Submarine Volcanoes Explained: The Hidden Majority
The most famous volcanoes are the ones we can see, but they are a small minority. The vast majority of Earth's volcanic activity takes place underwater, hidden beneath kilometres of ocean. Submarine volcanoes build new crust along the planet's longest mountain range, raise islands from the deep, and occasionally remind us of their power with eruptions that breach the surface. To understand volcanism on Earth, you have to look beneath the waves.
The mid-ocean ridges
The single greatest concentration of volcanic activity on Earth runs along the mid-ocean ridges, a continuous chain of underwater volcanic mountains that winds for tens of thousands of kilometres around the globe. Here, tectonic plates pull apart and magma rises to fill the gap, erupting as basaltic lava on the seafloor and constantly creating new oceanic crust. The Mid-Atlantic Ridge is the most famous example, rising above the waves only in Iceland.
Pillow lava and the deep eruption
When lava erupts under deep water, the immense pressure and rapid cooling produce a characteristic form known as pillow lava — rounded, bulbous masses of basalt that stack like sandbags on the seafloor. The water quenches the lava's surface almost instantly while molten rock continues to ooze from within, building these distinctive pillows. Vast areas of the ocean floor are paved with this rock, the most abundant volcanic rock on the planet.
Seamounts: undersea mountains
Beyond the ridges, the ocean floor is studded with seamounts — isolated underwater volcanoes that can rise thousands of metres without ever reaching the surface. There are likely tens of thousands of them, ranging from small cones to vast edifices. Many are extinct, but some are active, and they form important habitats, their slopes hosting rich marine ecosystems nourished by upwelling nutrients.
From seamount to island
When a submarine volcano grows tall enough, it breaks the surface to become an island. The Hawaiian Islands, the Canary Islands, the Azores, and countless others all began as submarine volcanoes. The process can be witnessed in action: eruptions such as those that built the island of Surtsey off Iceland in the 1960s showed the world how new land is born from the sea in a matter of weeks.
Explosive shallow-water eruptions
While deep eruptions are subdued by pressure, eruptions in shallow water can be violently explosive. When hot magma meets seawater near the surface, the water flashes to steam and shatters the lava into ash in fierce blasts. The 2022 eruption of Hunga Tonga in the South Pacific was a dramatic example, producing one of the most powerful explosions ever recorded and a tsunami that crossed oceans.
Hydrothermal vents and life
Submarine volcanism does more than build rock. Along the ridges, seawater seeps into the hot crust, is heated, and rises again as mineral-rich fluid through hydrothermal vents — the famous black smokers. These vents support extraordinary ecosystems that thrive without sunlight, fuelled by chemical energy. They are among the most important discoveries in modern ocean science and may hold clues to the origin of life itself.
Monitoring the unseen
Studying submarine volcanoes is extraordinarily difficult, since they lie hidden beneath the sea. Scientists use research ships, sonar mapping, deep-sea submersibles, and networks of seafloor sensors to detect eruptions and map the ocean floor. Even so, most submarine eruptions go entirely unobserved, detected only by the acoustic waves they send through the water or the floating pumice they sometimes produce.
Explore on the map
From the Mid-Atlantic Ridge surfacing in Iceland to the island-building volcanoes of the Pacific, submarine volcanism is the hidden engine of Earth's surface. Explore the volcanic islands and ridges on the interactive map — filter by region to see where the deep ocean's fire reaches the surface around the world.