A. Determination of the Nature of Eruptions

• Magama Composition
• Magma Temperature
• Amount of Dissolved Gases

• each of these affects the magma’s "viscosity" - the magma’s resistance to flow

• heating makes the magma less viscous (more fluid); as the magma cools it congeals and its mobility decreases

• viscosity is directly related to silica (SiO₂) content - high silica = high viscosity; low silica = low viscosity

• magmas which produce basaltic rocks contain ~ 50% silica; granitic magma contains ~ 70% silica; andesitic magma contains ~ 60% magma
• e.g., Hawaiian volcanoes - basaltic magma - very fluid lava (can travel 150 miles)

• the gas content of a magma determines its explosiveness

• basaltic magmas = low (1-2%) gas content
• andesitic magmas = intermediate (3-4%) gas content
• granitic magmas = high (4-6%) gas content

• at the higher pressures deeper inside the Earth, gases are dissoved in the magma
• as the magma moves to the near-surface environment, it encounters lower pressures and the dissolved gases are released and expand
• the basaltic (less viscous) magma does not impede (as greatly) the movement of these gases up and outward from the near-surface environment
• in contrast, the very viscous granitic magmas tend to prevent the gases from escaping the near-surface environment, and pressures build... until the volcano explodes !!!! (boom) e.g., Mt. St. Helens

• make-up 1-6% of magmas (by wt.)
• thousands of tons/day
• water vapor is greatest constituent
• outgassing of water vapor formed oceans

• Hawaiian Volcanoes: 70% water vapor; 15% carbon dioxide; 5% Nitrogen; 5% Sulfur > Sulfuric Acid

• Gases propel magma & create conduits for their escape to the surface - connect magma chamber to the surface

D.  Volcanic Landforms and Materials

• volcano - a landform at the end of a conduit or pipe which rises from below the crust & vents to the surface

• crater - a circular surface depression formed by volcanism; usually located at a volcanic vent or pipe

• lava (molton rock) - the magma which issues from the volcano

• tephra - pulverized rock and clastic material (rock fragments) ejected violently during a volcanic eruption

• > 1300 volcanoes world wide; < 600 are active; 70 (mostly inactive) volcanoes along western North America

• cinder cone - a landform comprised of tephra & scoria (a cindery rock), usually small & cone shaped - ht. not more than 450 m (1500 ft) - a truncated top

• formed from cinders during the course of a moderately explosive volcano
• caldera - the interior sunken portion of a volcanic crater; usually steep-sided & circular - sometimes contains a lake

• forms when the summit material on a volcanic mountain collapses inward after an eruption

E. Types of Volcanic Eruptions
 

Factors Affecting the Erruption:

• magma chemistry
• magma viscosity (resistance to flow)

• a relatively gentle eruption

• produce enormous amounts of lava

• characterized by low viscosity, basaltic magma (< 50% silica - thus, mafic); magma has a low gas content - gas readily escapes

• not explosive, thus little tephra

• as gases expand, jets of lava shoot upward

• effusive eruptions typically form "shield volcanoes" - a symmetrical mountain landform; gently sloped, gradually rising from the surrounding landscape to a summit crater - typical of the Hawaiian Islands (e.g., Mauna Loa)

• in rift areas - effusive eruptions send magma up through elongated fissures to the surface where it spreads out in vast sheets - e.g., the Columbia Plateau in the N.W. U.S. - "plateau basalts"

• are associated with subduction zones

• are violent & unpredictable eruptions

• the result of magma that is thicker, stickier, higher in gas content & silica (magma is more felsic)

• tends to produce blockages in the volcano’s conduit, trapping & compressing gases - leading to higher pressures inside the volcano > explosive eruption

• produces the "composite volcano" landform

• composite volcanoes have steep sides & are more conical than shield volcanoes

• when the explosion takes place, the top & sides of the mountain are often blown off - less lava, lots of "pyroclastics" (explosively ejected rock fragments)

F. Overview of Locations of Volcanic Activity

• a function of plate tectonics & "hot spots"

• oceanic plate - cont. plate (e.g., Mt. St. Helens)

• oceanic plate - oceanic plate (e.g., Japan)

• (e.g., Iceland) & continental rift zones

• (e.g., rift zone in East Africa)

(3) at hot spots (e.g., Hawaiian Islands)
 

G. Origin of Magma

• magma originates from solid rock in the crust and mantle

• how do we generate magma from solid rock?

