(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Scientists at the USGS Hawaiian Volcano Observatory routinely collect lava samples from Kīlauea and use the chemistry of these samples to infer the temperature of magma (molten rock below Earth’s surface).

Over time, these measurements have shown that magma temperatures rise and fall during an eruption, perhaps as the supply of magma to the volcano changes, or when the plumbing system is disrupted. Consequently, magma temperatures estimated from lava samples provide a fundamental means for tracking changing magma conditions and pathways within the volcano.

So, what’s the real scoop on magma temperatures? What temperature is deemed “hot” — and what’s not?

Magma rises from the mantle, a region deep within the Earth, into the “roots” of Kīlauea at temperatures of around 1500 degrees Celsius (2700 degrees Fahrenheit). From there, the magma eventually makes its way to a primary storage chamber that’s about 3.5 km (2 mi) beneath the summit of Kīlauea — a trip that takes about 8 years, according to a recently published study on the volcano’s magma chemistry.

By the time magma reaches Kīlauea’s summit storage chamber, it has cooled considerably. Samples collected from the lava lake within Halema‘uma‘u, which is a window into the summit storage chamber, indicate that temperatures within the chamber are around 1200 degrees Celsius (2200 degrees Fahrenheit).

If lava erupted at Kīlauea is 1200 degrees Celsius (2200 degrees Fahrenheit) or hotter, it is truly “hot.” This means the eruption likely tapped directly into Kīlauea’s summit magma storage chamber or regions even deeper within the volcano.

Lower eruption temperatures can result from magma stalling, cooling, and mixing as it moves out of the summit storage chamber and through the volcano’s shallow plumbing system.

For example, at the Pu‘u ‘Ō‘ō eruption site, magma has been transported underground from Kīlauea’s summit and through the East Rift Zone, a distance of about 19 km (12 mi). During this trip, it mixes with cooler magma stored in pockets along the rift.

The result is that lava samples collected at the Pu‘u ‘Ō‘ō vent now indicate magma temperature of about 1150 degrees Celsius (2100 degrees Fahrenheit), roughly 50 degrees Celsius (about 100 degrees Fahrenheit) cooler than magma in the volcano’s summit storage chamber.

A drop from 1200 to 1150 degrees Celsius (about 2200 to 2100 degrees Fahrenheit) may seem small. However, a significant amount of change to the magma occurs during that minor decrease in temperature. At 1200 degrees Celsius (2200 degrees Fahrenheit), magma in the summit storage chamber has already cooled enough to crystallize a bit.

At that point, it’s mixture of liquid magma and minor amounts of olivine and spinel, high-temperature mineral crystals. But, by the time the magma reaches Pu‘u ‘Ō‘ō, it has cooled and crystalized even further, adding pyroxene and plagioclase, slightly lower temperature mineral crystals, to the mix.

But how cool can Kīlauea magma get? As it turns out, it can’t cool too much before it solidifies, which occurs at a temperature of around 1000 degrees Celsius (about 1830 degrees Fahrenheit). Of course, at this temperature the magma would still be glowing hot. But since it’s no longer a liquid, it can’t flow as one.

So, within Kīlauea, magma can have temperatures from around 1200 degrees Celsius (2200 degrees Fahrenheit) down to about 1000 degrees Celsius (about 1830 degrees Fahrenheit). The former temperature is indicative of molten rock within the summit storage chamber, and the latter temperature suggests solidified, but still very hot rock.

On Kīlauea Volcano’s East Rift Zone, where magma is being steadily transported underground from the summit to Pu‘u ‘Ō‘ō, temperatures hover around 1150 degrees Celsius (2100 degrees Fahrenheit).

Exactly how we infer magma temperatures from the chemistry of lava samples collected at Kīlauea will have to be the topic of another Volcano Watch. But the short explanation is that we use experimentally-calibrated equations that model chemistry changes during cooling and crystallization. These are called geothermometers.

So, whether magma is hot, or not so hot, measuring its temperature through lava samples provides a window into the inner workings of a volcano.

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