Some volcanoes form under the sea due to the tectonic activity of plates on the ocean floor. When the Earth's crust separates, magma rises to the surface and forms underwater volcanoes.
Beneath the oceans, the Earth's crust is not fixed but divided into immense pieces called tectonic plates. These plates move slowly, sometimes pulling apart and sometimes coming together. When two oceanic plates pull apart, they allow magma from the Earth's mantle to rise. As this hot magma rises, it gradually cools upon contact with the cold water, progressively forming underwater volcanic chains: this is what is known as mid-ocean ridges. Conversely, when an oceanic plate sinks beneath another (a process called subduction), it melts the rock deep underground, creating magma that seeks to rise and forms underwater volcanoes. These tectonic movements are thus directly responsible for the birth of oceanic volcanoes that we cannot see, yet continuously shape the sea floor.
Under the ocean, when two tectonic plates diverge or overlap, it creates a breach in the Earth's crust. Through this crack, magma, an ultra-hot molten rock from the mantle located beneath the crust, slowly rises. This ascent of magma is possible due to its lower density compared to the surrounding rocks, just like oil rises to the surface of water. Upon coming into contact with cold water, this magma cools rapidly, solidifies, but often continues to push from below. The result is successive layers that gradually form a true underwater volcanic mountain, sometimes impressive. Sometimes, underwater pressure also triggers explosions, violently releasing gas and lava into the water.
When the blazing magma bursts forth and meets the cold water of the deep sea, it really heats up. The thermal shock causes spectacular reactions: the magma rapidly solidifies, forming an outer layer of basaltic rock, sometimes called "pillow lava." This type of cooled crust acts as an insulating shell around the still-fluid magma inside, allowing it to continue flowing gently. Seawater also seeps into the cracks of these new rocks and, upon contact with the heated rock, undergoes specific chemical reactions, creating small solid particles rich in minerals, often referred to as hydrothermal deposits. These phenomena fuel sometimes astonishing underwater ecosystems, full of strange life adapted to these extreme conditions.
Underwater, there are several types of volcanoes. Some resemble rounded underwater mountains called seamounts, formed by slow and steady eruptions over a long period. When they manage to break the surface, they create volcanic islands like those in Hawaii. Otherwise, there are also fissure-type volcanoes, which look like long cracks on the ocean floor where lava flows out in sheets, to simplify. You often find these along mid-ocean ridges where tectonic plates are separating. Then there are submarine explosive volcanoes, which can cause pretty strong explosions underwater due to the violent contact of hot magma with cold water, which is a bit more spectacular. Others form types of steep pinnacles or volcanic cones with well-inclined slopes, depending on how the lava flows and its chemical composition.
The eruptions of underwater volcanoes significantly alter their environment. When the burning magma comes into contact with cold water, it creates chemical reactions that release gases such as carbon dioxide or sulfur. This affects the local acidity of the water, disrupting nearby marine life. But there's good news too: these plumes of smoke and minerals often become oases for certain specialized organisms, like the famous giant tube worms. Underwater volcanoes also influence local temperatures, creating temporary warm currents that slightly disrupt the surrounding biodiversity. At great depths, far from sunlight, life around these volcanoes can thrive by directly harnessing the released chemical energy, without having to depend on the sun.
When the lava from underwater volcanoes cools rapidly upon contact with cold water, it can produce unique shapes known as pillow lavas.
Sometimes, a submarine volcano can grow large enough to emerge from the water and form a volcanic island, like Surtsey Island in Iceland, which suddenly appeared in 1963.
Some underwater volcanoes can reach impressive heights of several kilometers, but remain completely unknown as they never rise above the water's surface.
The Hawaiian archipelago, famous for its spectacular volcanoes, actually originates from a series of underwater volcanoes formed above a hotspot located on the ocean floor.
Submarine volcanoes host specific ecosystems around hydrothermal vents, containing various adapted organisms such as extremophile bacteria, albino crabs, giant tube-less worms, and other unique species that are found nowhere else on Earth.
The majority of active underwater volcanoes are located at great depths, significantly reducing immediate risks. However, those that approach the surface or emerge can pose risks related to eruptions and the release of toxic gases, disrupting maritime navigation and creating hazards for human life.
Submarine volcanoes are generally detected through bathymetric surveys (mapping of the seabed), acoustic sounding, or by monitoring thermal and chemical anomalies in the water indicating volcanic activity.
The prediction of underwater eruptions is still very complex and limited. Primarily, seismic surveys, chemical analyses of water, and the detection of topographical changes are used to identify any potential increased activity that may signal an eruption.
No, many underwater volcanoes remain completely under the sea. Only some manage to accumulate enough volcanic material to reach the surface and become islands, like Iceland or Hawaii.
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