A lot of people have ‘seeing lava’ somewhere on their bucket list, but for geologists it is almost mandatory. So when a family holiday in Hawaii was in the planning stages, one of the caveats was the need for a side trip to the Big Island to see Kilauea which has been erupting since 1983. The Pu’u ‘O’o vent produces a stream of lava that reaches the southern coast of the island in a spectacular fashion that only 900ºC lava meeting sea water can. Very amazing and all, but, in a sobering twist, the cliff-top viewing area we stood on a few weeks before collapsed on New Year’s Eve.
Although being ‘only three weeks from death’ doesn’t quite have the dramatic ring it was a reminder of the fragile relationship between humanity and geology. Well, calling it a relationship is a stretch. Geology doesn’t know or care about us and we have utterly no means to change or even influence geology. All we can do is watch, measure, and hope we can deal with the consequences…
In Geology 101 courses the Hawaiian volcanoes are the classic shield volcanoes produced by high temperature low-silica basaltic lava. The lava is hot and fluid, flowing relatively easily albeit locally devastating (the Pu’u ‘O’o flows have destroyed some 200 houses since the 1990’s). This is usually contrasted with ‘explosive’ volcanoes made of high-silica rhyolitic lava which doesn’t flow easily and traps gas leading to catastrophic eruptions after long periods of building pressure. These are the big volcanoes that can erupt so violently they completely destroy themselves, devastate large areas, and send enormous volumes of fine ash into the atmosphere that can affect weather around the world for years. While a Hawaiian-style volcanic eruption seems relatively quiet in comparison, a dark destructive capability lurks. The viewing area collapse was a glimpse of what is possible.
Rocks are solid, sturdy things right? Check out the picture of the lava fields.
Although it is very hot, the lava is at the surface cools rapidly to form a crust. This partially insulates the lava underneath so, as can be seen in the photo, as the fluid lava still moves downhill it breaks and drags the crust about to form piles of broken rock with some amazing intricate features.
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There is another consequence of the rapid cooling – the lava becomes glass. (Or, technically, an amorphous solid because the atoms in the liquid don’t have the chance to form the complex structures we recognise as minerals inside a rock).
Walking over this stuff is like walking over broken glass (with shoes on of course) with the constant tinkle of glass shards being crushed and broken underfoot.
All this adds up to a lava delta like the one at Komokuna being a loose pile of glassy rubble. And as New Year’s Eve event demonstrates it doesn’t take much to make it all collapse. Again that may be seen as a fairly localised event, obviously devastating for the few that might happen to be in the wrong place at the wrong time, but it is not a global catastrophe. But – here’s the catch – the inherent structural weakness in the pile of glassy rubble scales up. All of the Hawaiian islands are, as Ken Hon of the University of Hawai’i at Hilo describes it, built on ‘watermelons and marbles‘.
Sonar mapping of the seafloor around the Hawaiian Islands in the 1980’s discovered enormous submarine landslides originating from the islands. The submarine fan spreading north from Moloka’i and O’ahu is particularly impressive because it basically shows that large chunks of each island have collapsed into the ocean. The result is most stark on Moloka’i which is around 50 km long and looks like a knife has sliced off the northern half of the island. For Australian players this would be the equivalent of all the eastern coastal suburbs of Sydney from La Perouse to Palm Beach falling into the Tasman Sea and the residents of Surry Hills just needing a short walk for their morning surf.
The collapse of half an island obviously doesn’t go without consequences. There is evidence that enormous tsunami around Hawaii have thrown large boulders and marine debris hundreds of of metres above sea level. Numerical modelling of the waves that could be produced by a such island-sized landslides suggests tsunami waves up to 70 metres could have reached the Californian coastline.
This isn’t just ‘once every one hundred thousand years’ idle conjecture. Returning back to the southern coast of the Big Island, a 40 km wide area known as the Hilina Slump, which encompasses the current lava flows, is creeping into the ocean by around 10 cm per year. The slump slipped in large earthquakes in 1868 and 1975 with locally devastating tsunami. What triggers the sudden and catastrophic movement of the slump is not clear, although there is speculation that the Lo’ihi Seamount – a submarine volcano south of the Big Island (and the youngest volcano in the Hawaiian chain) – could actually be forming a buttress holding up the slump. No human engineering effort could hope to do the same.
What to do in the face of such a potential cataclysm? Take heart that although the five km walk or ride from Kalapana to Komokuna is across a barren and eerily silent landscape there are signs of life returning.
There were little ferns like this poking out of cracks all over the place, and it was clear that they and other pioneering species were being helped along with bits of mulch and low stone walls to trap moisture and provide wind protection. Because although we might not be able to prevent nature’s worst, people can be every bit as stubborn.
