Lava wave
If I had to save myself by surfing down a molten lava flow, what would I be able to stand on that wouldn’t melt from the heat of the lava?
• Just take an old surfboard, punch lots of holes through it and connect them to a water tank placed on top of the board. Water escaping through the holes will create the same effect that you can observe when spitting on a hot iron plate: the droplets dance on the plate for quite a long time because they are separated from the plate by a thin layer of steam, which is a bad heat conductor.
This effect would allow you to surf on the lava wave, because the board would be cushioned from the lava by the steam layer. The friction between the board and the lava would be virtually zero. I think that this creation should be known internationally as the sizzleboard!
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Radko Istenic
Ljubljana, Slovenia
• A lava surfboard must not only be melt-proof but it should also be less dense than the lava underneath and it should provide insulation for your feet.
If you are trapped at the summit of a volcano and you need to surf in order to escape, then you must use indigenous materials. Fortunately, volcanoes not only produce lava but also solid materials that are of roughly the same geological composition as lava but are less dense and more insulating because they contain gas bubbles.
A slab of this material, say 50 centimetres thick by 1 metre wide and 2 metres long, would float on molten lava and, just as important, it would melt quite slowly. I suspect that you could travel a mile or more before you would have to abandon it. Hopefully, by then, you would have been able to negotiate your way to an area of dry, cool ground.
However, if you know in advance that you will need to float on molten lava, you would be better off making a boat from a heat-resistant, or refractory material that would not melt and would last as long as you needed it. The vertical sides of the boat would also protect you from the heat radiated by the lava far better than a surfboard would.
The temperature of molten lava is usually about 1400 °C although it can be as high as 1650 °C depending on its chemical composition, so the best material to use for a boat is high-purity alumina insulating refractory concrete. This is made of aluminium oxide, which melts at 2000 °C, hardly reacts with molten lava and contains hollow bubbles, so it is slightly less dense than the molten lava it will have to float on, and is a good insulator too.
To construct your boat, make a mould by digging a pit in the ground of the shape you want the outside of the boat to be, then pound the soil in the pit until it is compact and smooth. You should then line the mould with plastic sheeting and mix the dry concrete with just enough water to form a stiff paste. Cover the plastic with a 10-centimetre layer of paste, line the concrete with another plastic sheet and fill the remaining cavity with water to press the concrete into place while it sets. The boat will be ready after a week.
Ross Firestone
Winnetka, Illinois, US
• If the only consideration is melting point, your correspondent would not have much difficulty. Different types of lava melt at different temperatures, rhyolite at up to 900 °C, dacite at up to 1100 °C, andesite at up to 1200 °C and basalt at up to 1250 °C. Steel, with a melting point of 1400 °C, would be fine, but to be really safe, how about tungsten, with a melting point of 3422 °C?
However, your feet would get a bit hot, so it would be better to use a non-metallic insulating substance such as the following ceramics: Cr2O3 melts at about 2250 °C and even Al2O3 at about 2050 °C would be safe, and both would provide insulation for your feet.
I suspect, though, that the six-year-old questioner might have difficulty getting hold of such materials, so I suggest using oak. All woods, and especially oak, form a protective carbonised layer when burnt, which slows further combustion. Indeed, when designing a timber structure one can allow for this layer to provide fireproofing. Timber structures are always designed a little over the size that is actually required so that their structural integrity is retained in case of fire. A thin steel plate lining the outside of the board would protect against abrasion if the surfer wanted to go back to the top to repeat the experiment.
Malcolm Nickolls
Aylesbury, Buckinghamshire, UK
• The problem is not just keeping the heat away. When I was in the Sahara I saw those lovely sand dunes and thought that surfing down one of them would be fun. Sadly sand and wood do not have the right coefficient of sliding friction, and you don’t so much surf as sit, even on the steepest dunes.
You can calculate coefficients of sliding friction using tables, and you should be looking for the sort of figures typical of waxed skis on snow. My tables are a little old, and there are no figures available for lava, so maybe one of your correspondent’s first jobs if he chooses to become a volcanologist would be to calculate these coefficients of friction experimentally for different materials, so that a suitable escape craft can be designed.
Of course, the designer would have to be a quick and accurate worker or their career may not be quite as long as they might hope.
Peter Brooks
Cape Town, South Africa
This week’s question
Kava on the brain
I recently drank some of the interesting root-based drink kava on the Pacific island of Vanuatu. I’m happy to report that it had some odd effects. What exactly did it do to my brain?
Robert Steers
Galston, New South Wales, Australia