🌎Plate Tectonics Unit 8 – Volcanoes and Igneous Activity

Key Concepts

• Magma molten rock beneath Earth's surface contains dissolved gases and is less dense than surrounding solid rock
• Lava magma that reaches the surface during a volcanic eruption can flow and solidify into various landforms
• Volcanic eruptions occur when magma rises through cracks or weaknesses in Earth's crust and reaches the surface
• Plate tectonics theory explains the distribution and behavior of volcanoes along plate boundaries (divergent, convergent, transform)
• Volcanoes form in three main tectonic settings: divergent boundaries (mid-ocean ridges), convergent boundaries (subduction zones), and hotspots (mantle plumes)
• Igneous rocks form from the cooling and solidification of magma (intrusive/plutonic) or lava (extrusive/volcanic)
  • Intrusive igneous rocks (granite, gabbro) form beneath Earth's surface and have large, interlocking crystals
  • Extrusive igneous rocks (basalt, rhyolite) form on Earth's surface and have small crystals or glassy texture

Types of Volcanoes

• Shield volcanoes broad, gently sloping volcanoes built from fluid basaltic lava flows (Mauna Loa, Hawaii)
  • Characterized by low viscosity lava that can flow great distances from the vent
• Stratovolcanoes tall, conical volcanoes composed of alternating layers of lava flows, volcanic ash, and cinders (Mount Fuji, Japan)
  • Steep-sided due to high viscosity lava that doesn't flow far from the vent
• Cinder cones small, steep-sided volcanoes built from ejected lava fragments called cinders or scoria (Parícutin, Mexico)
• Lava domes rounded, dome-shaped protrusions resulting from viscous lava piling up around the vent (Mount St. Helens, USA)
• Calderas large circular depressions formed by the collapse of a volcano's summit or the emptying of its magma chamber (Yellowstone, USA)
• Volcanic fields clusters of small volcanoes and lava flows covering a large area (San Francisco Volcanic Field, Arizona)
  • Often characterized by cinder cones, lava domes, and lava flows

Volcanic Eruptions

• Volcanic eruptions occur when magma rises through the Earth's crust and reaches the surface
• Eruption types depend on magma composition (silica content), gas content, and magma chamber depth
  • Effusive eruptions characterized by the outpouring of fluid lava with low viscosity (basaltic magma)
  • Explosive eruptions involve the violent fragmentation of magma and ejection of ash, pumice, and gas (rhyolitic magma)
• Volcanic gases (water vapor, carbon dioxide, sulfur dioxide) can contribute to the explosivity of eruptions and pose health risks
• Pyroclastic flows fast-moving, ground-hugging avalanches of hot ash, pumice, and gas can travel at speeds over 100 km/h
• Lahars volcanic mudflows formed by the mixing of volcanic ash and debris with water can inundate valleys and cause destruction
• Lava flows can vary in speed, thickness, and distance traveled depending on the lava's composition and viscosity
  • Pahoehoe lava flows have smooth, ropy, or wrinkled surfaces and are less viscous
  • A'a lava flows have rough, jagged, and clinkery surfaces and are more viscous

Igneous Rocks and Formation

• Igneous rocks form from the cooling and solidification of magma (intrusive) or lava (extrusive)
• Magma composition (silica content) determines the type of igneous rock formed
  • Felsic magma (high silica) forms light-colored rocks (granite, rhyolite) with high viscosity and explosive potential
  • Mafic magma (low silica) forms dark-colored rocks (gabbro, basalt) with low viscosity and effusive eruptions
• Cooling rate affects crystal size and texture in igneous rocks
  • Slow cooling (intrusive) allows large crystals to form, resulting in phaneritic texture (visible crystals)
  • Rapid cooling (extrusive) results in small crystals or glassy texture (aphanitic)
• Bowen's Reaction Series describes the crystallization sequence of magma as it cools and helps predict mineral composition
• Partial melting of the mantle or crust can generate magmas of different compositions
  • Basaltic magma forms from the partial melting of peridotite in the upper mantle
  • Granitic magma forms from the partial melting of basaltic oceanic crust or continental crust

