The activity of the volcano is on increasing trend. A first true pyroclastic flow descended this morning from the growing lava dome.
Early this morning at around 5 am local time, a larger rockfall disintegrated into a turbulent and highly mobile mixture of hot rocks, ash particles, and hot gasses, released from from the breaking lava particles,- a glowing avalanche that traveled approx. 1500 m downhill into the Kali Krasak valley. Ash plumes from rising fine material separating from the flow rose about 500 m.
Such avalanches are known by volcanologists under the terms of nuée ardente, block-and-ash flow, pyroclastic flow or pyroclastic density current, and represent the deadliest volcanic hazard of volcanoes such as Merapi, which is a notorious producer of them during most of its eruptions.
While this morning’s flow was still a small one and still stopped relatively far from inhabited areas, the length of such pyroclastic flows can easily reach many kilometers, and larger ones should be expected to occur at any time now.
During the past few days, the new lava dome has been showing intense glow and near-constant incandescent rockfalls, suggesting that the rate of its growth has accelerated. Seismic activity has also shown a strong increase lately, which likely means that more magma is on its way up to the crater and into the dome.
As its mass increases, so does the risk that partial collapses from its sides generate more voluminous rockfalls and increasingly dangerous hot avalanches.
For now, and likely the foreseeable near future, they seem to be confined to emerging from the notch in the southwestern crater walls leading into the deep Krasak ravine. The Krasak valley and its banks are the highest risk zones at the moment.
Time-lapse video of this morning’s pyroclastic flow:
