🌋Physical Geology Unit 4 – Igneous Rocks and Volcanic Activity

Key Concepts and Definitions

• Igneous rocks form from the cooling and solidification of magma (molten rock beneath the Earth's surface) or lava (molten rock above the Earth's surface)
• Magma composition influences the type of igneous rock formed and consists of a mixture of molten silicates, dissolved gases, and other elements
• Texture of igneous rocks determined by the rate of cooling and crystallization
  • Coarse-grained (plutonic) rocks (granite) cool slowly beneath the Earth's surface, allowing large mineral crystals to form
  • Fine-grained (volcanic) rocks (basalt) cool rapidly at or near the Earth's surface, resulting in small mineral crystals
• Mafic igneous rocks contain high amounts of magnesium and iron (basalt), while felsic igneous rocks are rich in silica and lighter elements (rhyolite)
• Viscosity, a measure of a fluid's resistance to flow, influences the behavior of magma and lava
  • High-viscosity magmas (rhyolitic) tend to trap gases, leading to explosive eruptions
  • Low-viscosity magmas (basaltic) allow gases to escape easily, resulting in effusive eruptions
• Volatiles, such as water and carbon dioxide, lower the melting point of rock and contribute to the formation and behavior of magma

Formation of Igneous Rocks

• Igneous rocks form through the cooling and crystallization of magma or lava
• Partial melting of mantle rocks produces primary magmas, which can undergo further differentiation to create a range of magma compositions
• Magma generation occurs at various tectonic settings, including divergent boundaries (mid-ocean ridges), convergent boundaries (subduction zones), and hotspots (mantle plumes)
• Magma rises through the crust due to its lower density compared to surrounding rocks, leading to the formation of magma chambers and potential volcanic eruptions
• Bowen's reaction series describes the crystallization sequence of minerals from magma as it cools, with high-temperature minerals (olivine, pyroxene) forming first, followed by lower-temperature minerals (quartz, muscovite)
  • Continuous branch represents the continuous solid solution between plagioclase feldspar compositions
  • Discontinuous branch represents the distinct crystallization of mafic to felsic minerals
• Fractional crystallization occurs when early-forming minerals are removed from the magma, altering its composition and leading to the formation of different igneous rock types
• Assimilation involves the incorporation of surrounding rock into the magma, which can modify its composition and properties

Types of Igneous Rocks

• Igneous rocks classified based on texture and composition
  • Phaneritic (coarse-grained) rocks include granite, diorite, and gabbro
  • Aphanitic (fine-grained) rocks include rhyolite, andesite, and basalt
• Intrusive (plutonic) igneous rocks form from the slow cooling of magma beneath the Earth's surface (granite, diorite, gabbro)
• Extrusive (volcanic) igneous rocks form from the rapid cooling of lava at or near the Earth's surface (rhyolite, andesite, basalt)
• Porphyritic texture occurs when larger crystals (phenocrysts) are embedded in a fine-grained groundmass, indicating a two-stage cooling history
• Pyroclastic rocks form from the consolidation of volcanic ash, lapilli, and bombs ejected during explosive eruptions (tuff, volcanic breccia)
• Ultramafic rocks (peridotite, dunite) are rich in magnesium and iron, representing mantle compositions or cumulates from mafic magmas
• Pegmatites are exceptionally coarse-grained igneous rocks that form from the late-stage crystallization of water-rich magmas, often containing rare minerals

Volcanic Structures and Landforms

• Shield volcanoes (Mauna Loa) characterized by broad, gently sloping flanks built from successive layers of low-viscosity basaltic lava flows
  • Lava plateaus and plains can form from extensive, far-reaching basaltic lava flows
• Stratovolcanoes (composite volcanoes) (Mount Fuji) steep-sided, conical volcanoes composed of alternating layers of lava flows, volcanic ash, and pyroclastic material
  • Often associated with subduction zones and exhibit a range of eruptive styles
• Cinder cones (Parícutin) small, steep-sided volcanic hills built from the accumulation of ejected volcanic fragments (scoria) around a central vent
• Lava domes (Novarupta) formed by the extrusion of viscous, silica-rich lava that piles up around the vent
• Calderas (Yellowstone) large, circular depressions formed by the collapse of a volcano's summit or the emptying of its magma chamber
  • Resurgent domes can form within calderas as magma pushes upward after collapse
• Volcanic necks (Ship Rock) erosional remnants of solidified magma that once filled a volcano's conduit
• Lava tubes (Thurston Lava Tube) form when the outer surface of a lava flow cools and solidifies while the interior continues to flow, creating a hollow tunnel

