Volcanoes: Types and Eruptions

Volcanoes: Types, Structure, and Eruptions

• A volcano is an opening in the Earth's surface where molten rock, gases, smoke, and ashes are erupted.
• Volcanoes are classified by their frequency of eruption:
  • Active: Erupted within the last 600 years or shows signs of potential eruption.
  • Inactive: Has not erupted for the last 10,000 years; their physical form may have been altered by weather and erosion.
  • Extinct: A volcano that will no longer erupt.
• Volcanoes are also categorized by cone shape:
  • Composite (Stratovolcanoes): Tall, symmetrical, steep-sided, built from layers of lava and pyroclastic materials; dangerous and explosive eruptions, sometimes involving slow-moving lava. Known to have caused the most human deaths.
  • Cinder Cone: Circular or oval; formed quickly due to the short eruption lifespan of building up from cooled lava fragments (cinders) and ash around the vent.
  • Shield Volcanoes: Broad, gentle slopes; shaped like a shield; formed due to numerous effusive (gentle) eruptions of runny lava.

Parts of a Volcano

• Magma Chamber: A chamber beneath the surface; contains magma (molten rock).
• Main Vent: The weak points in the Earth's crust; where magma rises and reaches the surface.
• Crater: The mouth of the volcano; surrounds the main vent.
• Side Vent: An opening on the side of a volcano.
• Parasitic Cone: A cone-shaped accumulation of organic materials that aren't part of the central vent.
• Gas & Volcanic Ash: Volcanic materials expelled during eruptions.

Types of Volcanic Eruptions

• Phreatic (Hydrothermal): A steam-driven eruption when hot rocks come into contact with water; short-lived, characterized by ash columns; can be a precursor to a larger eruption.
• Phreatomagmatic: A violent eruption; caused by contact between water and magma; creates large columns of fine, high-speed pyroclastic materials (called base surges).
• Strombolian: A weak to violent eruption, characterized by lava fountains.
• Vulcanian: Tall eruption columns reaching heights of 20 km with pyroclastic flow and ashfall tephra; small to moderate explosions lasting seconds to minutes.
• Plinian: Extremely explosive eruption of gases and pyroclastic material. A high rate of magma discharge and widespread ash dispersion.

Why do Volcanoes Erupt?

• Tectonic Plates: Massive slabs of solid rock gliding over the Earth's mantle. Volcanoes frequently occur at the boundaries between these plates.
• Earth's Structure: The Earth has layers, including the crust, mantle, outer core, and inner core. Volcanoes form through differing conditions, pressures, and magma flows.

Magma Viscosity

• Magma Viscosity: a measure of a fluid's resistance to flow. Factors affecting magma viscosity include:
  • Temperature
  • Chemical composition
  • Amount of dissolved gases

Types of Lava

• Blocky Lava: Cool and stiff; doesn't travel far
• Pahoehoe Lava: Slow-moving, smooth, and resembling dripping wax; forms a glassy surface
• ʻAʻā Lava: Jagged, sometimes slow-moving underwater surface
• Pillow Lava: Rounded, bubble-like lumps; forms underwater

The Ring of Fire

• A zone around the Pacific Ocean with many volcanoes and frequent earthquakes
• The Philippines is in this zone.

Assignment Questions

• 1. What type of volcano has a broad base and gentle slopes formed by lava flows? Shield volcano
• 2. Which of the following is an example of a composite volcano in the Philippines? Mt. Mayon
• 3. What type of volcanic eruption produces lava that flows steadily and calmly? Non-explosive eruption
• 4. Which term describes a volcano that has not erupted in thousands of years but may erupt again? Dormant
• 5. What is the smallest type of volcano, characterized by steep slopes and made from fragments of solidified lava?  Cinder cone
• 6. What is the name of the molten rock that comes out of a volcano during an eruption? Magma
• 7. Which type of volcano has steep sides and is made of layers of ash and lava? Composite (stratovolcano)
• 8. Which of the following is NOT a product of a volcanic eruption? Rain
• 9. What do you call a volcano that will never erupt again? Extinct
• 10. Which Philippine volcano had a massive eruption in 1991? Mt. Pinatubo