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  • A volcano is an opening in Earth’s crust that allows molten rock from beneath the crust to reach the surface. This molten rock is called magma when it is beneath the surface and lava when it erupts or flows from a volcano. Along with lava, volcanoes also release gases, ash, and rock. It’s a super hot mix that can be both incredibly destructive and creative.

    Volcanoes form at the edges of Earth’s tectonic plates. These huge slabs of Earth’s crust travel atop the partly molten mantle, the layer beneath the crust. If you could see the plates, you might think they look like pieces of a puzzle because the edges fit together. But these puzzle pieces move, usually at the unnoticeable pace of only a few inches every year. Sometimes, though, plates collide with one another or pull apart, and it’s at these active zones where volcanoes form. Volcanoes may also erupt in areas called hot spots where the crust is thin.

    Volcanoes erupt in different ways, producing different landforms. Steep, cone-shaped volcanoes form when plates collide. All the pressure and heat of the collision make for a violent eruption. The cone forms when lava and other material eject and build up around the opening. This type of volcano is known as a stratovolcano, and Mt. Rainier is a good example. Sometimes an eruption is so violent that the top of the volcano collapses, leaving a huge pit or caldera. You can see calderas in Yellowstone National Park and Crater Lake. When plates pull apart, lava escapes through the rift. This more gentle flow creates new crust on the seafloor and wide, rounded volcanoes on the surface called shield volcanoes. Hawaii’s Kilauea is a shield volcano. It is also an example of a volcano that formed over a hotspot.

    Volcanic eruptions can be deadly. Eruptions of Krakatau and Tambora in Indonesia caused the deaths of more than 100,000 people. Dangerous as they are, volcanoes also build and shape the land, creating mountains and new seafloor and depositing minerals and nutrients that enrich soil.

    Satellite images in the gallery are courtesy Geo Eye.

    • About 90% of the world's volcanoes can be found in the Ring of Fire, where tectonic plates meet around the rim of the Pacific Ocean.
    • Worldwide there are 1,500 active volcanoes that could erupt at any time. Other volcanoes are dormant; they are quiet for now but could become active. There are even extinct volcanoes, which are never expected to erupt again.
    • Volcanoes arent unique to Earth. The largest volcano in our solar system is Olympus Mons on Mars. It is three times taller than Mt. Everest, Earth's highest elevation. Mercury, Venus, and Jupiter's moon Io also have or have had volcanic activity, with Io being the most active of all.
    • The eruption of Mt. Vesuvius wiped out the Roman cities of Pompeii and Herculaneum in the year 79, killing thousands of people. Many victims were covered in ash, which hardened to solid rock over time. As the bodies decayed, hollow impressions remained in the rock. Archaeologists poured plaster into the hollows to form detailed casts of the victims, including animals.
  • Term Part of Speech Definition Encyclopedic Entry
    active volcano Noun

    volcano that has had a recorded eruption since the last glacial period, about 10,000 years ago.

    archipelago Noun

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

    Encyclopedic Entry: archipelago
    caldera Noun

    large depression resulting from the collapse of the center of a volcano.

    Encyclopedic Entry: Types of Calderas
    composite volcano Noun

    steep volcano made of hardened lava, rock, and ash. Also known as a stratovolcano.

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    extinct volcano Noun

    volcano that will no longer erupt.

    geyser Noun

    natural hot spring that sometimes erupts with water or steam.

    Encyclopedic Entry: geyser
    lagoon Noun

    shallow body of water that may have an opening to a larger body of water, but is also protected from it by a sandbar or coral reef.

    Encyclopedic Entry: lagoon
    lava Noun

    molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

    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
    shield volcano Noun

    large, gently sloping volcano made from fluid lava.

    tectonic plate Noun

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

    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