1 Volcanoes and Volcanic HazardsChapter 5
2 The Nature of Volcanic EruptionsAll eruptions involve magma Magma is molten rock that usually contains some crystals and varying amounts of dissolved gases Lava is erupted magma The behavior of magma is determined by: Temperature of the magma Composition of the magma Dissolved gases in the magma The above three factors control the viscosity of a magma, which in turn controls the nature of an eruption
3 The Nature of Volcanic EruptionsViscosity is a measure of a material’s resistance to flow The more viscous the material, the greater its resistance to flow Ex: syrup is more viscous than water Factors affecting viscosity Temperature – hotter magmas are less viscous Composition – silica (SiO2) content Higher silica content magmas are more viscous (e.g., rhyolitic and andesitic magmas) Lower silica content magmas are less viscous (e.g., basaltic lavas) Dissolved gases Dissolved water in magma reduces viscosity Gases expand within a magma as it nears Earth’s surface due to decreasing pressure The violence of an eruption is related to how easily gases escape from magma
4 Magma Type Chemical Composition Temperature (degrees C) Viscosity Gas Content Basaltic 45-55% SiO2; High in Fe, Mg, Ca; Low in K, Na. Low Andesitic 55-65% SiO2; Intermediate Fe, Mg, Ca, Na, K Intermediate Rhyolitic 65-75% SiO2; Low in Fe, Mg, Ca; High in K, Na High
5 The Nature of Volcanic EruptionsQuiescent Versus Explosive Eruptions Quiescent Hawaiian-Type Eruptions Involves fluid basaltic lavas Eruptions are characterized by outpourings of lava that can last weeks, months, or even years Explosive Eruptions Associated with highly viscous magmas Eruptions expel particles of fragmented lava and gases at supersonic speeds that evolve into eruption columns
6 Materials Extruded During an EruptionLava Lava Flows ~90 percent of lava is basaltic lava <10 percent of lava is andesitic lava ~1 percent of lava is rhyolitic lava Aa and Pahoehoe Flows Composed of basaltic lava Aa flows have surfaces of rough jagged blocks Pahoehoe flows have smooth surfaces and resemble twisted braids of rope Pillow Lavas Composed of basaltic lavas extruded underwater: found in places like Hawaii Flow is composed of tubelike structures stacked one atop the other
7 A Typical Aa Flow A Pahoehoe Lava Flow
8 Pahoehoe
9 Aa
10 Lava tube: A tube formed by cooling and solidifying of the lava walls while fluid lava continued to flow inside.
11 Materials Extruded During an EruptionLava (continued) Block Lavas Composed of andesitic and rhyolitic lava Upper surface consists of massive, detached blocks Gases Gases make up 1%‒6% of the total weight of a magma As the magma reaches the surface and the pressure is reduced, the gases expand and escape
12 Materials Extruded During an EruptionPyroclastic Materials Volcanoes eject pulverized rock and lava fragments called pyroclastic materials Particles range in size from fine dust, to sand-sized ash, to very large rocks Tephra:Pyroclastic material Volcanic ash – fine glassy fragments Welded tuff – fused ash Lapilli – walnut-sized material Cinders – pea-sized material
13 Materials Extruded During an EruptionPyroclastic Materials (continued) Blocks – hardened or cooled lava Bombs – ejected as hot lava
14 Materials Extruded During an EruptionPyroclastic Materials (continued) Pumice – light gray or pink porous rock from frothy andesitic and rhyolitic lava Scoria – reddish-brown porous rock from frothy basaltic and andesitic lava
15 Anatomy of a Volcano
16 Anatomy of a Volcano General FeaturesConduit – a fissure that magma moves through to reach the surface Vent – the surface opening of a conduit Volcanic cone – a cone of material created by successive eruptions of lava and pyroclastic material
17 Anatomy of a Volcano General Features (continued)Crater – a funnel-shaped depression at the summit of most volcanic cones, generally less than 1 km in diameter Caldera – a volcanic crater that has a diameter of >1 kilometer and is produced by a collapse following a massive eruption Parasitic cones – a flank vent that emits lava and pyroclastic material Fumaroles – a flank vent that emits gases
18 Shield Volcanoes General Features Broad, slightly dome-shapedCovers large areas Produced by mild eruptions of large volumes of basaltic lava Most begin on the seafloor as seamounts; only a few grow large enough to form a volcanic island Examples include the Hawaiian Islands, the Canary Islands, the Galapagos, and Easter Island Mauna Loa is the largest shield volcano on Earth
19 Shield Volcanoes
20 Cinder Cones General Features Built from ejected lava fragmentsSteep slope angle Rather small size Frequently occur in groups Sometimes associated with extensive lava fields Paricutin (located 320 km west of Mexico City) is an example of a cinder cone
21 Cinder Cones
22 Composite Volcanoes General Features Also called stratovolcanoesLarge, classic-shaped volcano (symmetrical cone, thousands of feet high and several miles wide at the base) Composed of interbedded lava flows and layers of pyroclastic debris, thus have pyroclastic eruptions Many are located adjacent to the Pacific Ocean in the Ring of Fire Mount St. Helens and Mount Etna are examples
