Volcanoes

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Volcanoes

• The Global Distribution of Volcanoes

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Mid-Ocean Ridges

• mid ocean ridges
• The following video talks about how mid-ocean
  ridges form volcanoes, and repave the Earths
  surface.

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I. Volcanoes Beneath the Sea

• A. Evidence of volcanic rocks on the oceans floor
  indicate there are more volcanoes under water
  than on land.
• B. Mid-Ocean Ridge A continuous mountain range
  extending through the North and South Atlantic
  Oceans, the Indian Ocean, and the South Pacific
  Ocean.

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I. Volcanoes Beneath the Sea (contd)

• C. Rift Valley The deep central cleft in the
  mid-oceanic ridge.
• D. Magma Naturally occurring molten rock
  material generated within the Earth.
• i. The magma rises and forms submarine
  (underwater) volcanoes.
• ii. Some places along the mid-ocean ridges
  have so much volcanic activity, they can form
  islands.

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II. Volcanoes on Land

• A. The Cascades mountain range along the western
  U.S. are formed by volcanoes.
• B. The Ring of Fire is a notable belt of
  volcanoes that is spread throughout the Pacific
  Ocean.
• i. The Pacific plate is sliding beneath
  continental plates and that forms the Ring of
  Fire. These plates are called convergent plates.

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II. Volcanoes on Land (contd)

• ii. When a plate travels downward, it creates
  heat. Water escapes the melting rock, and
  eventually magma is formed. The magma rises
  because it is less dense than the existing rock.
  
• iii. 4/5s of volcanoes on land are formed when
  one plate slides under another.

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III. Volcanoes Formed by Rifting on the
Continents

• A. Some volcanoes (like the ones in Eastern
  Africa) are because the plates are pulling away
  from each other. This is called a divergent
  plate.
• i. The plate is stretched and broken, and
  it opens up a rift valley. This rift valley
  allows magma to escape.

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III. Volcanoes Formed by Rifting on the
Continents (contd)

• B. There is evidence of continental rifting
  along the Eastern U.S. too.

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IV. Volcanoes at Hot Spots

• Hot spot A fixed source of abundant rising
  magma that forms a volcanic center that has
  persisted for tens of millions of years.
• The Hawaiian Islands are evidence of hot spot
  volcanoes.

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Convergent Plate Boundaries
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Convergent Plate Boundaries
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Divergent Plate Boundaries

• Most are at mid-ocean ridges
• Mantle melts as pressure is reduced
• Melt is basaltic in composition
• Cooling basalt forms new oceanic crust

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Activity 2Volcanic Landforms

• 1. Topography of Volcanic Regions
• i. Contour lines - A line on a map that
  connects points of equal elevation of the land
  surface.
• a. Contour lines never cross.
• b. The closer together the contour
  lines, the steeper the slope.
• Details and pictures of contour lines

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Topographic Maps

• Topographic map - A map showing the topographic
  features of the land surface.

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Volcanoes

• National Geographic volcanoes

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II. Magma Composition

• A. Magma is a mixture of liquid, melted rock,
  and dissolved gases.
• B. The most abundant chemical elements in magma
  are silicon and oxygen.
• C. Silicon and oxygen are the building blocks of
  most of the common minerals.
• Lava Flows and Pyroclasts

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II. Magma Composition (contd)

• D. Silica - the combination of the most common
  elements, oxygen and silicon, that are found in a
  volcano. The oxide is SiO2.
• a. More silica means more viscous, or
  more of a violent explosion.
• b. High silica will not form as much lava
  as low silica, but it probably will cause the
  pyroclastic flow to be huge.

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III. Types of Volcanoes

• A. Composition Cone - A volcano that is tall and
  has steep slopes because the lava does not flow
  easily.
• a. A volcano that is constructed of
  alternating layers of lava and pyroclastic
  deposits.
• B. Shield Volcano - The are generally the
  largest in the world. They are not that tall as
  the lava slowly moves out of them.

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III. Types of Volcanoes (Contd)

• A. Shield volcanoes are not explosive.