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Microscope: Fire vs lava

By Mike, 29 August 2013 News

Welcome to Double Helix magazine’s Q&A section – Microscope. We take a close look at small questions full of big ideas.

Q: I want to know which is hotter, fire or lava? Or are they both the same?
Double Helix reader, Mical from New South Wales

Stylised image of an erupting volcano

Which is hotter – fire or lava?

Shaking is something you do when you’re feeling cold. But, it’s also what tiny particles do when they’re hot.

All atoms, big and small, bounce and jiggle about randomly. Heat describes how energy makes them move this way. On the other hand, temperature is a measurement of the amount of bounce and jiggle there is in bunches of atoms. So, which has bouncier particles – fire or lava? The answer might surprise you.

Fire is a chemical reaction caused by a fuel, such as wood, gas or oil, combining with oxygen. When the molecules of fuel recombine with oxygen, other chemicals, such as carbon dioxide and water, are formed. This swapping of atoms releases a lot of the fuel’s energy in the form of light and heat. The energy in this heat and light depends on the chemicals in the fuel and how quickly they combine with oxygen.

If the fire is a burning candle, parts of the flame can be as hot as 1400 °C! However, since the wick is small and thin, the hot area is tiny, so it doesn’t contain a lot of heat energy.

How does that compare with lava? Molten rock in the mantle under Earth’s crust is under a lot of pressure. Radioactive particles also knock the molten rock’s atoms around, making them hot. Close to Earth’s core, the mantle’s temperature can be as high as 4000 °C. But near the surface, the molten rock (or magma) is a lot cooler. In fact, as it spills out onto Earth’s surface as lava, the runny rock’s temperature is only a little lower than the hottest part of a candle flame: about 1200 °C. On the other hand, a big blob of lava contains many more bouncing particles than a small candle flame, so it has a lot more heat energy.

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