1 Chapter 6: Volcanoes and Other Mountains 1. The Volcano Commandos 2. Magma Viscosity 3. Magma Sources and Magma Composition 4. The Mount St. Helens Eruption 5. Products of Volcanic Eruptions 6. Volcanoes and Volcanic Landforms 7. Mountains: Why are They There? 8. The Rise and Fall of Mountains and Temperatures Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2 Volcanoes and Other Mountains As a scientifically literate citizen, what 3 questions would you ask about this volcano if you moved to the city in the foreground (Tacoma, Washington)?
3 The Volcano Commandos 500 million people live near active volcanoes Fewer than 200 volcanoes have instruments to assess potential for eruption 1,500 active volcanoes worldwide a third have records of previous eruptions 2 or 3 eruptions per decade are major disasters
4 The Volcano Commandos Volcanic mudflows from the eruption of Nevado del Ruiz volcano, Colombia killed more than 23,000 people Geologists of the Volcano Disaster Assistance Program (VDAP) help identify risks of potential eruptions
5 The Volcano Commandos Distribution of older lava flows gave scientists some idea of the potential eruption products of Nyiragongo. VDAP scientists traveled to Congo to examine eruption of Nyiragongo volcano (2002) Lava flowed through the nearby city of Goma (450,000 residents) Fortunately, toxic volcanic gases trapped in Like Kivu were not released
6 The Volcano Commandos Lava from Nyiragongo was several meters thick and burned and destroyed buildings in Goma
7 The Volcano Commandos Poisonous volcanic gases from Lake Nyos, Cameroon, killed 1,700 people in surrounding villages (1986)
8 Go back to the Table of Contents Go to the next section: Magma Viscosity
9 Magma Viscosity Escaping gases drive volcanic eruptions How easily gases escape from magma is controlled by magma viscosity Magma molten rock below the surface Lava molten rock at the surface Viscosity = resistance to flow Viscosity depends upon temperature and magma composition Magma composition varies with plate tectonic setting
10 Volcanoes and Other Mountains Conceptest Place the following 4 materials maple syrup, milk, peanut butter, frozen yoghurt in the correct positions (A, B, C, D) for their relative viscosity.
11 Magma Viscosity Viscosity = resistance to flow Viscosity of materials decreases with increasing temperature Viscosity varies with composition
12 Magma Viscosity Escaping volcanic gases drive eruptions Gases are dissolved in magma Water vapor, carbon dioxide, sulfur dioxide Pressure on magma decreases as magma rises toward surface Gases are released as pressure decreases Carbonated drink analogy
13 Volcanoes and Other Mountains Conceptest All other factors being equal, which magma would flow the fastest? A. High viscosity magma B. Low viscosity magma C. Neither, magma does not have viscosity
14 Volcanoes and Other Mountains Conceptest How would the viscosity of motor oil in your car s engine change from the time you turn the key in the ignition to when you have driven 50 km (30 miles)? A. Viscosity would increase B. Viscosity would decrease C. Viscosity would stay the same
15 Magma Viscosity Viscosity depends upon magma composition (silica content) Silica is combination of oxygen and silicon that combines with other elements (e.g., sodium, potassium) to form minerals Elements combine to form simple structures in minerals with less silica = low viscosity Elements combine to form more complex structures in minerals with more silica = high viscosity
16 Magma Viscosity Viscosity depends upon magma composition (silica content)
17 Volcanoes and Other Mountains Conceptest Which type magma most likely has the lowest viscosity? A. High silica, high temperature B. High silica content, low temperature C. Low silica content, high temperature D. Low silica content, low temperature
18 Magma Viscosity More viscosity = More violent eruptions Gases escape easily during mild eruptions with low viscosity magma, e.g., Hawaii Gases escape explosively from high viscosity magma, e.g., Alaska Mild or violent eruption?
19 Volcanoes and Other Mountains Conceptest Imagine taking a straw and bubbling air through liquids in two glasses milk and a milkshake. Which liquid has the higher viscosity? A. Milk B. Milkshake Hint: Gas (air) would escape more readily from which liquid?
