Why do volcanoes have different eruptive styles??? styles??? Factors influencing eruptions dependant on the magma’s viscosity dependant on the magma’s viscosity high viscosity –”pasty” explosive high viscosity –”pasty” explosive low viscosity –”fluid” flows easily low viscosity –”fluid” flows easily Factors influencing eruptions dependant on the magma’s viscosity dependant on the magma’s viscosity high viscosity –”pasty” explosive high viscosity –”pasty” explosive low viscosity –”fluid” flows easily low viscosity –”fluid” flows easily Factors influencing viscosity Temperature of magma Temperature of magma T viscosity = fluid flow T viscosity = fluid flow T viscosity = pasty flow T viscosity = pasty flow Factors influencing viscosity Temperature of magma Temperature of magma T viscosity = fluid flow T viscosity = fluid flow T viscosity = pasty flow T viscosity = pasty flow Chemical composition Chemical composition SiO 2 content (high or low) SiO 2 content (high or low) Chemical composition Chemical composition SiO 2 content (high or low) SiO 2 content (high or low) mafic composition: (50% SiO 2 )=“fluid” flow intermediate comp.: (60% SiO 2 ) felsic composition: (70% SiO 2 ) =“pasty” flow high viscosityhigh viscosity high SiO 2high SiO 2 felsicfelsic “pasty”“pasty” explosiveexplosive high viscosityhigh viscosity high SiO 2high SiO 2 felsicfelsic “pasty”“pasty” explosiveexplosive low viscositylow viscosity low SiO 2low SiO 2 maficmafic “fluid”“fluid” non-explosivenon-explosive low viscositylow viscosity low SiO 2low SiO 2 maficmafic “fluid”“fluid” non-explosivenon-explosive A B
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On Sunday, May 18, 1980, the largest volcanic eruption On Sunday, May 18, 1980, the largest volcanic eruption to occur in North American historic times transformed a to occur in North American historic times transformed a picturesque volcano into a decapitated remnant. On picturesque volcano into a decapitated remnant. On this date in southwestern Washington State, Mount St. this date in southwestern Washington State, Mount St. Helens erupted with tremendous force. Helens erupted with tremendous force. What happened?? What happened?? • Approximately 1 km Approximately 1 km 3 3 of ash erupted. of ash erupted. • Summit decreased by 1,350 feet. Summit decreased by 1,350 feet. • Claimed 59 lives Claimed 59 lives • Ash propelled 11 miles into the atmosphere. Ash propelled 11 miles into the atmosphere. • Ash covered surrounding areas of Yakima, Tri- Ash covered surrounding areas of Yakima, Tri- cities, and cities, and northern Oregon for 3 days – Noon felt like northern Oregon for 3 days – Noon felt like night. night. • Feb. 1981- highest birth rate in Portland and Feb. 1981- highest birth rate in Portland and surrounding areas –TRUE FACT surrounding areas –TRUE FACT Advice from the authorities: Advice from the authorities: Mt. St. Helens Mt. St. Helens Before Before After After
Transcript
On Sunday, May 18, 1980, the largest volcanic eruption to occur in On Sunday, May 18, 1980, the largest volcanic eruption to occur in North American historic times transformed a picturesque volcano into North American historic times transformed a picturesque volcano into a decapitated remnant. On this date in southwestern Washington a decapitated remnant. On this date in southwestern Washington State, Mount St. Helens erupted with tremendous force.State, Mount St. Helens erupted with tremendous force.
What happened??What happened??•Approximately 1 kmApproximately 1 km3 3 of ash erupted.of ash erupted.•Summit decreased by 1,350 feet.Summit decreased by 1,350 feet.•Claimed 59 livesClaimed 59 lives•Ash propelled 11 miles into the atmosphere.Ash propelled 11 miles into the atmosphere.•Ash covered surrounding areas of Yakima, Tri-cities, and Ash covered surrounding areas of Yakima, Tri-cities, and northern Oregon for 3 days – Noon felt like night.northern Oregon for 3 days – Noon felt like night.• Feb. 1981- highest birth rate in Portland and surrounding Feb. 1981- highest birth rate in Portland and surrounding areas –TRUE FACTareas –TRUE FACT
Advice from the authorities: Advice from the authorities: If there is another major eruption, put your head between yourIf there is another major eruption, put your head between yourlegs and kiss your ash goodbye!legs and kiss your ash goodbye!
Mt. St. HelensMt. St. HelensBeforeBefore AfterAfter
The “buzzword” is VISCOSITYWhat is viscosity?
Viscosity = how well a material flowsmore viscous – flows very slowly
(high viscosity)
less viscous – flows quickly(low viscosity)
Does glass have viscosity?55
Why do volcanoes have different eruptiveWhy do volcanoes have different eruptivestyles???styles???
