What Happens to Sea Life When a Volcano Erupts Underwater?

Volcanoes in the Ocean

Did you know that most volcanic eruptions occur in the ocean? Seventy percent of the Earth’s surface is covered by ocean, and beneath that water lies a network of volcanoes hidden from view. Volcanoes are common in areas of tectonic activity, or places where the Earth’s crust shifts and moves. Scientists refer to these places as subduction zones and spreading centers, where magma from deep within the Earth comes up to the surface.

Volcanic eruptions on land are hard to miss, but volcanoes in the ocean may be a bit harder to spot. If a volcano is deep underwater, seismic readings, water discoloration, and floating volcanic rocks might be all we have to show an eruption has occurred. If a volcano is in shallow water, its eruption might break through the sea surface and reach the atmosphere. While it is very rare to see the effects of an underwater volcanic eruption, recent scientific discoveries have shed light on how volcanic activity might impact life at the bottom of the ocean.

The Hunga Tonga-Hunga Ha’apai Eruption

In April 2022, scientists aboard the research vessel Thomas G. Thompson saw the effects of a massive volcanic eruption just four months after it happened. In January 2022, a shallow-water volcano - Hunga Tonga-Hunga Ha’apai - erupted near the small island nation of Tonga. The volcanic boom was heard as far away as Canada and the ash cloud rose 36 miles into the atmosphere. Not since the Krakatoa eruption in Indonesia in 1883 had the world seen such a violent volcanic eruption.  

The Kingdom of Tonga is located in the Southwest Pacific Ocean with a population of just over 100,000 people. About 80 percent of Tonga’s population (~86,000 people), were estimated to have been within 60 miles of the eruption. For 10 hours on January 15th, ash rained down from the sky. In total across the kingdom, four people lost their lives and 14 were injured. Post-disaster damages and costs have been estimated at $90 million dollars – roughly 19 percent of Tonga’s gross domestic product – including structural damages, agricultural losses, and ash removal.

When researchers aboard the Thompson got to the Lau Basin - an area about 50 miles west of Tonga – they were hoping to study the deep-sea hydrothermal vent communities that thrive there. Hydrothermal vents, like volcanoes, are driven by geological activity, but instead of expelling lava, they expel boiling water that has been superheated by magma beneath the ocean floor. This fluid is used by animals harboring chemosynthetic (chemical-eating) bacteria – like Alviniconcha snails, Bathymodiolus mussels, or Riftia tubeworms – which may live happily nearby. 

Instead of finding thriving vent communities at the Lau Basin in 2022, however, the scientists found devastation. Layers of sediment up to 5 feet thick blanketed nearly everything in sight. The seafloor had been hit with a blizzard of glass. Once-thriving communities of snails and mussels were now gone, replaced with more mobile or resilient animals like crabs, squat lobsters, and whelk snails. Even on vent chimneys that rose high above the ash, there were few survivors.

Effects of Underwater Eruptions

The devastation at the Lau Basin was not only caused by ash, but also by underwater landslides. Like an avalanche off a mountain, the Hunga eruption loosened over 2 square miles of sediment from the side of the volcano, which slid down into the nearby Lau Basin and collected even more sediment as it passed. At some sites, plankton in the water column were swept out of their normal currents by these landslides, bringing extra carbon and nitrogen to the deep ocean. At other sites, ash settled out of the atmosphere and sank slowly back to the seafloor.

This type of eruption is considered “explosive,” with high pressures building up before releasing in a blast of ash and rock. But there are also “effusive eruptions,” characterized by steady lava flows. In April 2025, the effects of an effusive eruption were seen at a hydrothermal vent in the Eastern Pacific Ocean. Scientists diving in the deep-sea submersible Alvin were shocked to find the thriving vent communities they had seen the day before plastered over with hardened lava. Since 1991, four other effusive eruptions have been seen in this area, all of which paved over hydrothermal vent communities with volcanic lava.

Undersea eruptions can also leach oxygen out of ocean water through chemical processes. Through a chemical reaction called oxidation, volcanic chemicals react with oxygen in the surrounding seawater and deplete the water of the oxygen that animals need to breathe. Such deoxygenation is thought to have been partly responsible for the deaths at the Lau Basin, as well as the mass marine die-offs of the mid-Cretaceous period, a time period between 121 and 90 million years ago.

How Do Deep-Sea Animals Survive?

Much like humans, hope lies with the kids. Most deep-sea animals have free-floating babies, which can drift through ocean currents for days or months before finding a place to settle down and grow - sometimes landing tens or hundreds of miles away from their parents. This means that nearby areas with similar communities can bring new life to sites hit the hardest by volcanic eruptions. In certain cases, ecosystems can recover in just a few years! Studies have also found that chemosynthetic species - like Riftia tubeworms – may release their chemosynthetic bacteria back into the water column when they die, which bide their time in the environment before finding a new host.  

We are just beginning to understand the effects of volcanic eruptions in the ocean, thanks in large part to advances in ocean science and technology. With every new undersea eruption, scientists learn more about how life in the ocean recovers and repairs itself. What they are finding is that - just like communities on land - communities in the ocean can be a lot more resilient than one might think.