Volcanoes Nature of Volcanic Eruptions Factors Affecting Eruptions
- Slides: 19
Volcanoes Nature of Volcanic Eruptions
Factors Affecting Eruptions • Volcanoes erupt with different severities • Primary factors that determine whether a volcano erupts quietly or violently are: • (1) magma composition • (2) magma temperature • (3) amount of dissolved gases in magma
Magma Composition and Temperature • Viscosity refers to a substance’s resistance to flow • EX: syrup is more viscous than water and flows more slowly • Temperature increase = less viscous liquid
Magma Composition and Temperature • Chemical composition of magma greatly affects type of volcanic eruption • Viscosity of magma is related to its silica (sand) content • High silica content = greater viscosity
Dissolved Gases in Magma • During eruptions, gases trapped in magma provide force to eject molten rock from the vent (opening to the surface) • Highly viscous magmas slow the upward motion of expanding gases. The gases collect in pockets until they increase in size and explosively erupt
Volcanic Material • Many different materials are ejected from volcanoes during eruptions • (1) Lava flows • (2) Gases • (3) Pyroclastic Materials
Lava Flows • Basaltic lavas have low silica content, and are not very viscous • Two types of formations occur as a result of basaltic lava flows: • (1) pahoehoe – wrinkled/ropy lava flow • (2) aa – rough jagged block surface with spiny projections and sharp edges Pahoehoe Aa
Gases • Magma contains varied amounts of dissolved gases • Gases are confined to molten rock by pressure; once pressure is reduced, the gases begin to escape
Pyroclastic Materials • Pyroclastic material is the name given to particles that are produced during volcanic eruptions, appear as a fast moving deadly cloud of ash • Cinders – range from size of small beads to walnuts • Blocks – particles larger than 64 mm in diameter and composed of hardened lava • Bombs – glowing lava larger than 64 mm in diameter Cinder Blocks Bomb
Anatomy of a Volcano • Volcanic activity begins with fissure/crack that develops as magma is forced towards surface • Gas-rich magma moves up through fissure ending at a vent • Located at summit of volcano is a steep-walled depression called a crater
Anatomy of a Volcano • When a volcano violently erupts, the crater and mouth of the volcano will be ejected and destroyed. • A caldera forms, which is a large volcanic crater
Types of Volcanoes • • Three main types of volcanoes: (1) Shield volcanoes (2) Cinder cones (3) Composite or Stratovolcano Composite/Stratovolcano Cinder Cone Shield Volcano
Shield Volcanoes • Produced by the accumulation of fluid basaltic lava • Broad, slightly domed structure that resembles a warrior’s shield • Most have grown from ocean floor to form islands • EX: Hawaiian Islands and Iceland
Cinder Cones • Cinder cones mostly composed of gas-rich basaltic magma • Usually the product of a single eruption that lasts a few weeks • Once eruption ends, magma in pipe connecting the vent to the magma chamber solidifies so the volcano can never erupt again • Smaller in size • EX: Lava Butte, Newberry National Volcanic Monument, Oregon
Composite Volcano • Most dangerous volcanoes • Large symmetrical structures composed of lava and pyroclastic deposits • Most explosive volcanoes due to high silica content of magma • Most are located in Pacific Ocean Rim in an area called the “Ring of Fire” • EX: St. Helens
How Volcanoes are Created… • Plate Tectonics- subduction zones, rift valleys, hot spots
Negative Effects of Volcanoes • Ash fall, landslides, lahar (mud flows) – all lead to death and destruction • Volcanic gases-lower surface temperature by blocking out the sun, released sulfur gases lead to the creation of acid rain • Acid rain destroys leaves and can kill trees
Positive Effects of Volcanoes • Created up to 80% of Earth’s surface – we need land to live on! • Creates rich, fertile soil for agriculture • Heat can be harvested for renewable energy source EX: geothermal energy in Japan
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