Deep seafloor exposures of subaerial volcanic lithofacies on the Tonga trench slope: Ground-truthing forearc structure

Volcanic sequences exposed on the Tonga forearc and trench slope were investigated using a submersible to document seafloor outcrops and characterise volcanic lithofacies. These observations were used to establish lithofacies relationships and interpret depositional environments and volcanic processes. At ~5080 m below sea level, the sequences record both effusive and explosive volcanism, including lava flows, pumice fall and pyroclastic density current deposits. When integrated with over 70 years of regional geological data, these lithofacies are interpreted as Eocene subaerial rhyolitic volcanism, forming part of the forearc basement. Visual observation of seafloor exposures can help ground-truth, bridging the gap between sampling biases of dredging, the limited spatial coverage of drilling campaigns, and large-scale geophysical surveys. The data provide new documentation of volcanic lithofacies across previously inaccessible depths of the Tonga forearc and trench, offering rare insights into volcanic deposits that may otherwise be poorly preserved in the geological record or difficult to recover. The presence of subaerially derived volcanic sequences at these depths supports non-accretionary forearc models and highlights the value of targeted submersible investigations. • Direct submersible observations at ~5060 m below sea level on the Tonga forearc and trench slope have documented volcanic lithofacies at the seafloor. • Volcanic sequences record subaerially formed lava flows, pumice fall, and pyroclastic density current deposits, reflecting both effusive and explosive volcanism. • Integration with regional data indicates that these sequences represent Eocene subaerial rhyolitic volcanism forming part of the forearc basement. • Evidence supports non-accretionary forearc models and demonstrates long-lived, compositionally diverse volcanic activity in the region. • Targeted submersible observations provide high-priority sampling sites, offering rare access to volcanic deposits that are otherwise poorly preserved or inaccessible.