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Volcanic Activity

Volcanic Activity. Volcanoes: An opening in the earth's crust through which molten lava, ash, and gases are ejected. All volcanoes are fueled by magma deep beneath Earth’s surface. Derived from Vulcan , the Roman god of fire. Volcanic Activity.

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Volcanic Activity

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  1. Volcanic Activity • Volcanoes: An opening in the earth's crust through which molten lava, ash, and gases are ejected. • All volcanoes are fueled by magma deep beneath Earth’s surface. • Derived from Vulcan, the Roman god of fire.

  2. Volcanic Activity • Magma: a mixture of molten rock, suspended mineral grains, and dissolved gases deep beneath the Earth’s surface. • Magma is formed when temperatures are high enough to melt the rocks involved. • Temperature, pressure, and the amount of water present all influence whether rock will melt to become magma.

  3. Volcanic Activity • Composition of magma: • Magma consists of the same elements found in the Earth’s crust: oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and sodium. • Silica (SiO2) is the most abundant compound in magma and has the greatest effect on its characteristics.

  4. Volcanic Activity • Types of magma: • Magma is classified according to the amount of silica (SiO2) it contains. • Basaltic: 50 percent silica • Andesitic: 60 percent silica • Rhyolitic: 70 percent silica • The amount of silica affects the melting temperature of magma and also impacts how quickly it flows.

  5. Volcanic Activity • Characteristics of each type of magma • Basaltic: • Source material: upper mantle • Viscosity (resistance to flow): low • Gas content: 1-2% • Explosiveness: low • Location of magma: both oceanic and continental crust • Example: Hawaiian volcanoes, Kilauea and Mauna Loa

  6. Volcanic Activity • Andesitic: • Source material: Oceanic crust and oceanic sediments • Viscosity: intermediate • Gas content: 3-4% • Explosiveness: intermediate • Location of magma: continental margins associated with subduction zones • Example: Mount St. Helens, Tambora (Indonesia)

  7. Volcanic Activity • Rhyolitic: • Source material: continental crust • Viscosity: high • Gas content: 4-6% • Explosiveness: high • Location of magma: continental crust • Example: dormant volcanoes in Yellowstone National Park, Wyoming.

  8. Volcanic Activity • Magma intrusion • Since magma is less dense than the surrounding rocks, it wants to move upward and come in contact with the crust. • Sometimes the magma intrudes into the crust above • The intruding magma can force cracks to open in the crust and can fill those cracks

  9. Volcanic Activity • Types of magma intrusions • Plutons: • Intrusive igneous rock bodies formed when magma cools • Batholiths: • The largest plutons • Irregularly shaped masses of coarse-grained igneous rocks that cover at least 100 km2 • Common in the interiors of major mountain chains

  10. Volcanic Activity • Stocks: • Irregularly shaped plutons similar to batholiths but smaller in size • Both batholiths and stocks generally form 10-30 km below the surface • Laccoliths: • Formed when magma intrudes into parallel rock layers close to Earth’s surface. • The rocks above bow upward • Usually only about 10-15 km wide

  11. Volcanic Activity • Sills: • A pluton formed when magma intrudes parallel to layers of rock. • Can be from a few centimeters to hundreds of meters thick • Dykes: • A pluton formed when magma cuts across preexisting rocks • Form when magma invades cracks in surrounding rock areas

  12. Volcanic Activity • Plutons and tectonics • Many plutons are formed as a result of mountain-building processes • Many mountain chains were formed at continental-continental boundaries • Theory: these plate collisions caused crust to be pushed down into the upper mantle. The crust then melted and intruded into the overlying rocks. • Batholiths may form by the same process at oceanic-oceanic boundaries.

  13. Volcanoes • Anatomy of a volcano • Lava erupts through an opening in the crust called a vent. • As lava flows out onto the surface, it cools and solidifies around the vent. • Over time, lava accumulates to form a volcano.

  14. Volcanoes • Anatomy of a volcano • Crater: bowl-shaped depression at the top of a volcano • Volcanic craters are usually less than 1km in diameter • Calderas are larger craters – up to 50km in diameter

  15. Volcanoes • Types of volcanoes • Volcanoes classified according to two criteria: • The type of material that forms the volcano • The type of eruptions that occur • Three major types of volcanoes have been identified: • Shield volcanoes • Cinder-cone volcanoes • Composite volcanoes

  16. Volcanoes • Shield volcanoes • A mountain with broad, gently sloping sides and a nearly circular base. • Formed when layer upon layer of basaltic lava accumulates during non-explosive eruptions • Examples: Hawaiian volcanoes

  17. Volcanoes • Cinder-cone volcanoes • Formed when material ejected high into the air falls back to Earth and piles up around the vent. • They have steep sides and are generally small – less than 500 meters high. • Tend to be more explosive than shield volcanoes due to the higher amounts of water, silica, and gases.

  18. Volcanoes • Composite volcanoes • Form when layers of volcanic fragments alternate with lava. • Much larger than cinder-cone volcanoes. • High amounts of water, silica, and gases. • Typically very explosive. • Examples: Mount St. Helens, Mount Rainier

  19. Volcanoes • Volcanic material • Tephra • Rock fragments thrown into the air by a volcanic eruption • Can be cooled lava, mineral grains, or pieces of the volcanic cone • Classified by size as: dust, ash, lapilli

  20. Volcanoes • Pyroclastic flows • Rapidly moving volcanic material – usually clouds of gas, ash, and other tephra • Can travel at speeds up to 200 km/h and can contain hot, poisonous gases • The temp of pyroclastic flows can exceed 700 degrees Celsius.

  21. Volcanoes • Where volcanoes occur • Convergent volcanism • 80 percent of volcanoes are found along convergent boundaries. • Magma forced upward through crust at a subduction zone • Examples: The Pacific Ring of Fire (or Circum-Pacific Belt) and the Mediterranean Belt

  22. Volcanoes • Divergent volcanism • 15 percent of volcanoes are found along divergent boundaries • Magma forced upward into fracture formed when plates separate. • Most happen underwater at ocean ridges. • Example: Volcanoes on Iceland (Iceland lies directly on the Mid-Atlantic Ridge)

  23. Volcanoes • Hot spots • 5 percent of volcanoes are found away from plate boundaries • These volcanoes occur over hot spots – unusually hot regions of Earth’s mantle where high temperature plumes of mantle material rise toward the surface. • Examples: Hawaiian volcanoes

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