Sierra Negra: Eruption 13 years in the making

Scientists have tracked the behaviour of one of the world’s most active volcanoes over 13 years, revealing the first-ever detailed description of the lead-up to its eruption.

The volcano – Sierra Negra in the Galápagos archipelago – erupted in 2018, triggered by an earthquake that raised the ground two metres in an instant. The event was captured in rare detail by an international research team using seismometers, GPS, satellite observations and chemical analysis of the lava – providing vital insights that will help forecast future eruptions.

Their results are published in Nature Communications.

“The power of this study is that it’s one of the first times we’ve been able to see a full eruptive cycle in this detail at almost any volcano,” says co-author geologist Peter La Femina, from Penn State University in the US.

“We’ve monitored Sierra Negra from when it last erupted in 2005 through the 2018 eruption and beyond, and we have this beautiful record that’s a rarity in itself.”

Located 1000 kilometres west of mainland Ecuador, the Galápagos Islands are dotted around the equator in the Pacific Ocean. Their unique geography – isolated, tropical, but receiving cold ocean currents – has formed an ecosystem well-known for its evolutionary oddities.

But the islands’ geological history is equally fascinating. They are formed by a “hotspot” under the Earth’s crust, where a plume of boiling magma (molten rock) is pushing up to the surface. As tectonic movements over the last five million years shifted the crust over the plume, volcanic eruptions have created a chain of islands from cooling lava.

The westernmost islands are volcanically active, with a remarkable 50 eruptions occurring in the past 200 years. The largest island, Isabela, was created by six volcanoes that have flowed into each other. Five remain active; one – Sierra Negra – was the focus of this new research.

Its 2018 eruption was a spectacular event, spewing lava for over two months. But the remarkable thing about this study was its monitoring of the volcano pre-eruption, watching as its magma chamber – emptied by the 2005 eruption – slowly refilled, stressing the surrounding crust and triggering earthquakes.

“Based on constant monitoring of activity of Galápagos volcanoes, we detected a dramatic increase of seismicity and a steady uplift of crater floor at Sierra Negra,” explains co-author Mario Ruiz, director of the Ecuador Geophysical Institute, the country’s national monitoring agency.

Then, in June 2018, one final violent earthquake triggered the eruption.

But the volcano didn’t behave exactly as expected. Volcanic eruptions often result in the creation of a caldera: a bowl-like depression formed as the magma chamber empties and the ground sinks. Instead, the eruption of Sierra Negra actually uplifted the ground (an event termed “caldera resurgence”), where an area ten kilometres in diameter was left around two metres higher in elevation.

“Resurgence is typical of explosive calderas at volcanoes like Yellowstone, not the kind of shield volcanoes we see in the Galápagos or Hawaii,” says La Femina.

This is the first time such a resurgence has been observed at this level of detail. Understanding it is important for understanding when and where eruptions happen.

“The data will be invaluable in improving volcano monitoring in Galápagos, where eruptions pose a risk to the unique and fragile ecosystem,” says Michael Stock, co-author from Trinity College Dublin.

“However, it also has far-reaching global implications, demonstrating that not all volcanoes are created equally – our current understanding of volcano monitoring data is largely based on well-studied eruptions in Iceland and Hawaii and may need to be urgently reassessed to effectively manage volcanic hazards in other locations.”