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Chapter 5: Magma And Volcanoes. Introduction: Earth’s Internal Thermal Engine. Magma is molten rock beneath Earth’s surface. Because liquid magma is less dense than surrounding solid rock, and obviously more mobile, magma, once formed, rises toward the surface.
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Introduction: Earth’s Internal Thermal Engine • Magma is molten rock beneath Earth’s surface. • Because liquid magma is less dense than surrounding solid rock, and obviously more mobile, magma, once formed, rises toward the surface. • Magma that reaches the surface does so by erupting through vents we call volcanoes.
Volcanoes • The term volcano comes from the name of the Roman god of fire, Vulcan. • There are different types of volcanoes. • Eruption vary from gentle flows (Hawaii and Iceland) to catastrophic explosions (Mount St. Helens, Mt. Pinatubo, Soufriere Hills). • The majority of eruption never make the news because they occur beneath the ocean, unobserved.
Magma • Magma has a wide range of compositions, but silica (SiO2) always dominates the mix. • Magma has high temperatures. • Magma is fluid—it has the ability to flow. Most magma actually is a mixture of liquid (often referred to as melt) and solid mineral grains.
Composition of Magmas and Lavas • The composition of magmas and lavas is controlled by the most abundant elements in the Earth—Si, Al, Fe, Ca, Mg, Na, K, H, and O. • Three distinct types of magma are more common than others: • Basaltic, containing about 50 percent SiO2. • Andesitic, about 60 percent SiO2. • Rhyolitic, about 70 percent SiO2.
Basaltic Magmas • Basaltic magmas are erupted by approximately 80 percent of volcanoes worldwide (the seafloor worldwide is mostly basalt). • Magma from Hawaiian volcanoes such as Kilauea and Mauna Loa is basaltic. • The entire island of Iceland is basaltic.
Andesitic and Rhyolitic Magmas • Andesitic magmas are about 10 percent of the total magma. • Magma from Mount St. Helens in Washington State and Krakatau in Indonesia is usually andesitic. • Rhyolitic magmas are about 10 percent of the total magma. • Magmas erupted from volcanoes that once were active at Yellowstone Park are mostly rhyolitic.
Gases Dissolved in Magma • Small amounts of gas (0.2 to 3% by weight) are dissolved in all magma. • The principal gas in water vapor, which, together with carbon dioxide, accounts for more than 98 percent of all gases emitted from volcanoes.
Temperature of Magmas and Lavas • Magma temperatures range from 1000o to 1200oC. • Magma temperatures can reach 1400oC under some conditions.
Viscosity of Magmas and Lavas (1) • The internal property of a substance that offers resistance to flow is called viscosity. • The more viscous a magma, the less easily it flows. • Viscosity of a magma depends on temperature and composition (especially the silica and dissolved-gas contents).
Viscosity of Magmas and Lavas (2) • The higher the temperature, the lower the viscosity, and the more readily magma flows. • The smooth, ropy-surfaced lava in Hawaii, formed from a very hot, very fluid lava is called pahoehoe. • The rough-looking lava formed from a cooler lava having a high viscosity is called aa (ah ah).
Viscosity of Magmas and Lavas (3) • The greater the silica content, the larger is the polymerized group. • For this reason, rhyolitic magma (70% silica) is always more viscous than basaltic magma (50% silica). • Andesitic magma has a viscosity that is intermediate between the two (60% silica).
How Buoyant Magma Erupts on the Surface (1) • Magma is less dense than the solid rock from which it forms. • The pressure is proportional to depth (thickness of overlying rock). • Therefore, as magma rises upward, the pressure on it decreases.
How Buoyant Magma Erupts on the Surface (2) • Pressure controls the amount of gas a magma can dissolve—more at high pressure, less at low. • Gas dissolved in an upward-moving magma comes out of solution and forms bubbles.
Eruption Style—Nonexplosive or Explosive? (1) • Nonexplosive eruptions occur notably in Hawaii, Iceland, and the seafloor. • They are relatively safe. • The difference between nonexplosive and explosive eruptions depends largely on magma viscosity and dissolved-gas content. • Low viscosity magmas and low dissolved gas contents produce nonexplosive eruptions.
Eruption Style—Nonexplosive or Explosive? (2) • Nonexplosive eruptions may appear violent during their initial stages. • The reason is that gas bubbles in a low-viscosity basaltic magma will rise rapidly upward, like the gas bubbles in a glass of soda. • If a basaltic magma rises rapidly, spectacular lava fountains will occur.
Eruption Style—Nonexplosive or Explosive? (3) • Because heat is lost quickly at the surface of the flowing lava, the surface solidifies into a crust, beneath which the liquid lava continues to flow in well-defined channels called lava tubes. • The very fluid lava initially forms thin pahoehoe flows. • With increasing viscosity the rate of movement slows and the stickier lava may be transformed into a rough surfaced aa flow that moves very slowly.
Vesicles and Amygdules • When lava finally solidified to rock, the last-formed bubbles become trapped; these bubble preserved in the rock are called vesicles. • Vesicles filled by secondary minerals are called amygdules.
Explosive Eruptions (1) • In viscous andesitic or rhyolitic magmas, gas bubbles can rise only very slowly. • When confining pressure drops quickly, the gas in a magma expand into a froth of innumerable glass-walled bubbles called pumice.
Explosive Eruptions (2) • In many instances, instead of forming pumice, small bubbles expanding within a huge mass of sufficiently gas-rich, viscous magma will shatter the magma into tiny fragments called volcanic ash. • Volcanic ash is the most abundant product of explosive eruptions.
