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“On the landward side, a fearful black cloud was rent by forked and quivering bursts of flame,
and parted to reveal great tongues of fire, like flashes of lightning magnified in size.”
That’s what Pliny the Younger, nephew of the naturalist Pliny the Elder, wrote
after watching Mount Vesuvius erupt and cover the city of Pompeii with ash in 79 CE.
And that line ends with the first known report of a phenomenon called volcanic lightning,
where lightning streaks across the black clouds of ash and smoke above an explosive eruption,
even without a thunderstorm in sight.
But despite centuries of scientific study, and millennia of first-hand observations,
we still don’t fully understand why volcanic lightning happens in the first place.
Though it probably has something to do with all that ash moving through the air.
Volcanic lightning generally happens during explosive eruptions that release
huge ash clouds high into the atmosphere,
so the most popular explanation is that all that ash somehow causes the lightning.
Like we’ve talked about before here on SciShow, lightning in a regular thunderstorm happens
when water vapor, dust, and ice crystals rub against each other.
Electric charges jump to some molecules more easily than others,
so all that rubbing creates a huge charge imbalance,
which eventually gets evened out by a sudden burst of moving charge in the form a lightning bolt.
Scientists still disagree on some of the details, though,
since lightning can be hard to study directly.
But studying regular lightning is a piece of cake compared to studying volcanic lightning.
The explosive eruptions that cause it also tend to come with
noxious fumes, earthquakes, and clouds of super-hot ash.
But scientists, and photographers, are persistent.
And most of those scientists studying volcanic lightning think that it happens
directly because of those huge clouds of ash.
The idea is that the ash rubs together in the air just like water does in a normal thundercloud,
creating a similar sort of charge imbalance.
And once the imbalance is big enough, you get a bolt of lightning.
Laboratory experiments have reproduced the effect on a small scale,
and scientists have even been able to track ash mixing as the
charge builds up around it above an eruption.
Which seems like a pretty conclusive case that ash causes volcanic lightning,
but some researchers have said that we shouldn’t be so quick to jump on the ashy bandwagon.
Since water causes regular lightning,
they think we should take another look at the water involved in an eruption.
The idea of water in a volcano might seem kind of weird at first, but enormous volumes
of water can be dissolved in the molten rock that bubbles to the surface during an eruption.
That water boils in the heat of the eruption, which could lead to water molecules rubbing
together in volcanic clouds just like they do in normal thunderstorms.
Or all that water could coat the ashy particles flying around,
changing how the clouds charge up.
And a growing group of scientists think that this is the primary source of volcanic lightning.
Now, claiming that water causes volcanic lightning isn’t new; it goes back at least to the
late 1700s, when scientists knew far less about volcanoes than they do now.
But research in the last couple decades has breathed new life into the idea.
Many clouds leading to volcanic lightning seem to balloon out at similar altitudes
as normal, water-based clouds, which means that water is likely an important component
of them, and helps determine their behavior.
On top of that, charges tend to be distributed in those clouds
just like they are in normal thunderclouds.
So based on the evidence we have right now,
it seems like ash is the primary cause of most volcanic lightning,
with water and other conditions of the atmosphere playing something of a supporting role.
But like anything in science, that might change as we continue to learn more.
And if you were ever to find yourself in Pliny’s position, near an eruption that seems
big enough to end the world, and then lightning starts flashing around you,
you should probably not stop to wonder if it’s ash or steam to blame.
Just, you know, get out of there!
Because there have been some cases of volcanic lightning causing destruction
down here on the ground,
like the strikes during the eruption of Mount St. Helens in 1980 that caused forest fires.
There are even a few very rare cases of volcanic lightning killing people.
But even though some volcanic lightning can strike about a hundred kilometers
from the eruption, it usually stays much closer.
So if you’re close enough to a volcano that you might be hit by lightning coming straight
down from the ash cloud, you probably have bigger concerns, anyway.
Thanks for watching this episode of SciShow!
If you like learning about weird weather phenomena,
check out our episode on how animals can rain down from the sky.
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