• raise the temperature above the rock’s melting point
• one source of heat is that which is released by the decay of radioactive minerals found in the Earth’s mantle & crust
• additionally, as pressure decreases, the melting point decreases
• when water (and other fluids) are introduced, melting temperatures are lowered
• recall that silica rich rocks melt at a lower temperature than basaltic rocks - "partial melting"
• partial melting of rock often produces a granitic magma - produces most, if not all, magma

H. Plate Tectonics and Volcanic Activity

• most active volcanoes are associated with plate boundaries - spreading center, subduction zone, intraplate
• Earth’s ~ 600 active volcanoes are predominately near convergent plate boundaries

• heat from radioactive decay in the crust & mantle melts subducting oceanic crust, forming basaltic-andesitic magma, which moves upward (less dense) and:

• when it occurs in an ocean, it can produce a chain of volcanoes - "Island Arcs" (e.g., Aleutian Islands)
• when it occurs beneath continents, the magma assimilates silica-rich crustal material, forming a granitic-andesitic magma (e.g., Andes Mountains of South America)
• volcanoes that ring the Pacific ("Ring of Fire") are associated with subducted oceanic crust that contains abundant water (which lowers the melting point of rock); the resultant magma has a high water vapor content, which leads to "explosive" volcanoes - such as those of the Cascade Range in the Northwestern U.S. (eg., Mt. St. Helens)

• occurs in the middle of continental and oceanic plates (perhaps where the plates have been stretched/weakened)
• most likely the source is the partial melting of mantle rocks
• the basaltic magma may come from rising plumes of hot mantle material (e.g., Hawaii, Columbia Plateau basalts)
• or, when the extruded lavas are granitic in composition, silica rich continental crust may have been melted/incorportated in the rising plumes of basaltic magma from the mantle (e.g., Yellowstone region)

(3) Spreading Center Volcanism

• oceanic spreading centers produce the greatest volume of volcanic rock

• as the lithosphere pulls apart, the pressure on the underlying rocks is reduced, lowering the melting point of the mantle rocks (peridotite), and producing large quantities of basaltic magma, which moves upward through the cracks and fissures to the ocean floor and spreads out, or builds a volcanic cone (e.g., Surtsey)
• spreading center volcanism can also occur on continents - e.g., African Rift Zone

II. Volcanic Hazards
 
 

• Volcanoes pose risks to people and property

• The severity of the risk depends on the:

• Volcano and Lava type
• Climate
• Topography
• Population Density

• Humans and their property can be impacted by:

• Lava flows
• Tephra falls
• Blasts of gassy incandescent

• Volcanic mudflows (lahars)
• Poisonous gases

1. Lava Flows

• Are rarely life-threatening because they move so slowly

• They can however cause tremendous environmental and property damage

1. Tephra

• Tephra is pyroclastic material "(fire-broken") -- more specifically, it is airborne pyroclastic material. Tephra is comprised of lava fragments.

• Tephra forms in masses or drops of magma that quickly cool and solidify in the air before they can fall to the ground

• Tephra can also include pieces of older rock that were broken from the volcano interior during the explosion.

• Tephra can also include glassy particles

• Tephra varies in size from sand-sized (volcanic ash), up to boulder-sized pieces (volcanic bombs)

• Fine tephra (ash) can be deposited by the wind hundreds of kilometers to areas downwind of the eruption

• The volcanic glass in tephra is very abrasive

• Explosive eruptions that produce lots of Tephra can be very hazardous, causing:

• Structural damage to buildings
• Mechanical failures of engines in vehicles and airplanes
• Human burial and suffocation

• The damage from tephra falls is both immediate and long-lived

1. Pyroclastic Flows (Nuees Ardentes -- glowing cloud)

• Are an incinerating mixture of volcanic gas and volcanic debris

• Its temperature ranges between 700 and 1000 degrees Celsius

• It's most commonly associated with violent eruptions of andesitic volcanoes

• Pyroclastic flows are ground-hugging, and can move at velocities ranging to 90 mph

• They may extend vertically from the volcanoes summit, or laterally from a weak area on the flank of the cone (most dangerous because the hot gas is concentrated in one direction)

1. Lahars (Volcanic Mudflows)

• Lahars are mudflows comprised of a mixture of water and volcanic debris that move downslope

• Their viscosity is a function of their water content

• Viscous lahars have more force; less viscous lahars travel more quickly

• Water in lahars comes from, rain, groundwater, ice and snow, summit crater lakes, and water converted to water vapor and then precipitated out as rain (volcanic ash acts as condensation nuclei)

• Lahars move down slopes and along stream valleys, transporting trees, structural debris, and tremendous volumes of sediment, which often chokes river and stream channels

1. Volcanic Gases

• Gas is emitted during all eruptions, whether gentle or explosive

• Most of the gas released is water vapor, however, other gases include: carbon dioxide, sulfur dioxide, sulfur trioxide, carbon monoxide, hydrogen sulfide

• Sulfur dioxide and sulfur trioxide combine with water vapor to form sulfurous acid and sulfuric acid, respectively

• Carbon dioxide (because it's heavier than air) can be fatal when it pools in valleys

• Sulfur dioxide is converted to sulfuric acid downwind to produce volcanic fog (vog)

• Lava entering the sea reacts with seawater to produce hydrochloric acid haze (lava haze -- laze)

1. Predicting Eruptions

• To help predict eruptions volcanologists look for precursors -- minor events that warn of a major eruption to come

• No single precursor can predict accurately when a volcano will erupt; the more precursors, the greater the probability of an eruption

• Precursors:

• Bulging and tilting of the ground surface
• An increase in surface temperature
• The occurrence of earthquakes caused by magama movement or hydrothermal pressure
• Changes in volcanic gas composition

1. Volcanic Hazard Mitigation

• Evacuation is a necessity
• Lava diversion by explosion and barriers
• Cooling lava fronts