The sky above an erupting volcano in Indonesia has turned a stunning electric purple during a lightning storm.

Spectacular images captured the spectacle as Mount Sinabung in Karo, North Sumatra, continued to spew a column of ash and smoke up to 16,400ft (5km) into the sky as lightning bolts crackled above.

Sinabung leapt back into life last week, with 13 separate blasts on Tuesday alone, and it continued to shoot volcanic debris for several days afterwards.

The purple skies – which can be caused by the scattering of white light due to moisture, haze and dust in the air – were captured by photographers on Friday using long exposures.

Below are some images from lightning strikes on volcanoes:

The stunning phenomenon has been little studied, so scientists can’t say with certainty what causes it. For a start, it’s difficult to study. According to German photographer Martin Rietze, who has photographed the phenomenon, lightning activity is at its highest at the start of an eruption, meaning if you’re not already at the right place at the right time, it will probably be too late by the time you get there.

One study suggests that ash particles are responsible for lightning that strikes near the ground. Researchers found that at Sakurajima volcano in Japan, particles were rubbing together in the dense ash clouds, creating static electricity, according to a study in the journal Geophysical Research Letters. For that study, the scientists recorded video of volcanic lightning at Sakurajima — one of the world’s most active volcanoes.