Download
1 / 41
410 likes | 414 Views
Volcanoes Chapter 6. You will learn. What volcanoes are, where they occur, and why they form The different types of volcanoes and the hazards associated with each type How scientists study volcanoes and come to understand and predict their eruptions
E N D
You will learn What volcanoes are, where they occur, and why they form The different types of volcanoes and the hazards associated with each type How scientists study volcanoes and come to understand and predict their eruptions How people can lower the risks posed by volcanoes
Over the Volcano Figure 6-2 Over the Volcano If you fly over the Pacific rim, you will probably find yourself looking down on volcanoes. This erupting volcano, in the Aleutian Islands of Alaska, is on the main flight path between Asia and North America.
What Volcanoes Are • Magma: (molten rock underground) rises from great depths to the upper level of the crust and erupts (as lava) onto the surface. • hot and less dense (more buoyant) than surrounding solid rock • quickly cools and solidifies on the surface—or at relatively shallow depths in the crust • magma contains dissolved gases (e.g. H2O vapor, CO2, SO2) • As magma rises: • confining pressure of the overlying rocks decreases • dissolved gases bubble out of solution and release the pressure • the more dissolved gas, the more explosive the magma
Lava Flow Figure 6-3 Volcanoes Are Rock Factories Magma quickly cools and solidifies into igneous rock. The surface of this lava flow in Hawaii is changing to a gray color as it solidifies.
Numbers of Volcanic Eruptions • How many volcanoes have been active in the last 10,000 years? • 1300–1500 volcanoes on land with probable eruptions • only about 550 volcanic eruptions eyewitness accounts • 50–70 volcanoes erupt on land in an average year • many more volcanic eruptions occur in the deep oceans
Potentially Active Volcanoes ofthe Western U.S. Figure 6-4 Many people do not realize that even in the continental United States there are several potentially active volcanoes dangerously close to large population centers.
Types of Magmas • Eruptions range from spectacular explosions to oozing red-hot lava flows. • This difference reflects the fact that not all magmas are the same. • different temperatures • compositions • dissolved-gas contents
Types of Magmas (cont.) • Viscosity = resistance to flow, depends on temperature, composition • Composition (e.g, SiO2 content) has greatest influence on magma viscosity • high SiO2 = more viscous • magmas with high SiO2 solidify at lower temperatures • their viscosity increases even more • Amount of SiO2 depends on: • composition of source rocks • % partial melting of source—low % partial melt = higher SiO2
Gas Content in Lavas • H2O—most abundant gas dissolved in magma, along with CO2, SO2, and others • lowers viscosity (by reducing the tendency of SiO4 ions to combine) • Gases may separate from magma during ascent due to: • the magma may partially crystallize (and thus hold less liquid) • temperature may go down • being trapped, accumulating in more viscous magmas • decrease in the confining pressure • gas (vapor) pressure increases • Explosive eruptions!
Shield Volcanoes of the Big Island of Hawaii • Kilauea is the currently active volcano on the island. Loihi is just beginning to form underwater to the South. Figure 6-6The Big Island of Hawaii consists of five overlapping shield volcanoes formed at different times over the last few hundred thousand years.
