Tonga – Despite the crushing pressure, extreme variations in temperature and complete lack of sunlight, live still thrives at hydrothermal vents in the deep sea. This is a diffuse flow site dominated by snails and sea anemones.  Credit:​ ​SOI​ ​/​ ​Ken​ ​Rubin

“Submarine volcanism is one of the fundamental processes that has affected both the composition of our oceans and the shape of the seabed, as well as the development of life on our planet,” said Ken Rubin, Principal Investigator and Professor at the School of Ocean and Earth Science and Technology (SOEST), University of Hawai‘i. “It is hard to pick the most exciting things that we have learned on this expedition because we have learned so much. However, perhaps the top four are the number of recent eruptions in the area, the amount of deep sea explosive volcanic deposits, the wide range of volcanic styles on small, closely-spaced volcanoes, and the number and diversity of hydrothermal​ ​systems​ ​and​ ​habitats​ ​in​ ​the​ ​area.”

Most of Earth’s volcanoes are in the oceans, yet scientists know very little about them compared to volcanoes on land. The core goal of this expedition was to understand more about submarine volcanic activity, as well as their impacts on ocean chemistry and ecosystems. By scrutinizing the geological and magmatic evolution of each volcano as a single individual – then studying all of them as a group – researchers are developing a stronger understanding of volcano history, volcanic eruption styles, and how submarine volcanoes grown over time. Additionally, the relationship of volcanic activity to magma generation in the mantle below – across a wide array of volcanic settings – was an important focus. By approaching with a multidisciplinary team, researchers investigated many aspects of submarine volcanism, studying how geological, biological, and chemical processes interact at these seamounts. Vent biology, fluid chemistry, and chimney sampling were also components of the research, ingredients to​ ​uncode​ ​the​ ​relationships​ ​between​ ​volcanoes​ ​and​ ​hydrothermal​ ​vent​ ​ecosystems.

Tonga – One highlight of the expedition was the discovery of three new hydrothermal venting sites. Here a “black smoker” chimney releases hydrothermal fluid into the deep sea.  Credit:​ ​SOI​ ​/​ ​Ken​ ​Rubin

Although these volcanoes are very closely-spaced (their bases are just 0.6 km apart on average), the stark volcanic differences between them were key findings. Four of the volcanoes have active hydrothermal systems (and another has an inactive site), but the style of the hydrothermal activity, the shapes and spacing of the chimneys, their heights and the biological communities living among them are all​ ​very​ ​different​ ​from​ ​site​ ​to​ ​site.

Researchers​ ​discovered​ ​that​ ​recent​ ​volcanic​ ​eruptions​ ​are​ ​not​ ​restricted​ ​to​ ​the​ ​summits​ ​of​ ​the​ ​Mata volcanoes​ ​-​ ​several​ ​have​ ​been​ ​active​ ​at​ ​their​ ​flanks​ ​and​ ​bases.​ ​One​ ​of​ ​these​ ​eruptions​ ​started​ ​as​ ​a​ ​very unusual​ ​intrusion​ ​into​ ​sediment,​ ​pushing​ ​up​ ​and​ ​blistering​ ​the​ ​sea​ ​floor,​ ​followed​ ​by​ ​lava​ ​oozing​ ​out​ ​of the​ ​base​ ​of​ ​this​ ​uplift​ ​mound.​ ​These​ ​various​ ​eruption​ ​styles​ ​and​ ​locations​ ​provide​ ​new​ ​insights​ ​into​ ​the magma​ ​plumbing​ ​systems​ ​of​ ​the​ ​volcanoes​ ​and​ ​how​ ​they​ ​grow​ ​over​ ​time.​ ​The​ ​team​ ​also​ ​recorded​ ​an unexpectedly​ ​large​ ​range​ ​of​ ​lava​ ​types​ ​(both​ ​slow​ ​and​ ​fast​ ​moving),​ ​and​ ​interior​ ​textures​ ​of​ ​lava​ ​rocks recovered​ ​(including​ ​the​ ​numbers,​ ​shapes​ ​and​ ​sizes​ ​of​ ​crystals​ ​and​ ​frozen​ ​gas​ ​bubbles​ ​known​ ​as vesicles).​ ​These​ ​lava​ ​rock​ ​samples​ ​provide​ ​important​ ​clues​ ​about​ ​the​ ​wide​ ​variety​ ​of​ ​eruption​ ​styles​ ​at these​ ​volcanoes.

The​ ​team​ ​was​ ​able​ ​to​ ​closely​ ​observe​ ​dacite​ ​lava​ ​flows​ ​in​ ​two​ ​sites​ ​that​ ​are​ ​enormous​ ​in​ ​area​ ​and volume,​ ​with​ ​flow​ ​lengths​ ​up​ ​to​ ​10​ ​km.​ ​These​ ​are​ ​the​ ​largest​ ​known​ ​dacite​ ​lava​ ​flows​ ​on​ ​the​ ​planet. Some​ ​of​ ​the​ ​flows​ ​do​ ​not​ ​seem​ ​to​ ​be​ ​associated​ ​with​ ​any​ ​obvious​ ​volcano,​ ​which​ ​is​ ​puzzling​ ​for​ ​flows of​ ​this​ ​size.​ ​Researchers​ ​determined​ ​the​ ​eruptions​ ​that​ ​created​ ​these​ ​huge​ ​dacite​ ​lava​ ​flows​ ​likely included​ ​an​ ​explosive​ ​phase.​ ​This​ ​is​ ​a​ ​surprising​ ​finding,​ ​striking​ ​to​ ​see​ ​in​ ​the​ ​deep​ ​ocean:​ ​the​ ​force needed​ ​to​ ​produce​ ​explosive​ ​activity​ ​under​ ​the​ ​pressure​ ​of​ ​more​ ​than​ ​two​ ​kilometers​ ​of​ ​water​ ​is massive.

Millions of viewers watched daily as the expedition broadcast livestreams of the dives over YouTube and Facebook. The live video was captured by Remotely Operated Vehicle SuBastian at depths of up to 2,900 meters (1.8 miles), then fed to the ship by way of a cable tether before being shared worldwide via a​ ​satellite​ ​internet​ ​connection.

Accomplishments​ ​of​ ​the​ ​expedition​ ​include:

• 21​ ​ROV​ ​dives​ ​at​ ​11​ ​Submarine​ ​volcanoes
• 250+​ ​Lava​ ​samples
• 40​ ​volcanic​ ​sediment​ ​samples
• 30+​ ​Sulfide​ ​samples
• 60+​ ​Vent​ ​fluid​ ​samples
• 470+​ ​Biological​ ​samples
• 12,000+​ ​km2​ ​​ ​of​ ​seafloor​ ​mapped

For​ ​more​ ​information​ ​about​ ​the​ ​expedition,​ ​please​ ​visit: https://schmidtocean.org/cruise/underwater-fire-studying-submarine-volcanoes-tonga/

This​ ​expedition​ ​was​ ​a​ ​partnership​ ​between​ ​Schmidt​ ​Ocean​ ​Institute,​ ​the​ ​University​ ​of​ ​Hawaii’s Department​ ​of​ ​Geology​ ​and​ ​Geophysics​ ​in​ ​the​ ​School​ ​of​ ​Ocean​ ​and​ ​Earth​ ​Science​ ​and​ ​Technology, NOAA’s​ ​Earth-Ocean​ ​Interactions​ ​Program​ ​at​ ​the​ ​Pacific​ ​Marine​ ​Environmental​ ​laboratory,​ ​plus​ ​other international​ ​partners.