Introduction to Volcanoes and Their Formation

Study Notes

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).

Description

This quiz explores the dynamic features of volcanoes, focusing on their formation, behavior, and the processes that lead to eruptions. It covers concepts such as volcanic arcs, plate boundaries, and the role of volcanologists in studying these geological phenomena. Test your understanding of how and where volcanoes form both on land and underwater.