Types of Volcanoes • Shield volcanoes • wide and gently sloping (like an upturned warrior’s shield) • almost entirely composed of layers of solidified mafic (basaltic) lava • mafic magma is hot and less viscous, so it flows long distances • largest on Earth and build over hundreds of thousands of years • Example: island of Hawaii • 5 successively younger, overlapping shield volcanoes • the island Hawaii is the largest mountain on Earth • high point ≈10,200 meters (33,500 ft) above seafloor
Types of Volcanoes (cont.) • Flood basalts • mafic magmas that erupt from fissures—cover extensive areas with basalt • form stacks (over thousands of years) of basaltic flows • the amount of magma erupted can be tremendous • Columbia River flood basalt—covers 130,000 km2(50,000 mi2) • 100,000 km3 (24,000 mi3) of basalt was erupted ≈ 5 million years ago. • Cinder cones • from mafic magma, rich in gas, spew globs of lava into air • erupt at shield volcanoes, flood basalt areas, and stratovolcanoes • pieces solidify and rain down as cindersand larger blocks (lava bombs) • this loose debrisfalls down around the vent to form cone-shaped piles • typically short-lived and low volume
Types of Volcanoes (cont.) • Stratovolcanoes • towering, steep-sloped, and frequently symmetrical mountains • complexly alternating layers of lava and other volcanic debris • intermediate to felsic in composition (some mafic magma) • many stratovolcanoes commonly caped with glaciers • magmas are viscous and gas-rich, and can erupt explosively
Pyroclastics (Tephra) Figure 6-13 Pyroclastic debris ranges from (a) fine ash (small crystals, rock fragments, and bits of glassy frozen magma), (b) pumice (glassy solidified magma with many holes created by gas bubbles), (c) to large blocks of rock; (d) glowing lava bombs and cinders
Large Calderas • Calderas are large, circular-to-oblong depressions that form when magma chambers erupt their contents and the volcanic mountain above collapses into the empty magma chamber. • Caldera-forming eruptions can expel tens to thousands of cubic kilometers (cubic miles) of ash that completely blankets whole regions. (Volcanic ash that becomes lithified is tuff.) • Crater Lake National Park formed from the collapse of Mount Mazama, forming the caldera—which subsequently filled with water.
Huge Caldera Eruptions • La Garita Caldera (New Mexico) erupted 5000 km3 (1200 mi3) of tuff —28 million years ago • Toba Caldera (Sumatra in Indonesia) erupted 2800 km3 (670 mi3) of tuff 75,000 years ago = largest volcanic eruption during the last 2 million years • Yellowstone Caldera— eruptions at: • 2.0 million years ago • 1.2 million years ago • .64 million years ago—Lava Creek Tuff 1000 km3 (240 mi3)
The Volcanic Explosivity Index (VEI) • The VEI assigns a volcano a number from 0 to 8 based on: • volume of material ejected by past eruptions • height of eruption column • style of the eruption (such as lava flows vs. explosive eruptions) • duration of the eruption
Benefits of Volcanoes • Ultimate source of valuable natural resources • metal deposits are found in the roots of extinct volcanoes • solid debris (e.g., pumice and ash) mined for construction materials and abrasives • Mountain scenery that draws tourists (Mount Fuji in Japan) • Ski resorts (Mount Hood in Oregon) • Geothermal springs/spas (Yellowstone National Park) • Geothermal power (ancient and current) • Weathering transforms ash and lava into fertile soil that supports agriculture in many lands (Mount Vesuvius in Italy)
Volcanoes: Divergent Plate Boundaries • About 62% of the magma erupted on Earth is at mid-ocean ridges. • Mantle material decompresses as it rises below divergent plate boundaries. • As the mantle material rises—pressure and melting point decrease • At 100 km (62 mi), temperature of the mantle is 1200°–1400°C • Melting doesn’t occur—pressures keep the melting point high • Material rises to lower-pressure regions, where the melting point is lower than the temperature of the mantle material • Melting produces a mafic magma, derived from partially melted ultramafic mantle rock
Volcanoes: Convergent Plate Boundaries • About 26% of the magma erupted on Earth is generated at subduction zones—where oceanic lithosphere sinks into the mantle • descending plate carries water • trapped in oceanic crust and sediments • in the mantle, the oceanic plate recrystallizes, releases water and other volatiles, such as carbon dioxide • these rise into the overlying mantle and lower its melting point • overlying mantle begins to melt producing mafic (basaltic) magma • BUT the magma that erupts at subduction-zone volcanoes is primarily intermediate in composition (60% silica), making it an andesite. HOW? • mafic magma may differentiate • heat may partially melt crust
Volcanoes: Within Plates • Volcanoes that are far from plate boundaries (Hawaii; Yellowstone) • Hypothesis: • caused by local anomalies in the mantle called hot spots • voluminous mantle material rises and melts to form mafic magma • crust above hot spots melts to produce intermediate and felsic magma • Example: Hawaiian Islands—or Emperor-Hawaiian Seamount Chain • >80 volcanoes—progressively younger from N to S (80 mya to now), change in age along the chain has been attributed to progressive movement of the Pacific Plate over a fixed mantle hot spot. • However, melting and magma movement in the mantle can be influenced by: • mantle’s thermal structure • mantle composition (larger % of lower-melting-point material)
The Hawaiian Islands • Recent research is suggesting that hot spots may not all be stationary. Figure 6-21 (a) The progressive change in age of the Emperor-Hawaiian seamount chain—getting younger from north to south—has been interpreted to reflect the movement of the Pacific plate over a “fixed” mantle hot spot, as shown in part (b).
