VolcanoesVolcanoes

Parkland High SchoolParkland High School

Volcanoes and Plate TectonicsVolcanoes and Plate Tectonics

• Connection: plate motions provide the mechanisms Connection: plate motions provide the mechanisms by which mantle rocks melt to generate magmaby which mantle rocks melt to generate magma

• Most form along plate boundariesMost form along plate boundaries• 80% along convergent boundaries80% along convergent boundaries• 15% along divergent boundaries15% along divergent boundaries• 5% far from boundaries (intraplate volcanism) as a 5% far from boundaries (intraplate volcanism) as a

result of hot spots (ex: Hawaiian islands) result of hot spots (ex: Hawaiian islands) • Unusually hot regions of the mantle (mantle Unusually hot regions of the mantle (mantle

plumes)plumes)• As plates move over them, a trail of progressively As plates move over them, a trail of progressively

older volcanoes formolder volcanoes form

Nature of Volcanic EruptionsNature of Volcanic Eruptions

• Factors that affect the magma extrusion (violent Factors that affect the magma extrusion (violent or gentle)---see Table 8.1 on pg. 229or gentle)---see Table 8.1 on pg. 229• Magma’s compositionMagma’s composition• Its temperatureIts temperature• Amount of dissolved gases it contains and the Amount of dissolved gases it contains and the

ease at which they can escapeease at which they can escape• Volatiles provide force that extrudes the molten rock Volatiles provide force that extrudes the molten rock

• Magma’s viscosity (sticky)Magma’s viscosity (sticky)• More viscous More viscous greater resistance to flow greater resistance to flow• Higher temperature Higher temperature lower viscosity lower viscosity• Directly related to silica content (more silica Directly related to silica content (more silica greater greater

viscosityviscosity

ExtrusionsExtrusions

• Lava flowsLava flows• GasesGases• Pyroclastic MaterialsPyroclastic Materials

Lava FlowsLava Flows

• Basaltic lavas are very fluid (low silica)—can Basaltic lavas are very fluid (low silica)—can travel 10-300 m/hr down steep slopestravel 10-300 m/hr down steep slopes• Two typesTwo types

• PahoehoePahoehoe• Smooth, braided ropeSmooth, braided rope• ThinThin• Fast moving Fast moving

• AaAa• Rough, jagged, spiny projectionsRough, jagged, spiny projections• Cool, thickCool, thick• Slow movingSlow moving

Aa LavaAa Lava

Pahoehoe LavaPahoehoe Lava

GasesGases

• Magma contains various amounts of Magma contains various amounts of volatiles held in by confined pressure volatiles held in by confined pressure (think of a soda bottle)---as soon as (think of a soda bottle)---as soon as pressure is reduced, gases escapepressure is reduced, gases escape

• Made up of 1-6% of total weight of Made up of 1-6% of total weight of magma (most is water vapor)magma (most is water vapor)

• May cause air pollution (gases form May cause air pollution (gases form sulfuric acid)sulfuric acid)

Pyroclastic MaterialsPyroclastic Materials

• Gases may propel lava into the air—Gases may propel lava into the air—may land near volcano or be carried by may land near volcano or be carried by windwind

• Rhyolitic magma contains high Rhyolitic magma contains high quantities of gases and blows quantities of gases and blows fragments from the volcanofragments from the volcano

• Fragments called pyroclastic materialsFragments called pyroclastic materials• Range in size from dust to pieces that weight Range in size from dust to pieces that weight

tonstons

Continued…Continued…

• Dust (less than 0.25 mm)Dust (less than 0.25 mm)• Ash (less than 2mm)Ash (less than 2mm)• Lapilli or Cinders (2-64mm)Lapilli or Cinders (2-64mm)• Blocks (larger than 64mm ejected in solid Blocks (larger than 64mm ejected in solid

state)state)• Bombs (larger than 64mm ejected in Bombs (larger than 64mm ejected in

molten statemolten state

Anatomy of a VolcanoAnatomy of a Volcano

• Fissure---crack in the crust; begins volcanic Fissure---crack in the crust; begins volcanic activity as magma moves toward surfaceactivity as magma moves toward surface

