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    From Hawaii to Indonesia to Iceland, hundreds of islands across the globe have been formed by submarine volcanoes. Submarine volcanoes are exactly what they sound like—volcanoes located beneath the ocean’s surface. 

    Because they erupt into water instead of air, submarine volcanoes behave quite differently than terrestrial volcanoes. For instance, it’s uncommon for submarine volcanoes to have explosive eruptions. The sheer weight of the water above them creates very high pressure, usually resulting in what are known as passive lava flows along the seafloor. Most submarine eruptions do not disturb the ocean surface. 

    Charles Mandeville is the program coordinator for the Volcano Hazards Program of the United States Geological Survey (USGS). He and his colleagues are responsible for monitoring all 169 active volcanoes—all terrestrial—in the United States. Prior to joining the USGS, Mandeville focused his research on submarine volcanology, becoming an expert on the famous 1883 eruption of the island of Krakatoa in Indonesia. 

    According to Mandeville, there are two main factors that contribute to submarine volcanoes eventually forming islands: the supply of magma and tectonic activity

    “The first thing you need is a supply of magma,” he says. “Typically, in the origin of most oceanic island volcanoes or submarine volcanoes, you need to melt the Earth’s mantle.”

    Most volcanic islands originate from passive lava flows on the seafloor. These passive flows harden into rock and build up the height of the underwater mountain over millions of years. Eventually, some volcanoes reach heights above the seafloor where lower pressure allows for explosive eruptions. Submarine volcanoes that do not reach sea level are called seamounts. 

    In addition to magma supply, plate tectonics play a large part in determining which submarine volcanoes will eventually form islands. Tectonic activity can sometimes “take the island volcano away from the source of magma that originates in the mantle, because the tectonic plate that the volcano is growing upon is moving,” says Mandeville. 

    Volcanic Island Ecosystems

    Formed from nothing but rock, volcanic islands have surprisingly vibrant ecosystems. These ecosystems evolve over millions of years, along with the island itself. Life on volcanic islands starts with the most basic building blocks—autotrophic bacteria

    “There are bacteria that can make their food from just the chemical elements and parts of the elements that are being emitted from volcanoes,” Mandeville says. “ . . . Once you have a microscopic ecosystem established, you have enough of a food supply to support larger life.”

    Species from nearby landforms also contribute to the developing ecosystem. Passing birds may stop to nest on the new island, bringing seeds and spores from the mainland or other islands. Plant life can float through the ocean to end up on the island’s shores. 

    Because they evolve in such an isolated environment, many organisms are considered endemic species—unlike any others in the world. The finches endemic to the Galapagos Islands, described by naturalist Charles Darwin in the 19th century, are one famous example of this. These birds are found only in the isolated Galapagos Islands. The Hawaiian Islands, even more isolated, have over 1,000 endemic plant species alone. 

    World’s Youngest Island

    One of the world’s most recently formed volcanic islands is part of the island nation of Tonga, in the South Pacific Ocean. Tonga is an archipelago of 170 volcanic islands. The new landmass formed in March 2009 as an explosive eruption sent steam, volcanic gases, and volcanic ash roughly 800 meters (2,625 feet) into the sky, covering the uninhabited island of Hunga Ha’apai—63 kilometers (39 miles) away—in black, volcanic ash. 

    Days later, a second, smaller eruption from a vent between Hunga Ha’apai and the new landmass combined with rock and debris from the initial eruption to fill the space between the two. The result was a single landmass nearly double the original size of Hunga Ha’apai. 

    Though Hunga Ha’apai previously had rich plant and animal life, the ash from the eruption devastated its ecosystem. It’s unclear whether the new island will evolve to be able support larger life forms.  

    “The wind and the waves are constantly trying to erode that island back below sea level. The only thing that’s going to outpace the effects of the wave and storm erosion is if the magma supply produces enough lava flows and explosive deposits to keep pace with that erosion,” Mandeville says.

    Increasing the height of the island above sea level—through lava flows and eruptions—is critical to allowing birds from nearby islands to “seed the new island with life,” he says. 