The Long Valley Caldera Erupted the Bishop Ash 760,000 Years Ago FIGURE 6-22 (a) This volcanic eruption covered most of the southwestern United States, including Los Angeles, with ash. The continued volcanic activity, which produced Mammoth Mountain (b) with its popular ski slopes, is still active and is being closely monitored by the U.S. Geological Survey.
Mount St. Helens Erupts Figure 6-24 Ash, steam, water, and debris were blasted to a height of 18,300 meters (60,000 ft) during the May 18, 1980, eruption of Mt. St. Helens.
Pyroclastic Flows and Surges • Hot ash/solid debris—>800oC (1500oF), up to160 km/hr (100 mph) • Causes: • 1) Gravitational collapse of ash column above a stratovolcano • hot ash, gas in the column rise—reach cooler altitudes, lose buoyancy • column falls back in on itself, spreads out on ground as a pyroclastic flow • 2) Lateral eruption • side of a volcano may collapse, releasing pressure on magma, initiating a lateral (sideways) pyroclastic eruption (1980, Mount St. Helens, WA) • 3) Collapse of a dome plugging a vent • Base flow = dense, ground-hugging mass of ash, cinders, blocks of rock • Pyroclastic surge = envelope of hot gas and ash around the base flow, lighter; behaves like a fluid—can detach from the main flow and flow up and over ridges
Volcanic Ash Figure 6-27 Unhealthy to Breathe Volcanic ash contains tiny particles of glassy, solidified magma (shown here in a scanning electron micrograph) and small rock and crystal fragments. The size of this particle is about 0.01 cm. Ash is abrasive and can cause respiratory problems. Figure 6-28
Lahars Gravity-driven, wet debris flow heat from eruption melts snow and ice, which begins to flow slurry of ash, lava debris, and water very dense, massive, fast-flowing lahars can kill long after a volcanic eruption Example: Nevado del Ruiz, Colombia, November 13, 1985 relatively small eruption melted snow and ice water-saturated loose flank debris triggered lahars destroyed >5000 homes and killed >23,000 people Example: Mount Pinatubo, Phillipines, 1991 lahars can kill long after a volcanic eruption loose flank debris mixed with water (e.g., from heavy rains) after the 1991 eruption, lahars continued to flow for years, burying whole villages
Stratovolcano Hazards FIGURE 6-32 Stratovolcano Eruptions Can Create Many Hazards
After the Eruption of Mount St. Helens in 1980 FIGURE 6-34 (a) Surrounding forests were devastated by the 1980eruption of Mount St. Helens. Over four billion board feet of usable timber was damaged or destroyed. (b) Lahars severely damaged property and stream drainages. The lahar that devastated the Muddy River drainage carried this huge boulder along with it.
