🌋Volcanology Unit 10 – Volcanoes and Plate Tectonics

Key Concepts and Terminology

• Volcanology studies the formation, eruption, and hazards associated with volcanoes
• Plate tectonics describes the large-scale motion of Earth's lithosphere, which is divided into rigid plates that move relative to each other
• Magma refers to molten rock beneath Earth's surface, while lava is magma that reaches the surface during a volcanic eruption
• Volcanic edifice is the structure built by the accumulation of erupted material around the vent (stratovolcano, shield volcano, cinder cone)
• Pyroclastic flows are high-speed, ground-hugging avalanches of hot ash, pumice, rock fragments, and volcanic gas
  • Can travel at speeds up to 700 km/h and reach temperatures of 1,000°C
• Lahar is a destructive mudflow or debris flow composed of volcanic ash and water from a volcano
• Tephra includes all solid volcanic material ejected into the atmosphere during an eruption (ash, cinders, pumice)
• Viscosity measures a fluid's resistance to flow, with high-viscosity magmas being more explosive than low-viscosity magmas

Plate Tectonic Theory Basics

• Earth's lithosphere is broken into several large plates that move and interact with each other
• Plates are composed of the crust and the uppermost mantle, known as the lithosphere
• Plate motion is driven by convection currents in the mantle, with hot material rising and cooler material sinking
• Plates move at rates ranging from 1-10 cm per year, with the fastest rates occurring at mid-ocean ridges
• Interactions between plates occur at plate boundaries, which can be convergent, divergent, or transform
• Earthquakes and volcanic activity are concentrated along plate boundaries
• The theory of plate tectonics unifies concepts of continental drift, seafloor spreading, and convection in the mantle
• Hotspots are areas of persistent volcanism not associated with plate boundaries, believed to be caused by mantle plumes (Hawaii, Yellowstone)

Types of Plate Boundaries

• Divergent boundaries occur where two plates move away from each other, often resulting in the formation of new oceanic crust (mid-ocean ridges)
  • Characterized by shallow earthquakes, basaltic volcanism, and rift valleys
• Convergent boundaries occur where two plates collide, resulting in subduction, mountain building, and volcanism
  • Oceanic-continental convergence leads to subduction of the denser oceanic plate beneath the continental plate (Andes, Cascade Range)
  • Oceanic-oceanic convergence results in the subduction of one plate beneath the other, forming island arcs and deep-sea trenches (Mariana Islands)
  • Continental-continental convergence leads to the formation of large mountain ranges (Himalayas)
• Transform boundaries occur where two plates slide past each other horizontally, often resulting in significant earthquakes (San Andreas Fault)
• Plate boundary types influence the style of volcanism and the composition of magmas produced
• Divergent boundaries typically produce basaltic magmas, while convergent boundaries can produce a range of magma compositions from basaltic to rhyolitic
• Intraplate volcanism, such as hotspots, occurs within a plate rather than at a plate boundary

Volcano Formation and Structure

• Volcanoes form when magma rises through the crust and erupts onto the surface
• The structure of a volcano depends on the composition of the magma, the eruption style, and the environment in which it forms
• Stratovolcanoes are steep-sided, conical volcanoes composed of alternating layers of lava flows, volcanic ash, and cinders (Mount Fuji, Mount St. Helens)
  • Typically associated with subduction zones and have a wide range of magma compositions
• Shield volcanoes are broad, gently sloping volcanoes built from multiple layers of fluid lava flows (Mauna Loa, Olympus Mons)
  • Typically associated with hotspots or mid-ocean ridges and have basaltic magma compositions
• Cinder cones are small, steep-sided volcanoes built from ejected lava fragments called cinders or scoria (Parícutin, Mexico)
• Calderas are large, circular depressions formed by the collapse of a volcano's summit or the emptying of its magma chamber (Yellowstone, Crater Lake)
• Volcanic vents are openings at the Earth's surface through which volcanic materials (lava, tephra, and gases) are erupted
• Magma chambers are large reservoirs of molten rock beneath the surface that feed volcanic eruptions

Magma Composition and Properties

• Magma composition refers to the chemical makeup of the molten rock, which influences its properties and eruption style
• The three main types of magma are basaltic, andesitic, and rhyolitic, which differ in their silica content, viscosity, and gas content
• Basaltic magma has low silica content (45-52%), low viscosity, and low gas content, resulting in fluid lava flows and effusive eruptions (Kilauea, Hawaii)
• Andesitic magma has intermediate silica content (52-63%), moderate viscosity, and moderate gas content, often associated with stratovolcanoes and explosive eruptions (Mount Merapi, Indonesia)
• Rhyolitic magma has high silica content (>63%), high viscosity, and high gas content, leading to highly explosive eruptions and the formation of thick, viscous lava flows or domes (Yellowstone, USA)
• Magma viscosity increases with increasing silica content and decreasing temperature, affecting the ease with which magma can flow and degas
• Gas content and composition (primarily water vapor, carbon dioxide, and sulfur dioxide) influence the explosivity of an eruption
• Magma differentiation processes, such as fractional crystallization and assimilation, can change the composition of magma over time