Factors influencing eruptionsFactors influencing eruptions• dependant on the magma’s viscositydependant on the magma’s viscosity
Dissolved gasses – influencing the movement of magmaDissolved gasses – influencing the movement of magma(volatiles – water, CO(volatiles – water, CO22, SO, SO22….) ….) Silica contentSilica content and and volatilesvolatiles erupt two types of materials: erupt two types of materials:
Dissolved gasses (volatiles)Dissolved gasses (volatiles)• 1-6% of total magma wt.1-6% of total magma wt.• contributes to atmospherecontributes to atmosphere
PahoehoePahoehoe AaAa
Types of Basaltic Lava FlowsTypes of Basaltic Lava Flows(low silica (SiO(low silica (SiO22) content)) content)
• very fluid, thin, very fluid, thin, broad sheetsbroad sheets• flows 10-300 km/hrflows 10-300 km/hr (30-900 ft/hr)(30-900 ft/hr)•high volatile gas contenthigh volatile gas content• smooth “skin,” ropeysmooth “skin,” ropey type flowtype flow
• very “pasty,” sticky,very “pasty,” sticky, thick, cool flowsthick, cool flows• flows 5-50 m/hrflows 5-50 m/hr (15-150 ft/hr)(15-150 ft/hr)•low volatile gas contentlow volatile gas content• rough, blocky, sharp,rough, blocky, sharp, angular type flowangular type flow
Pyroclastic materialsPyroclastic materials
VolcanicBombs
BombsBombs
Lahars mud flows
LaharsLahars
Ash
AshAsh
Nuee-Ardente
Nuee-ArdenteNuee-Ardente
Ryholitic magmasRyholitic magmas• high silicahigh silica• very explosivevery explosive• thick, pastythick, pasty• high viscosityhigh viscosity• pyroclastic ejectionspyroclastic ejections
Four (4) types of Volcanoes
shield
composite(stratovolcano)
cinder cone
Explain the differences.
•Volcano type is dependant on SiO2 content.
plugged dome
Shield Volcano - HawaiiShield Volcano - Hawaii
Shield VolcanoesShield Volcanoes•Hawaiian Islands, Iceland, Galapagos IslandsHawaiian Islands, Iceland, Galapagos Islands•commonly rise from the deep ocean floorcommonly rise from the deep ocean floor
•formed by the accumulation of fluid basaltic flowsformed by the accumulation of fluid basaltic flows•low silica content (basaltic composition)low silica content (basaltic composition)
•low viscositylow viscosity•less than 1% pyroclastic debrisless than 1% pyroclastic debris•non-explosive eruptionsnon-explosive eruptions
•Western U.S. coast, Western South American coast, JapanWestern U.S. coast, Western South American coast, Japan• typically form in the ocean along continent convergent boundariestypically form in the ocean along continent convergent boundaries• found along the ring of firefound along the ring of fire
Steep high angle flanks
•Formed from Formed from layeringlayering deposits of deposits of ash, lava, and pyroclastic flowsash, lava, and pyroclastic flows•High silica content (70%)- (Rhyolitic High silica content (70%)- (Rhyolitic composition)composition)
•deadly airborne debrisdeadly airborne debris•Explosive eruptions – very hazardousExplosive eruptions – very hazardous
Cinder ConesCinder Cones• Exist all over the Earth’s surfaceExist all over the Earth’s surface• Typically, located in volcanic fields (Flagstaff AZ-600+) Typically, located in volcanic fields (Flagstaff AZ-600+)
•Formed by gas rich basaltic flows (low viscosity, low silica)Formed by gas rich basaltic flows (low viscosity, low silica) producing small sized material. Commonproducing small sized material. Common rock scoria and volcanic glassrock scoria and volcanic glass•Single eruptive episode lasting Single eruptive episode lasting a short time a short time •Composed of scoria and loose Composed of scoria and loose pyroclastic materialpyroclastic material• Commonly forms along the Commonly forms along the flanks of preexisting volcanoesflanks of preexisting volcanoes
Cinder Cones
Lava Dome (plugged dome volcano)Lava Dome (plugged dome volcano)• results from viscous lava extruding into the crater.results from viscous lava extruding into the crater.• high degassing of extrusion high degassing of extrusion “pasty – sticky” lava “pasty – sticky” lava• most preserved domes are chemically high SiOmost preserved domes are chemically high SiO22 content content
• commonly rhyolitic or dacitic in composition commonly rhyolitic or dacitic in composition
• Proclaimed has a National Proclaimed has a National Monument in May, 1907Monument in May, 1907• Eruption of Lassen Peak Eruption of Lassen Peak May, 1914May, 1914• Subsequent eruptions betweenSubsequent eruptions between 1914 – 19211914 – 1921• Lassen Volcanic National ParkLassen Volcanic National Park established August 9, 1916established August 9, 1916
The Ring of FireThe Ring of Fire
Cascade Mt. Range Cascade Mt. Range StratovolcanoesStratovolcanoes
1717
Baker
Rainier
St. HelensAdams
HoodJefferson
Three SistersNewberry VolcanoNewberry Volcano
Crater LakeMcLaughlin
Medicine Lake VolcanoShasta
Lassen Peak
Pacific PlatePacific Plate
NorthNorthAmericanAmericanPlatePlate
Oceanic plate is subducted beneath continental plate. Melting plate ascends upward mixing with continental material.
