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Volcanism and Other Igneous Processes 1 On Sunday, May 18, 1980, the largest volcanic eruption to occur in North American historic times transformed a picturesque volcano into a decapitated remnant. On this date in southwestern Washington State, Mount St. Helens erupted with tremendous force. Before After What happened?? Mt. St. Helens •Approximately 1 km3 of ash erupted. •Summit decreased by 1,350 feet. •Claimed 59 lives •Ash propelled 11 miles into the atmosphere. •Ash covered surrounding areas of Yakima, Tri-cities, and northern Oregon for 3 days – Noon felt like night. • Feb. 1981- highest birth rate in Portland and surrounding areas –TRUE FACT Advice from the authorities: If there is another major eruption, put your head between your legs and kiss your ash goodbye! 2 Goals for mastering this chapter: •Describe factors that affect the nature of volcanic eruptions. •What factors affect viscosity of magma? •Describe the various types of materials associated with volcanic eruptions. •Describe the major types of volcanoes recognized by volcanologists and their eruptive styles – the geomorphology. •Describe magmatic differentiation and how it is related to Bowen’s reaction series. •Describe various intrusive bodies associated with plutonic rocks. 3 •Understand the general relationship between volcanic activity and plate tectonics. The “buzzword” is VISCOSITY What is viscosity? Viscosity = how well a material flows more viscous – flows very slowly (high viscosity) less viscous – flows quickly (low viscosity) Does glass have viscosity? 5 Why do volcanoes have different eruptive •high viscosity styles??? 4 •high SiO2 Factors influencing eruptions •felsic • dependant on the magma’s viscosity •“pasty” •explosive • high viscosity –”pasty” explosive • low viscosity –”fluid” flows easily A •low viscosity •low SiO2 •mafic •“fluid” •non-explosive B Factors influencing viscosity • Temperature of magma • T • T viscosity viscosity = fluid flow = pasty flow • Chemical composition • SiO2 content (high or low) mafic composition: (50% SiO2)=“fluid” flow intermediate comp.: (60% SiO2) felsic composition: (70% SiO2) =“pasty” flow Dissolved gasses – influencing the movement of magma (volatiles – water, CO2, SO2….) Silica content and volatiles erupt two types of materials: Gas charged lava expands 100 times its volume lava fountains Gas charged expands 100 times its volume very explosive lava flows fluidly volatiles easily migrate upward magma low in SiO2 Volatiles migrate upwards with difficulty magma high in SiO2 6 sulfur dioxide > 1% SO2 5% Carbon dioxide water vapor 70% 15% 5% volatiles Dissolved gasses (volatiles) • 1-6% of total magma wt. Magma chamber • contributes to atmosphere 7 I VOLCANOES. Discuss with a friend: 1. What factors affect viscosity? 2. How does silica content (SiO2) influence the consistency of magma? 3. Name various volatiles that are typically emitted from a volcanic eruption. I will get an A on my exams and quizzes. 8 Types of Basaltic Lava Flows (low silica (SiO2) content) Pahoehoe Aa • very fluid, thin, broad sheets • very “pasty,” sticky, thick, cool flows • flows 10-300 km/hr (30-900 ft/hr) • flows 5-50 m/hr (15-150 ft/hr) •high volatile gas content •low volatile gas content • smooth “skin,” ropey type flow • rough, blocky, sharp, angular type flow 9 Pyroclastic materials Ash Volcanic Bombs Bombs Nuee-Ardente Lahars Lahars mud flows Ash Ryholitic magmas • high silica • very explosive Nuee-Ardente • thick, pasty • high viscosity • pyroclastic ejections 10 Why are there various types of volcanoes??? What factors govern the structure of volcanoes?? Features of a typical volcano: crater flank flank conduit Magma Chamber 11 I VOLCANOES. Discuss with a friend: 1. Describe the differences between aa and pahoehoe type basaltic flows. 2. Describe a pyroclastic type flow. Describe 3 types of pyroclastic flows. 3. Draw a typical volcano and label the common features. 