Volcanic Eruptions and Hazards

• Effusive eruptions characterized by the relatively gentle flow of low-viscosity lava (basaltic) from the vent, often resulting in lava fountains and lava flows
• Explosive eruptions involve the violent fragmentation and ejection of magma and rock due to the rapid expansion of dissolved gases in high-viscosity magmas (rhyolitic)
  • Pyroclastic density currents (Mount Pelée) are ground-hugging flows of hot ash, pumice, and gas that can travel at high speeds and cause significant destruction
  • Lahars (Nevado del Ruiz) are destructive mudflows or debris flows that occur when volcanic ash and debris mix with water from melting snow, ice, or rainfall
• Volcanic gases, such as water vapor, carbon dioxide, and sulfur dioxide, can contribute to air pollution, acid rain, and climate change
• Volcanic ash consists of fine particles of pulverized rock and glass ejected during an explosive eruption, which can cause respiratory issues, damage infrastructure, and disrupt air travel
• Lava flows can destroy property, infrastructure, and ecosystems in their path, but their slow advance often allows time for evacuation
• Volcanic edifice instability can lead to landslides or sector collapses, potentially triggering tsunamis if they occur in coastal areas

Plate Tectonics and Volcanism

• Plate tectonics theory explains the distribution and characteristics of volcanoes worldwide
• Divergent plate boundaries (mid-ocean ridges) associated with the upwelling of mantle material and the formation of new oceanic crust
  • Submarine volcanoes and hydrothermal vents are common features along mid-ocean ridges
• Convergent plate boundaries (subduction zones) involve the subduction of oceanic lithosphere beneath another plate, leading to the formation of volcanic arcs (Aleutian Islands)
  • Partial melting of the subducting slab and overlying mantle wedge generates magmas that feed stratovolcanoes
• Hotspots (mantle plumes) are areas of persistent volcanism that are independent of plate boundaries (Hawaii, Yellowstone)
  • Mantle plumes bring hot material from the deep mantle to the surface, causing melting and the formation of volcanic chains as plates move over the stationary hotspot
• Intraplate volcanism occurs within plates, often related to extensional tectonics or the reactivation of ancient rifts (East African Rift System)
• Back-arc basins (Mariana Trough) form behind volcanic arcs in subduction zones due to the extension of the overriding plate, resulting in the formation of spreading centers and associated volcanism

Case Studies and Famous Volcanoes

• Mount St. Helens (USA) 1980 eruption demonstrated the destructive power of explosive eruptions and the importance of volcanic monitoring and hazard assessment
  • Lateral blast, pyroclastic flows, and lahars caused extensive damage and loss of life
• Kilauea (Hawaii) is a shield volcano known for its persistent effusive eruptions and lava flows
  • Provides opportunities to study basaltic volcanism and the formation of volcanic landscapes
• Mount Vesuvius (Italy) famous for its 79 CE eruption that buried the Roman cities of Pompeii and Herculaneum
  • Highlights the long history of human interaction with volcanoes and the need for preparedness
• Krakatoa (Indonesia) 1883 eruption was one of the most violent in recorded history, generating tsunamis and global climatic effects
• Yellowstone Caldera (USA) is a supervolcano with a history of large, explosive eruptions and ongoing geothermal activity
  • Offers insights into the behavior of silicic magmatic systems and the potential for future eruptions
• Mount Pinatubo (Philippines) 1991 eruption demonstrated the successful use of volcanic monitoring and early warning systems to mitigate the impact on surrounding communities

Practical Applications and Research

• Geothermal energy utilizes heat from magmatic systems to generate electricity and provide direct heating
  • Iceland and New Zealand are examples of countries that heavily rely on geothermal resources
• Volcanic ash and tephra deposits can provide valuable information about past eruptions and help reconstruct the eruptive history of a volcano
• Volcanic soils (andisols) are often fertile due to their high content of nutrients and favorable physical properties, supporting agriculture in many regions
• Volcanic rocks and minerals are used as building materials (pumice, pozzolana) and for industrial purposes (sulfur, copper)
• Volcanoes and their associated geothermal systems can be tourist attractions, contributing to local economies (Yellowstone National Park, Iceland)
• Volcanic hazard assessment and monitoring involve the use of seismology, ground deformation measurements (GPS, InSAR), gas emissions, and other techniques to predict and mitigate the impact of eruptions
• Interdisciplinary research in volcanology combines geology, geophysics, geochemistry, and remote sensing to better understand volcanic processes and improve hazard management strategies