    In the years since the 2009 eruption, the young island has maintained itself above sea level and experienced significant growth after a series of eruptions in late 2014 and early 2015 added to its landmass. It is still attached to Hunga Ha’apai and is in the very early stages of developing an ecosystem. Other submarine volcanoes near Tonga remain active. 

    Geology of the Deep
    Submarine volcanoes rarely erupt with explosive force.
    Heat Wave
    A large number of autotrophic bacteria—bacteria that produce their own food—live near hydrothermal vents and submarine volcanoes. These bacteria are considered chemosynthetic, meaning they produce food from chemical reactions usually involving carbon dioxide, oxygen, or hydrogen. Scientists have identified species of chemosynthetic bacteria that can survive in temperatures of up to 350 degrees Celsius (662 degrees Fahrenheit). 
    Survival Mode
    "The wind and the waves are constantly trying to erode that island back below sea level. The only thing that’s going to outpace the effects of the wave and storm erosion is if the magma supply produces enough lava flows and explosive deposits to keep pace with that erosion."
    —Charles Mandeville, program coordinator for the USGS Volcano Hazards Program
  • Term Part of Speech Definition Encyclopedic Entry
    archipelago Noun

    a group of closely scattered islands in a large body of water.

    Encyclopedic Entry: archipelago
    autotroph Noun

    organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis.

    Encyclopedic Entry: autotroph
    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.

    Encyclopedic Entry: Bacteria
    Charles Darwin Noun

    (1809-1882) British naturalist.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    devastate Verb

    to destroy.

    ecosystem Noun

    community and interactions of living and nonliving things in an area.

    Encyclopedic Entry: ecosystem
    emit Verb

    to give off or send out.

    endemic Adjective

    native to a specific geographic space.

    erode Verb

    to wear away.

    evolve Verb

    to develop new characteristics based on adaptation and natural selection.

    finch Noun

    small, common bird.

    initial Adjective


    island Noun

    body of land surrounded by water.

    Encyclopedic Entry: island
    Krakatoa Noun

    island in Indonesia, site of major volcanic eruption in 1883. Also called Krakatau.

    landform Noun

    specific natural feature on the Earth's surface.

    Encyclopedic Entry: landform
    magma Noun

    molten, or partially melted, rock beneath the Earth's surface.

    Encyclopedic Entry: Magma's Role in the Rock Cycle
    mantle Noun

    middle layer of the Earth, made of mostly solid rock.

    Encyclopedic Entry: mantle
    microscopic Adjective

    very small.

    monitor Verb

    to observe and record behavior or data.

    naturalist Noun

    person who studies the natural history or natural development of organisms and the environment.

    passive lava flow Noun

    non-violent volcanic eruption, where lava oozes or seeps from a fissure or vent.

    pressure Noun

    force pressed on an object by another object or condition, such as gravity.

    seafloor Noun

    surface layer of the bottom of the ocean.

    sea level Noun

    base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

    Encyclopedic Entry: sea level
    seamount Noun

    underwater mountain.

    shelter Noun

    structure that protects people or other organisms from weather and other dangers.

    spore Noun

    reproductive unit of many organisms, such as plants and bacteria, similar to a seed.

    steam Noun

    water vapor.

    submarine Adjective


    tectonic activity Noun

    movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

    tectonic plate Noun

    massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

    terrestrial Adjective

    having to do with the Earth or dry land.

    USGS Noun

    (United States Geological Survey) primary source for science about the Earth, its natural and living resources, natural hazards, and the environment.

    Encyclopedic Entry: Surveying the United States
    vent Noun

    crack in the Earth's crust that spews hot gases and mineral-rich water.

    vibrant Adjective


    volcanic ash Noun

    fragments of lava less than 2 millimeters across.

    Encyclopedic Entry: Human and Environmental Impacts of Volcanic Ash
    volcanic gas Noun

    gas such as water vapor or carbon dioxide that is released into the atmosphere by a volcano.

    volcanic island Noun

    land formed by a volcano rising from the ocean floor.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: Plate Tectonics and Volcanic Activity
    volcanology Noun

    the study of volcanoes. Also called vulcanology.


This material is based in part upon work supported by the National Science Foundation under Grant No. DRL-1114251. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.