Hazards of Shield Volcanoes Lava flows—velocity is controlled by gravity and viscosity Figure 6-35 Pahoehoe flows are fast, hot mafic fluid and develop smooth, hummocky, or ropy surfaces when they solidify. Aa flows, which are lower in temperature and more viscous than pahoehoe, develop a rough surface of broken lava blocks. Here an aa flow is advancing over a previously solidified pahoehoe flow.
Volcanic Gases • Hazards to people • CO2 build-ups in volcanic lakes • geological disturbances churn up clouds of colorless, odorless CO2 gas from the depths of volcanic lakes • gas bubbles out of solution—flows out of lake (heavier than air) • Lake Nyos (Cameroon, 1986)—1700 persons and numerous wildlife and livestock; clouds spread over 300 km2 (116 mi2) • VOG (volcanic fog)—sulfur dioxide (SO2) and other gases react chemically with moisture and oxygen to form a type of air pollution
Volcanic Gases (cont.) • Hazards to plant life • Sulfur gases combine with moisture in air, generating sulfuric acid (acid rain) • Climate changes • SO2 gas erupts and combines with water to form aerosols of sulfuric acid • aerosols reduce sunlight reaching surface, lowering global temperatures
Living with Volcanoes • Worldwide, about 500 million people live in volcanic hazard zones. • Strategies for protection: • Dropping bombs to divert lava flows (in 1935, 1942, 1975–1976 in Hawaii) • Pump water onto flow to cool and solidify lava flows (Heimaey, Iceland—36,000 tons of seawater per hour pumped onto the encroaching lava flows—for 5 months!) • Mount Etna, Sicily • Blasting through walls of channels feeding flows (1983) • Build earth barriers to slow flows • Dig channels to divert flows into old lava tubes (1992) or into artificial channels (2001)
Hazard Assessments • Geologic mapping and dating of lava and ash from past eruptions can reveal approximately how often a volcano has large eruptions. • Hazard mapping involves locating and describing geological deposits near the volcano to identify them as lava, ash, pyroclastic flows, lahars, etc. • Laboratory work can date these deposits, sometimes precisely, down to individual lava flows and the calendar year they erupted. • Volume and thickness of individual deposits provides clues to the magnitude of past eruptions, which in turn puts some constraints on how large a future eruption could be. • Long-range forecasts—state the probability that an eruption of a certain size will occur within a certain time period.
Monitoring Volcanic Activity Figure 6-40 Volcanologists and other scientists frequently monitor volcanoes to determine when an eruption may be imminent. They may visit active volcanoes to gather data collected by sensors, take samples of magma or volcanic gases, and make close-up observations of ongoing activity.
Monitoring Eruption Precursors • Earthquakes • magma pushing upwards shatters surrounding rocks • increase in small- to medium-sized earthquakes at shallow depths • specific patterns, associated with changes in volcanic “plumbing” • Monitoring ground deformation • mechanical tilt meters—monitor inflation as magma pushes upwards • Global Positioning System (GPS) receivers, or field surveying • remote sensing techniques, such as satellite radar mapping • Gas composition and concentration • monitoring done from a distance with a correlation spectrometer • (COSPEC) • Other precursors • minute changes in the force of gravity on a volcano’s slopes • fluctuations in electrical conductivity reflect changes in the magma
SUMMARY • Volcanic activity is commonplace on Earth • Most volcanoes form along divergent and convergent plate boundaries • The most explosive magmas are rich in silica, viscous, and gas-rich • Mafic magmas are silica-poor, less viscous, and erupt as lava flows • tend to form shield volcanoes • Lava flows from shield volcanoes destroy property, but are associated with less loss of life than eruptions at stratovolcanoes
SUMMARY (cont.) • Stratovolcanoes are the most explosive and the most dangerous • create pyroclastic flows, lahars, and ash plumes in the atmosphere • Volcanic gases, released by all types of volcanoes, can be extremely hazardous, and in large eruptions can influence the global climate • Scientists study eruption history, map distribution and character of erupted material, identify hazards, and predict future eruptions
