Mount Merapi, Indonesia's most active volcano, spews rocks and gas into Wednesday's morning sky.

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Mount Merapi, Indonesia's most active volcano, spews rocks and gas into Wednesday's morning sky. / AFP via Getty Images

Mount Merapi, Indonesia's most active volcano, erupted Wednesday, emitting a river of lava onto the mountain below and gas clouds into the sky.

The eruption set off the volcano's longest lava flow since the danger level for Merapi was raised in November, Hanik Humaida, the head of Yogyakarta's Volcanology and Geological Hazard Mitigation Center, told The Associated Press. Sounds of the eruption could reportedly be heard almost 18 miles away.

The 9,737-foot Mount Merapi volcano sits on the densely populated island of Java. No residents were evacuated Wednesday morning, but officials are closely monitoring the volcano's activity.

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The 9,737-foot Mount Merapi volcano sits on the densely populated island of Java. No residents were evacuated Wednesday morning, but officials are closely monitoring the volcano's activity. / AFP via Getty Images

The 9,737-foot volcano sits on the densely populated island of Java and near the ancient city of Yogyakarta. It has repeatedly erupted recently, keeping local officials and residents living nearby on a state of alert.

In November, local authorities evacuated nearly 2,000 people who lived in the Java mountain districts of Magelang and Sleman after Merapi erupted. Earlier this month, authorities evacuated more than 500 people in Magelang after the volcano spewed hot clouds of ash.

No residents were evacuated as of 5:30 a.m. EST, but Indonesian authorities are closely monitoring the volcano's activity. People were told to stay out of the 3-mile danger zone around the crater.

Merapi's last major eruption in 2010 killed 347 people.

Mount Merapi's 2010 eruption killed 347 people.

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Mount Merapi's 2010 eruption killed 347 people. / Xinhua News Agency/Getty Images

Indonesia, an archipelago of 270 million people, sits along the Pacific "Ring of Fire," a horseshoe-shaped series of seismic fault lines around the ocean. That location leaves the country prone to earthquakes, volcanic activity and tsunamis.

In August, Indonesia's Mount Sinabung, located on Sumatra Island, spewed a plume of ash several miles into the sky posing health and aviation risks for days.

A series of eruptions in 2018 at the Anak Krakatau volcano, which also triggered a deadly tsunami, caused serious damage and forced authorities to reroute flights.

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