Volcanoes and Volcanic Eruptions: Types, Shapes, and Magma Properties

Volcanic Eruptions and Volcano Types

Types of Volcanic Eruptions

• Fissure Eruptions:
  • Occur where multiple fractures in the Earth's crust allow magma to well up from the magma chamber or mantle.
  • Common at spreading zones where crust is fractured and fragmented by extension.
  • Often lead to the creation of flood basalts.
    • Thick deposits of basalt (cooled volcanic lava).
    • Can be $\text{several meters}$ thick and cover $\text{hundreds of miles}$.
    • Example: The Columbia River Basalt Group in Washington and Oregon, representing large and long-lived fissure eruptions.
• Vent Eruptions:
  • Result in the more characteristic volcanic cone shape.
  • All magma erupts from a single vent or crater.
  • These are the types of eruptions that form volcanoes.

Classification of Volcanic Mountains

• Volcanoes are usually classified based on their shape or profile and their size.

Shield Volcanoes

• Profile: Broad volcanic mountains with gentle slopes, resembling a shield.
• Size: Can be very large; the largest volcanic mountains. The largest known shield volcano is about $10 \text{ kilometers}$ tall.
• Formation: Built up by successive eruptions of low-viscosity lava, occurring over millions of years.
• Example: The Hawaiian Islands are prominent examples of shield volcanoes.

Volcanic Domes

• Profile: Characterized by very steep slopes and do not spread out much laterally.
• Eruptions: Pressure inside can build up, leading to explosive eruptions.
• Often found associated with a crater from past explosive events.
• Example: A volcanic dome forming within the crater of Mount Saint Helens.

Cinder Cone Volcanoes

• Size: Generally the smallest volcanic mountains, usually only several $\text{hundred meters}$ tall.
• Profile: Have a characteristically steep-sided cone shape.
• Formation: Form immediately around the erupting vent.
• Composition: Primarily made of ejected explosive material like ash and small particles, referred to by geologists as pyroclastic flow.

Stratovolcanoes (Composite Volcanoes)

• Prevalence: Most common type of volcano above subduction zones.
• Profile: Characterized by a concave cone shape.
• Structure: Have a central vent, similar to cinder cones, but are generally much taller.
• Composition: Built up from alternating layers of lava flows and pyroclastic layers. This makes them stronger, allowing them to resist erosion for longer and grow into taller mountains.
• Example: Mount Rainier in Washington state, near Seattle, is a typical stratovolcano with very steep slopes.

Associated Volcanic Landforms

• Craters:
  • Simply bowl-shaped pits surrounding the central vent of a volcano.
• Calderas:
  • Large, basin-shaped depressions that form when a magma chamber collapses in on itself.
  • This typically occurs after a large volcanic eruption empties the underlying magma chamber.
  • Calderas can range from a few kilometers to about $50 \text{ kilometers}$ wide.
  • Example: Crater Lake National Park is a lake formed within a very large caldera, indicating it sits on a very old volcano that experienced a massive eruption and subsequent collapse.

Why Some Eruptions are Explosive and Others are Not: The Role of Magma Viscosity

• The key factor determining whether eruptions are explosive or effusive (smooth and constant) is the viscosity of the magma.

Viscosity Defined

• Viscosity is a measure of a liquid's resistance to flow.
  • Low viscosity liquids: Flow easily (e.g., water).
  • High viscosity liquids: Flow less easily (e.g., molasses).
• Different magmas possess different viscosities.

Factors Influencing Magma Viscosity

1. Temperature:
   • The hotter a magma is, the lower its viscosity, meaning it flows more easily.
   • Analogy: Refrigerated honey is viscous and hard to pour; heating it (e.g., in a microwave) lowers its viscosity, making it flow more easily.
2. Chemical Composition (Silicate Crystal Structures):
   • The arrangement of silicate tetrahedra within minerals significantly influences viscosity.
   • Mafic Magmas (Lower Viscosity):
     • Composed of minerals rich in magnesium (mafic minerals).
     • These minerals generally have less complex crystal structures (e.g., isolated tetrahedra or single chains).
     • Fewer 'tangles' or long molecular chains mean less resistance to flow, resulting in less viscous magma.
   • Felsic Magmas (Higher Viscosity):
     • Contain minerals rich in elements like sodium, calcium, or potassium.
     • These minerals typically have more complicated crystal structures (e.g., framework silicates, where each silicate tetrahedron is bound to others).
     • On a molecular level, these complex structures can get tangled with each other in the liquid, slowing down the magma's flow and creating higher viscosity.
3. Amount of Volatiles (Gases and Water):
   • Volatiles can influence viscosity in different ways:
     • Water: The addition of water to magma can decrease its viscosity. Water weakens chemical bonds and dissolves some minerals, reducing resistance to flow.
     • Other Gases: The addition of certain gases may increase viscosity.
   • Therefore, volatiles can both increase and decrease viscosity depending on their type.

Viscosity and Eruption Behavior

• Low Viscosity Magmas:
  • Flow readily and do not cause large explosions.
  • Gases can easily escape from the magma, preventing pressure buildup.
  • Example: The Hawaiian Islands exhibit low-viscosity lava flows, forming characteristic lava types like a'a and pahoehoe as it cools.
• High Viscosity Magmas:
  • Normally lead to large, explosive eruptions.
  • Gases build up within the magma but cannot escape because the high viscosity prevents their release.
  • This trapped gas increases pressure, eventually resulting in violent, explosive eruptions.
  • Examples:
    • The $1902$ eruption of Mount Pelée on the island of Martinique in the Caribbean, which completely destroyed the city of Saint-Pierre.
    • The $1980$ explosion of Mount Saint Helens, known for its high-viscosity magma.