Underwater Volcanoes and Seamounts: Geology, Biodiversity, Hazards, and Conservation

Underwater, sometimes referred to as submarine or undersea, volcanoes share some characteristics with their land-based counterparts in that they are generally located where the earth’s plates meet. However, significant differences also speak to their geology and the biological habitats they create.

Volcanoes found underwater and on land are best imagined as pressure valves that periodically release heat, gases, and magma—molten or partially molten rock. As such, they are essential for creating and recycling the earth’s crust. On land, these valves are mainly located near subduction zones—the region where one of the tectonic plates is pushed below another—and create massive peaks that have the potential to wreak havoc on the populations residing in their proximity. While such threatening giants influence the earth’s history and geography, most volcanoes are below the sea.

Underwater volcanoes can be divided into three types. The first ones are those located along rift zones, where tectonic plates are formed and separate. The best example of this would be the mid-Atlantic Ridge. Like their land counterparts, the second type is the subduction volcano. They emerge in subduction zones, where plates are pushed under one another and melted. The last type is when significant plumes rise through the earth’s crust. For instance, this “hot spot” volcanism gave rise to the Hawaiian Archipelago. Until recently, it was assumed that underwater volcanic activity was relatively benevolent and that the ocean’s temperature and water pressure would dissipate into slow lava flows hardly noticed on the surface. To make matters worse, while submarine volcanoes are most abundant, they are also challenging to access and remain Earth’s least studied geological feature. However, recent devastating effects, such as the Hunga volcano in Tonga, have shattered the image of the benign undersea volcano. A closer scientific study reveals that some eruptions release megaplumes, hot, chemically rich water, that carry ash to the surface, like an atmospheric plume on land.

It is believed that such discharges gave rise to seamounts or volcanoes that are either isolated or part of a chain, among the least studied geological features. It is estimated that close to 100,000 seamounts can be found on the bottom of the ocean, sometimes only located by chance. For instance, when the nuclear submarine USS San Francisco collided with a seamount in the Pacific in 2005, it caused death and injury among the crew, forcing the US Navy to invest in sonar surveys of the ocean floor to avoid similar occurrences. This endeavor led to the mapping of roughly 24,000 seamounts. Similarly, the less deadly collision of the USS Connecticut with a seamount, also in the Pacific, in 2021 resulted in a high-resolution ocean floor scan revealing an additional 19,000 mounts. Despite such efforts, an estimated 20,000-50,000 mountains remain to be uncovered in the depths of the oceans. A new initiative called Seabed 2030 hopes to create more encompassing maps and reveal the seamounts’ accurate locations.

But there is more to seamounts than their occasional role as navigational hazards. Scientists have pointed out that seamounts are often zones of hotspots or oases of biodiversity. The volcanic activity quickly attracts tiny organisms that can harvest energy from inorganic matter expelled by the vents. Such autotrophic bacteria can produce their food through chemical reactions and survive in the extremely hot neighborhood of a submarine volcano where temperatures around 350 degrees C—over 650° F—are standard. These bacteria stand at the beginning of expanding food webs that attract larger marine animals over time as volcanic activity subsides. The slowly emerging seamount provides surfaces for attaching coral and sea sponges, further drawing sea life. Acting as barriers to deep-water currents, seamounts divert such nutrient-rich currents upwards, further supporting their teeming biodiversity. Marine biologists estimate that 20 percent of all aquatic organisms surrounding seamounts are endemic to the region.

Hunga, the Submarine Volcano that Almost Consumed a Nation. In 2015, two remote islands of the Tongan archipelago—Hunga Tonga and Hunga Ha'apai—merged through the activity of an underwater volcano. The resulting new volcano took on the name of the two islands but became known as Hunga for short. The over 100,000 inhabitants of Tonga, a kingdom of over 170 islands in the Pacific Ocean, were no strangers to volcanic activity since the country was near the Kermadec-Tonga subduction zone. Given the proximity of Hunga’s caldera to the ocean surface, the Tongan Geological Survey kept a watchful eye on its activities, especially in December of 2021 when Hunga experienced several small explosions. When these flare-ups subsided, the government proclaimed the area safe. Roughly a month later, on January 15, Hunga erupted violently. The inhabitants of Nukualofa, the main island, noticed a rapid receding of the ocean’s waters and rightly suspected the sudden arrival of a tsunami. Moving inland, waves as high as 50 feet soon covered Nukualofa and many islands in the archipelago. The explosion sent plumes of smoke and ash into the atmosphere. The tsunami and seismic activity severed the submarine cable providing phone and internet access. The thick plume covering Tonga prevented satellite imaging, and outsiders feared the worst for the nation, with some predicting a catastrophe like Krakatoa’s eruption in the late nineteenth century. When relief vessels from other countries reached Nukualofa, they found the island devastated by Hunga’s effect. Fortunately, despite the magnitude of Hunga’s fury, only four people perished, mainly because the tight-knit family structure of the archipelago served as an early warning and support system during the catastrophe. The lesson of Hunga, however, has government agencies closely monitoring submarine volcanoes worldwide. Rainer F. Buschmann

The rich concentration of marine life in the waters surrounding seamounts did not escape the attention of global fishing fleets that target the upper layer of the food webs. As elsewhere in the oceans, the excessive removal of desired fish and shellfish species creates imbalances that affect the food webs in general. Moreover, some fishing vessels’ bottom trawling methods result in highly disruptive and destructive interventions into the seamounts’ environment: catching a few selected species while crashing and scraping coral and sponges in their path.

Guarding former underwater volcanoes from such devastation has resulted in several protective zones, such as the large US-protected Marine National Monuments in the Pacific Ocean. However, the number of protected seamounts is tiny compared to those zones still suffering from potential overfishing.

FURTHER READING: Morgan, Lance. 2020. “Inside the Fight to Save Underwater Volcanoes.” Marine Conservation Institute. https://marine-conservation.org/on-the-tide/inside-the-fight-to-save-underwater -volcanoes/. Accessed October 18, 2023.

Rubin, Kenna Harmony, S. Adam Soule, William W Chadwick Jr., Daniel J. Fornari, David A. Clague, Robert W Embley, Edward T. Baker, Michael R. Perfit, David W Caress, and Robert P Dziak. 2012. “Volcanic Eruptions in the Deep Sea.” Oceanography 25 (1): 142-57.

Sigurson, Haralder, Bruce Houghton, Hazel Rymer, and John Stix. 2015. The Encyclopedia of Volcanoes, 2nd Edition. Amsterdam, NL: Elsevier Science.

Voosen, Paul. 2023. “‘It’s Just Mind-Boggling.’ More than 19,000 Undersea Volcanoes Discovered.” Science. https://www.science.org/content/article/it-s-just-mind-boggling-more-19 -000-undersea-volcanoes-discovered. Accessed October 16, 2023.