Introduction to Volcanoes and Their Formation

Introduction

• Volcanoes, dynamic features creating over 80% of Earth's surface above and below sea level, are found on every continent, including Antarctica.
• Volcanoes are locations where lava erupts onto Earth's surface, either underwater (subaqueous eruption) or on land (sub-aerial eruption).
• Volcanologists study volcanic behavior by monitoring emitted volcanic gases, ground deformation, and seismic activity.

Forming Volcanoes

• Volcanoes form above areas where mantle melts due to decompression melting or water addition.
• Most active volcanism occurs at divergent plate boundaries, especially mid-ocean ridges.
• Land volcanism occurs along subduction zones and above mantle plumes.
• Volcanoes form along a curved line (volcanic arc) at subduction zones, where the subducted slab releases chemically bound water into the mantle.
• Island arcs form when the overriding plate is oceanic crust; continental arcs form when it's continental crust.
• Hot spot volcanoes are the only type that form outside plate boundaries.
• Continental volcanoes are explosive, erupting felsic lava; oceanic volcanoes are effusive, erupting mafic lava.

Types of Volcanoes

• Scoria cones (Cinder cones):
  • Smallest type (12-1,000 ft tall), characterized by high dissolved gas content and rapid growth.
  • Powered by dissolved gas pressure, which solidifies in the new rock.
  • Eruptions are spattery and explosive, forming ash and cinders.
  • Made of loose cinders, easily eroded (e.g., Parícutin, Mexico).
• Composite volcanoes (Stratovolcanoes):
  • Typically form at subduction zones, especially continental arcs.
  • Can reach up to 8,000 ft tall with steep slopes.
  • Alternating layers of lava flows and tephra with a central vent.
  • May produce lava flows, forming lava domes that plug the vent..
  • Can erupt explosively due to high water content in the lava.
  • Explosive eruptions cause pyroclastic flows (hot, fast-moving masses of ash, rock, and gas) (e.g., Mount St. Helens).
• Shield volcanoes:
  • Among the largest volcanoes, often formed by mantle plume activity.
  • Extrusion of runny basaltic lava from small vents and fissures.
  • Gently sloping, with diameters of 3-4 miles and heights up to 2,000 feet (e.g., Hawaiian Islands).
• Large igneous provinces (LIPs):
  • Massive igneous rock deposits (millions of cubic kilometers) erupted lava.
  • Often associated with mass extinction events (e.g., Siberian Traps, end-Permian; Deccan Traps, end-Cretaceous).
  • Formed by mantle plumes (e.g., Columbia River Basalt).
• Seafloor volcanism:
  • Occurs along mid-ocean ridges and rift zones.
  • Primarily basaltic lava erupted from linear volcanoes or fissures.
  • A primary source of Earth's lava.
  • Influences ocean chemistry via hydrothermal circulation.
• Kimberlite pipes:
  • Deep-seated magma conduits, bringing ultramafic magma from deep Earth.
  • Violent explosive eruptions rapidly carrying mantle material.
  • Frequently contain diamonds and high-pressure minerals.
  • Provide samples of the asthenosphere to geologists.

Volcanic Structure and Activity

• Volcanoes are primarily layered ash and lava flows, with eruptions occurring from vents.
• Less consolidated materials (like ash) are often weak and prone to collapse.
• More solidified volcanoes (made mostly of lava flows) usually are more structurally sound.
• Two lava types:
  • Pahoehoe: Less viscous, higher-temperature lava from basaltic volcanoes.
  • A'a: More viscous, blocky lava, flows slowly, often breaks apart.
• Besides lava, volcanoes erupt solid material (tephra), from ash to volcanic bombs.
• Explosivity:
  • Explosive volcanoes erupt silica-rich felsic lava with high dissolved gases.
  • Effusive volcanoes erupt mafic lava at higher temperatures with lower dissolved gases.

Volcanic Hazards

• Sulfuric gas emissions: Cause respiratory problems and acid rain.
• Tephra: Airborne sharp ash damages buildings and infrastructure.
• Ash: Lung-filling ash leads to breathing difficulties and health problems.
• Molten lava: Destroys property and causes damage.
• Pyroclastic flows: Hot, fast-moving ash-gas flows, extremely dangerous.
• Lahars: Fast-moving mudflows (volcanic ash and water), cause widespread destruction.

Monitoring Volcanic Activity

• Volcanologists monitor volcanoes for eruptions by:
  • Using seismometers.
  • Measuring ground deformation with satellites.
  • Detecting gas leaks.
  • Observing increased seismic activity.
  • Identifying phreatic eruptions (steam released from volcanic vents).