The seabed is famously unexplored and is depicted in much less detail than the surfaces of Mars, the Moon and Venus.
Draining the water from the oceans would reveal a vast and largely unknown volcanic landscape. In fact, most of the Earth’s volcanic activity takes place underwater and at depths of a few kilometers in the deep ocean.
But unlike terrestrial volcanoes, even detecting that an eruption has occurred on the seabed is extremely difficult.
Therefore, much remains for scientists to learn about submarine volcanism and its role in the marine environment.
Now our new study of offshore eruptions, published in Natural Communications, gives important insights.
Scientists did not realize the true extent of oceanic volcanism until the 1950s, when they discovered the global mid-ocean ridge system. This finding was essential to the theory of plate tectonics. The network of volcanic ridges spans more than 60,000 kilometers around the world.
Subsequent research has led to the discovery of “black smoker” vents, where mineral-rich “hydrothermal” fluids (heated water in the earth’s crust) are ejected into the deep ocean.
Driven by heat from the lower magma, these systems influence the chemistry of the entire oceans. The vents also host “extremophiles” – organisms that survive in extreme environments that were once thought to be incapable of survival.
But many questions remain. It has long been thought that deep sea eruptions themselves are quite uninteresting compared to the variety of eruptive styles observed on land.
Terrestrial volcanoes that produce similar types of magma to those on the seabed, such as Hawaii or Iceland, often produce spectacular eruptive eruptions, spreading volcanic ash (called tephra). This type of eruption was thought to be highly unlikely in the deep ocean due to the pressure of the upper water.
But data collected by remotely powered submarine vehicles showed that tephra deposits are surprisingly common on the seabed. Some marine microorganisms (foraminifera) even use this volcanic ash to build their shells.
These eruptions are likely to be driven by expanding bubbles of carbon dioxide. Vapor, which is largely responsible for explosive eruptions on earth, cannot form at high pressures.
Scientists have also sporadically detected massive regions of hydrothermal fluid in the ocean over volcanic ridges. These enigmatic regions of heated, chemical-rich water are known as megaplumes.
Their size is really huge, with volumes that can exceed 100 cubic kilometers – equivalent to more than 40 million Olympic pools.
But although they appear to be linked to seabed eruptions, their origin has remained a mystery.
In our study, we used a mathematical model to explain the spread of submarine tephra across the ocean. Thanks to a detailed mapping of a volcanic ash deposit in the northeast Pacific, we know that this tephra can spread up to a few kilometers from the site of an eruption.
This cannot be easily explained by tides or other ocean currents. Our results instead suggest that the feathers should be very energetic. Like the atmospheric feathers seen on terrestrial volcanoes, these initially rise upward through the water before spreading horizontally.
The heat transfer needed to drive this current, and bring the tephran with it, is surprisingly large at about one terabyte (double what it takes to power the entire United States at once). We calculated that this should create feathers of a similar size, which was indeed measured.
Our work gives strong evidence that mega-feathers are linked to active seabed eruptions and that they form very quickly, probably after a few hours.
So, what is the specific source of this intense input of heat and chemicals that ultimately create a megaplum? The most obvious candidate of course is the newly erupted molten lava. At first glance our results seemed to support such a hypothesis.
They show that megaplum formation occurs simultaneously with the eruption of lava and tephra. But when we calculated the amount of lava needed for this, it was unrealistically high, about ten times larger than most submarine lava flows.
Our best conjecture at present is that while the creation of megaplumes is closely linked to seafloor eruptions, they are mainly due to their origin to draining reservoirs of hydrothermal fluids that are already present in the oceanic crust. As magma strengthens its way up to feed seabed eruptions, it can push this hot (> 300 ° C) fluid with it.
We now know that various microorganisms live in rocks below the surface. As surprising as the discovery of extremophile life forms around hydrothermal vents, this discovery drove our ideas about what life is and where it might exist, even more so.
The fact that our research suggests that mega-feathers originate from the crust is consistent with the detection of such bacteria in some mega-feathers.
The rapid outflow of fluids associated with megaplum formation may indeed be the main mechanism that separates these microorganisms from their underground origin. If so, then offshore volcanic activity is an important factor influencing the geography of these extremophile communities.
Some scientists believe that the unusual physical and chemical conditions associated with seafloor systems may have provided a suitable environment for the origin of life on Earth. Megaplumes may therefore have been involved in spreading this life across the ocean.
If life is to be found elsewhere in our solar system, then hydrothermal vents, such as those believed to exist on the moon Saturn’s Enceladus, would be a good place to look.
Lacking other sources of food and light, these species of organisms – perhaps the first to exist on our planet – owe their existence to the heat and chemicals delivered by the magma that rises up to feed seafloor volcanoes.
Because deposits of volcanic ash transported by megaplumes appear to be common at offshore volcanoes, the results of our research suggest that the spread of life by megaplum emissions may be extensive.
While being able to personally observe an offshore eruption remains unlikely at present, efforts are being made to collect data on submarine volcanic events.
The most notable of these is the observatory at Axis Volcano in the Pacific. This set of seabed instruments can transmit data in real time, capturing events as they occur.
Through such efforts, along with continued mapping and sampling of the seabed, the volcanic character of the oceans is slowly unfolding.
David Ferguson, Volcanic Process Researcher, University of Leeds and Sam Pegler, Academic Fellow in Applied Mathematics, University of Leeds.
This article is republished by The Conversation under a Creative Commons license. Read the original article.