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Raining sand

By Michael Welland

It was a double-dose of adrenalin: watching a violently growing volcanic eruption while retaining a firm grip on my twelve-year old daughter to prevent her sliding off the rolling boat and plummeting into the turbulent waters of the Sunda Strait. The boat was a rickety old tub, the Sumatran helmsman grinning cheerfully. The volcano was Anak Krakatoa. The “child of Krakatoa” remains exactly that, the diminutive offspring of its infamous parent that in 1883 blew itself up in one of the most violent eruptions of recent history – but adolescent volcanic hormones were stirring. Long periods of tranquillity would suddenly be disrupted by the appearance of an eruption cloud swelling into the sky, the deep explosive booming sound hitting the chest a few seconds later; rocks the size of buses were hurled into the air, and the billowing ash cloud spread with the wind. We were, relatively sensibly, a few kilometres away, and securely upwind. Until the wind shifted, and suddenly it was raining, a shower of black grit drifting to earth, onto the waves, the boat, and us.

It was raining sand. For the term “sand” describes not simply the stuff we find on beaches or the golf course, but any natural material that falls in a defined range of grain sizes, larger than dust and smaller than gravel. It can be composed of almost anything – including the cooled fragments of molten material hurled out of a volcano (not to mention the pellets of fish excrement that are just the right size to form certain romantic tropical beaches). Out of the cloud of a volcanic eruption, the big fragments, the cars and buses, fall first, hurtling down the flanks of the volcano – they can be seen in the photograph. The wind and the power of the eruption can carry sand further, but, inevitably, it soon falls back to earth. We were prudently well outside the bus realm of Anak Krakatoa, but, as it turned out, within the falling sand zone. And we were being showered with some the most newly minted material on the planet, tiny black and rusty fragments of cooled lava, some of them glass, some displaying the remains of minute, gas-filled, bubbles.

In the recent, relatively minor, but chaos-inducing eruption of the unpronounceable Icelandic volcano, it was only the dust that was light enough to be borne by the winds and swirl in the UK’s atmosphere – but a lot of sand has been deposited on Iceland.

And this is not the only way that sand can be formed from volcanic activity. Lava, cooled and battered by the waves and the elements, breaks into sand-sized particles that form, for example, some of Hawaii’s famous beaches.

The sands can be starkly black or green or rusty red – or a mixture of these colours. The green grains are the apple-coloured mineral olivine, a common ingredient of Hawaiian lavas. Even the most apparently nondescript-looking sand, viewed down a microscope, will open up into a glittering world, and Hawaii’s sands are spectacular. Most of our planet’s sands originate from natural processes other than volcanoes, and the majority are perhaps not as photogenic as Hawaii’s, but none are humble and all have stories to tell.

Michael Welland is a geologist. He obtained a BA and PhD in geology from Cambridge, and an MA from Harvard, and went on to a career with the British Geological Society in Oman, followed by university teaching and research in the US, and 20 years in technical and management positions in the international energy industry. A Fellow of the Geological Society of America, and the Royal Society for the Arts and Commerce, he now divides his time between London, France, and international projects, most recently in Central Asia and Egypt. He is the author of Sand: A Journey Through Science and the Imagination. This post first appeared on the BBC Focus/Oxford University Press microsite.

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