Plate Tectonics and Volcanism

• Plate tectonics theory explains the global distribution of volcanoes and their relationship to plate boundaries
• Divergent boundaries (mid-ocean ridges) characterized by seafloor spreading and the formation of new oceanic crust
  • Basaltic magma rises to fill the gap created by diverging plates, forming submarine volcanoes and lava flows
• Convergent boundaries (subduction zones) occur where denser oceanic crust subducts beneath less dense continental or oceanic crust
  • Subducting plate releases water, lowering the melting point of the overlying mantle and generating magma
  • Stratovolcanoes form along the volcanic arc parallel to the subduction zone (Andes, Cascades)
• Hotspots volcanic regions not associated with plate boundaries, caused by mantle plumes rising from deep within the Earth
  • As the plate moves over the stationary hotspot, a chain of volcanoes forms (Hawaiian Islands)
• Transform boundaries where plates slide past each other can experience volcanism if the boundary is complex or has step-overs
• Intraplate volcanism can occur within plates due to localized mantle upwelling or extension (East African Rift)

Volcanic Hazards and Risks

• Volcanic eruptions pose significant hazards to nearby communities and can have global impacts
• Lava flows can destroy infrastructure, bury land, and start fires but generally move slowly enough for people to evacuate
• Pyroclastic flows are fast-moving, deadly avalanches of hot ash, pumice, and gas can incinerate and bury anything in their path
• Volcanic ash fine particles of pulverized rock ejected during explosive eruptions can cause respiratory issues, damage machinery, and disrupt aviation
  • Ash fall can blanket large areas, collapsing roofs and damaging crops
• Lahars volcanic mudflows can inundate valleys, destroy bridges and buildings, and pose a risk long after an eruption
• Volcanic gases (sulfur dioxide, carbon dioxide) can cause respiratory problems, acid rain, and contribute to climate change
• Indirect hazards include tsunamis triggered by submarine eruptions or flank collapses, and flooding from melting glaciers
• Risk assessment considers the likelihood and potential impacts of volcanic hazards on nearby populations and infrastructure
  • Hazard maps delineate zones of varying risk based on historical eruptions and modeling

Monitoring and Prediction

• Volcano monitoring involves the use of various techniques to detect signs of unrest and impending eruptions
• Seismicity increased earthquake activity can indicate magma movement and rock fracturing beneath a volcano
  • Volcanic earthquakes (long-period, hybrid, tremor) have distinct signatures from tectonic earthquakes
• Ground deformation (uplift, subsidence) measured using GPS, tiltmeters, and satellite radar (InSAR) can reveal magma intrusion
• Gas emissions (sulfur dioxide, carbon dioxide) can be monitored using spectrometers and provide insight into magma degassing
• Thermal anomalies detected by satellite imagery can indicate increased heat flow and impending eruptions
• Hydrology changes in water chemistry, temperature, and level in nearby springs and wells can signal magmatic unrest
• Eruption forecasting combines monitoring data, historical records, and models to estimate the likelihood and timing of an eruption
  • Short-term forecasts (hours to days) based on accelerating unrest and past eruption patterns
  • Long-term forecasts (months to years) consider a volcano's eruption history and magma recharge rates

Real-World Examples

• Mount Vesuvius (Italy) buried the Roman cities of Pompeii and Herculaneum in 79 AD, preserving them in volcanic ash
• Mount Pinatubo (Philippines) 1991 eruption was the second-largest of the 20th century, causing global cooling and extensive lahars
• Eyjafjallajökull (Iceland) 2010 eruption disrupted European air travel for weeks due to the widespread dispersal of volcanic ash
• Kilauea (Hawaii) ongoing effusive eruptions since 1983, with lava flows, lava lakes, and occasional explosive events
  • 2018 lower East Rift Zone eruption destroyed over 700 homes and created new land along the coast
• Mount St. Helens (USA) 1980 eruption was the deadliest and most economically destructive in U.S. history, causing a massive lateral blast and lahars
• Yellowstone Caldera (USA) a supervolcano with the potential for catastrophic eruptions, though the likelihood is low in the near future
  • Hydrothermal features (geysers, hot springs) and seismic activity are closely monitored
• Tambora (Indonesia) 1815 eruption was the largest in recorded history, causing global cooling and the "Year Without a Summer" in 1816