Mount Agung erupts: Different stages of a volcano eruption

A fisherman steers a traditional boat as Mount Agung erupts, seen from the Kubu sub-district in Karangasem Regency on Indonesia's resort island of Bali on Nov 28, 2017. PHOTO: AFP

Volcanic activity from Mount Agung, a volcano on Indonesia's Bali island, was detected from Nov 21 and the alert was raised to the maximum level on Tuesday (Nov 28).

There are about 100,000 people living on the slopes of the volcano. Of these, only about 40,000 residents have evacuated and Indonesian authorities must forcibly evacuate the remainder, local reports said.

Over the past week, pictures of the crater on the holiday island have shown it spewing clouds of smoke and glowing red.

What are the different stages of a volcanic eruption and what does each stage mean?

The Straits Times breaks it down.

Phase 1: Preparation phase

Mount Agung's crater as seen in a photo taken on Oct 19. PHOTO: AFP/INDONESIAN NATIONAL BOARD OF DISASTER MANAGEMENT

During this phase, magma rich in dissolved gas fills the magma chamber of the volcano, which can be tens of kilometres underground, with temperatures up to more than 1,000 deg C.

Tremors are often felt at the surface, and the volcano expands. There is also increased seismic activity.

At this point, steam may be seen rising from the volcano, as water inside the volcano evaporates when in contact with hot magma.

Phase 2: Eruption phase

Mount Agung in Bali erupted in black smoke on Nov 21 at 5.35pm local time. PHOTO: BNPB INDONESIA

As the pressure builds in the magma chambers, energy is gained for an eruption. Last Tuesday, Mount Agung began spewing thick ash and steam in a phreatic eruption, United States-based volcanologist Janine Krippner told the BBC in a report.

Such an eruption refers to the expulsion of pressurised steam from inside the volcano. It occurs due to the heating up of water by the magma inside the volcano.

Mount Agung spewing hot volcanic ash as seen from Karangasem, Bali, on Nov 27. PHOTO: EPA-EFE

The resulting expansion and explosion of the steam can shatter rocks, hurl large blocks and volcanic ash hundreds of metres in the air, and excavate a crater in the ground.

Finally, the explosion of pent-up gases - such as water vapour, carbon dioxide, and sulfur dioxide - pushes the magma up the conduit or opening channels in the volcano. Fused particles fall as ash, and the pressure enlarges the conduits.

Mount Agung erupting, seen at night from Kubu sub-district in Karangasem Regency, Bali, on Nov 28. PHOTO: AFP

A red or orange glow may be seen as the hot magma reaches the surface. As the magma flows out of the volcano, it cools and turns into lava.

Phase 3: Ending phase

In the ending phase, the magma chamber of the volcano empties, and the walls of the conduit are weakened and collapse, creating a crater or caldera.

Other phenomena that occur during volcanic eruptions

The volcano spews hot volcanic ash, as seen from the Besakih Temple in Karangasem, Bali, on Nov 28. PHOTO: EPA-EFE

Eruptions can create ash clouds that reach up to 32,000m in height. These clouds consist of tiny mineral particles that can travel hundreds of kilometres.

Even if the actual activity within the volcano has ceased, volcanic mud flows known as lahars can still be dangerous.

Villagers watch a river overflowing with water mixed with volcanic ash on Nov 28. PHOTO: REUTERS

Images from Bali show thick grey volcanic mud flows snaking through greenery - these lahars are created when ash and rock deposits from the volcano combine with rainwater.

Such volcanic debris flows can produce locally destructive flooding in drainage systems below the eruptive vents.

SOURCES: AFP, BBC,, United States Geological Survey

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