Mount Agung: Bali volcano eruption photos explained

Some of the images coming out of Mt Agung, the volcano in Bali which could be on the brink of a major eruption, have been spectacular. But the pictures also tell the story of what is going on inside, as volcanologist Janine Krippner explains.

Over the last two months Mt Agung has seen increased seismicity. This is the increased fracturing of rocks inside the volcano as magma, a volatile mixture of molten rocks, fluids and gases, moves from deep within the earth right up to the top.

In the past week we have seen thick plumes of steam and ash being belched out, as well as lava glowing at the surface of the crater, and flowing rivers of cold mud down river valleys.

Steam rising up

In most of the first pictures of the mountain two months ago, you wouldn't have been able to tell there was an eruption brewing. That information came to us from the data within the mountain that detected its increased seismicity - and from tremors that began shaking the area.

Image copyright BAY ISMOYO steam
The most you would have seen is steam rising up, which was simply the water inside the volcano heating up and coming out of its surface. The mix of volcanic ash and lava that make up the mountain is like a sponge - and in rainy Indonesia the water soaks onto it and is held there until it gets heated up.

Since then it has been relatively quiet and this is typical for a volcano. These fluctuations in activity are what makes it so hard to predict.

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Dense plumes of ash

The volcano first began belching thick ash and steam last Tuesday, its first eruption in more than 50 years.