20 Volcanoes and Other Mountains Conceptest How would you classify the viscosity of the magma that produced the eruption of Nyiragongo and the violence of the eruption? A. Low-viscosity magma; violent eruption B. High-viscosity magma; violent eruption C. High-viscosity magma; mild eruption D. Low-viscosity magma; mild eruption
21 Magma Viscosity Viscosity depends upon magma composition (silica content) Q: Why does silica content vary? A: Silica content controlled by magma source (plate tectonics)... See next section
22 Go back to the Table of Contents Go to the next section: Magma Sources and Magma Composition
23 Magma Sources and Magma Composition Most active volcanoes (top) found near convergent plate boundaries (bottom) Others associated with divergent boundaries (Iceland, East Africa) or form above hot spots (Hawaii)
24 Magma Sources and Magma Composition Different plate settings generate magma from melting different source rocks 1 2. Rhyolitic magma - melting of parts of continental crust 2 1. Basaltic magma partial melting parts of asthenosphere below oceanic ridge or hot spots 80% of all magma), also continental divergent boundary and hot spot
25 Magma Sources and Magma Composition Different plate settings yield different magmas from different source rocks Divergent Boundary Oceanic Ridge Basaltic magma Oceanic Hot spot Basaltic magma Divergent Boundary Continental Rift Basaltic & Rhyolitic magma
26 Magma Sources and Magma Composition Different plate settings generate magma from melting different source rocks 3. Andesitic magma partial melting of mantle rocks (with water) at subduction zone 3
27 Magma Sources and Magma Composition Different plate settings yield different magmas from different source rocks Convergent Boundary Oceanic Trench/ Subduction Zone Andesitic magma Convergent Boundary Oceanic Trench/ Subduction Zone Andesitic magma
28 Magma Sources and Magma Composition Silica content is controlled by partial melting of rocks at magma source Partial melting occurs when some minerals in a rock melt while others remain solid Minerals with lowest melting temperatures will melt first Silica-rich minerals have lowest melting temperatures Partial melting generates a more silica-rich magma than the parent rock
29 Magma Sources and Magma Composition Three types of magma defined by silica content Partial Melting Of Asthenosphere/Mantle wedge/continental crust... generates Basaltic/Andesitic/Rhyolitic magma with... Low/Intermediate/High silica content Different magma types form in different plate tectonic settings
30 Volcanoes and Other Mountains Conceptest Which image best illustrates the source of most magma from active volcanoes? Magma has a source in the molten rocks of the outer core Magma comes from a source layer in the mantle Magma comes from isolated sources below volcanoes
31 Volcanoes and Other Mountains Checkpoint 6.6 Use the Venn diagram to compare and contrast the compositions and sources of the 3 types of magma. 1. Low silica content 2. From a mantle source 3. Example: Aleutian Island volcanoes, Alaska
32 Volcanoes and Other Mountains Checkpoint Examine the map and identify 5 volcanoes formed above subduction zones. 2. Name a volcano that generates low-viscosity magma. 3. Other than Mauna Loa, which volcano is most likely to have formed above a hot spot?