Eruption Styles and Hazards

• Volcanic eruptions can be broadly classified as effusive or explosive, depending on the magma composition, viscosity, and gas content
• Effusive eruptions involve the relatively gentle outpouring of fluid lava, typically associated with basaltic magmas (Kilauea, Hawaii)
  • Hazards include lava flows, which can destroy infrastructure and vegetation, and lava fountains, which can eject molten material hundreds of meters into the air
• Explosive eruptions involve the violent fragmentation of magma and the ejection of tephra into the atmosphere, typically associated with andesitic to rhyolitic magmas (Mount Pinatubo, Philippines)
  • Hazards include pyroclastic flows, lahars, volcanic ash, and volcanic bombs
• Pyroclastic flows are high-speed, ground-hugging avalanches of hot ash, pumice, rock fragments, and volcanic gas that can travel hundreds of kilometers from the volcano
• Lahars are destructive mudflows or debris flows composed of volcanic ash, rock, and water from a volcano, often triggered by heavy rainfall or the melting of snow and ice
• Volcanic ash is fine-grained material ejected into the atmosphere during an explosive eruption, which can cause respiratory issues, damage infrastructure, and disrupt air travel
• Volcanic gases, such as sulfur dioxide and carbon dioxide, can pose health risks and contribute to climate change
• Secondary hazards, such as volcanic earthquakes and tsunamis, can also occur as a result of volcanic activity

Monitoring and Prediction Methods

• Volcanic monitoring involves the use of various techniques to detect changes in a volcano's behavior that may indicate an impending eruption
• Seismic monitoring uses seismometers to detect and locate earthquakes associated with magma movement and volcanic activity
  • Increased seismicity, particularly in the form of volcanic tremor, can signal the rise of magma towards the surface
• Ground deformation monitoring uses GPS, tiltmeters, and satellite imagery (InSAR) to measure changes in the shape of a volcano, which can indicate magma intrusion or chamber inflation
• Gas monitoring involves measuring the composition and emission rates of volcanic gases, such as sulfur dioxide and carbon dioxide, which can provide insights into the depth and volume of magma
• Thermal monitoring uses infrared cameras and satellite imagery to detect changes in heat flux, which can indicate the presence of magma near the surface
• Hydrological monitoring involves tracking changes in nearby water sources, such as springs, streams, and wells, which can be influenced by volcanic activity
• Geologic mapping and stratigraphic analysis provide insights into a volcano's eruptive history and can help identify patterns and recurrence intervals
• Eruption forecasting combines monitoring data with statistical models and expert judgment to estimate the likelihood and timing of future eruptions
  • However, precise predictions of eruption timing, duration, and magnitude remain challenging due to the complex nature of volcanic systems

Real-World Case Studies

• Mount St. Helens, USA (1980): A catastrophic explosive eruption that caused the largest landslide in recorded history and resulted in 57 deaths
  • Demonstrated the importance of volcanic monitoring and hazard assessment
• Eyjafjallajökull, Iceland (2010): An explosive eruption that produced an ash cloud that disrupted air travel across Europe for several weeks
  • Highlighted the far-reaching impacts of volcanic ash on modern society
• Kilauea, Hawaii (2018): An effusive eruption that caused significant damage to infrastructure and homes due to the slow-moving but persistent lava flows
  • Showcased the challenges of managing long-duration volcanic crises in populated areas
• Mount Pinatubo, Philippines (1991): The second-largest volcanic eruption of the 20th century, which produced massive pyroclastic flows and ash fall
  • Successful evacuation of thousands of people due to timely warnings based on monitoring data
• Nevado del Ruiz, Colombia (1985): An explosive eruption that triggered destructive lahars, resulting in the deaths of over 23,000 people in the town of Armero
  • Emphasized the need for effective risk communication and emergency preparedness in volcanic regions
• Yellowstone Caldera, USA (last eruption 640,000 years ago): A supervolcano with the potential for a catastrophic eruption in the future
  • Ongoing monitoring efforts aim to detect signs of renewed activity and assess the likelihood of future eruptions
• Taal Volcano, Philippines (2020): A phreatomagmatic eruption that generated a volcanic lightning storm and forced the evacuation of thousands of people
  • Demonstrated the complex interplay between magma, water, and the atmosphere in certain volcanic systems