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Why a Volcano Has Erupted Almost Every Hour for 94 Years

The Santiaguito lava dome under Guatemala’s Santa Maria Volcano is one of the most regular erupting features in the world. Here's how it works.

TIL: This Volcano Has Erupted Almost Every Hour For 94 Years

Almost like church bells or a clock tower, a town in the western highlands of Guatemala has been able to practically tell the time by the regular eruptions of a local volcano. In the town of Quetzaltenango, volcanic eruptions have occurred so frequently that they have nearly marked every passing hour of every day for 94 years.

The source: Santiaguito, one of the world’s most active lava-dome complexes, which is situated at the base of the stratovolcano Santa Maria. The complex formed about 20 years after a massive explosion in 1902.

“[Since then], almost continuous minor explosions have accompanied the growth of the Santiaguito,” says Stephanie Grocke, a National Geographic Explorer and volcanologist. “There has not been an extensive period of time since 1922 when Santiaguito was quiet.”

Scientists are trying to unravel the internal structure of the volcano to figure out what is causing the regular activity at Santiaguito, Grocke adds.

“What we know for sure is that magma and gas are steadily rising from deep within the system, allowing the volcano to continuously show signs of life,” Grocke says.

Before an explosion, the vent at the top of the Santiaguito dome is sealed, trapping hot gases and magma below. As the gas builds up, the pressure increases and eventually tears through the dome’s sealed vent, releasing the pent-up gases and sending magma and loose rocks skyward. Santiaguito eventually runs out of fuel, causing eruptions to cease. The materials that were spit out cover the dome, resealing Santiaguito’s vent and starting the process over again.

The size of the eruptions are never the same, Grocke says. Most often, they tend to be small, producing mainly ash plumes that can reach a height of nearly 1,640 feet, or lava flows that travel about two miles from the summit. However, there are some eruptions that prove more dangerous.

“At times, larger explosions occur, producing ash plumes that rise less than 3 kilometers above the vent,” Grocke says. Such instances can result in fast-moving pyroclastic flows, or hot flows made up of gas, ash, and rock fragments, that can be deadly to local communities. Volcanic mudflows, or lahars, are also another serious hazard.

What makes Santiaguito unique isn’t its episodic eruptions, Grocke says, as there are plenty of volcanoes around the world that exhibit this sort of behavior.

“It is Santiaguito’s regularity that makes the system so unique,” Grocke says. “Even though we do observe cycles in the way that Santiaguito inflates and deflates, for example, the system is not perfect and we cannot predict exactly when an explosion will occur.”