23 Composite Volcanoes
24
25 Volcanic Hazards Pyroclastic Flows Lahars Other hazardsVolcano-related tsunamis Volcanic ash Volcanic gases – Effects of volcanoes on climate
26 Volcanic Hazards Pyroclastic FlowsA pyroclastic flow is a mixture of hot gases infused with incandescent ash and lava fragments that flows down a volcanic slope Also called a nuée ardente Propelled by gravity and move similarly to snow avalanches Material is propelled from the vent at high speeds (can exceed 100 km [60 miles] per hour) Pyroclastic flows are typically generated by the collapse of tall eruption columns A surge is a small amount of ash that separates from the main body of the pyroclastic flow Occasionally, these surges have enough force to knock over buildings and move automobiles In 1902, the town of St. Pierre was destroyed by a pyroclastic flow from Mount Pelée
27 Pyroclastic Flows
28 Volcanic Hazards LaharsA lahar is mudflow on an active or inactive volcano Volcanic debris becomes saturated with water and rapidly moves down a volcanic slope Some lahars are triggered when magma nears the surface of a volcano covered in ice and snow and causes it to melt In 1985, lahars formed during the eruption of Nevado del Ruiz, killing 25,000 poeple
29 Lahars
30 Lahar
31 Volcanic Hazards Volcano-related tsunamis Volcanic ash Volcanic gasesOther hazards Volcano-related tsunamis Destructive sea waves can form after the sudden collapse of a flank of a volcano Volcanic ash Jet engines can be damaged when flying through a cloud of volcanic ash In 2010, the eruption of Iceland’s Eyjafjallaöku created a thick plume of ash over Europe, stranding hundreds of thousands of travelers Volcanic gases Volcanoes can emit poisonous gases, endangering humans and livestock
32 Volcanic Hazards Effects of volcanoes on climateAsh particle released from volcanoes can reflect solar energy back into space The ash from the eruption of Mount Tambora in 1815 led to the “year without summer” (1816)
33 Other Volcanic LandformsCalderas Calderas are circular, steep-sided depressions with a diameter >1 km Three different types Crater Lake-type calderas: Form from the collapse of the summit of a large composite volcano following an eruption; these calderas eventually fill with rainwater Hawaiian-type calderas: Form gradually from the collapse of the summit of a shield volcano following the subterranean drainage of the central magma chamber Yellowstone-type calderas: Form from the collapse of a large area after the discharge of large volumes of silica-rich pumice and ash; these calderas tend to exhibit a complex history
34 Formation of Crater Lake-Type Calderas
35 Hawaiian-Type Calderas
36 Formation of Yellowstone-Type Calderas
37 Practice Question Which of the following choices best explains the difference between a crater and a caldera? A) A crater is a very large circular depression over 1 km across; A caldera is a small funnel-shaped depression. B) A crater is a small, funnel-shaped depression; A caldera is a large depression that has a diameter of over 1 km. C) A crater is a fissure in the crust; A caldera is a collapsed summit. D) A crater is a large circular depression over 1 km across; A caldera is a small, parasitic cone growing on the flank of the volcano. Answer: B
38 Other Volcanic LandformsLarge Igneous Provinces Large igneous provinces cover a large area with basaltic lava Basaltic lava extruded from fissures blanket a large area, called a large igneous provinces or basalt plateaus The Colombia Plateau and the Deccan Traps are two examples
39 Other Volcanic LandformsLava Domes A lava dome is a small dome-shaped mass composed of rhyolitic lava Volcanic Necks and Pipes A volcanic neck is the remains of magma that solidified in a volcanic conduit Shiprock, New Mexico, is an example
40 Plate Tectonics and Volcanic ActivityVolcanism at convergent plate boundaries Occurs at subduction zones, where two plates converge and the oceanic lithosphere descends into the mantle Volcanic arcs develop parallel to the associated subduction zone trench The Aleutians, the Tongas, and the Marianas are examples of volcanic island arcs The Cascade Range is an example of a continental volcanic arc Most active volcanoes are found along the circum-Pacific Ring of Fire Eruptions tend to be explosive and associated with volatile- rich, andesitic magma
41 Convergent Plate Volcanism
42 Plate Tectonics and Volcanic ActivityVolcanism at divergent plate boundaries 60% of Earth’s yearly output of magma is from spreading centers Characterized by a vast outpouring of fluid, basaltic lavas
43 Plate Tectonics and Volcanic ActivityIntraplate volcanism: Hot Spot Volcanoes that occur thousands of kilometers from plate boundaries Occurs when a mantle plume ascends towards the surface Examples include the Hawaiian Islands, the Columbia River Basalts, and the Galapagos Islands
44 Until 1996 Loihi was thought to be an inactive seamount.It began erupting in 1996 and the eruptions were preceded by a cluster of small earthquakes indicating the movement of magma.
