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Volcanoes. OBJECTIVES: Types of Magma Anatomy of a Volcano Types of Volcanoes. Viscosity. Viscosity : Resistance of a liquid to flow There are 3 types of magma. They range from very runny (low viscosity) to very sticky (high viscosity). Anatomy of a Volcano.

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Types of Magma

Anatomy of a Volcano

Types of Volcanoes

  • Viscosity: Resistance of a liquid to flow
  • There are 3 types of magma. They range from very runny (low viscosity) to very sticky (high viscosity).
anatomy of a volcano
Anatomy of a Volcano
  • Lava erupts through an opening called a vent.
  • Over time, the cooled & solidified lava forms a mountain known as a volcano.
  • At the top of the volcano, there is a bowl-shaped depression that can vary in size:
    • Crater: Smaller in size
    • Caldera: Larger in size.
      • Over 1 km in size
      • Forms when top of volcano collapses into the magma chamber.
lava vs magma
Lava vs Magma
  • Magma – molten rocks located within the interior of the Earth
  • Lava – molten rocks located on the surface/exterior of the Earth


anatomy of a volcano1
Anatomy of a Volcano
  • Volcanic fragments thrown into the air during eruptions are called tephra.
    • Can be newly cooled & hardened lava, crystallized mineral grains, or pieces of volcanic cone.
    • Categorized based on size.
      • Dust Size: Small
      • Volcanic Bombs: Large, rounded tephra.
      • Volcanic Blocks: Large, angular tephra.
anatomy of a volcano2
Anatomy of a Volcano
  • Pyroclastic flow are clouds of gas, ash, & other tephra down a slope at incredible speeds.
    • A.K.A. Rapidly moving volcanic material!
    • Can cause death & tremendous damage!
types of volcanoes
Types of Volcanoes
  • Shield
  • Cinder-Cone
  • Composite (Stratovolcano)
shield volcano
Shield Volcano
  • Gently sloping sides
  • Nearly circular base
  • Forms when layers of basaltic lava accumulate during non-explosive eruptions
  • Low viscosity magma: Basaltic Magma
  • Ex. Hawaiian Islands
cinder cone volcano
Cinder-Cone Volcano
  • Forms when material ejected high into the air falls and piles up around the vent.
  • Usually has steep sides
  • Generally small in height
  • Has more silica than shield and large volumes of gases
  • Explosions are more severe than shields: Andesitic Magma
composite volcanoes
Composite Volcanoes
  • Forms when layers of volcanic fragments alternate with lava.
  • Large amounts of silica and gas: Rhyolitic magma.
  • Much larger than cinder-cone.
  • Violently explosive: Harmful to humans & environment
  • Ex. Mt. Saint Helens
where do volcanoes occur
Where Do Volcanoes Occur?
  • Most occur at plate boundaries
  • 80% along convergent
  • 15%along divergent
  • 5% occur far from plate boundaries.
convergent volcanism
Convergent Volcanism
  • Circum-Pacific Belt
    • A.K.A. “Ring of Fire”
    • Volcanoes of the Western U.S., Japan, Philippines, & Indonesia
    • Ex. Mt. Saint Helens
  • Mediterranean Belt
    • Volcanoes of Italy
    • Mt. Vesuvius
divergent volcanism
Divergent Volcanism
  • Most occur along ocean ridges.
  • Plates are pulling apart and magma wells up into the gap, producing volcanoes and lava flows.
hot spots
Hot Spots
  • The volcanoes that are formed far from plate boundaries are formed because of hot spots.
  • Hot Spots: areas of intense magma temperatures.
  • As a plate moves over a hot spot, the temperatures melt the crust and the new magma rises to form volcanoes.
  • When this happens, you get a string of volcanoes
  • Ex. Hawaiian Islands