Volcanoes: Plate Tectonics, Lava Composition, and Hazards

Volcanoes

Scenario: Consider a geographic location for a volcano. What type of plate setting is it located in? What kind of volcano is it (composite, dome, or shield)? What kind of lava does it produce (rhyolite, andesite, or basalt)? Is the lava predominantly silicate, feldspar, or ferromagnesian? Why? Is the lava felsic, mafic, or a combination in origin? Why? Describe the characteristics of a volcanic eruption.

Plate Tectonics:

• Divergent (mid-ocean ridges = volcanoes)
• Rift zones = volcanoes
• Subduction zones = volcanoes
• Hot spots = volcanoes

Lava

Composition:

• Result of the geologic setting, which determines available source material.
• Extent of melting, which determines what becomes the source material.
• A mix of SiO4, Al, Fe, Mg.

• Rich in SiO4: more felsic in origin (quartz, feldspar)
• Weak in SiO4, rich in Fe, Mg, and Al: more mafic (ferromagnesian)

• Ridges: mafic. Lavas are andesitic to basaltic.
• Rift zones: mafic. Magma is andesitic to rhyolitic.
• Subduction zones: felsic near continental crust. Is andesitic to rhyolitic.

• Mafic: thin, low viscosity.
• Felsic: thick, high viscosity. Felsic has more vapor trapped from the silica, preventing gases from escaping and causing explosions.

• Mafic: low viscosity. Volcanic eruptions can be effusive.
• Felsic: high viscosity. Volcanic eruptions can be catastrophic.

• Hot Spots: Shield Volcanoes. Basaltic magmas from mafic origins. Eruptions are not particularly explosive.
• Divergent Oceanic Spreading Centers (Ridges): Shield Volcanoes. Basaltic magmas from mafic origins. Eruptions are not particularly explosive.
• Divergent Continental Spreading Centers (Rifts): Shield Volcanoes. Andesitic to Basaltic magmas from mafic origins. Eruptions can be explosive if SiO4 is abundant in the melt.
• Subduction Zones: Composite Dome Volcanoes. More Rhyolitic from felsic origins. Always explosive and dangerous.

• Caldera: an enlarged volcanic summit crater formed either by the explosion or collapse of the volcano.
• Cinder cone: if the pyroclastic material is small and falls close to the cone, the result is this.

• Active: has erupted recently.
• Dormant: not erupted recently.
• Extinct: no recent eruptive history and appears eroded.

Number of active volcanoes: 300-500

Volcanoes erupt every 220 years on average. 20% erupt less than once every 1000 years. 2% erupt less than once every 10000 years.

Primary Hazards

• Lava: some land building. If mafic, control is sometimes possible (e.g., Iceland, Hawaii).
• Pyroclastics: fragments of hot rock and spattering lava, ash. Silicic origins (e.g., Costa Rica, Mt. St. Helens).
• Lahars: flows of water-laden pyroclastics (e.g., Mt. Vesuvius, Nevado del Ruiz, Pinatubo, Mt. St. Helens).
• Pyroclastic flows: thick clouds of SO2, pyroclastic, CO2 rolling downhill (e.g., Mt. St. Helens, Mont Pelée).
• Toxic gases: SO2, CO2, water vapor.
• Phreatics: steam explosion. Volcanic activity near water as island (e.g., Krakatoa, Indonesia).

Secondary Hazards

• Pyroclastic ash cloud cover: 1 km spread distance, scattering sunlight, cooling the earth.
• Ozone depletion: SO2 release to greatly reduce ozone levels.
• Acid rain: SO2 mixes with rain and produces sulfuric acid (H2SO4).

Volcanic Precursors

• Seismic activity increases.
• Bulging, lifting, tilting of the volcano's surface.
• Change in the mixtures of gases by the volcano (+SO2).
• Ground surface temperature increase.
• Animal activity changes.