45 The modern active island rests close to the hot spot and its shield volcanoes are fed from the magma that the hot spot generates.
46 Monitoring Volcanic ActivityEfforts aimed at detecting movement of magma from a subterranean reservoir Changes in patterns of earthquakes Inflation of the volcano related to rising magma Changes in the amount and/or composition of gases released from the volcano Increase in ground temperature Remote sensing devices aid in monitoring limited- accessibility volcanoes A volcano must be monitored for a long time to recognize a difference between “resting state” and “active state”
47 Monitoring Volcanic ActivityMonitoring gas emissions Measuring magma movement Measuring ground temperature
48 A surface bulge appeared on Mt. St. Helens prior to its eruption.Surface tilting: recognition of changes in the land surface due to building pressure in the conduit. A surface bulge appeared on Mt. St. Helens prior to its eruption. April 8, 1980 April 26 May 2
49 Hazard zones have been distinguished around MtHazard zones have been distinguished around Mt. Shasta based on topography and past experience with eruptions. Zone 1: areas likely to be affected most frequently. Most future flows from summit eruptions probably would stay within this zone. Zone 1
50 Hazard zones have been distinguished around MtHazard zones have been distinguished around Mt. Shasta based on topography and past experience with eruptions. Zone 1: areas likely to be affected most frequently. Most future flows from summit eruptions probably would stay within this zone. Zone 2: areas likely to be affected by lava flows erupted from vents on the flank of the volcano or that move into zone 2 from zone 1. Zone 2
51 Hazard zones have been distinguished around MtHazard zones have been distinguished around Mt. Shasta based on topography and past experience with eruptions. Zone 1: areas likely to be affected most frequently. Most future flows from summit eruptions probably would stay within this zone. Zone 3: areas likely to be affected infrequently and then only by long lava flows that originate at vents in zones 1 and 2 Zone 3 Zone 2: areas likely to be affected by lava flows erupted from vents on the flank of the volcano or that move into zone 2 from zone 1.
52 Earthquakes: generated as the magma moves up the feeder conduit to the vent.When viscous magma becomes stuck in the conduit strain energy builds as more magma tries to push out. Movement takes place in a series of “jerks” as the rock material breaks. Each “jerk” produces an earthquake. Magnitudes generally less than 5 M. The more earthquakes the further the magma has moved.
53 The 1992 Eruption of Crater Peak VentMount Spurr, Alaska: The 1992 Eruption of Crater Peak Vent USGS Black bars: earthquake frequency. Red lines: volcanic eruptions.
54 A combination of approaches is likely the key to short-term prediction.
55 The impact of volcanic eruptions
56 A series of eruptions of Tambora (Indonesia) extruded up to 150 km3 of magma (solid equivalent), much of it into the atmosphere. Tambora (1815 eruption) was followed in 1816 by the “year without a summer”. Average global temperature is estimated to have been reduced by 3 degrees Celsius.
57 In June of 1816 there was widespread snowfall throughout the eastern United States.The normal growing season experienced repeated frosts as cold air extended much more southerly than normal. Food shortages and starvation are attributed to the deaths of 80,000 people. The global population was about 1 billion people in 1816. Our current population is a little over 6 billion. The 1816 fatality rate would have resulted in a death toll of nearly 500,000 people due to starvation.
58 Approximately 260,000 people have been killed by volcanoes in historic times…most by a handful of individual eruptions.
59 Yellowstone Caldera Known for its hot springs and geysers, Yellowstone National Park, is likely the most popular super volcano in the world. The park sits on an active caldera that rises and sinks in response to magma movement and pressure fluctuations within the Earth. Over recent years the surface has risen by as much as a metre and sunk back by 1/3 of a metre. Thousands of small earthquakes are produced as earth surface moves.
60 The magma chamber is only 5 to 13 km below the land surface.The caldera is 80 km long and 50 km wide.
61 The caldera and its magma chamber are due to a hot spot in the mantle that has moved several hundred kilometres over the past 12.5 million years. The movement is due to the drift of the north American continent over the hot spot. Ancient, inactive calderas mark the path of the hot spot.
62 The current caldera was formed with an eruption 640,000 years ago (the Lava Creek Eruption).This eruption ejected 1,000 km3 of pyroclastic debris. An earlier eruption (the Huckleberry Ridge Eruption, 2 million years ago) ejected 2,500 km3 of pyroclastic debris. A smaller eruption happened 1.3 million years ago, releasing 280 km3 of debris.
63 Eruptions appear to have a 600,000 year period (that long between eruptions) so we’re overdue for another one. Previous eruptions spread ash over thousands of km2 across the US.
64 Heightened monitoring of the Yellowstone Caldera in recent years has led to media concern of an impending eruption. Government officials and geologists indicate that there have been no clear indicators of high risk at this time. If such an eruption were to take place, North America and the rest of the world could experience another “Dark Ages”.