Vulcanicity: Intrusive, Extrusive, and Shield Volcanoes

• Vulcanicity refers to volcanic activity on or within the Earth's crust.

Intrusive Vulcanicity

• Refers to volcanic activity within the Earth's crust
• Involves magma movement along lines of weakness like faults and joints.
• Cooling magma forms igneous rocks with features like sills, dykes, laccoliths, batholiths

Extrusive Vulcanicity

• Refers to volcanic activity on the Earth's surface.
• Involves emission of volcanic materials through vents or fissures
• Extruded materials: gases (sulfur dioxide, hydrogen, steam etc), liquid lava, and solids (rock fragments, ash)

Shield Volcanoes

• Special volcanoes built from fluid lava flows
• Named for their large shield-like shape
• Gentle eruptions with highly fluid lava that travels far

Viscous Lava

• Thick, sticky lava which clogs easily
• Tends to resist flow, making it virtually immobile
• Entrapped gases in vesicles (bubbles) often lead to explosive eruptions

Basic Lava

• High in iron and magnesium, low in silica (40%)
• Fluid at around 1200 degrees Celsius
• Flows farther than acidic lava, forms gently sloping shield volcanoes like basalt or granite

Acidic Lava

• High in silica (over 60%)
• Viscous and virtually immobile
• Solidifies quickly to form steeply sloping dome volcanoes

Fissure & Vent

• Fissure: a long vertical crack in the Earth's crust, caused by faulting
• Vent: an opening in the Earth's crust for volcanic material emission

Crater & Parasitic Cone

• Crater: bowl-shaped depression at a volcano's summit, the exit point for materials
• Parasitic Cone: a subsidiary cone formed from a fracture, not the central vent

Magma Chamber & Pyroclastic Flow

• Magma Chamber: a large underground reservoir of molten rock under pressure
• Pyroclastic Flow: a fluidized mixture of solid/semi-solid volcanic material and hot gases flowing rapidly down a volcano

Tephra & Lahar

• Tephra: fragmented rock ejected during explosive eruptions
• Lahar: a hot or cold mixture of water and volcanic materials flowing down a volcano, like wet concrete

Nue’ Ardente & Plutonic Rocks

• Nue’ Ardente: a destructive, fast-flowing mass of gas-enveloped particles down a volcano
• Plutonic Rocks: igneous rocks formed underground from magma intrusion

Volcanic Bombs & Pumice

• Volcanic Bombs: fragments of fluid lava/rock (>64 mm) blasted into the air
• Pumice: solidified frothy lava created from super-heated, pressurized rock

Volcano Definition & Location

• Volcano: a natural vent/fissure linking the Earth's interior
• Volcanoes occur at convergent/divergent plate margins and 'hot spots'

Stages of a Volcano's Life Cycle

• Active Volcano: constantly erupting/frequent eruptions with seismic activity, examples: Soufriere Hills, Kick-em-Jenny
• Dormant Volcano: not erupted in a long time but still capable, examples: Mauna Kea, Mount Misery
• Extinct Volcano: not erupted in historic times, no seismic activity, will not erupt again, examples: Morne Diablotins, Kilimanjaro

Types of Volcanic Cones

• Some geographers classify volcanic cones into shield volcanoes and composite volcanic cones, but there are three main types

Ash and Cinder Cone

• Steeply sloping, made of ash and cinder particles
• Formed from volcanic rocks thrown into the air during eruptions

Composite Volcanic Cone (Stratovolcano)

• Tall, conical, with alternating layers of lava and volcanic ash
• Often contains tephra and pumice
• Formed from violent and gentle eruptions

Lava Cones

• Consists of two types
• Basic Lava Cones: Made of basic lava, shield volcanoes such as Mauna Loa
• Viscous Lava Cones: Formed from Acidic lava and are dome volcanoes such as Soufriere Hills

Intrusive Volcanic Features

• A Sill: An intrusion of magma along the bedding plane of rocks, forms a sheet-like mass of magma
• A Dyke: Intrusion of magma across the bedding planes of volcanic rock, forms a wall-like mass of magma
• A Laccolith: A mass of magma which lies relatively close to the earths surface
• A Batholith: Large irregular shaped mass of igneous rock, usually granite

Extrusive Volcanic Features

• A Caldera: Huge crater-like depression which marks the site of a volcano
• Hot (Sulphur) Springs: Are formed when super heated water flows naturally from rocks
• A Lava Plateau: Is formed during fissure eruption
• A Geyser: Is a jet of superheated water and stream

Volcano Eruption

• Occurs when pressure builds up in the magma chamber
• Magma is forced upwards to the surface where weakness exists
• Magma is forced through a vent
• Eruption size depends on: magma amount, gas amount, silica amount and water amount

Gentle Eruption

• Low water and silica content = gentle eruptions
• Little water means no explosive potential
• Lack of silica results in fluid, mobile lava

Explosive Eruption

• High silica content: viscous lava plugs the vent, pressure builds and is released explosively
• High water content: creates steam leading to explosive eruptions if silica content is high
• Pumice accumulation can block magma pipes and lead to a violent eruption when pressure is released