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Where Does Magma Come From?• Earth’s interior is hot (25 C/km near surface
= 1000 C at 40 km)
• Pressure inhibits melting– Mantle is solid– Never far below melting point
• Volcanoes fed by small pockets 0-100 km deep– Rising hot material may melt– Water can lower melting point
Why Igneous Rock Classification Matters
• Silica Content = Viscosity• Silica Content Governs Violence of
Eruptions– Silica Poor (Basalt): Fluid lavas, generally little
explosive activity– Intermediate Lavas (Andesite): Pasty lavas,
explosive eruptions common– Silica-Rich Lavas (Rhyolite): Extremely
viscous lava and explosive eruptions
Basalt (45-52% SiO2)• Slightly modified planetary raw material• Derived directly from mantle
– Oceanic crust– Hot Spots and Flood Basalts– Oceanic volcanic arcs– Early stage of continental volcanic arcs– Rift zones with rapid spreading
• Fluid lava with little explosive activity• Shield volcanoes, Cinder Cones
Andesite (52-66% SiO2)
• Mixture of mantle material and continental crust
• Continental volcanic chains
• Pasty lava with significant explosive activity
• Stratovolcanoes
Rhyolite (>66% SiO2)
• Mostly remelted continental crust
• Settings where magma has a long time to react with continental crust– Late stage of continental volcanic arcs– Slow-spreading Continental Rifts– Continental Hot Spots (Yellowstone)
• Catasrtophic explosive activity common
• Obsidian domes, magma chamber collapses
Some Igneous Rocks Are Named on Textural Criteria
• Pumice - Porous
• Obsidian - Glass
• Tuff - Cemented Ash
• Breccia - Cemented Fragments
Classes of Eruption
Effusive• Icelandic• HawaiianExplosive• Strombolian• Vulcanian• Plinian• Caldera-Forming (Ultra-Plinian)• Phreatic:
Classes of EruptionType Lava Volcano Effects
Icelandic Basalt None or Shield Fissure Flows
Hawaiian Basalt Shield
Strombolian Basalt-Andesite
Small Stratovolcano
Mild, Continuous
Vulcanian Andesite Stratovolcano Large eruption cloud
Plinian Andesite – Rhyolite
Stratovolcano Pyroclastic Flows
Caldera-Forming Rhyolite Stratovolcano or None
Large Pyroclastic Flows
Phreatic Any Any Steam Blast
Products of Eruptions
Lava Flows
Pyroclastic Debris
• Bombs
• Lapilli
• Ash
Mudflows
Landslides
Gases
• Steam
• Carbon Dioxide
• H2S
• SO2
• HCl
• HF
Environmental Hazards of Volcanoes
Pollution • SO2, HCl in
Water Lava Flows Falling Ejecta Ash Falls • Building Collapse • Crop Destruction
Mudflows • Direct Damage
(Colombia, 1985) • Floods (Several Types)Blast (Mt. St. Helens, 1980) Pyroclastic Flow (St. Pierre,
1902) Gas (Lake Nyos,
Cameroon, 1986)
Nyiragongo, Congo• At least 34 eruptions since 1982• Semi-permanent lava lake• Area accounts for 40% of Africa’s historic
eruptions• Steep-sided but unusually fluid lava: unique• 1977: Lava lake drains at night, killing 70-
hundreds• 2002: Lava invades city of Goma: 400,000
evacuated, 45 killed, 4500 buildings destroyed, 120,000 homeless
Volcanic Explosivity IndexVEI Classification Description Plume Ejecta
volume Frequency Example
0 Hawaiian non-explosive < 100 m < 104m³ daily Mauna Loa
1 HawaiianStrombolian gentle 100-1000 m > 104 m³ daily Stromboli
2 StrombolianVulcanian explosive 1-5 km > 106 m³ weekly Galeras 1993
3 Vulcanian /Pelean severe 3-15 km > 107 m³ yearly Lassen 1915
4 Pelean/Plinian cataclysmic 10-25 km > 0.1 km³ ≥ 10 yrs Soufrière Hills 1995
5 Plinian paroxysmal > 25 km > 1 km³ ≥ 50 yrs St. Helens 1980
6 Plinian/Ultra-Plinian colossal > 25 km > 10 km³ ≥ 100 yrs Pinatubo 1991
7 Plinian/Ultra-Plinian super-colossal > 25 km > 100 km³ ≥ 1000 yrs Tambora 1815
8 Ultra-Plinian mega-colossal > 25 km > 1,000 km³ ≥ 10,000 yrs Toba (73,000 BP)
Volcanoes and Climate
• Stratospheric Ash
• Sulfuric Acid Aerosols– Colorful sunset effects– Large amounts can block sunlight
• Carbon Dioxide
Dating Large Remote Eruptions
• Historical Records of Unusual Cold
• Optical Effects
• Persistent “Dry Fog”
• Frost Rings in Trees
Recorded Large Distant Eruptions
• 1627 BC: Thera?
• 536 AD: Krakatoa?
• 626: Unknown
• 934: Eldgja, Iceland
• 1258: Unknown
• 1783: Laki, Iceland
• 1815: Tambora, Indonesia
Tambora 1815
1816: “Year Without A Summer”
• 100 cubic km of ash erupted
• Global sunset color effects for months
• New England– Snow in June and August, Frost in July– Exodus to Midwest
• Europe: High prices, food riots
Flood Basalts
• Siberian Traps and Permian Mass Extinction?
• High Sulfur Content– Aerosols may block significant sunlight– Surface crust may trap sulfur
Supervolcanoes?
• Magma Chamber Collapse (Yellowstone?)– Destruction of crops– Destruction of high technology– Economic Disruption– Climatic Effects
• Flood Basalts– Climatic Effects– Toxicity