The underwater volcano that erupted on Friday, causing tsunami waves to hit Tonga, could erupt again, according to a geological expert, but without active monitoring of the volcano the likelihood of another eruption cannot be determined.

The Hunga-Tonga-Hunga-Ha’apai submarine volcano, located about 30 kilometers southeast of the island of Fonuafo’ou in Tonga, erupted sending ash, steam and gas 20 kilometers in the air.

Tsunami waves overwhelmed low-lying areas of the Pacific island nation on Saturday, forcing Tongans to flee to higher ground as water crashed into homes.

Marco Brenna, a senior lecturer at the University of Otago School of Geology, said the impact of the latest eruption was “relatively mild”, but another eruption with a much larger impact could not be excluded.

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The Hunga Volcano is part of the chain of volcanoes forming the Tonga Arc, formed by the subduction of the Pacific tectonic plate under the Indo-Australian tectonic plate.

Its hallmark is a summit caldera – a large depression formed when a volcano erupts and collapses – which formed in large eruptions around 900 years ago. The volcano has erupted several times over the past century, including in 1912, 1937, 1988, 2009 and 2014.

Little is known about those from the early 1900s, but eruption volumes during the 1988, 2009 and 2014 eruptions appear to have increased, Brenna said.

“There is a reservoir of magma about 5-6 km deep that has fed previous eruptions, and it is likely that the current event is fed by the same reservoir,” he said.

But because the volcano was not actively monitored, it was not possible to say for sure if another eruption was imminent.

“[Because of a lack of monitoring] it is unclear how much magma was introduced into the shallow magma reservoir below the summit. Usually on monitored volcanoes there are inclinometers and other instruments that can gauge how much the volcano is swelling to estimate the amount of magma that has been injected into the shallow magma chamber. But without that information, it’s really hard to predict what might happen next.

Brenna said the reasons the volcano was not monitored could be due to a lack of funding and resources in Tonga, or because past volcanic eruptions were relatively minor, it was not considered a priority for monitor it.

“There are no satellite images yet of what remains after Friday’s event, but given its scale, it’s possible that maybe even the whole island is gone. And that may be -be that, or it could lead to something even bigger.

Brenna said satellite images showed the eruption plume extending over a wide area, including Tonga, which meant ash fallout was likely.

Despite its name, volcanic ash is not like ash from a fire but rather a dense mixture of soil, rock and mineral particles. “If it covers a landscape and a few inches of ash ends up on the roofs of buildings, it can weigh heavily on structures and cause them to collapse.”

The chemicals in the ash are toxic and can destroy vegetation and contaminate water sources, while airborne ash can cause respiratory problems.

Brenna said it was vital for the Tongan government to inform people of the risks, including the possibility of further tsunamis and the risk of inhaling ash or drinking contaminated water.