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7.2: Volcanoes at Plate Boundaries

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    5426
  • What good are plate boundaries?

    Plate boundaries create natural resources, such as gold. But plate boundaries also create beautiful scenery like mountains. Volcanoes at convergent plate boundaries can be beautiful to look at. They can also be beautiful to visit or even climb!

    Convergent Plate Boundaries

    Converging plates can be oceanic, continental, or one of each. If at least one is oceanic, it will subduct. A subducting plate creates volcanoes. Many of the world's volcanoes are the result of subduction at a convergent plate boundary. A map of the world's active volcanoes is pictured below (Figure below).

    World map of active volcanoes

    World map of active volcanoes.

    Melting

    Mantle melting can come about three ways: (1) when the temperature rises, (2) if the pressure lowers (which lowers the melting point), and (3) if water is added, which lowers the melting point. Two of these (1 and 3) might explain why there are volcanoes at convergent plate boundaries: the temperature of the subducting plate increases as it sinks into the hot mantle (1). Sediments lying on top of the subducting plate contain water. As the sediments subduct, the water rises into the overlying mantle material. This lowers the melting temperature of the mantle (3). When the mantle above the subducting plate melts, volcanoes form above it. This leads to volcanoes in an island arc or continental arc.

    Pacific Rim

    Convergent plate boundaries are found around much of the Pacific Ocean basin. Trenches mark subduction zones. Volcanoes are found in an arc above those subduction zones.

    The Cascades (Figure below) are a chain of volcanoes at a convergent boundary. In this region, an oceanic plate is subducting beneath a continental plate.

    Map of volcanoes on the Cascade Range

    The Cascade Range is formed by volcanoes created from subduction of oceanic crust beneath the North American continent.

    The Cascades have been active for 27 million years. The mountains we see today are no more than 2 million years old. The volcanoes receive a lot of winter snow, and many are covered by glaciers (Figure below).

    Picture of Mt. Rainier in Washington

    Mt. Rainier, Washington.

    Divergent Plate Boundaries

    At divergent plate boundaries, hot mantle rock rises into the space where the plates are moving apart. As the hot mantle rock rises, it goes from higher to lower pressure. This is because there is no longer the weight of all the rock above it. Lower pressure lowers the melting temperature of the rock (2 in the previous section). This is why there is volcanism at divergent plate boundaries. Lava erupts through long cracks in the ground, or fissures.

    Mid-Ocean Ridges

    Volcanoes erupt at mid-ocean ridges, such as the Mid-Atlantic ridge. Seafloor spreading creates new seafloor in the rift valleys. This type of volcanism makes all of the ocean floor! Where a hotspot is located along the ridge, such as at Iceland, volcanoes grow high enough to create islands (Figure below).

    A volcanic eruption on Iceland

    A volcanic eruption on Iceland, where the Mid-Atlantic Ridge breaches the ocean surface.

    Continental Rifting

    Eruptions are found at divergent plate boundaries as continents break apart, known as continental rifting. The Mount Gahinga volcanoes (Figure below) are in the East African Rift between the African and Arabian plates. Baja California is breaking apart from mainland Mexico, also by continental rifting.

    Mount Gahinga, a mountain in Uganda, located in the East African Rift valley

    Mount Gahinga, a mountain in Uganda, located in the East African Rift valley.

    Summary

    • Subduction at convergent plate boundaries leads to melting and volcanoes.
    • Convergent plate boundaries line the Pacific Ocean basin so that volcanic arcs line the region.
    • Melting at divergent plate boundaries is due to release of pressure.
    • At mid-ocean ridges seafloor is pulled apart and new seafloor is created. Rifting in a continent can create a new ocean eventually.

    Review

    1. What are the two sources of melting at convergent plate boundaries?
    2. Why are there so many volcanoes around the Pacific Ocean basin?
    3. What causes melting at divergent plate boundaries?

    Explore More

    Use the resource below to answer the questions that follow.

    1. Where does the melted mantle rock rise?
    2. What does spreading cause?
    3. What happens where there is plate convergence?
    4. How is carbon dioxide released from the rock?
    5. How is carbon dioxide returned to the atmosphere?