Researchers have documented a January eruption of the underwater volcano Hunga-Tonga-Hunga-Haʻapai that sent waves of rock, ash, and water-saturated debris across the seafloor at remarkable speeds. In places, density currents surged through the ocean depths at speeds approaching 122 kilometers per hour, carving paths along the seabed and reshaping the underwater landscape.
These density currents, sometimes called debris flows beneath the sea, severed long stretches of submarine cables, interrupting internet connectivity in the kingdom and causing notable disruptions to local biodiversity. The moving masses overwhelmed marine life and altered habitats as they coursed across the ocean floor, leaving a lasting mark on the region’s underwater ecosystem.
It should be noted that the eruption column itself has largely dissipated, leaving behind a record-setting event with several notable milestones:
– The explosion column reached a height of about 58 kilometers.
– The eruption generated the largest atmospheric disturbance recorded in history.
– It produced an extraordinarily intense electrical storm, with lightning activity measured at approximately 2,600 strikes per minute.
Scientists estimate that roughly six cubic kilometers of rock and ash were expelled into the atmosphere and then settled back toward the ocean. Although much of this material reentered the sea, researchers were able to map and measure the submarine trajectory and speed of the debris once it settled.
To gather this data, seafloor samples were collected to determine the debris pathways and to compare the timing of the explosion with the moment the cables failed. Two cables ran near the volcano: a national cable about 50 kilometers away that connected Tonga to the internet, and an international link about 70 kilometers away that served broader communications for the region.
The national cable was the first to fail, roughly 15 minutes after the main explosion began. The international cable experienced failure about an hour later, as reported by major news outlets. Researchers from the United Kingdom’s National Oceanography Centre concluded that the density current responsible for cutting the local cable moved at speeds between 73 and 122 kilometers per hour, with the possibility of 47 to 51 kilometers per hour at the distance of the international cable.
Lead author Mike Clare described the mechanism to the BBC World Service: fragments of rocks and ash were hurled from the towering eruption column and fell into the ocean. When this jet-like material hit the 40-degree submarine slopes, it fractured larger pieces and formed an even denser mass. The current then followed the terrain, striking the national cable and, after veering around bends, reaching the international cable.
When comparing these speeds to other density flows, mountain snow avalanches can reach around 250 kilometers per hour, and explosive pyroclastic flows from terrestrial volcanoes can exceed 700 kilometers per hour. Those terrestrial events primarily involve suspended particles interacting with air. In contrast, the Tonga submarine currents moved through water, revealing their power through an underwater surge rather than air-embedded debris.
The incident has implications for the networks that carry most international data. More than 99 percent of cross-continent data traffic traverses submarine cables, including daily financial transactions worth billions of dollars. Cables cross paths near many underwater volcanos, especially in the Pacific and Caribbean regions.
While Tonga’s international link required about five weeks to repair, replacing the national cable demanded roughly eighteen months. Experts emphasize improved seabed mapping as a critical step in predicting and mitigating future events, noting the ocean floor remains poorly charted in many regions. Better data about the seabed would help operators understand risk and readiness in a changing environment.
This event underscores the need for robust submarine cable infrastructure and proactive monitoring of the ocean floor. Ongoing research continues to refine models of underwater density currents and their potential impact on critical communications infrastructure. The cited findings contribute to a growing body of knowledge about how underwater volcanic activity interacts with seabed geology and submarine networks. Researchers acknowledge that there is still much to learn about the unseen layers beneath the waves and the ways they shape global connectivity.
Reference work: a recent study in a leading science journal details the measurements and interpretations described here. Source materials and data have been synthesized to provide a coherent picture of how the Hunga-Tonga-Hunga-Haʻapai eruption affected underwater currents and cable networks. Endnotes and data sources are available through institutional publications and peer-reviewed reports.
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