Volcanoes

What Creates a Volcano?

Molten rock from the Earth’s interior is ejected via the central vent. Layers of lava, pyroclastic material, ash or a combination of all build up around a central vent.

• Subduction zones (10%) - Melt produced by fiction of the subducting plate rises up through the crust to produce stratovolcanoes
• Spreading Centers (80%) - Mid Ocean Ridges produce oceanic crust that covers 70% of Earth. .
• Hot Spots (10%) -
  • Oceanic Hot Spots - Basalt erupts at the seafloor and eventually forms a seamount or a island
  • Continental Hot Spots - two types
    • Basaltic magmas – Makes up the mantle plume and is usually the first magma to be erupted.
    • Rhyolitic magmas – Basalt melts the granitic crust it passes through, which changes the composition of the magma
    • Example: Yellowstone
      • Flood basalts from the Yellowstone Hot Spot extend from the current location all the way to the Pacific Ocean. These basalts can be seen along the Columbia River Gorge.
      • The current location is is Idaho. An eruption 640,000 a created a 100 km wide caldera.
      • Yellowstone has erupted 3 times in the last 2.1 million years
        • 2.1 million years ago - 600 million cubic miles of material ejected
        • 1.3 million years ago - 67 million cubic miles of material ejected
        • 640,000 years ago - 240 million cubic miles of material ejected
        • Average interval between eruptions? 730,000 years (give or take a bit)
        • A Long, Long Time Ago: Geologic History of Yellowstone

 

Volcanic Eruptions

Factors that determine the violence of an eruption

• Temperature of the magma
• Composition of the magma
• Dissolved gases in the magma

The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption .

Rock Type	Basalt	Andesite	Rhyolite
SiO2 content	45-55%	55-65%	65-122%
Magma temperature	1,000 – 1,250ºC	800 – 1,000 ºC	600-900 ºC
Viscosity	Low	increasing	High
Gas escape from magma	Easy	increasing	Difficult
Eruptive style	Peaceful	increasing	Explosive

 

Eruptive Style

• Effusive eruptions - Produce lava flows.
• Explosive eruptions - Blows up.

Controls:

• Viscosity – Controls the ease of lava flow. Gas
• Pressure – Greater P favors explosive style.
• Environment – Where eruption occurs important.

Effusive eruptions - Produces lava flows. Typically basaltic magma, create shields volcanoes

Explosive eruptions - Typically produce pyroclastic flows, and creates stratovolcanoes. Andesitic and rhyolitic compositions predominate.

Phreatomagmatic eruptions - These eruptions occur when magma interacts with water. Some eruptions can be cataclysmic. Examples:  Santorini, Krakatau.

 

Tectonic Settings of Volcanoes (revisited)

Why more volcanic activity at spreading centers?	Why less volcanic activity at subduction zones?
Low SiO2 content High temperature Low pressure as plates pull apart              More fluid = easier to erupt!	High SiO2 content Lower temperatures Higher pressures              Less fluid = harder to erupt!