13 Three types of Volcanoes •Volcano type is dependant on SiO2 content. Shield Composite (stratovolcano) Explain the differences. Cinder cone 14 Shield Volcano - Hawaii Broad, low angle flanks Shield Volcanoes •Hawaiian Islands, Iceland, Galapagos Islands •commonly rise from the deep ocean floor •formed by the accumulation of fluid basaltic flows •low silica content (basaltic composition) •low viscosity •less than 1% pyroclastic debris •non-explosive eruptions •pahoehoe flows •aa flows Stratovolcano Composite Cones (stratovolcanoes, stratacomposite) •Western U.S. coast, Western South American coast, Japan • typically form in the ocean along continent convergent boundaries • found along the ring of fire Steep high angle flanks Pyroclastics •formed from layering deposits of ash, lava, and pyroclastic flows •high silica content (70%)- (Rhyolitic composition) •high viscosity flows •abundant pyroclastic activity •deadly airborne debris •explosive eruptions – very hazardous The Ring of Fire Cascade Mt. Range Stratovolcanoes 17 Old lahars 18 Cinder Cones •Exist all over the earth’s surface (by the 1000’s) •Located in volcanic fields (Flagstaff, AZ– about 600) Very high, steep angled flanks 30-40 degrees Averages 100 ft – 1000 ft high •Formed by gas rich basaltic flows (low viscosity, low silica) producing small sized material. Common rock scoria and volcanic glass •Single eruptive episode lasting a short time •Composed of scoria and loose pyroclastic material 19 Cinder Cones 20 I see extensive lava flows, but where are the volcanoes? Fissure type eruptions and lava plateaus • very fluid basaltic lava erupted from fractures in the earth’s crust • lava fountains along “linear” fractures spreading out over wide areas • extrudes voluminous amounts of low silica basaltic lava • single flows can travel 100’s of kilometers Columbia River basalts My study Area Linear cracks (fissures) • 11 m.y. flows • very extensive –single flows from Idaho to Portland, Ore. • 1 mile thick in southwest Washington 21 I VOLCANOES. 1. Fill in the following blanks given the chart below: Pyro- Volcano SiO2 type Composition content Viscosity clastics Mafic basaltic Intermediate Felsic rhyolitic Low <50% Low None Approx Intermediate some 60% High 70% high high Shield type Flood basalts Composite Cider-cone pyroclastic Composite type 22 Divergent plate volcanism Plates separate resulting from basaltic magma ascending into fractures. Shield type volcanoes form ridges and mountains below the ocean. • • • • Very fluid eruptions Less than 50% SiO2 content Shield type volcanoes Basalt rocks 23 Ocean – Ocean plate convergence •Very fluid eruptions • Less than 50% SiO2 • Shield type volcanoes • Basalt rocks Oceanic plate subducts beneath oceanic plate. Melting subducted plate ascends upward forming shield type volcanoes in the form of island arc systems “mountainous arcs” that rise above the ocean floor. – Japan, Aleutian Islands 24 Baker Pacific Plate Rainier St. Helens Adams Hood Jefferson North American Plate Three Sisters Newberry Volcano Crater Lake McLaughlin Medicine Lake Volcano Shasta Lassen Peak 25 Ocean to Continent convergence Oceanic plate is subducted beneath continental plate. Melting plate ascends upward mixing with continental material. • High SiO2 – High viscosity • explosive volcanoes • “pasty” lava flows • composite type volcanoes • andesite/rhyolite rocks I VOLCANOES. Discuss with a friend: 1. Draw a diagram that shows how the shield and composite type volcanoes are related to their respective plate boundary or “plate tectonic setting.” I will get an A on my exams and quizzes. 