Midterm 2 Flashcards
Veins
Veins are mineral deposits within
rock fractures in the country rock
that come from the magma
Pegmatites
Pegmatites are extremely coarse-grained veins that cut across finer-grained country rock • Happens when there is slow cooling of magma extra rich in water (dissolved in the magma) • The water allows elements to rapidly diffuse (move through) the magma to add to crystals so they grow very large -almost always felsic
Parts of a volcano
- Magma Chamber
- Flank Eruption
- Central Vent
- Crater
- Lava Flows
- Volcanic Debris
• Three main compositions of lava and
corresponding volcanic igneous rocks:
• Three main compositions of lava and corresponding volcanic igneous rocks: • Basaltic lava / basalt • Andesitic lava / andesite • Rhyolitic lava / rhyolite
Magma viscosity is controlled by 3
• Magma viscosity is controlled by temperature, composition,and gas content
• The higher the temperature of a magma or lava, the less
viscous it is e.g., as you heat up honey, it gets runnier – less
viscous
• Felsic lava has _____ silica content
and is more viscous than basaltic lava
higher
Basaltic Eruptions are the
Hawaiien Style, 10-100 m output, effusive
Pahoehoe vs Aa
Pahoehoe • Thin, glassy layer forms at surface of fluid lava • Layer is twisted and coiled as underlying lava is transported
• Aa • Lava degasses and forms bubbles, and becomes more viscous as it cools so the bubbles are trapped in the lava • Flows more slowly and solid layer breaks into rough, jagged blocks
pillow basalts/lava
• pillow-like blocks of basalt form when basaltic lava erupts under water • Outer skin of the pillow basalt cools fast and inner lava cools more slowly – outer skin is glassy, inner rock is crystalline
Andesitic Lavas
• Produced in volcanoes above subduction zones • Intermediate silica content • Lower temperatures • More viscous • Flow more slowly • Can produce explosive eruptions with large ash plumes – Vulcanian or Plinian eruptions • E.g., Mount St. Helens, erupted in 1980 (a Plinian eruption)
Rhyolitic Lavas
• Magma produced when large volumes of continental crust
are melted
• Highly viscous, can erupt at lower temperature (650-750˚C)
• High silica content
• Rich in potassium and sodium
• Typically flows 10x more slowly than basaltic lava and tends
to pile up in rounded deposits
• Gases are easily trapped causing large pressure increases
as the gasses expand
• Can produce most explosive of all volcanic eruptions!
vesicular textures caused by
• Gases trapped in lava during cooling produces vesicular
textures (bubbles)
tephra
• Tephra includes all pyroclastic debris - airborne rock and
volcanic dust ejected during a volcanic eruption
central vent volcanoes vs Large scale volcanic terrains
Central Vent Volcanoes • central vent • summit crater • flank eruptions • fissure eruptions
Large-scale Volcanic Terrains • no central vent • network of source material • extend over a large area • E.g. Mid-Atlantic Ridge
Stratovolcanoes
Stratovolcanoes (aka Composite Volcanoes)
• Form around vents that eject lava and pyroclasts
• Alternating layers form cone-shaped volcanoes, steep sided
in comparison with shield volcanoes
• Lava solidifies in core
and radiating dikes
• Commonly found
above subduction
zones – andesitic
composition
ex: cotopaxi
volcanic dome
Volcanic Dome
• Mounds that form in vents when viscous lava erupts slowly
• Associated with andesitic and rhyolitic magmas
• Remember: higher silica content = higher viscosity
• Plug vents and trap gases leading to pressure increases
within vent
shield volcanoes
Shield Volcanoes
• Mafic, low silica, low gas magma originates in the mantle
• Basaltic lava results in “Aa” and “Pahoehoe”
• Low viscosity creates broad, gentle slopes
• Lava tubes are common
ex: Kilauea, Hawaii
Cinder cone
Cinder Cones
• formed from fountains of basaltic lava
• commonly on the flanks of shield volcanoes
• composed of pyroclastic debris from a single vent
• Usually maximum of 300 m high
• short-lived features - sources often cut off after short period of
time (weeks to years)
T OR F
Large-scale volcanic terrains lack a central vent
T
Large Igneous Province3s
Large Igneous Provinces
• Large volumes of mafic intrusive and extrusive igneous rock
• created by processes other than seafloor spreading
• No central vent
• Fissure eruption that produced the Siberian Traps occurred
at time of the Permian mass extinction ~251 Ma
Fissure Eruptions in Large Igneous
Provinces
- Have produce the largest eruptions in Earth’s history
* Magma ejected from near vertical fractures in lithosphere
Flood Basalts
• Basaltic lava erupting from volcanic fissures that spread
over large areas of flat terrain
• Have occurred on continental scales, creating large
plateaus and mountain ranges
• Large Igneous Provinces
• Eruption ~16 Ma buried large portions of what are now
the states of Washington, Oregon and Idaho
• Form the Columbia Plateau, covers area of ~160,000 km2
eight types of volcanoes
• Cinder cone • Stratovolcano • Rhyolite caldera complex • Shield volcano • Maar vents and diatremes • Monogenetic field • Mid-ocean ridge • Large igneous province
Monogenetic field
• Poorly understood • Multiple maar vents and cinder cones • Erupt at different times • usually grow laterally from single magma source • Form fields of smaller vents and cones instead of mountains
ex:San Francisco Volcanic Field, Arizona
Wells Gray-Clearwater Volcanic field, BC
Kinds of volcanoes in Canada?
- Monogenetic field
2. Hospot
Most volcanoes are associated with:
• Spreading centres – spreading centre volcanism • Subduction zones – arc volcanism (island arcs and volcanic arcs) • Hotspots - intraplate volcanism -• Chains of volcanic islands form as oceanic lithosphere is transported over hot spots
volatiles
Volcanoes emit volatiles in addition to pyroclasts and lava • Volatiles are chemical compounds with low boiling points • Volatiles include: • Water vapor (H2O) – accounts for 75 to 90 % of volatiles • Carbon dioxide (CO2 ) • Sulfur dioxide (SO2 ) • Nitrogen (N2 ) • Hydrogen sulfide (H2S) • Volatiles may be emitted for centuries following initial eruption
Aerosols
Aerosols (tiny particles of dust or water)
intercept sunlight and the layer nearest the
Earth (the troposphere) cools
• Sulfuric acid, silicate dust (ash)
• Chlorine can also enhance ozone depletion
ex:Example: Mount Pinatubo
Volcanic Hazards
- Eruption clouds
- Lahars
- Flank collapse
- Caldera collapse
- Toxic gases
Deadliest volcano was RUIZ
Pyroclastic flows
Pyroclastic Flows
• Hot volcanic ash and gases ejected in cloud that moves
down the volcano’s side at high speed
• Solid particles are lifted up by hot gases – limited friction,
move very quickly
flank collapse
• Volcanoes constructed of layers of lava and ash
• If sides of volcano become too steep, weak ash layers may
cause flank to collapse
• Material can be very destructive
caldera collapse
• Potentially one of the most destructive natural phenomena
on Earth – have not occurred during recorded history
• Faulting leads to formation of secondary vents
• Caldera becomes insufficiently supported
• Collapse may trigger catastrophic eruption
