After 30 years in Fairbanks, Alaska, we finally wimped-out and went to Hawaii at Christmas instead of our cabin. The cabin is in a remote mountain valley and gets no direct sun this time of year, and the temperature hovers around -20°F. Truth be told, on most mid-winter trips to the cabin we sleep a lot. Hawaii offered a more wakeful vacation. At the Kailua/Kona airport I recognized the other Alaskans by their unhealthy pallor and the fact that they were ill at ease in sandals and shorts. We all quickly dispersed to our various hotels, beaches, boats, kayaks, and surf boards, marveling at the exotic greenery and the sun.
In addition to soaking up sun, I had two things I wanted to see on the Big Island: flowing lava, and the stone stripes on the summit of Mauna Kea. Lots of people want to see the former; few know or care about the latter. Because lava viewing is popular, there are at least 3 ways of doing it. One is to fly by helicopter from Hilo and circle over where the lava plunges into the sea. This will set you back about $180 and the closest you get to the lava is a couple of hundred feet. The second way is by boat, which looks spectacular from the websites, but still leaves you a hundred feet from the lava. The third method is to hike across 4 ½ miles of hot black lava (Fig. 1), which takes about 6 to 8 hours round trip, but has the advantage that you can burn up your shoes walking on hot lava if you really want to. Despite what seems a fairly concerted effort by the Park Service to discourage people from the hike, it appeared to be quite popular on the day we were there, with perhaps as many as 50 hiking groups. Some were armed with water, stout boots and powerful headlights and knew what they were doing, while others appeared to be casual tourists in flip-flops and shorts without a clue.
My wife and I had headlights, water, rain jackets and good shoes, but little idea of what to expect or where to find it. After hiking about 3 hours, with the sun setting in the ocean to our right, we could see heat waves shimmering above the lava floes ahead. Our pace increased. I was just about to step up onto the next floe when I saw in a crack that the inside of the floe was glowing red-hot. Indeed, all about us there were glowing red cracks (Fig. 2). Soon we located a place were the lava was flowing on the surface forming lobes and tongues (called lava toes), congealing, then breaking out somewhere nearby (Fig. 3). The heat was intense, and there were fumes, but the scene was primordial and mesmerizing. Several other groups of lava pilgrims had arrived and we all stood in awe as the orange lava pulsed and flowed and glowed more intense as it got darker. Near the sea cliffs, waterfalls of orange lava were sliding into the sea where six-foot waves crashed against the sea cliffs. New land! This was the hike of a lifetime.
It was dark when we headed back across the lava floes. I had temporized in my mind that the lava, which was so new it was actually shiny in the daylight, would be a good reflector of headlights in the dark, but this was not the case. The crescent moon provided almost no light, and soon a rain squall blew in obscuring what little light there was. We stumbled our way back over the pahoehoe floes. Other lava pilgrims were painfully working their way back too, their headlights winking in the dark. We fell in with a couple from Whitehorse, Yukon Territory, and collectively our 4 headlights were just sufficient to pick a way. By 9 PM we had made it back to our car largely intact. Dozens of cars were still parked there, suggesting that many lava pilgrims, well-prepared or otherwise, were still out there in the dark.
The lava we saw had come all the way from Pu`u `O`o crater (Fig. 4). Based on its color (yellow to red) it was probably still over 1700°F at the hottest, which meant it had cooled a mere 300°F during its 7 mile journey to the ocean. This minimal cooling attests to the fine insulating value of cooled lava, since the trip probably took more than a month. It is also why I was able to walk on lava floe that was still red hot inside without burning my shoes, and it helps explain the prevalence of lava tubes. The lava toes we saw (perhaps we could see half a dozen) had a cumulative discharge rate on the order of 0. 05 m3/s, while Pu`u `O`o crater was probably spewing lava at 1 to 10 m3/s of, indicating there were dozens if not hundreds of other lava toes oozing out that we could not see. Perhaps the aerial view from the helicopter would have confirmed all this. Still, there is something to be said for standing 3 feet from an active lava floe without expert supervision. (For more info see here and here).
Two days later we headed up Mauna Kea (Fig. 5). It may be the tallest mountain on Earth when measured from it base in the abyssal deeps (about 10,000’ below sea level) to its top (13,796’). The summit is home to several large telescopes and astronomical observatories, and just below the summit is Lake Waiau at 13,020’. The sacred lake was frozen. On a slope of volcanic scoria above the lake we could see the distinctive stone stripes, finer-grained brown stripes alternating with darker gray stripes of gravel and cobbles. I had heard of these stripes from a colleague, Dr. Bernard Hallett from the University of Washington. They are a striking example of self-organization in Nature arising from random processes. They are also part of a continuum of cyrospheric features that includes sorted circles, polygonal patterned ground, and sorted stripes. Self-organization has always intrigued scientisists, so there is a considerable literature on the topic.
The ingredients needed for making stone stripes is a diamicton (a mixture of pebbles and cobbles in a matrix of finer soil) (Fig. 6), a slope (somewhere between 5° and 15°) (Fig. 7), abundant soil moisture (15 to 25%), and frequent freeze-thaw cycles. The agent in making the stripes is needle ice (Figs. 8 and 9). This exotic looking ice is actually quite common throughout the world (see here for some nice needle ice photos as well as a related phenomenon called frost flowers). First described by Le Conte in 1850, it has been reported widely in mid-latitudes and from numerous alpine and high-latitude locations. It forms during cold clear nights due to radiative cooling of the soil surface.
The exact processes in play have been debated for more than 60 years, but basically water moves upward through pores in the soil due to either capillary suction or the downward progression of the freezing front pressurizing the deeper soil moisture. Just below the soil surface the water is super-cooled (it is below 32°F) so that when it emerges it freezes very rapidly. The small diameter of the pores in the soil ensures that the upward seep takes place as a series of very thin water filaments, which produce needle-like spicules of ice. The needles grow at their base, pushing the frozen parts higher and higher. The process can easily lift soil and rock particles. Laboratory growth of needle ice indicates that amount of soil moisture, the amount of fines in the soil, and the temperature structure in the soil all influence whether the needles will grow, and how fast and how high.
The needles alone, however, are insufficient to produce the stone stripes. For that a sorting mechanism is needed. It is supplied by thawing ice needles and gravity. Because the needles grow perpendicular to the general surface (which is about 15° for Mauna Kea), when they thaw and weaken during the warm daylight hours, they bend and preferentially fall down-slope (Fig. 10). Stones that were initially supported by the needles fall downslope as was well. Through this process some areas become stone-rich while others become stone-poor.
Subsequent needle growth is inhibited in the stone-rich areas because of their low moisture content, but it remains vigorous in the fine-grained areas, eventually leading to a slight difference in elevation between the two. Now the local slope is actually both downhill and away from fine-grained soil areas. This feedback effect ensures that in subsequent needle ice events, the stones move even more efficiently to the sides of soil stripes.
As we drove back down Mauna Kea toward the beach, I was struck by the fact that there were actually some striking similarities in the processes that create stone stripes and lava loops. Both are produced by the freezing of a liquid, though very unusual circumstances are required for rock to be found in its liquid state at the surface of the Earth. In both cases, cooling rates are very important, determining whether the stripes or loops will form, how quickly they will do so, and the form and shape they will take. Slope too is important ingredient, though the stripes need a healthy slope while pahoehoe needs a much lower slope. And both processes have produced unusually striking geologic features on the beautiful island of Hawaii.
Images: The map: USGS. Author photo: Elizabeth Sturm. All the rest of the images: Matthew Sturm