Visualizing Earth Science By Z. Merali and B. F. Skinner Chapter 9 Volcanism and Other Igneous Processes Volcanoes types and effects of eruption Chapter Overview Melting and cooling of rocks Geological formations from igneous activity Volcanoes and Eruption Types Categories of Molten Rock Lava: Surface molten rock Magma: Molten rock inside the earth Volcano: vent through which rocks and debris erupt 1
Volcanoes and Eruption Types Magma viscosity depends on: Temperature: higher temperature lower viscosity Silica content: More silica greater viscosity Dissolved gas content of Magma depends on viscosity less viscous magma lets gas escape more easily Nature of volcanic eruption depends on gas content pressure of dissolved gas leads to explosive eruptions Volcanoes and Eruption Types Low viscosity lava and magma Hawaiian eruptions Shield volcanoes Flood basalts and Basalt plateaus Volcanoes and Eruption Types Strombolian eruptions Cinder or Spatter cones of loose volcanic rock Vulcanian and Plinian eruptions Tephra Pyroclastic flows: pyroclasts Volcanic ash Viscous Lava and Magma 2
Volcanoes and Eruption Types Constructive phase Vulcanian and Plinian eruptions layers of: Silica rich lava and pyroclasts Volcanic ash from pyroclastic flows and ash columns Destruction Explosive Plinian eruption Extinction Summit collapse Life of a Stratovolcano Crater outlet depression of a volcano Caldera large depression from collapse due to emptying magma chamber Thermal springs, geysers and fumaroles hot water and gases from a fuming magma chamber Other volcanic features Craters, calderas, geysers and fumaroles Global distribution of Volcanic Activity Greater density of volcanic features located along plate tectonic boundaries The Pacific Ring of Fire convergent plates African rift valley volcanoes convergent plates Intraplate volcanism The Hot Spot hypothesis The Hawaiian island chain 3
Primary effects of volcanic activity Lava flows Pyroclastic flows Secondary effects Fires Flooding and Tsunami Earthquakes and debris landslides Mudflows lahar Effects of Volcanic Activity Volcanic Hazards Tertiary effects Landscape Effects of Volcanic Activity Reshaping of mountains Stream blockage and re- direction Tertiary effects Climate Effects of Volcanic Activity Volcanic ash in the atmosphere Acid rain Prolonged darkness and lower temperatures 4
Replenishes rich volcanic soil Provides potential source of geo-thermal energy Rising magma produces near surface mineral deposits Diamonds found in volcanic vents Effects of Volcanic Activity Beneficial Effects Identification Is the volcano active, dormant or extinct? What type of volcano is it? Composition of lava from previous eruptions Observation of activity Gas discharge Bulging of surrounding rock Increase in seismic activity Changes in rock temperature Predicting Volcanic Eruptions Melting of Rocks A magma chamber contains molten rock inside the earth Understanding volcanism requires understanding the melting of rocks Environmental factors affecting the melting of rocks Temperature Pressure Presence of water 5
Melting of Rocks Effect of Temperature, Pressure and Moisture Crustal temperature increases with depth Rate of increase greater for oceanic crust than continental Melting of Rocks Effect of Temperature, Pressure and Moisture Pressure in the rock increases with depth Dry rocks melt at higher temperatures at higher pressures Moisture in rock lowers melting temperature Melting of Rocks Subsurface Rock and Fractional Melting Fractional melting of rock occurs At the melting points of the minerals it contains Melting points are reached when Temperature increases Pressure decreases Fractionation is the separation of molten portion of rock 6
Cooling and Crystallization of Magma Hundreds of kinds of igneous rocks exist on Earth Different rocks are formed from the same Magma Differences in the cooling process lead to different kinds of rocks This is magmatic differentiation Cooling and Crystallization of Magma Partial separation of solid crystals from liquid melt is fractional crystallization Magma minerals crystallize At different temperatures and pressures In differing environmental conditions In similar magma this leads to Differences in fractional crystallization Formation of different rocks Cooling and Crystallization of Magma Bowen s Reaction Series Bowen s hypothesis: All magmas originate from basaltic magmas Among Bowen s experiments Create a sodium rich magma Create a rhyolitic magma from a basaltic magma Through fractional crystallization Diffusion of ions from solid crystal to the surrounding melt Melt reacting with crystallized rock 7
Plutons Igneous bodies Different from surrounding older rock (intrusive) Originate underground as magma Exposed at the surface due to geological processes Batholiths and Stocks Largest Plutons Cut across existing rock layers Stocks are smaller than Batholiths Plutons and Plutonism Dikes Solidified magma in a fracture cutting across existing layers Plutons and Plutonism Sills Solidified magma squeezed into layers between existing layers Chapter Summary Volcanoes: Types and Effects The production of igneous rock Melting of mantle rock Crystallization of magma Plutons Bodies of igneous Rock 8
Properties of Magma and Lava Magma is composed of Mostly Silica - SiO 2 (45-75%) Temperatures of Erupting Lava Between 800 o C-1200 o C Magmas with higher silica content are more viscous Magmas and lavas at higher temperatures are less viscous 9