26 Why and How Rocks Melt Granite/Shale (common rocks) typically begin to melt at 8000C and turn to liquid at 12000C. Why a temperature range? The range of temperature for complete melting of a rock is due to various individual mineral melting point characteristics. Amphibole (hornblende) melts at around 9000C. Feldspar (orthoclase) melts at around 7000C. Quartz melts at around 6000C. 27 What factors influence the melting points of rocks? • Temperature: • geothermal gradient • Pressure: • influence of pressure at depth • pressure/temperature relationship • Presence of water in the rocks: • water in the subduction zone • how water influences the melting point 28 Temperature inside the earth 0 500 1000 Geothermal gradient 1500 2000 • the rate at which temperature increases with depth Continent gradient 5,000 200 10,000 29 • In thicker crust, gradient increases. • average 7oC/km rate • temperature increases gently Oceanic gradient 300 400 Pressure (mpa) Depth (km) 100 15,000 • Below the ocean floor, temperature increases rapidly. • average 130C/km How does pressure influence the melting point of rock? Pressure inside the earth with depth. • Pressure/Temperature increase with depth. • Mantle (mesosphere) reaches temperatures beyond rock melting point, but the mantle remains “solid!” • Increased pressures raise melting points. • At a depth of 100 km, pressure is 35,000 times greater than at sea level. • Many minerals will begin melting at temperatures above melting points at sea level. 30 How does the presence of water influence the 31 rock’s melting point? The presence of water or water vapor the rock’s melting point. • water present in subduction zones The introduction of water decreases melting points and creates melted plate material that moves upward. Magma stays liquid due to the decrease in pressure as magma rises. (decompression melting) Introduction of water Cooling and Crystallization of Magma • What is magmatic differentiation? • What is the significance of Bowen’s reaction series? • How do you get a rhyolitic composition from a basaltic magma? Put the following igneous rocks in the order of their compositional equivalence. 32 granite basalt diorite rhyolite gabbro andesite Is it possible to create a granite composition from a basaltic magma? Which minerals form first, second, third…….etc.? Bowen’s reaction series Gabbro Diorite Granite 35 33 Magmatic differentiation: • the formation of many kinds of igneous rocks from a single magma Simple example SiO2 Fe SiO2 Fe SiO2 Mg SiO2 Mg SiO2 Fe Fe SiO2 Mg Mg SiO2 SiO2 SiO2 Liquid magma FeSiO2 Cooling liquid SiO2 SiO2 SiO2 FeSiO2 MgSiO2 MgSiO2 FeSiO2 FeSiO2 MgSiO2 solid Part liquid/solid How has the liquid magma changed composition? As the liquid magma begins to cool, various minerals precipitate as solids and become separated from the liquid melt. This separation of various chemistries changes the composition of the original magma. Three ways crystals separate from a melt: 34 Filter pressing: • Remaining melt is pushed through a fracture and separated from xlized melt. Crystal settling: • The first minerals to xlize are denser and sink to the bottom. Crystal floatation • The first crystals are less dense and rise to the top. I crystallizing. Discuss with a friend: 1. Why do rocks typically have a temperature range to become completely melted? 2. Describe magmatic differentiation. 3. Describe the three ways crystals are separated. 4. What is Bowen’s reaction series? I will get an A on my exams and quizzes. 36 What types of features are formed when magma cools below the surface? 37 Dikes Tabular Tabular intrusive bodies forming below the earth’s surface 38 Intrusive Bodies: Batholith: intrusive body GREATER than 40 mi2 Stock: intrusive body LESS than 40 mi2 Dike: intrusive body cutting across strata (disconcordant) Sill: intrusive/extrusive body parallel to strata (concordant) Laccolith: “mushroom-shaped” intrusive body forming a dome-like structure 39 Intrusive Bodies Sill Loccolith Stock Dike 40 Melting magma rises and mixes with continental material (high SiO2) and solidifies beneath the surface. Sierra Nevada Batholith Granite/Diorite 41 I intrusive rocks. 1. Given the block diagram below, describe the following plutonic (intrusive) type features: 42