Intro to Geology

⛏️Intro to Geology Unit 4 – Volcanoes and Volcanic Hazards

Volcanoes are Earth's fiery vents, allowing magma and gases to escape from below. These geological wonders form when molten rock rises through cracks in the crust, with pressure building until eruption occurs. Volcanic activity shapes landscapes and impacts climate. Different volcano types exist, from gently sloping shield volcanoes to steep stratovolcanoes. They form at plate boundaries or hotspots, each with unique structures and eruption styles. Understanding volcanoes is crucial for assessing hazards and harnessing their benefits, from fertile soils to geothermal energy.

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"


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.