Eruption Columns and Tephra Falls (1) • The largest and the most violent eruptions are associated with silica-rich magmas having a high dissolved-gas content. • This hot, turbulent mixture rises rapidly in the cooler air above the vent to form an eruption column that may tower as high as 45 km in the atmosphere.
Eruption Columns and Tephra Falls (2) • A violent eruption of this kind is called a plinian eruption, named after the Roman author and statesman, Pliny, who lost his life in the A.D. 79 eruption of Mt. Vesuvius. • The particles of debris rain down in a tephra fall and eventually accumulate on the ground as tephra deposits.
Pyroclastic Flows (1) • When the mixture of hot gases and pyroclasts is more dense than the atmosphere, the turbulent mixture flows down the side of the volcano rather than forming an eruption column. • A hot, highly mobile flow of tephra that rushes down the flank of a volcano during a major eruption is called a pyroclastic flow (the most devastating and lethal forms of volcanic eruption).
Pyroclastic Flows (2) • Pyroclastic flows are also known as nuée ardente (glowing cloud). • Historic observations indicate that pyroclastic flows can reach velocities of more than 700 km/h. • In 1902, a pyroclastic flow rushed down the flanks of Mont Pelee Volcano at an estimated speed of 200 KM/h, instantly killing 29,000 people.
Lateral Blast—Mount St. Helens • In 1980, Mount St. Helens, a volcano in Washington, erupted violently. • As magma rose under the volcano, the mountain’s north flank began to bulge upward and outward. • The initial blast was sideways rather than upward. • 600 km2 of trees in the once-dense forest were leveled.
Volcanoes • There are two broad families of volcanoes: • Those formed by eruptions from a central vent. • Those that erupt through a long fissure. • Central-vent eruptions build mounds of the kind most people associate with volcanoes. • Fissure eruptions build plateaus.
Central-vent Volcanoes • Based on their size and shape, there are three broad classes of central-vent volcanoes: • Shield volcanoes. • Tephra cones. • Stratovolcanoes.
Shield Volcanoes (1) • A shield volcano produces a broad, dome-shaped mountain with an average surface slope of only a few degrees. • Low-viscosity basaltic lavas can flow for kilometers down gentle slopes. • The accumulated lava from repeated eruptions of low-viscosity lava build a shield volcano.
Shield Volcanoes (2) • The farther lava flows down the flank, the cooler and more viscous it becomes, so the steeper the slope must be for it to flow. • Large shield volcanoes rise as islands in the ocean (Hawaiian Islands, Tahiti, Samoa, the Galapagos, and many others).
Shield Volcanoes (3) • Mauna Loa volcano, for example, rises to a height of 4169 m above sea level, but if measured from the seafloor the height is 10,000 m, making Mauna Loa the tallest mountain on Earth.
Tephra Cones • Tephra cone is built by shower of pyroclastic debris around a volcanic vent. • The slopes of tephra cones are steep, typically about 30o.
Statovolcanoes (1) • Some volcanoes (andesitic composition) emit both viscous lava flows and tephra. • The emissions tend to alternate, forming alternating strata of lava and tephra, building a stratovolcano. • Stratovolcanoes are: • Large. • Conical. • Steep-sided.
Statovolcanoes (2) • Near the summit, a stratovolcano’s slope may reach 40o. • Toward the base, the slope flattens to about 6o to10o. • As a stratovolcano develops, lava flows act as a cap to slow erosion of the loose tephra.
Statovolcanoes (3) • The volcano becomes much larger and steeper than a typical tephra cone. • Mount Fuji (Japan), Mount Rainier, Mount Baker in Washington State, Mount Hood in Oregon, Mt Mayon in the Philippines are stratovolcanoes.
Other Features of Central Eruptions (1) • Craters: • Funnel-shaped depressions with steep-sided walls that open upward. • Craters form in two ways: • By the collapse of the steep sides of the vent. • By an explosive eruption. • In subsequent eruptions, pressure blasts open the vent, removing both the solidified magma from the previous eruption and part of the crater wall. • A crater can grow slowly larger, eruption by eruption.
Other Features of Central Eruptions (2) • Lava domes: • If the magma is very viscous (as in a rhyolitic or andesitic magma), it squeezes out to form a lava dome.
Other Features of Central Eruptions (3) • Calderas: • Caldera is from the Spanish word for cauldron. • A roughly circular, steep-walled basin about a kilometer in diameter or larger. • Calderas are created by collapse of the surface rock following an eruption and partial emptying of the underlying magma chamber. • Crater lake in Oregon occupies a circular caldera 8 km in diameter.
Other Features of Central Eruptions (4) • Resurgent domes: • Often, more magma enters the chamber and lifts the collapsed caldera floor to form a resurgent dome. • Diatremes: • Volcanic pipes filled with a rubbles of broken rock. • The walls are vertical, or very nearly so. • A famous diatreme is the diamond mine in Kimberly, South Africa.
Fissure Eruptions (1) • Fissure eruptions extrude lava along an elongate fracture in the crust. • When fissure eruptions occur on land, the low-viscosity basaltic lava tends to spread widely and to create flat lava plains. • Such lavas are called plateau basalts.
Fissure Eruptions (2) • The Laki eruption, in Iceland in1783, occurred along a 32 km long fracture. Lava from it flowed 64 km from one side of the fracture and nearly 48 km from the other, covering 588 km2. • The Laki eruption is the largest lava flow of any kind in historic times. • Famine followed and more than 9000 died (20 percent of the Icelandic population).
Fissure Eruptions (3) • Pillow basalts: • When the basaltic magma erupts under the ocean, seawater cools it so rapidly that pillow-shaped masses of basalt, ranging from a few centimeters to a meter or more in greatest dimension form. • Fissure eruptions of andesitic or rhyolitic magma are much less common than fissure eruptions of basaltic lava.
