EES 1030 Lecture 4 Outline of Last Lecture I. Igneous RocksII. Crystallization of magma/lavaIII. The nature of magma/lavaIV. Classifying Igneous rocksV. Igneous TexturesVI. PumiceVII. Minerals in Igneous RocksVIII. Common MineralsIX. Origin of MagmaX. Decompression MeltingXI. Fluid-Induced MeltingXII. Magmatic DifferentiationXIII. Magma Compositional VariationsOutline of Current Lecture XIV.Lecture 4: Volcanoes and other Igneous ActivityXV. Nature of Volcanic EruptionsXVI. Types of Volcanic DepositsXVII. Basaltic lava flow typesXVIII. Basaltic VolcanoesXIX. Pyroclastic MaterialXX. Intermediate-Rhyolitic VolcanoesXXI. Mt. St. Helens 1980XXII. Pyroclastic Falls & FlowsXXIII. Lahars: Volcanic mud flowsXXIV. Volcanoes and ClimateXXV. Intrusive Igneous ActivityXXVI. Hydrothermal SystemsCurrent LectureWhat controls the style of a volcanic eruption? Effusive lava flow vs. explosive eruption?Magma composition, temperature, gas contentNature of Volcanic EruptionsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.These factors control viscosity of magma.This measures a material’s resistance to flow.Temperature: Hotter magmas are less viscous (more runny)Composition: High SiO2 content  high viscosity (sticky)Dissolved gases: Violence of an eruption is related to how easily gases escape from magma, which depends on viscosity. Viscous magma mean gas trapped in bubbles.Pressure builds up as gases expand as magma nears Earth’s surface.Basaltic lavas = mils eruptionsRhyolitic or andesitic lavas = explosive eruptionsTypes of Volcanic DepositsNon-explosive eruptions: Lava flows mainly basaltic and intermediate compositions.Explosive eruptions: Pyroclastic deposits mainly intermediate to rhyolitic compositions.Basaltic lava flows: very fluid behavior. Can flow at rate typically 10-200 meters/hour. Travel distance of several 100 km.Rhyolitic lava flows: very sticky behavior. Travel distance of few km. at most.Basaltic lava flow typesPahoehoe: ropy texture.Aa: Rough, jagged blocky texture. Change from pahoehoe to aa with loss of gas and temperature, increasing viscosity.70% of the lava volume erupted at the Earth’s surface every year occurs under water at mid-ocean ridges: submarine eruptionsBasaltic VolcanoesShield Volcano: large, broad, circular shape with gentle dipping slopes. (3 to 5 degrees). Built by successive fluid lava flows from a central vent. Flows travel long distances.Cinder cones: also known as Scoria cones. Built from ejected gas-rich lava fragments. Steep slope angle (30 to 40 degrees) of loose cinders. Small size (30-300m).Short eruption: weeks to years.Fissure eruptions: 1783 Laki eruption, Iceland: Erupted 15 km3 in two years. Largest historic lava flow.Floor Basalt Plateaus: Huge stacks of thick fissure-fed basalt lavas. Example: Columbia River Plateau: flows 100-500 km3Pyroclastic MaterialFrothy, gas-rich material: vesicular texture. Greater than 60% vesicles or voids.Basaltic composition = Scoria or CindersIntermediate/Rhyolitic composition = PumicePyroclastic = “fire fragments”Intermediate-Rhyolitic VolcanoesComposite Cones or Stratovolcanoes: interbedded lava flows and pyroclastic debris. Potentially most dangerous volcano type, complex histories.Caldera: Volcano sitting above magma chamberExample: Crater Lake, OregonLava Dome: SiO2 rich lava is very viscous and can hardly flow. Thick lava squeezed out of vent like toothpaste.Mt. St. Helens 1980Largest eruption in historic times in USA, Summit lowered by more than 400 m.Over 1 km3 of ash and rock debris erupted.Pyroclastic Falls & FlowsPyroclastic material falls from eruption cloud of material explosively ejected from the vent.Pyroclastic flows are gravity-controlled surface flows from collapse of eruption column or lava dome.Lahars: Volcanic mud flowsMixture of volcanic debris and water.Eruption-triggered melting of snow & glaciers.Heavy rainfall saturates loose ash deposits.1991 eruption of Mt Pinatubo, Philippines coincided with a major typhoon. 1985 Nevado de Ruiz, Columbia 23,000 people killed.Mt. Rainer: potential catastrophe4000m summit with largest alpine glaciers in the Cascades, at least 50 major lahars down river valleys.Volcanoes and ClimateApril 1815: eruption of Mt Tambora, Indonesia.Largest eruption in historic times > 100 km3.100 x bigger than 1980 Mt St Helens eruption.Intrusive Igneous ActivityOver 80% of magma produced never reaches the Earth’s surface: Cool & solidify to IntrusionsIntrusion types:Shape: tabular (sheet-like) vs. massiveOrientation: with respect to the host (surrounding) rockHydrothermal SystemsCirculation of water around a magma chamberLeaches elements from surrounding rocks.Can form economically important metal