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Chapter 8 Section 2 Types of Volcanoes

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Chapter 8 Section 2

Types of Volcanoes

  • The process of magma formation is different at each type of plate boundary.

  • Therefore, the composition of magma differs in each tectonic setting.

  • Tectonic settings determine the types of volcanoes that form and the types of eruptions that take place.

Volcanoes at Divergent Boundaries

  • At a divergent boundary, the lithosphere becomes thinner as two plates pull away from each other.

  • Hot mantle rock rises to fill these cracks.

  • As the rock rises, a decrease in pressure causes hot mantle rock to melt and form magma.

  • The magma that reaches Earth’s surface is called lava.

  • Lava that flows at divergent boundaries forms from melted mantle rock.

  • This lava is rich in the elements iron and magnesium. It is relatively poor in silica.

  • The term mafic describes magma, lava, and rocks that are rich in iron and magnesium.

  • Because it is low in silica, mafic lava is runny and not sticky.

  • This type of lava generally produces nonexplosive eruptions.

  • Mid-ocean ridges are underwater mountain chains that form where two tectonic plates are moving apart.

  • Some of the magma erupts as basaltic lava on the ocean floor.

  • The magma and lava cool to become part of the oceanic lithosphere.

  • As the plates continue to move, older oceanic lithosphere moves away from the mid-ocean ridge.

  • New cracks form, and new lithosphere forms in the rift zone.

  • The mid-ocean ridge called the Mid-Atlantic Ridge is unusually active.

  • This activity has built part of the ridge into a large island known as Iceland.

  • Long linear cracks called fissures have formed where the Atlantic and Eurasian plates are moving apart.

  • Basaltic magma rises to Earth’s surface through these fissures and erupts nonexplosively.

  • Icelandic volcanoes, such as Krafla, are often associated with large, connected fissure systems.

  • Lava erupts frequently through these fissures. As a result, Iceland is continually getting bigger.

Volcanoes at Hot Spots

  • A hot spot forms in a tectonic plate over a mantle plume.

  • Plumes are thought to originate at the boundary between the mantle and the outer core.

  • When the top of a mantle plume reaches the base of the lithosphere, the mantle rock spreads out and “pools” under the lithosphere.

  • Because pressure on the rock is low at this shallow depth, the rock melts.

  • Large volumes of magma are released onto the ocean floor.

  • Continuous eruptions may produce a volcanic cone.

  • Because lava at hot spots comes from the mantle, it is mafic and fluid.

  • Most eruptions at hot spots are nonexplosive.

  • The type of rock that forms from this lava depends on the temperature, gas content, flow rate, and slope of the lava flow.

Shield Volcanoes

  • Shield volcanoes usually form at hot spots.

  • Shield volcanoes form from layers of lava left by many nonexplosive eruptions.

  • The lava is very runny, so it spreads out over a wide area.

  • Over time, the layers of lava create a volcanic mountain that has gently sloping sides.

  • The sides of shield volcanoes are not very steep, but the volcanoes can be very large.

Parts of a Volcano

  • Most volcanoes share a specific set of features.

  • At Earth’s surface, lava is released through openings called vents.

  • Before erupting as lava, magma rises from the magma chamber to Earth’s surface through cracks in the crust.

  • This movement of magma causes small earthquakes that can be used to predict an eruption.

  • Lava may erupt from a central summit crater of a shield volcano.

  • Lava may also erupt from fissures along the sides of the shield volcano.

  • A lava flow is a long river of molten rock.

  • Often the flow will cool and solidify on top while lava in the interior continues to flow.

  • Flowing lava in the interior travels through long, pipelike structures known as lava tubes.

Volcanoes at Convergent Boundaries

  • At a convergent boundary, a plate that contains oceanic lithosphere may descend into the mantle beneath another plate.

  • The descending lithosphere contains water.

  • As the lithosphere descends into the mantle, temperature and pressure increase.

  • The subducting lithosphere releases water into the surrounding mantle and overlying crust.

  • The magma that forms rises through the crust and erupts. These eruptions form a chain of volcanoes parallel to the plate boundary.

  • Magmas at convergent boundaries are composed of melted mantle rock and melted crustal rock.

  • Therefore, fluid mafic lava and lava rich in silica and feldspar form at these boundaries.

  • Lavas rich in silica and feldspar cool to form light-colored rocks.

  • The term felsic is used to describe magma, lava, and rocks that are rich in silica and feldspars.

  • Silica-rich magma tends to trap water and gas bubbles.

  • This causes enormous gas pressure to develop within the magma.

  • As the gas-filled magma rises to Earth’s surface, pressure is rapidly released.

  • This change in pressure causes a powerful explosive eruption.

  • Pyroclastic materials are released during the eruption.

  • Pyroclastic material forms when magma explodes from a volcano and solidifies in the air.

  • Pyroclastic material also forms when powerful eruptions shatter existing rock.

  • Four types of pyroclastic material include volcanic bombs, lapilli, volcanic ash, and volcanic blocks.

  • Pyroclastic flows are produced when a volcano ejects enormous amounts of hot ash, dust, and toxic gases.

  • This glowing cloud of pyroclastic material can race down the slope of a volcano at speeds of more than 200 km/h.

  • This speed is faster than the speed of most hurricane-force winds.

  • The temperature at the center of a pyroclastic flow can exceed 700°C.

  • At this high temperature, a pyroclastic flow burns everything in its path.

  • Extreme winds and temperatures make pyroclastic flows the most dangerous of all volcanic phenomena.

Cinder Cone Volcanoes

  • Cinder cone volcanoes are the smallest type of volcano.

  • They generally reach heights of no more than 300 m.

  • Cinder cone volcanoes are made of pyroclastic material.

  • Cinder cone volcanoes most often form from moderately explosive eruptions.

  • They have steep sides and a wide summit crater.

  • Unlike other types of volcanoes, cinder cone volcanoes usually erupt only once in their lifetime.

Composite Volcanoes

  • They form from both explosive eruptions of pyroclastic material and quieter flows of lava.

  • This combination of eruptions forms alternating layers of pyroclastic material and lava.

  • Composite volcanoes have a broad base and sides that get steeper toward the crater.

  • These volcanoes may generate many eruptions. However, eruptions may occur at intervals of hundreds of years or more.

  • Mount Fuji in Japan is a famous composite volcano.

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