Intro to Geology Unit 4 Review: Volcanoes and Volcanic Hazards

What's the Deal with Volcanoes?

• Volcanoes are openings or ruptures in Earth's surface that allow hot magma, volcanic ash, and gases to escape from below
• Occur when magma rises through cracks or weaknesses in the Earth's crust
• Magma is molten rock beneath Earth's surface that forms from partial melting of rock in the upper mantle or lower crust
• Pressure builds up beneath the surface causing magma to rise through the rock
• As magma rises, bubbles of gas form inside it leading to increased pressure
• Volcanoes erupt when the pressure becomes too great, forcing magma to the surface
• Volcanic eruptions can be explosive or effusive (non-explosive) depending on magma composition and gas content
• Explosive eruptions occur when magma is viscous and trapped gases build up pressure until the magma violently shatters into ash and pumice

Types of Volcanoes: Not All Fire Mountains Are the Same

• Shield volcanoes have broad, gently sloping flanks built by multiple eruptions of fluid lava flows (Mauna Loa, Hawaii)
  • Produce relatively gentle effusive eruptions with low-viscosity basaltic magma
  • Lava flows easily for long distances from the vent
• Stratovolcanoes (composite volcanoes) are steep-sided symmetrical cones built of alternating layers of lava flows, volcanic ash, and cinders (Mount Fuji, Japan)
  • Erupt periodically with viscous, gas-rich magma that produces explosive eruptions
  • Lava flows are thicker and shorter, alternating with layers of ash and cinders
• Cinder cone volcanoes are simple structures built from particles and blobs of congealed lava ejected from a single vent (Parícutin, Mexico)
  • Relatively small, usually less than 300 meters tall
  • Form from explosive eruptions of gas-charged magma that solidifies into cinders
• Lava domes are rounded, steep-sided mounds built by viscous magma that piles up around the vent (Lassen Peak, California)
  • Magma is too viscous to flow far so it piles up over the vent
  • Lava domes often form within the craters or on the flanks of stratovolcanoes
• Calderas are large circular depressions formed by the collapse of a volcano into an empty magma chamber (Crater Lake, Oregon)
  • Form after massive explosive eruptions drain the underlying magma chamber causing the volcano's summit to collapse inward

How Volcanoes Form: Earth's Fiery Kitchen

• Volcanoes form at plate boundaries where tectonic plates move apart or collide with each other
• Divergent boundaries occur where plates pull apart, allowing magma to rise up and erupt (mid-ocean ridges)
  • As plates separate, pressure is reduced on the mantle below allowing it to melt and form magma
• Convergent boundaries happen where plates collide, forcing one plate to subduct beneath the other
  • Subducting plate sinks into the mantle, releasing water from hydrated minerals
  • Water lowers the melting point of the overlying mantle wedge, generating magma
• Magma is less dense than surrounding rock so it rises through the mantle and crust
• As magma rises, decreasing pressure allows dissolved gases to form bubbles
• Increasing gas bubbles create pressure, potentially leading to an explosive eruption
• Hotspots are areas where plumes of hot mantle rock rise up far from plate boundaries (Hawaii, Yellowstone)
  • Hotspots remain stationary while the tectonic plate moves over them
  • Produce a chain of volcanoes as the plate slowly shifts over the hotspot

Anatomy of a Volcano: From Magma to Lava

• Magma chamber is a pool of molten rock beneath the surface where magma collects before erupting
• Vents are openings at the Earth's surface through which volcanic materials (lava, ash, gases) are erupted
  • Primary vents open downward to the magma chamber
  • Secondary vents are smaller and form on the volcano's flanks
• Central vent is the main opening at the summit of a volcano, often within a crater
• Crater is a bowl-shaped depression around the central vent formed by volcanic explosions or collapse
• Conduit (pipe) is the channel through which magma travels from the magma chamber to the surface
• Lava is magma that reaches the Earth's surface through a volcano or fissure
  • Lava flows are outpourings of lava that flow downslope from a vent or fissure
  • Lava composition (silica content) and temperature determine its viscosity and flow behavior
• Volcanic ash consists of fine particles of pulverized rock and glass created by explosive fragmentation of magma or rock
• Volcanic gases include water vapor, carbon dioxide, sulfur dioxide, and other volatiles released from magma before and during eruptions

Eruption 101: When Things Get Explosive

• Volcanic eruptions occur when magma and gases are discharged from a vent to the Earth's surface
• Eruption type depends on magma composition, viscosity, gas content, and how easily gases can escape
• Effusive eruptions involve gentle outflows of basaltic lava with low viscosity and gas content (shield volcanoes)
  • Lava flows easily forming thin, widespread sheets or rivers of molten rock
  • Gas escapes easily so pressure doesn't build up, resulting in a less explosive eruption
• Explosive eruptions happen when viscous, gas-rich magma fragments violently into ash and pumice (stratovolcanoes)
  • High silica content and trapped gases create tremendous pressure leading to explosive fragmentation
  • Volcanic ash, cinders, pumice, and gas are ejected high into the atmosphere
• Pyroclastic flows are ground-hugging avalanches of hot ash, pumice, rock fragments, and volcanic gas rushing down the side of a volcano (Montserrat, 1997)
  • Can travel at hurricane velocity (100-200 km/hr) and reach temperatures of 1,000°C
• Lahars (volcanic mudflows) are mixtures of volcanic ash and water from melted snow or heavy rains that flow down river valleys (Nevado del Ruiz, Colombia 1985)
  • Consistency of wet concrete, capable of carrying large boulders and destroying buildings/bridges
• Volcanic gases can be released during and between eruptions, sometimes causing localized asphyxiation (Lake Nyos, Cameroon 1986)
  • Common gases include water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, and hydrogen halides

