The Indonesian island Java’s tallest volcano, Semeru sent an ash cloud nearly one mile high this week – the latest in unrest from a mountain which emits ash and volcanic gases almost continuously.
The latest explosive activity is normal for Semeru. However, this event may have been exacerbated by rain. According to Indonesia’s National Disaster Management Agency, heavy seasonal rains had caused unstable lava flows near Semeru’s summit to collapse, causing a pyroclastic flow (collapse of ash and hot gas) and contributing directly to the eruption.
” It can be like opening a soda bottle,” says Sally Sennert (a U.S. Geological Survey volcanologist embedded with Smithsonian Institution’s Global Volcanism Program). Rains can wash away the products of previous eruptions, she explains. This reduces the pressure that traps hot gas under the surface and allows them to erupt. The volcano is constantly erupting small and medium gas explosions, which can cause new flows or collapses. Sennert states that it is difficult to determine what happened. “Sometimes [explosions or erosion] trigger each .”
Semeru rises 12,060 feet above Java, home to more than half of Indonesia’s population of nearly 280 million. The new explosion is the largest since December 4, 2021–exactly one year earlier. The 2021 blast killed 51 people, mostly because of lahars (rivers of ash, mud and rock) that buried nearby villages. The current eruption has not yet resulted in any deaths. MAGMA Indonesia, a country’s volcano-monitoring agency, reported that the latest pyroclastic flows traveled eight miles down Semeru’s slopes. Lahars can be cold or reach temperatures up to 212 degrees Fahrenheit, but their true destructive power comes from their speed and force. Some lahars move boulders of up to 33 feet in diameter.
Semeru may be busy, but it’s not the most active volcano in Indonesia. This distinction goes to Mount Merapi on Java. Semeru is close behind. The combination of ash and gases, pyroclastic flows, mud, and human presence make it a very dangerous mountain. The ASEAN Coordinating Center for Humanitarian Assistance on Disaster Management estimates that more than 61,000 people live within 6.2 miles of the volcano. Many people have been evacuated since then. Local authorities have warned not to travel within five miles of the summit due to the risk of “volcanic Bombs”, incandescently hot rocks ejected from an eruption. Authorities also cleared an area of the volcano’s southeastern flank, which is where the lava flows and pyroclastic flows have been flowing. This area includes the banks and riverbeds of the Besuk Kobokan River. Riverbeds can funnel deadly lethal lahars over long distances.
Lahars can be a danger near Semeru due to the large volumes and wet climate of Java. These flows occur here every wet season, with 80 percent triggered by rainstorms. And a single eruption can have long-ranging consequences: according to a 2010 paper published in Earth Surface Processes and Landforms, after a pyroclastic flow-forming eruption on Semeru, lahars are more frequent for a full five to seven years. The threat will be high for the next 1 to 2 months, according to the Head of the Center for Volcanology and Geological Hazard Mitigation at Indonesia’s Ministry of Energy and Mineral Resources . This is because heavy rains are likely to continue in the region.
Lahars are possible with little warning, according to Elizabeth Westby, a USGS geoologist at the agency’s Cascades Volcano Observatory who studies Mount Rainier in Washington State. Rainier, like Semeru is a stratovolcano (a classic cone-shaped mountain that has been erupted many times) that is susceptible to lahars. The USGS monitors the river channels in the region to warn communities downstream of a potential flow. “At Mount Rainier, large lahars have been associated with eruptions like the Osceola Mudflow,” Westby says, referring to a lahar that happened at the volcano 5,600 years ago. Lahars can also occur as large landslides, which can travel down river channels to densely populated areas. The Electron Mudflow for example, which occurred about 500 years ago, was not associated with an eruption, so it is possible that a lahar could occur during quieter periods.”
Semeru, like Rainier, is part of the Ring of Fire. This belt runs in a horseshoe pattern from New Zealand to Southeast Asia and Japan, Russia, and then on to Alaska and the western coasts of North America and South America. This belt is where tectonic plates collide, pushing under each other in a series subduction zones, giving rise both earthquakes as well as volcanism.
Semeru has been in a period of frequent activity since 1967, Sennert says, and this week’s explosion is part of an eruption that has been ongoing since April 2014. (The Global Volcanism Program considers eruptions to be continuous unless they pause more than three months. She says it’s hard to predict what Semeru will do next.
” I like to compare it with the weather,” Sennert said. “How do you determine if a storm is going to produce tornadoes? For a tornado to form, certain conditions must be met. They’re hard to forecast, and once they happen, it’s hard to know where they’re going to go or how long they’re going to last.”