33 Go back to the Table of Contents Go to the next section: The Mount St. Helens Eruption
34 The Mount St. Helens Eruption Cascade Mountains volcanic arc in Pacific Northwest Major cities within 100 km of active volcanoes Mount St. Helens eruption of May 18, 1980
35 The Mount St. Helens Eruption Cascade Mountains Volcanoes formed above subduction zone where Juan de Fuca plate slides beneath North America Mount St. Helens is most active volcano in conterminous US
36 Volcanoes and Other Mountains Conceptest What type of magma is associated with Mount St. Helens? A. Basaltic B. Rhyolitic C. Andesitic
37 The Mount St. Helens Eruption Prior Activity Early (March) unrest featured Minor eruptions Earthquakes Release of volcanic gases Followed by change in shape of cone (bulge on North flank) Increasing frequency of earthquakes
38 The Mount St. Helens Eruption May 18 Eruption A moderate earthquake triggered a massive landslide (debris avalanche) on the North side of the volcano Debris clogged streams Pressure released on near-surface magma Lateral blast produces an initial sideways eruption to North Later vertical eruption
39 The Mount St. Helens Eruption Measuring the Eruption Volcanic Explosivity Index (VEI) measures volume of erupted material 8 divisions on log scale (x10 increase between divisions)
40 The Mount St. Helens Eruption Measuring the Eruption Mount St. Helens May 18 eruption had a VEI = 5 Later eruptions were much smaller Eruption of VEI 5 or higher approximately every 22 years Loss of life from eruptions often associated with associated mudflows, tsunami
41 Go back to the Table of Contents Go to the next section: Products of Volcanic Eruptions
42 Products of Volcanic Eruptions Major products of volcanic eruptions: Airborne lateral blast, tephra, volcanic gases Flows on land lava, pyroclastic flows, lahars
43 Products of Volcanic Eruptions Eruption of Mount St. Helens reduced height of volcano by 400 meters Features near volcano were blown over or carried away by products of eruption Geologist David Johnston (right) died at this site (Johnston s Ridge) located 10 km from the volcano.
44 Products of Volcanic Eruptions Airborne Eruption Products Rare lateral blasts can destroy objects up to 12 km away and knock down trees more than 25 km distant Effect of lateral blast only seen on North flank of Mount St. Helens
45 Products of Volcanic Eruptions Airborne Eruption Products Tephra represents particles blasted into air by eruption Volcanic bombs and ash are found near and far from eruption source, respectively Blobs of magma solidify to form lava bombs Wind can transport fine volcanic ash for hundreds of kilometers downwind
46 Products of Volcanic Eruptions Airborne Eruption Products Tephra represents particles blasted into air by eruption Volcanic ash (particles of <2 mm diameter) Measurable ash deposits found hundreds of km from volcano Compare Mt. St. Helens ash fall to the Yellowstone super-eruption 640,000 years ago.
47 Products of Volcanic Eruptions Airborne Eruption Products Volcanic gases (water vapor, sulfur dioxide, carbon dioxide) may affect climate patterns Sulfur dioxide may block insolation, temporarily (up to 1 year) reducing global temperatures Trees killed by excessive carbon dioxide released by magma under Mammoth Mountain, California. Widespread release of carbon dioxide and higher temperatures due to faster rates of volcanic activity approximately million years ago
48 Products of Volcanic Eruptions Eruption Products on Land Low viscosity lava can flow up to 50 km from its source Lava transported to front of lava flows in long lava tubes Lava flows build up in a series of layers
49 Products of Volcanic Eruptions Eruption Products on Land Low viscosity lava can flow up to 50 km from its source Lava transported to front of lava flows in long lava tubes Lava flows build up in a series of layers Walter s Kalapana store, Hawaii, was buried in lava within a few weeks in 1990
50 Products of Volcanic Eruptions Eruption Products on Land Higher viscosity lava remains within volcano crater Lava dome formed in crater of Mount St. Helens
51 Products of Volcanic Eruptions Eruption Products on Land Pyroclastic flow dense cloud formed from combination of tephra and volcanic gases Fast moving, up to 700 C
52 Products of Volcanic Eruptions Eruption Products on Land Lahars mudflows formed when volcanic debris mixes with streams or melting ice Often confined to stream channels Lahar along Muddy River reached depths of 20 meters following Mount St. Helens eruption
53 Volcanoes and Other Mountains Conceptest Predict which product of the volcanic eruption traveled farthest from Mount St. Helens? A. Pyroclastic flow B. Lava C. Lahar
54 Volcanoes and Other Mountains Checkpoint 6.14 Compare and contrast the eruptions of Nyiragongo and Mt. St. Helens. Place the numbers in the appropriate locations on the diagram. 1. Located near a convergent plate boundary. 2. Located near an early stage divergent plate boundary. 3. Produced significant lava flows. 4. Eruption followed a century of inactivity. 5. Several eruptions in the last century. 6. Few monitoring instruments prior to unrest. 7. Volcanic gases released prior to main eruption. 8. Frequent earthquakes associated with unrest. 9. Unrest lasted for approximately 2 months before eruption. 10. Eruption occurred in daylight. 11. Volcano located within 20 km of large city. 12. Volcanic activity subsided after about one week. 13. Low viscosity magma. 14. USGS geologists aided in interpretation of volcanic activity. 15. Death toll less than Death toll more than Shape of volcano changed prior to eruption. 18. Eruption characterized by a massive lateral blast.
