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What You Need to Know About Bali's Rumbling Volcano

Rivers of lava, debris, and mud could be some of the deadly effects of an eruption from Bali's Mount Agung.

Residents of the Indonesian island of Bali are holding their breath as they wait for the imminent eruption of Mount Agung, an active volcano threatening to coat part of the island in ash, lava, and mudflows. Seismic tremors were detected as early as late September, indicating the volcano was rumbling to life, but scientists were unsure of when those rumblings would come to fruition.

Now, two months later, the volcano seems to be living up to its promise. Small eruptions began last Tuesday, but it wasn't until Saturday that eruptions began ramping up. Volcanologists say a large eruption could be imminent.

Read on for five things you need to know about this evolving danger.

1. Are people in harm's way?

By Sunday evening, the Indonesia government issued an evacuation order for 100,000 people living in northeast Bali. This comes in addition to the nearly 40,000 who have been displaced in their country since September.

Thousands of tourists have also fled from the popular island nation.

The last time Mount Agung erupted was 1963, and nearly 2,000 people died.

It's unclear exactly when Agung might erupt—volcanic eruptions are notoriously unpredictable—but recent activity suggests more fireworks are possible.

2. What is Mount Agung, anyway?

To understand what makes Mount Agung so powerful, it helps to know a little about its geologic history.

On the 13,000 small and large islands that comprise Indonesia, there are 78 active volcanoes. According to the Smithsonian's Global Volcanism Program, only Japan exceeds Indonesia in the number of eruptions, but by far, Indonesia has seen the largest number of deaths caused by volcanoes because people live in closer proximity to volcanically active regions. One study from 1600 to 1982 estimated 160,000 deaths were caused by an erupting Indonesian volcano.

3. Isn't Bali on the Ring of Fire?

Indonesia sits along an upside-down U-shaped region known as the Ring of Fire. This region, which arcs from eastern Australia, up to Alaska, and along the western coast of South America, is the most seismically active in the world. And where there's seismic activity, volcanoes follow. In fact, about 90 percent of the world's active volcanoes are associated with the Ring of Fire.

Agung, in particular, sits along a region of the Ring of Fire called the Sunda Arc, which curves along the lower half of the Sunda tectonic plate. To its south, the Indo-Australian plate is slowly being forced beneath it. This subduction zone sees considerable friction between the two plates, and over time, tension snaps and creates earthquakes. As part of the subducted plate melts in the Earth's inner heat it can create magma. Less dense than the surrounding rock, that magma rises toward the surface, eventually resulting in eruptions.

Volcanoes 101

4. How is the eruption progressing?

In an interview with news outlet RTE News at One, University of Adelaide volcanologist Mark Tingay said that magma in this part of the world is highly viscous. That can lead to more gas bubbles becoming trapped in it, creating more explosive conditions.

Tingay also noted that, since Saturday, the volcano has evolved from a phreatic eruption—explosions characterized by steam, water, and ash—to a magmatic phase in which hot magma billows up from below.

5. What can we expect next?

Volcano monitoring center MAGMA Indonesia has been locally updating residents on the volcano's activity over the past months. They note that, based on the 1963 eruption, four hazards could be potentially seen from Mount Agung: pyroclastic and lava flows, ashfalls, and mudslides.

According to the U.S. Geological Survey, pyroclastic flows are characterized by a hot flow of ash, gas, hot lava blocks, and pumice that move at high speeds down volcanic slopes. These flows typically have coarse fragments that flow along their bottom half and hot ashy gas floating above it. In addition to carrying hot debris, pyroclastic flows are fast. Some can move as fast as 50 miles per hour, and they have been known to destroy everything in their path.

By contrast, viscous lava flows move much slower but can potentially trap bubbles of gas that explode.

University of Pittsburgh volcanologist Janine Kippner noted on Twitter that ashfall as thick as 1.6 meters could coat everything within nine miles of Mount Agung's summit. Moving through this ashfall without protective masks can cause respiratory distress in people.

When coupled with rainfall, Kippner noted, ashfall can create a dangerous mudflow called a lahar. As Tingay noted in his interview, lahars are often more dangerous than the initial eruption. The USGS notes that lahars generally start small but grow in volume and speed as they move downhill. Like floodwaters made of moving concrete, lahars are extremely destructive and can't be outrun.

You can follow Indonesia's state disaster agency for updates.