• Conduit or pipe—magma flows up itConduit or pipe—magma flows up it• Vent---opening of pipe at the surfaceVent---opening of pipe at the surface• Lava and pyroclastic material---builds up the Lava and pyroclastic material---builds up the

structurestructure• Crater---steep-walled depression at the Crater---steep-walled depression at the

summitsummit• Parasitic cone----small secondary vent (may Parasitic cone----small secondary vent (may

be called fumaroles if only emit gases)be called fumaroles if only emit gases)

Types of VolcanoesTypes of Volcanoes

• Based on the composition of the Based on the composition of the contributing magmacontributing magma

• Three TypesThree Types• ShieldShield• Cinder coneCinder cone• CompositeComposite

Shield VolcanoesShield Volcanoes

• Produced by the accumulation of basaltic lavaProduced by the accumulation of basaltic lava• Form layer upon layer of lava during non-Form layer upon layer of lava during non-

explosive eruptionsexplosive eruptions• Shape: broad, gentle slope and slightly domedShape: broad, gentle slope and slightly domed• LargestLargest• Many form islands (Hawaii and the Galapagos) Many form islands (Hawaii and the Galapagos)

(largest volcano is Mauna Loa—read pg. 233) (largest volcano is Mauna Loa—read pg. 233) (most active is Kilauea---read pg. 234)(most active is Kilauea---read pg. 234)

Cinder Cone VolcanoesCinder Cone Volcanoes

• Also called scoria cones (why??)Also called scoria cones (why??)• Built from ejected pyroclastic fragments that Built from ejected pyroclastic fragments that

look like cinders (range in size again from look like cinders (range in size again from dust to bombs) that fall back and pile updust to bombs) that fall back and pile up

• Steep sides with large, deep cratersSteep sides with large, deep craters• Small because of short life span (only lasts a Small because of short life span (only lasts a

few weeks—magma in pipe solidifies) few weeks—magma in pipe solidifies) • ExplosiveExplosive• Can be found as parasitic conesCan be found as parasitic cones

Composite VolcanoesComposite Volcanoes

• Picturesque but dangerousPicturesque but dangerous• Also called stratovolcanoesAlso called stratovolcanoes• Most located in the Ring of Fire (circles the Most located in the Ring of Fire (circles the

Pacific Ocean)Pacific Ocean)• Includes Mount St. HelensIncludes Mount St. Helens• Formed when layers of pyroclastic fragments Formed when layers of pyroclastic fragments

alternate with lava (mostly rich in gas and alternate with lava (mostly rich in gas and andesitic in nature)---can occur andesitic in nature)---can occur simultaneouslysimultaneously

Composite Continued…Composite Continued…

• Thick lava that travels short distancesThick lava that travels short distances• Violent explosive eruptionsViolent explosive eruptions• Steep summit with gradually sloping Steep summit with gradually sloping

flanks (broad base)---early lava flows flanks (broad base)---early lava flows greater distances than latergreater distances than later

Life Cycle of a VolcanoLife Cycle of a Volcano

• 1.1. Active VolcanoActive Volcano- is a volcano that is - is a volcano that is erupting now, has erupted in the recent past, erupting now, has erupted in the recent past, or shows signs of erupting in the near future.or shows signs of erupting in the near future.

• 2.2. Dormant VolcanoDormant Volcano- is a volcano that has - is a volcano that has not erupted recently, but has during recorded not erupted recently, but has during recorded history. It is expected to erupt again in the history. It is expected to erupt again in the future. future.

• 3.3. Extinct VolcanoExtinct Volcano- is one that has not - is one that has not erupted in recorded history. It is unlikely to erupted in recorded history. It is unlikely to erupt again.erupt again.

CalderasCalderas

• When the side of the volcano collapses When the side of the volcano collapses or caves into the magma chamber, it or caves into the magma chamber, it forms a calderaforms a caldera

• Crater Lake, OR is located in a calderaCrater Lake, OR is located in a caldera• Cinder cone volcano in middle of lake is Cinder cone volcano in middle of lake is

called Wizard Islandcalled Wizard Island