55 Go back to the Table of Contents Go to the next section: Volcanoes and Volcanic Landforms
56 Volcanoes and Volcanic Landforms Three types of volcanoes Shield, stratovolcanoes, cinder cone Composed of different materials Volcano type and eruption style varies with plate setting Different sizes (note size of trees on volcano slopes) Shield volcano Stratovolcano Cinder cone volcano
57 Volcanoes and Volcanic Landforms Shield volcanoes (e.g., Hawaiian Islands) Broad, gentle slopes Built from many low viscosity lava flows (basalt) Relatively mild eruptions associated with hot spots, divergent plate boundaries
58 Volcanoes and Volcanic Landforms Stratovolcanoes (e.g., Osorno, Chile) Most common volcano type Steeper slopes built from alternating layers of tephra and medium viscosity lava (andesite) Form on plates overriding subduction zones at convergent plate boundaries
59 Volcanoes and Volcanic Landforms Cinder cone volcanoes Smallest volcanoes, up to 400 meters elevation Built from more viscous magma products (coarse tephra) May form on slopes of shield or stratovolcanoes Sunset Crater volcano, Arizona
60 Volcanoes and Other Mountains Conceptest What type of volcano is Mount St. Helens? A. Shield volcano B. Stratovolcano C. Cinder cone volcano
61 Volcanoes and Volcanic Landforms Other Volcanic Landforms - Caldera Giant crater formed from collapse of volcano into underlying magma chamber e.g., Yellowstone, Wyoming Caldera formed from collapse of previous volcano Crater of Karymsky volcano
62 Volcanoes and Volcanic Landforms Other Volcanic Landforms Lava Plateau Hundreds of low viscosity lava flows stack up to on top of each other Individual layers of basalt meters thick Plateau thousands of meters thick Form some of the largest volcanic eruptions e.g., Columbia River plateau
63 Volcanoes and Volcanic Landforms Lava Plateau Massive basalt eruptions formed lava plateaus and elevated atmospheric levels of carbon dioxide Myrs ago to produce global hothouse conditions
64 Volcanoes and Volcanic Landforms Geysers, Hot Springs, Fumeroles, Mud Volcanoes Form when water circulates near magma chamber Geyser water heated under pressure with volcanic gases Hot spring heated groundwater rises to surface Mud volcano chemical reactions convert rock to clay Fumerole volcanic gases escape in absence of water Geyser Hot Spring Mud volcano Fumerole
65 Go back to the Table of Contents Go to the next section: Mountains: Why Are They There?
66 Mountains: Why Are They There? Thickest crust found below mountains along convergent plate boundaries Himalayas, 70 km thick Andes, up to 60 km thick normal crust 40 km thick Himalayas - thickest continental crust and tallest mountains
67 Mountains: Why Are They There? Higher, younger mountains along present convergent boundaries (e.g., Himalayas) Lower, older mountain belts represent ancient convergent boundaries (e.g., Appalachians)
68 Mountains: Why Are They There? India India Eurasia Eurasia Reverse faults stack up and thicken the crust along convergent plate boundaries Additional thickening of crust where the northern margin of Indian continental crust wedged below southern margin of Eurasia
69 Mountains: Why Are They There? If the crust is 30 km thicker under the Himalayas, whey are they not 30 km higher than the rest of the continents? The elevation of mountains is tied to density and isostasy.