Ocean to Continent Convergence
• High SiO2 – High viscosity• explosive volcanoes• “pasty” lava flows• composite type volcanoes• andesite/rhyolite rocks
Black DaciteBlack Dacite Light DaciteLight Dacite
Banded PumiceBanded Pumice AndesiteAndesite
Common Volcanic Rock Types Common Volcanic Rock Types Lassen Peak National ParkLassen Peak National Park
Dacite eruption from 1915Dacite eruption from 1915• 63% to 68% SiO63% to 68% SiO22
• plagioclase feldspar, amphibole, pyroxeneplagioclase feldspar, amphibole, pyroxene• erupted around 800-1000 erupted around 800-1000 00CC
plagioclase feldspar
$0.25
Mount Tehama caldera (Brokeoff Volcano)• exploded and collapsed during Late Pleistocene leaving volcanic remnants • “Huge” composite volcano• 3 ½ mile wide volcanic caldera• Lassen Peak formed on the northern flank of Mount Tehama
(Mount Tehama)volcanic base ofMount Tehama
Mount Mount BrokeoffBrokeoff
MountMountDillerDiller
Projected cross-section ofProjected cross-section ofMount Tehama Mount Tehama
EstimatedEstimated11,000 feet +11,000 feet +
Bumpass Hell Hot SpringsBumpass Hell Hot SpringsSulfur WorksSulfur Works
• Hot springs, fumaroles, strong rotten egg odor indicates theHot springs, fumaroles, strong rotten egg odor indicates the presence of hydrogen sulphide (Hpresence of hydrogen sulphide (H22S)S)
• Represents the center of Brokeoff cone (Mount Tehama)Represents the center of Brokeoff cone (Mount Tehama)• Results of hydrothermal alteration:Results of hydrothermal alteration:
• hard gray-green andesite lava hard gray-green andesite lava bright colored clays bright colored clays• other volcanic rocks other volcanic rocks reduced to red iron oxides reduced to red iron oxides• presence of sulphuric acid (Hpresence of sulphuric acid (H22SOSO44) ) rapidly rapidly reducing volcanic rock to claysreducing volcanic rock to clays• High water acidity High water acidity pure opal (not gem quality) pure opal (not gem quality)
Bumpass Hell – Named for a cowboy that Bumpass Hell – Named for a cowboy that worked in the area in the 1860’s worked in the area in the 1860’s (Kendal Bumpass)(Kendal Bumpass)
Scalded his feet and when ask where he was, heScalded his feet and when ask where he was, hereplied ------ IN HELLreplied ------ IN HELL
Devil’s KitchenDevil’s Kitchen
• Strongly acidic resulting in holesStrongly acidic resulting in holes and pits eaten in bedrockand pits eaten in bedrock• Sulfur Works, Bumpass Hell, Devil’sSulfur Works, Bumpass Hell, Devil’s Kitchen associated with a north-west Kitchen associated with a north-west trending fault systemtrending fault system• Less civilized than Bumpass Hell andLess civilized than Bumpass Hell and 6000 feet elevation6000 feet elevation• Relatively small hike from BumpassRelatively small hike from Bumpass Hell with a 440 feet elevation gainHell with a 440 feet elevation gain• Stepping back into the Mesozoic!Stepping back into the Mesozoic!
Lassen Peak Statistics:Lassen Peak Statistics:• Stands 10,457 feet to summitStands 10,457 feet to summit• Located at northern end of the Sacramento valleyLocated at northern end of the Sacramento valley• Landmark for immigrants entering the valleyLandmark for immigrants entering the valley (around the 1800’s)(around the 1800’s)• Named after Peter Lassen, Danish blacksmithNamed after Peter Lassen, Danish blacksmith (1830)(1830)• Peter Lassen guided parties of immigrants intoPeter Lassen guided parties of immigrants into California using Lassen Peak, but frequently California using Lassen Peak, but frequently got lost ---- as the story goes.got lost ---- as the story goes.
