A volcanic eruption is terrifying on its own, but we just learned that, even more terrifyingly, the avalanche of hot rocks, ash, and gas that volcanoes spew can spawn giant tornado-like twisters.

This was caught on video after a pyroclastic flow from Mount Sinabung, a volcano in Indonesia that’s recently started spewing again after more than 400 years of dormancy.

Sinabung erupted again in September 2013, and has been sporadically erupting since.

The most recent eruption in January was by far the most violent and devastating to the Indonesian villages near the base of the volcano.

Volcano Spawns Terrifying Tornado-Like Twisters [GIFS]1▲ Mount Sinabung spews ash as it is pictured from Simpang Empat village in Karo district, Indonesia’s north Sumatra province, November 5, 2013.

On Feb. 1, the lava dome from the last eruption swelled so large that it eventually collapsed from the pull of gravity. The collapse generated what’s called a pyroclastic flow — a devastating avalanche of hot rocks and gas that destroys everything in its path.

The hot ash rises off the pyroclastic flow like this:

Volcano Spawns Terrifying Tornado-Like Twisters [GIFS]2

The ash warms the air above it. That hot air then rises and creates the tornado-like spirals. You can see that they really do look like giant tornadoes firing out of a volcano in the GIF below.

Volcano Spawns Terrifying Tornado-Like Twisters [GIFS]3

But technically speaking they aren’t tornadoes: Normal tornadoes form at the base of the cloud and spiral down to the ground, while the twisters arising from pyroclastic flows are built from the ground up. Fire tornadoes and dust devils form in similar ways.

Pyroclastic flows can reach speeds up to 450 mph and the gas can get as hot as 1,800 degrees. They incinerate anything in their path, and can form these twisters if the conditions are right.

You can watch these crazy pyroclastic flow twisters in action in the video below, uploaded to YouTube by Photovolcanica. The twisters start forming around the one minute mark. We first saw the video on Phil Plait’s Bad Astronomy blog:

Sadly for the country another Indonesian volcano just started spewing ash this week. Mount Kelud has forced evacuations of more than 200,000 people from Java, and closed three airports.
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