Lecture 10: Volcanism and the Chemistry of Magma Flashcards
Volcanology
Study of the origin and ascent of magma through the planet’s mantle and crust and its eruption at the surface
Extinct
A volcano that has not erupted during the last several thousand years and is not expected to erupt again; for example, Kilimanjaro, Tanzania
Dormant
Volcanoes that have erupted during the last several thousand years but have been quiet in historical times; for example, Mount Meager, British Columbia
Active
A volcano is active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions, or if it has erupted in historic time. It’s important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in Hawai`i, little more than 200 years.
Stratovolcano
Volcanoes constructed of alternating layers of pyroclastic debris and lava flows; also called composite volcanoes
Med/high viscosity, med/high volatiles, large volume, great slow angle
Andesite
Shield volcano
Very wide volcanoes built of low-viscosity lava
Hawaiian
Low viscosity, low volatiles, large volume, low slope angle
Basalt
Somma volcano
Volcanic caldera that has been partially filled by a new central cone
Caldera
A large (over two kilometres in diameter), basin-shaped volcanic depression, roughly circular in map view, that forms by a piston-like collapse of rock into a partially evacuated magma chamber Rhyolite
Cinder cone
Most common types of volcanic cones
Form after violent eruptions blow lava fragments into the air, which then solidify and fall as cinders around the volcanic vent
Viscosity
Resistance to gradual deformation by shear stress or tensile stress – Internal Friction
What affects magma viscosity? (3)
- Gas escape
- Water content
- Silica content
Most common molecules in magma (3)
O-2, Si+4, SiO2
Second most common molecules in magma (2)
Al+3, Al2O3
Third most common molecules in magma (4)
Fe+2/+3, CaO, FeO, Fe2O3
Affect on magma: Left side of period table
Weaker bonds, therefore weaker magma viscosity
Bond energy and breakage
Higher bond energy, harder to break
Andesite
52-66% silica
Lava domes and stratovolcanoes
Basalt
45-62% silica
Shield volcanoes
Dacite
> 63% silica
Caldera
Viscous: explosive
Rhyolite
> 66% silica
Caldera
Mafic
Basalt
45-52% silicon dioxide
Higher temperature, earlier crystals
Intermediate
Andesite
52-66% silicon dioxide
Felsic
Rhyolite
>66% silicon dioxide
Lower temperature, latest crystals
Quartz
Mid-ocean ridge volcanism
80% of the volcanism on Earth
Eruption on ridges at great depth
Takes time to build up to surface
Important hydrothermal activity
Icelandic eruption
Gentle eruption, non exposive Low silica content Low viscosity Low gas content Basaltic lava Volcanic plateau and small shield volcanoes
Hawaiian eruption
Gentle eruption, non explosive Low silica content Low viscosity Low gas content Basaltic lava Large shield volcanoes
Strombolian eruption
Gentle eruption, low explosive Low silica content Low viscosity Low gas content Basal to andesitic lava Pyroclastic fragments Scoria cones
Vulcanian eruption
Gentle eruption, moderate explosive Low silica content Low viscosity Low gas content Basalt to andesitic to rhyolite Pyroclastic fragments Scoria cones and stratovolcanoes
Vesuvian eruption
Explosive eruption High silica content High viscosity High gas content Adesitic lava Pyroclastic fragments
Kratatoan
Explosive eruption High silica content High viscosity High gas content Adesitic lava Pyroclastic fragments
Pelean
Explosive eruption High silica content High viscosity High gas content Rhyolitic lava Pyroclastic fragments
Plinian
Explosive eruption, high explosivity High silica content High viscosity High gas content Andesite to rhyolitic lava Pyroclastic fragments Stratovolcanoes
Volcanic eruptions in order of eruption force (8)
IHSVVKPP
Icelandic Hawaiian Strombolian Vulcanian Vesuvian Kratatoan Pelean Plinian
Direct volcano hazards
Lava Pyroclastic flow Nuee Ardentes Pyroclastic surge Ash fall Acid rain Bombs Lahar Debris avalanche Shock wave Lateral blast Gases
Indirect volcano hazards
Change in groundwater
Lack of vegetation
Lack of solid ground
Yellowstone
Resurgent Caldera Eruptions 2.1 Myr ago, 1.3 Myr ago, 0.64 Myr ago (when is the next one?) Lava Creek Tuff 640,000 years ago 1000 km3 of ash
Can we predict when a volcano will erupt?
Eruption recurrence history Infrared photos of cone temp. Temperature of steam Bulges and slope movement Seismic activity
Long Valley California
760,000 yr ago 400 km3 magma erupted Pyroclastic flows covered > 1500km2 “Bishop Tuff” Long Valley Caldera collapsed 2 km Modern magma chamber: 10 km diameter, 8 km deep 600, 200 years ago Associated with Mount St. Helens, 1980
Risk of incorrect prediction?
Mammoth Lakes is affluent resort town 20,000 residents during ski season May 1982 USGS issued vol. haz. warning House prices fell 40%, major economic impact ‘Cry wolf’ syndrome
Mt. Pinatubo
1 Million people
Magma rose from 32 km in 500 yrs
June 7, 1991 small initial eruption, June 12 millions of m3 erupted, June 15 more than 5 km3 erupted
Ash cloud to 35 km in air, 480 km across
Blocked sunlight, filled valleys, lahars
300 people died
$1.5M monitoring saved 20,000 lives and >$500M evacuated property
Canada’s volcano hazard
There are more than 200 potentially active volcanoes in Canada, 49 of which have erupted in the past 10,000 years
Latest Canadian eruption
Iskut-Unuk River Cones in 1904