Lassen Peak Geologic HistoryLassen Peak Geologic History • Radiometric dating shows the Radiometric dating shows the formation of Lassen Peak aroundformation of Lassen Peak around 31,000 years ago --- along the 31,000 years ago --- along the northern flank of Mount Tehamanorthern flank of Mount Tehama
• Streams of dacitic lava flows movedStreams of dacitic lava flows moved to the north reaching 1500 ft coveringto the north reaching 1500 ft covering 20 mi20 mi22
• 25,000 -31,000 years, Lassen grows25,000 -31,000 years, Lassen grows rapidly reaching 1800 ft in a fewrapidly reaching 1800 ft in a few years --- becoming the largest years --- becoming the largest plugged dome type volcano plugged dome type volcano
• 18,000 – 25,000 years, Lassen Peak18,000 – 25,000 years, Lassen Peak significantly altered by glaciationsignificantly altered by glaciation
• 30 additional smaller steep-sided30 additional smaller steep-sided dacitic domes form (Bumpass Mt,dacitic domes form (Bumpass Mt, Helen Ridge, Eagle Peak, Valcan’sHelen Ridge, Eagle Peak, Valcan’s Castle, Reading PeakCastle, Reading Peak
Lassen Peak Geologic HistoryLassen Peak Geologic History• 300 – 1100 years, several dacitic300 – 1100 years, several dacitic pumice domes form with abundantpumice domes form with abundant avalanches producing topographyavalanches producing topography similar to the Chaos Crags deposit.similar to the Chaos Crags deposit.
This resulted in the Chaos JumblesThis resulted in the Chaos Jumbles deposit and the damming of deposit and the damming of Manzanita LakeManzanita Lake
• Mid 18Mid 18thth century, formation of Cinder century, formation of Cinder Cone (NE-section of the park). AshCone (NE-section of the park). Ash falling on the streams of lava resultedfalling on the streams of lava resulted the formation of Painted Dunes. the formation of Painted Dunes.
A flow of A flow of quartz-studded basaltic lava quartz-studded basaltic lava flows from Cinder Cone damming Butte flows from Cinder Cone damming Butte and Snag Lakesand Snag Lakes
avalanche debrisavalanche debris
1914 -1921 Lassen Peak Activity“The Great Explosion”
May 22, 1915May 22, 1915
Subsequent eruptionSubsequent eruption19151915
Eruptions seen as far as 150 miles awayEruptions seen as far as 150 miles away
Explosions recurred at irregular intervals Explosions recurred at irregular intervals on Lassen Peak for most of 1914. on Lassen Peak for most of 1914. Later, on May 19, 1915, a mass of lava rose Later, on May 19, 1915, a mass of lava rose in the summit crater and spilled 1,000 feet in the summit crater and spilled 1,000 feet (300 m) down the western side of the (300 m) down the western side of the volcano.volcano. Extensive lahars (mudflows) were Extensive lahars (mudflows) were created on the northeastern side as snow created on the northeastern side as snow banks were melted. banks were melted.
A great explosion blasted out a new crater A great explosion blasted out a new crater three days later on May 22, 1915. A volcanic three days later on May 22, 1915. A volcanic cloud rose 40,000 feet (12,000 m) along with cloud rose 40,000 feet (12,000 m) along with flowing lava creating various dams and flowing lava creating various dams and lakes currently observed today. Volcanic lakes currently observed today. Volcanic activity declined, finally ending in 1921.activity declined, finally ending in 1921.
Lassen PeakLassen Peak
Eagle PeakEagle Peak
Bumpass MtnBumpass Mtn
Lassen PeakLassen Peak
Cinder cones producingEagle Peak and BumpassMountain. Photo taken inthe southwest direction.
Smaller cinder cones compared to the plugged dome Lassen Peak
Chaos CragsChaos Crags(older)(older)
Chaos JumblesChaos Jumbles(younger)(younger)
Oct, 1930Oct, 1930Sept 2006Sept 2006
Avalanche debris produced on Avalanche debris produced on the north side of Lassen Peakthe north side of Lassen Peakbetween 300-1100 yearsbetween 300-1100 years
Painted DunesPainted Dunes
The Painted Dunes are The Painted Dunes are composed of oxidized cinders composed of oxidized cinders lying over the Fantastic Lava lying over the Fantastic Lava Beds. In the distance, the Beds. In the distance, the concave flank (right side) of concave flank (right side) of Lassen Peak is where the Lassen Peak is where the lahars blew down the peak in lahars blew down the peak in 1915 to create the Devastated 1915 to create the Devastated Area. Area.
