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Volcanism. Volcanism. Volcanoes are fueled by magma. Once magma forms, it rises towards Earth’s surface because it is less dense than the surrounding mantle and crust. Volcanism describes all the processes associated with the discharge of magma, hot fluids, and gases.

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volcanism1
Volcanism
  • Volcanoes are fueled by magma. Once magma forms, it rises towards Earth’s surface because it is less dense than the surrounding mantle and crust. Volcanism describes all the processes associated with the discharge of magma, hot fluids, and gases.
  • Volcanoes will erupt in about 60 different places on Earth in any given year. However, these places on Earth’s surface are not random. Most volcanoes form at plate boundaries – both divergent and convergent. Only about 5% of volcanoes erupt far from a plate boundary.
convergent volcanism
Convergent Volcanism
  • Most volcanoes that form on land result from oceanic-continental subduction. These volcanoes are characterized by explosive eruptions. (Ex: Mount St. Helens)
  • The volcanoes associated with convergent plate boundaries forms a major belt known as the Circum-Pacific Belt (Pacific Ring of Fire).
    • Ex: Mount St Helens, Mount Fuji, Mount Pinatubo
    • Mediterranean Belt – Mount Etna, Mount Vesuvius
divergent volcanism
DIVERGENT VOLCANISM
  • Mostly occurs along ocean ridges. Here, the lava takes the form of giant pillows and it is called pillow lava. Unlike convergent volcanism, volcanism along divergent boundaries tends to be non-explosive with effusions of large amounts of lava.
hot spots
HOT SPOTS
  • Volcanism that occurs far from any plate boundaries. These hot spots are unusually hot regions of Earth’s mantle where high temperature plumes of magma rise to the surface.
    • Ex: Hawaii (explain how the island chain has formed) Meiji is near Kamchatka 80 million years old.
    • Daikakuji bend – change in the direction of the Pacific plate about 43 million years ago.
flood basalts
Flood Basalts
  • Sometimes, hot spots occur beneath continental crust. Flood basalts will form when lava flows out of long cracks in the Earth’s crust. Over hundreds or even thousands of years, lava can flow out of fissures and form large plateaus.
    • Ex: Columbia River Basalts – Northwestern United States has over 170,000km3 of basalt.
    • Ex: Deccan Traps – Occurred 65 mya in India has over 512,000km3 of basalt. Many geologists think that the eruption of the Deccan traps might have caused a global change in climate. Is the timing of this event significant?
anatomy of a volcano
Anatomy of a VOlcano
  • Magma Chamber – Origin of magma for a volcano
  • Conduit – Magma reaches the surface where it becomes lava via a conduit
  • Vent – Emerges through an opening
  • Crater – Around the vent at the top of the volcano
  • Caldera – a large depression that often forms after a large eruption. Occurs when the summit collapses into the empty magma chamber
types of volcanoes
Types of Volcanoes
  • Smallest of the 3 types of volcanoes
  • Steep-sloped and cone-shaped
  • Usually composed of basaltic lava
  • Explosive eruptions
  • Usually form at the edges of larger volcanoes
  • Ex: Lassen Volcanic Park cinder cone
types of volcanoes1
Types of Volcanoes
  • Shield Volcano
    • Largest of the three types of volcanoes
    • Long, gentle slopes
    • Composed of layers of solidified basaltic lava
    • Quiet eruptions
    • Ex: Mauna Loa, Hawaii
types of volcanoes2
Types of Volcanoes
  • Composite Volcanoes
    • Considerably larger than cinder cones
    • Tall, majestic mountains
    • Composed of layers of rock from explosive eruptions and lava flows
    • Cycle through periods of quiet and explosive eruptions
    • Ex: Mount Augustine, Mount St Helens
eruptions

Often, the activity of a volcano depends on the composition of the magma within it.

Temperature – Most rocks begin to melt at temperatures between 800 -1200 degrees Celsius. Temperature increases with depth.

Pressure – Pressure also increases with depth. As pressure increases, the temperature required to melt a rock also increases. This effect of pressure explains why most of the rocks in the lower crust and upper mantle do not melt.

ERUPTIONS
composition of magma

The composition of magma depends on the following factors:

    • Temperature
    • Pressure
    • Amount of dissolved gas
      • As the amount of gases in magma increases, the magma’s explosivity also increases
      • Important gases in magma include: water vapour, carbon dioxide, sulfur dioxide, and hydrogen sulfide
    • Amount of silica
      • The higher the silica content, the thicker and more viscous the magma
      • Due to the higher viscosity, magma with a lot of silica tends to trap more gases and, as a result, the eruptions are more explosive.
      • Magma with low silica content tends to flow easily and produce quiet, non-explosive eruptions.
Composition of Magma
types of magma

Basaltic Magma

    • Often occurs when rocks from the upper mantle melt
    • This magma is less than 50% silica
    • Low-viscosity magma
    • The resulting volcanoes are characterized by quiet eruptions
Types of Magma
types of magma1

Andesitic Magma

    • Magma is 50-60% silica
    • Andesitic magma is generally found along oceanic-continental subduction zones
    • Source material is either oceanic crust or oceanic sediments
    • Resulting volcanoes have intermediate viscosity and, therefore, intermediate explosivity
Types of Magma
types of magma2

Rhyolitic Magma

    • Occurs when molten material rises and mixes with the overlying continental crust that is rich in silica and water.
    • Magma is >60% silica
    • Due to its high viscosity, large volumes of gas are trapped in this type of magma. This results in very explosive volcanoes
    • The dormant volcanoes in Yellowstone National Park are good examples
Types of magma
explosive eruptions
Explosive eruptions
  • This occurs when lava is too viscous to flow freely from the vent of a volcano. The result is that pressure builds up until the volcano explodes throwing lava, rock and ash into the air. The erupted materials are called tephra.
pyroclastic flows
Pyroclastic Flows
  • Some tephra can cause a lot of damage and kill many people. Violent volcanic eruptions can send clouds of ash and other tephra down a slope at speeds in excess of 80km/h. These rapidly moving tephra and clouds of hot suffocating gases are called pyroclastic flows. These flows can have internal temperatures of more than 700 degrees Celsius.
  • A deadly pyroclastic flow occurred in 1902 on the island of Martinique in the Caribbean Sea. More than 29,000 people suffocated or were burned to death.