70 Mountains: Why Are They There? Density = mass per unit volume Density of water = 1 g/cm 3 Density of pine wood = 0.5 g/cm 3 What would happen if we used a block of oak with a density that is 80% that of water? Wood floats because it is less dense than water Density of pine is half (50%) the density of water Half of the pine block lies below the water surface
71 Mountains: Why Are They There? Density of oak is 80% density of water 80% of oak blocks lies below the surface Smaller blocks don t float as high but don t extend as far below surface Much of the difference in the size of the blocks is in the submerged root
72 Volcanoes and Other Mountains Checkpoint 6.23 Different types of wood have different densities. For example, the density of pine (0.5 g/cm 3 ) is less than ebony (0.9 g/cm 3 ) but more than balsa wood (0.14 g/cm 3 ). All would float in water but with different proportions of each block lying above and below the surface. 1. Draw a diagram to show what would happen if equal-sized blocks of each type of wood were added to a container full of water. 2. What would happen to the blocks if we replaced the water with a liquid with higher density like corn syrup?
73 Mountains: Why Are They There? Density of continental crust is 80% density of mantle Thicker continental crust rises higher but also extends farther below the surface Much of the difference in the thickness of the continental crust is in the crustal root Similar to deeper foundation for taller buildings
74 Mountains: Why Are They There? Elevation of mountains depends on Thickness of crust Density contrast with underlying mantle Isostasy balance between topography of Earth s surface and thickness and density of underlying rocks Higher mountains with thicker or less dense rocks
75 Volcanoes and Other Mountains Checkpoint 6.24 The following profile shows the topography of a continent. Draw the relative position of the base of the crust taking into account the principles of isostasy. Label continental crust, oceanic crust, and mantle. Explain your drawing.
76 Go back to the Table of Contents Go to the next section: The Rise and Fall of Mountains and Temperatures
77 The Rise and Fall of Mountains and Temperatures Changes in elevation depends on the relative density of crust and mantle The height of wood blocks and continents will decrease (float lower) as mass is removed The height of wood blocks and continents will increase (float higher) as mass is added Elevation only changes by 20% of added/removed material Most changes occur in crustal root below the surface
78 The Rise and Fall of Mountains and Temperatures Elevation only changes by 20% of added/removed material Most changes occur in crustal root below the surface Isostasy compensates for added material by building a bigger root or for lost material by raising the pile
79 Volcanoes and Other Mountains Conceptest How would the elevations of mountains differ if Earth s crust was composed of denser rocks? Mountains would be A. Higher B. Lower C. Unchanged in elevation
80 The Rise and Fall of Mountains and Temperatures For every 1,000 meters of rock eroded from mountains, isostasy results in just 200 meters decrease in elevation 800 meters of change accommodated by raising the crustal root Evenly distributed erosion causes uniform lowering of mountains Erosion concentrated in valleys, can cause peaks to become higher
81 The Rise and Fall of Mountains and Temperatures Mountains are long-lived features on Earth Still forming Himalayas began to form ~50 Myrs ago Lower Appalachian Mountains formed ~300 Myrs ago
82 The Rise and Fall of Mountains and Temperatures Mountains are long-lived features on Earth Approximate erosion rates mm/year Rate depends of rock types, climate, other factors 5-10 Myrs to erode 1 km of rock But isostasy will replace 80% of this erosion 200 meter change in elevation every 5-10 Myrs Myrs to lower mountains by 1 km Mountains ranges will persist on the landscape for hundreds of millions of years
83 Volcanoes and Other Mountains Checkpoint 6.26 Use the information from this section to explain: 1. Why are the ten tallest U.S. peaks all in Alaska? 2. Why are the Rocky Mountains taller than the Appalachian Mountains? 3. Why we can drive across former mountains in Canada without rising in elevation?
84 The Rise and Fall of Mountains and Temperatures Erosion of the Himalayan Mountains Warm, moist air from Indian Ocean rises over Himalayas to form monsoon rains Rain feed rivers that erode mountains Sediment deposited in Bay of Bengal and Arabian Sea
85 The Rise and Fall of Mountains and Temperatures Mountains influence climate patterns Monsoon rains strip carbon dioxide from atmosphere Removal of CO 2 has been going on for 20 Myrs Reduction of this greenhouse gas has lowered global temperatures by ~5 o C
86 Use information from this chapter to identify interactions between volcanoes and the earth system.
87 The End Go back to the Table of Contents