is still hot, producing a much needed (my opinion) veneer of pahoehoe. This is the situation attending the image on the right.
Shelly pahoehoe is just plain nasty. This form of lava is mostly confined to near vent regions, since highly gaseous lava is required for its formation. Often this form occurs with overflows from a near vent lava lake, as volumes of gas rich lava pours over confining dikes. As this lava flows downslope it forms huge knaps and rolls which drain out leaving thin, fragile tubes. The weight of a human on these tubes is generally sufficient to break through, especially for volcanologists as massive as your professor. The result is often serious lacerations on the lower extremities, of which I have quite a few. There is considerable shelly pahoehoe on the upper and middle slopes of Mauna Ulu adjacent to the ancient Pu'u Hulu Hulu cone.
Slabby Pahoehoe is somewhat common in the near vent region, though perhaps not as common as shelly pahoehoe. It seems to form from flows that are well above the melting temperature so that a fairly thick lid forms on the surface while the interior is still quite hot. I admit that this is somewhat speculative on my part. The image shown here was taken during the beginning of Episode 51 within about 100 m of Pu'u O'o. I was struck by the way the slabby surface had "plowed" up the tephra which is ubiquitous along the axis of the rift zone. I have occasionally found slabby pahoehoe on the coastal flats, but generally associated with what appeared to be high temperature break outs.
Spiney Pahoehoe is shown in the image on the right. This form of pahoehoe is sometimes called transitionary pahoehoe or semihoe. It seems to represent a distal transitory form between regular pahoehoe and a'a, and can be quite unpleasant -- something like walking over an array of swords pointing up with their handles buried in rock. If I remember my Camelot correctly, its supposed to be the other way around. Spiney pahoehoe is generally limited in horizontal extent.
Lava trees and tree moulds are closely related phenomena. Both form when high volume surface flows engulf a forest. As shown in the sketch on the left, a flow initially surrounds the trunk of a tree and chills. This not only insulates the tree and prevents it from burning, but also forms a structure within the flow that is not revealed until the lava drains away. Lava trees are the vertical structures that protrude from a flow that deflates after the eruption wanes. Frequently the effusion rates near a vent cause lava levels to build up faster than the lava can flow away. As the lava subsides, lava trees are left in mute testimony to the previously higher levels of the flow. Tree moulds form where there is little or no post-emplacement deflation. They appear as holes in the ground where a tree protruded through the flow and has subsequently burned away. Frequently the first flow to enter a forest leaves many of the trees either standing or lying unburned on the cooled surface of the flow. The reason for this is the insulating effect of the coating lava. The next flow tends to burn the slash left by the first, making movement over the flow much easier.
This is my favorite picture of a lava tree forming. At the time this picture was taken (by Dorian Weisel), part of the flow surface had collapsed providing a seldom seen view of a lava tree before its protrusion through the chilled surface. At the time this photograph was taken, the photographer was extremely hot and nearly burst into flames. Compare this picture to the sketch above.
This is a picture of a lava tree after the cooled surface has collapsed. It should be readily apparent how this situation evolves from the one shown in the preceding image.
Yet another picture of a recently formed forest of lava trees.
I call this picture a "Fine grove of fresh lava trees". I'm beginning to wonder why I have included so many pictures of lava trees. I guess I must like them, or something. Anyway, this image shows how the first flow into an area leaves many of the trees standing. Spatter, from the vent in the forground, has caught in the branches of some of the trees that remained standing. Similar featues can be seen on the trail to Mauna Ulu -- trees still protruding from lava trees that formed during the Mauna Ulu eruption (1969-1974).
Driblet spire atop broad tumulus
Fresh tumulus near coast, coated with black sand
Group of Mauna Ulu Hornitos
Active hornito (spatter cone)
Overflow of lava lake (kapaianaha)
Lava flooding into forest
Pillars in Wailuku River
Aerial: Pu'u o'o to the sea
Lava river tubing
Thurston Lava Tube
Spigot caused when bench collapse truncated tube
Littoral cone (active)
Litoral cone (landlocked)
Closeup of limu
Bubble forming limu
Pillows on wailuku river
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