JH

Volcanos Lecture Notes

VOLCANOES

Definition

  • A volcano is a mountain or hill with a conical shape and contains a crater or vent through which lava, rock fragments, hot vapor, and gases have erupted.

  • Volcanoes erupt when magma, formed from melting upper mantle or lower crust, rises through fissures to the surface.

  • Magma consists mainly of silicon and oxygen, with dissolved gases, especially water, that are released during an eruption.

Types of Volcanoes

  • Shield Volcanoes

  • Composite/Stratovolcanoes

  • Cinder Cone/Scoria Cone

Shield Volcanoes

  • Characteristics:

    • Largest and hottest type of volcano.

    • Resembles a warrior’s shield.

    • Low gas and silica content (50% silica, 0.5-2% gas).

    • Formation occurs primarily in oceans, with mafic/basaltic lava.

  • Lava Types:

    • Aa: Clinky and jagged appearance.

    • Pahoehoe: Smooth, ropy appearance; approximately 90% of basaltic lava is Pahoehoe.

    • Temperature range of 1,000°-1,250°C.

Composite/Stratovolcanoes

  • Characteristics:

    • Intermediate composition with andesitic magma.

    • Intermediate gas (3-4%) and silica (60%).

    • Most dangerous type of eruption found on continental edges.

    • Temperature range of 800°-1,050°C.

Cinder Cone/Scoria Cone

  • Characteristics:

    • Smallest type of volcano with short eruptive cycles.

    • Forms rapidly with felsic composition (granite, rhyolitic).

    • Highest gas (5-8%) and silica (70%) content, leading to explosive eruptions.

    • Typically found in continental interiors with a temperature range of 650°-900°C.

Characteristics of Lavas and Magmas

Felsic Lava

  • Composition: High in silica (quartz), feldspar.

  • Properties: Extremely viscous, associated with pyroclastic tephra.

Intermediate Lava

  • Composition: Lower in aluminum and silica; richer in magnesium and iron.

  • Properties: Hotter than felsic, moderately explosive.

Mafic Lava

  • High ferromagnesian content; lower in silica.

  • Properties: Flows easily with low viscosity; forms pillow lavas underwater.

Ultramafic Lava

  • Ancient type; rare today, containing over 18% magnesium oxide, eruptions exceeding 1,600°C.

Unusual Lava Types

  • Carbonatite: Found in Ol Doinyo Lengai volcano in Tanzania; erupts at temperatures never exceeding 510°C, has a unique dark appearance.

Types of Volcanic Eruptions

  • Icelandic: Characterized by long, parallel fissures with basaltic lava.

  • Hawaiian: Fluid lava flows from summit, forming shield volcanoes.

  • Strombolian: Moderate bursts of lava with expanding gases.

  • Vulcanian: Moderate explosive eruptions with ash clouds.

  • Pelean: Generates pyroclastic flows; extremely destructive.

  • Plinian: Intense, continuous eruptions; e.g., Mount Vesuvius eruption.

Tephra

  • General term for any erupted material.

  • Types categorized by size:

    • Dust/Ash: <2mm.

    • Cinder/Lapilli: 2-64mm.

    • Blocks/Bombs: >64mm.

Volcanic Hazards

Lahar

  • A type of mudflow composed of pyroclastic material and water, can be destructive.

Nuee Ardente

  • A hot cloud of gas and ash associated with pyroclastic flows.

Properties of Magma

  • Silica and gas content affect eruptive temperatures and viscosity.

  • Types include Basaltic (mafic), Andesitic (intermediate), and Rhyolitic (felsic).

Famous Volcanoes and Regions

  • Ring of Fire: A seismic region around the Pacific basin with numerous volcanoes.

  • Tamu Massif: Largest volcano on Earth located in the Pacific Ocean.

  • Olympus Mons: The largest known volcano in the solar system, located on Mars.

  • Siberian Traps: A large igneous province known for massive eruptions.

  • Deccan Traps: Associated with climate change and mass extinction.

Conclusion

  • Understanding volcano types, their behavior, and their impact on the environment is crucial for predicting eruptions and assessing hazards.

The geothermal gradient refers to the rate at which temperature increases with depth in the Earth's crust, typically about 25-30°C per kilometer in the upper parts of the crust but can vary with geological conditions.

Flood Basalts/Traps

  • Flood basalts occur when large volumes of magma are erupted over a short period, resulting in thick layers of basalt rock that cover extensive areas. These formations are often associated with large igneous provinces.

Hot Spots

  • Hot spots are volcanic regions that arise from mantle plumes, which are upwellings of hot material from the Earth's mantle that create volcanic activity independent of tectonic plate boundaries.

Mantle Plume

  • A mantle plume is a long, upwelling column of hot rock that provides the heat source for volcanic activity. These plumes can remain stationary while tectonic plates move over them.

Super Plume

  • Super plumes are exceptionally large mantle plumes that can cause extensive volcanic activity and have significant implications for plate tectonics and the geological features they create.

Diapir

  • A diapir is a type of intrusion where less dense material pushes through surrounding denser rock, often leading to the formation of volcanic features at the surface.

Lava Tubes

  • Lava tubes are natural conduits formed by flowing lava that moves beneath the surface, allowing molten rock to reach distant areas before cooling and solidifying.

Volcanic Diagram

  • A volcanic diagram typically illustrates the parts of a volcano, including the magma chamber, vent, crater, and various types of eruptive materials such as tephra and lava flows.

Plutons

  • Plutons are large, intrusive igneous rock bodies that have crystallized from molten magma beneath the surface. They can form in various shapes, including batholiths, stocks, and laccoliths.

Dikes and Sills

  • Dikes are vertical or steeply inclined sheets of igneous rock that cut through existing rock layers, while sills are horizontal sheets of igneous rock that form between layers. Both are formed from magma that solidifies after intruding into existing rock formations.