Bali authorities have issued evacuation orders for 100,000 people living within a six-mile radius of volcanic Mount Agung, the highest point on the Indonesian island.

Trouble has been brewing at the volcano for quite some time. Researchers recorded seismic activity at Agung beginning in August, with the unrest increasing in the following weeks, according to the Earth Observatory of Singapore. On September 22, authorities raised to the volcano’s status to level 4, its highest warning category. Then, last Tuesday the volcano began emitting plumes of smoke and mudflows streamed through local waterways. Over the weekend, the ash cloud reached 30,000 feet and magmatic eruptions began, reports the Associated Press. About 59,000 travelers are currently stuck on the island after the ash caused the international airport to close.

While authorities tell the AP they don’t expect a major eruption, the activity changed early this morning from emission of steam to magma. So officials are playing it safe. Last time Agung erupted in 1963, an estimated 1,100 people died. And since the 1963 catastrophe, population density has only intensified on Agung’s slopes.

So what makes Agung so dangerous? Blame its geology.

Agung is what's known as a stratovolcano. Also known as composite volcanoes, these formations occur at tectonic subduction zones, areas where two tectonic plates meet and one plate slides underneath another, geophysicist Jacqueline Salzer at the German Research Centre for Geosciences tells Fabian Schmidt at Deutsche Welle. The lava in those areas is usually thick and sticky, causing pressures to build within the steep cones, which results in highly explosive—and deadly—eruptions.

As Janine Krippner, a volcanologist at the University of Pittsburgh, writes for the BBC, Agung has gone through the predictable stages of a waking volcano. In August, small earthquakes were measured, but the mountain appeared unchanged. Then, in September, as rising magma heated the interior of the cone, plumes of steam were observed as the water in the mountain heated up.

Beginning last week, steam-driven or phreatic eruptions began. During this time, steam inside the volcano built up pressure causing small explosions to shoot ash, crystals and rock into the air. Now the magma has reached the surface—the point at which it is called lava—and its glow can be seen at the top of the mountain.

Authorities are hopeful the eruption won’t continue further but if it does, several types of disasters could unfold. The cloud of gas and steam will blow off larger pieces of the mountain off, shooting rock “bombs” into the air. Actual lava flows could also stream down the mountain for several miles. But the most dangerous element of the eruption is the pyroclastic flow, an explosion of hot gas and debris that follows valleys or low-lying areas. These flows can race down the mountain at 50 miles per hour, destroying everything in its path.

Another major concern is lahars which occur when volcanic debris and ash mixes with water, creating a slurry the consistency of wet concrete. Lahars can rush down slopes at up to 120 miles per hour and swell in volume, destroying any villages or structures in its path.

According to John Seach at VolcanoLive, during the 1963 Agung eruption, 820 people were killed by pyroclastic flows, 163 died from falling ash and rock and 165 were killed by lahars.

The 1963 eruption also had global consequences. Alle McMahon at the Australia Broadcasting Corporation reports that the sulphur dioxide blown into the atmosphere by that event temporarily cooled the Earth by 0.1-0.4 degrees Celsius by reflecting some of the sun’s ultraviolet radiation.

If Agung does have another major eruption, this miniscule amount of cooling is likely too small to be noticed. But the immediate consequences of such an eruption can be deadly, so authorities are encouraging locals to heed the evacuation notices.