Volcanic Hazards: Nature's Deadly Special Effects

• Lava flows are streams of molten rock that pour from a vent and travel downslope
  • Basaltic lava flows advance slowly allowing people to escape but destroy everything in their path
  • More viscous lava flows (andesite, rhyolite) move more slowly but are thicker and can create lava domes
• Pyroclastic density currents (pyroclastic flows and surges) are hot, fast-moving avalanches of rock, ash, and gas
  • Pyroclastic flows are ground-hugging, fluidized masses that travel at high speeds (100-700 km/hr)
  • Pyroclastic surges are more dilute, turbulent clouds that can surmount topographic barriers
• Tephra includes volcanic ash, cinders, and pumice ejected into the atmosphere during an explosive eruption
  • Volcanic ash consists of fine particles (<2 mm) of pulverized rock that can cause respiratory problems and damage aircraft engines
  • Larger tephra (cinders, pumice) falls back to Earth more quickly but can still cause damage
• Volcanic gases can be toxic, corrosive, or asphyxiating depending on their composition and concentration
  • Sulfur dioxide can react with water to form acid rain, damaging vegetation and infrastructure
  • Carbon dioxide is heavier than air and can accumulate in low-lying areas causing asphyxiation
• Lahars are volcanic mudflows formed by mixing volcanic ash with water, creating a concrete-like slurry
  • Can travel long distances down river valleys, destroying bridges, buildings, and infrastructure
• Volcanic earthquakes and tremors can occur before and during eruptions as magma forces its way to the surface
  • Can cause ground shaking, surface faulting, and trigger landslides or tsunamis

Living with Volcanoes: Risks and Benefits

• Over 500 million people worldwide live in the shadow of active volcanoes
• Volcanic soils are often fertile due to weathering of volcanic ash and minerals, supporting agriculture
  • Volcanic ash adds nutrients like potassium, phosphorus, and sulfur to the soil
  • Andisols (volcanic soils) cover 1% of Earth's surface but support 10% of the world's population
• Geothermal energy can be harnessed from volcanically active areas to generate electricity and heat homes
  • Iceland generates 25% of its electricity and heats 90% of its homes using geothermal resources
• Volcanic materials are used as building materials, such as volcanic ash for cement and pumice for lightweight concrete
• Volcanoes attract tourists, providing economic benefits to local communities (Hawaii Volcanoes National Park)
• Volcanic eruptions can have devastating impacts on nearby communities
  • Destroy homes, infrastructure, and agricultural land
  • Cause fatalities through direct impact or secondary hazards like lahars and famine
• Monitoring and early warning systems can help mitigate risks by providing time for evacuation
  • Seismic monitoring detects earthquake swarms that may indicate impending eruptions
  • Gas monitoring tracks changes in the composition and amount of gases released from a volcano
• Hazard maps delineate zones at risk from various volcanic hazards to guide land-use planning and evacuation routes
• Education and outreach help communities understand volcanic hazards and how to respond during an emergency

Famous Volcanoes: Earth's Hall of Fame (and Infamy)

• Mount Vesuvius, Italy: Buried the Roman cities of Pompeii and Herculaneum in 79 AD
  • Pyroclastic flows and ash fallout preserved buildings and human remains in remarkable detail
• Krakatoa, Indonesia: Explosive eruption in 1883 destroyed most of the island and created global cooling
  • Generated tsunamis up to 40 meters high and audible blast heard 4,800 km away in Australia
• Mount St. Helens, Washington: Catastrophic eruption in 1980 flattened forests and triggered massive lahars
  • Landslide from the collapse of the volcano's north flank was the largest in recorded history
• Kilauea, Hawaii: World's most active volcano, known for its continuous effusive eruptions and lava flows
  • Pu'u 'O'o eruption (1983-2018) added 570 acres of new land to the island and buried 48 sq miles
• Mount Pinatubo, Philippines: Second-largest volcanic eruption of the 20th century in 1991
  • Produced voluminous ash fallout and extensive lahars exacerbated by a typhoon following the eruption
• Yellowstone Caldera, Wyoming: Supervolcano with a history of massive explosive eruptions
  • Last supereruption 640,000 years ago ejected 1,000 cubic kilometers of ash and created the 45x85 km caldera
• Tambora, Indonesia: Largest volcanic eruption in recorded history (1815), causing global cooling and the "Year Without a Summer"
  • Eruption column reached 43 km high and ejected 150 cubic kilometers of ash and pumice
• Eyjafjallajökull, Iceland: 2010 eruption disrupted European air travel for weeks due to ash plumes
  • Pronounced "AY-yah-fyah-lah-YOH-kuul," the name translates to "island mountain glacier"