Lecture Outlines Natural Disasters 7 th edition Volcanic

Lecture Outlines Natural Disasters, 7 th edition

Volcanic Eruptions: Plate Tectonics and Magma

Vesuvius, 79 C. E. • Cities of Pompeii and Herculaneum buried by massive eruption which blew out about half of Mt. Vesuvius • Similar to 1991 eruption of Mt. Pinatubo in Philippines • Clouds of hot gas (850 o. C), ash and pumice enveloped city • Many tried to escape near sea, but were buried by pyroclastic flows Figure 8. 1 Figure 8. 3

Vesuvius, 79 C. E. • Vesuvius was inactive for 700 years before 79 CE eruption – People lost fear and moved in closer to volcano • After 79 CE, eruptions in 203, 472, 512, 685, 993, 1036, 1049, 1138 -1139 • 500 years of quiet, then 1631 eruption killed 4, 000 people • 18 cycles of activity between 1631 and 1944, nothing since then • 3 million people live within danger of Vesuvius today; 1 million people on slopes of volcano

The Hazards of Studying Volcanoes • Eruptive phases are often separated by centuries of inactivity, luring people to live in vicinity (rich volcanic soil) – 400, 000 people live on flanks of Galeras Volcano in Colombia • Many people killed each year by volcanoes, sometimes including volcanologists • Volcanoes may be active over millions of years, with centuries of inactivity

How We Understand Volcanic Eruptions • Understand volcanoes in context of plate tectonics • Variations in magma’s chemical composition, ability to flow, gas content and volume determines whether eruptions are peaceful or explosive

Plate-Tectonic Setting of Volcanoes • 90% of volcanism is associated with plate boundaries – 80% at spreading centers – About 10% at subduction zones • Remaining 10% of volcanism occurs above hot spots Figure 8. 5

Plate-Tectonic Setting of Volcanoes • Subduction carries oceanic plate (with water-rich sediments) into hotter mantle, where water lowers melting temperature of rock • Rising magma melts continental crust it passes through, changing composition of magma Figure 8. 6

Plate-Tectonic Setting of Volcanoes • No volcanism associated with transform faults or continent collisions • Oceanic volcanoes are peaceful • Subduction-zone volcanoes are explosive and dangerous – Subduction zones last tens of millions of years – Volcanoes may be active any time, with centuries of quiet Figure 8. 6

Chemical Composition of Magmas • Of 92 naturally occurring elements: – Eight make up more than 98% of Earth’s crust – Twelve make up 99. 23% of Earth’s crust – Oxygen and silicon are by far most abundant • Typically join up as Si. O 4 tetrahedron, that ties up with positively charge atoms to form minerals Figure 8. 7

Chemical Composition of Magmas • Mineral formation in magma: crystallization • Order of crystallization of different minerals in magma can be determined: – Iron and magnesium link with aluminum and Si. O 4 to form olivine, pyroxene, amphibole and biotite families – Calcium combines with aluminum and Si. O 4 until calcium replaced by sodium, to form plagioclase feldspar family; calcium and sodium are later replaced by potassium, to form potassium feldspar and muscovite families; finally only Si and O remain, forming quartz

Chemical Composition of Magmas Figure 8. 8

Chemical Composition of Magmas • Elements combine to form minerals • Minerals combine to form rocks • Different compositions of magma result in different igneous rocks • If magma cools slowly and solidifies beneath surface plutonic rocks • If magma erupts and cools quickly at surface volcanic rocks

Viscosity, Temperature and Water Content of Magmas • Viscosity: internal resistance to flow – Lower viscosity more fluid behavior • Water, melted ice-cream – Higher viscosity thicker • Honey, toothpaste • Viscosity determined by: – Higher temperature lower viscosity – More silicon and oxygen tetrahedra higher viscosity – More mineral crystals higher viscosity • Magma contains dissolved gases: volatiles – Solubility increases as pressure increases and temperature decreases

Viscosity, Temperature and Water Content of Magmas • Consider three types of magma: basaltic, andesitic and rhyolitic – Basaltic magma has highest temperatures and lowest Si. O 2 content, so lowest viscosity (fluid flow) – Rhyolitic has lowest temperatures and highest Si. O 2 content, so highest viscosity (does not flow) – Basaltic makes up 80% of magma that reaches Earth’s surface, at spreading centers, because it forms from melting of mantle – Melted mantle at subduction zones rises through continental crust before reaching the surface, incorporating continental high Si. O 2 rock as it rises, to become andesitic or rhyolitic in composition before it erupts

Viscosity, Temperature and Water Content of Magmas

Viscosity, Temperature and Water Content of Magmas • Water is most abundant dissolved gas in magmas • As magma rises, pressure decreases, water becomes steam bubbles – Basaltic magma has lower water content peaceful, safe eruptions – Rhyolitic magma has higher water content and high viscosity many steam bubbles form and can not escape through thick magma, so explode out violent, dangerous eruptions Figure 8. 9 Figure 8. 10

Plate-Tectonic Setting of Volcanoes Revisited • Spreading centers have abundant volcanism because: – Sit above hot asthenosphere – Asthenosphere has low Si. O 2 – Plates pull apart so asthenosphere rises and melts under low pressure, changing to high-temperature, low Si. O 2, low volatile, low viscosity basaltic magma that allows easy escape of gases peaceful eruptions

Plate-Tectonic Setting of Volcanoes Revisited • Subduction zones have violent eruptions because: – Magma is generated by partial melting of the subducting plate with water in it – Melts overlying crust to produce magmas of variable composition – Magma temperature decreases while Si. O 2, water content and viscosity increase violent eruptions Figure 8. 11

How a Volcano Erupts • Begins with heat at depth – Rock that is superheated (heated to above its melting temperature) will rise – As it rises, it is under less and less pressure so some of it melts (becomes magma) – Volume expansion leads eventually to eruption • Three things will cause rock to melt: – Lowering pressure – Raising temperature – Increasing water content • Lowering pressure is most common way to melt rock decompression melting

How a Volcano Erupts • Magma at depth is under too much pressure for gas bubbles to form (gases stay dissolved in magma) Figure 8. 12 • As magma rises toward surface, pressure decreases and gas bubbles form and expand, propelling the magma farther up • Eventually gas bubble volume may overwhelm magma, fragmenting it into pieces that explode out as a gas jet

How a Volcano Erupts Eruption Styles and the Role of Water Content • Concentration of water in magma largely determines peaceful or explosive eruption • Basaltic magma can erupt violently with enough water • Rhyolitic magma usually erupts violently because of high water content, high viscosity (secondary role) Figure 8. 14 • Styles of volcanic eruptions – Nonexplosive Icelandic and Hawaiian – Somewhat explosive Strombolian – Explosive Vulcanian and Plinian

How a Volcano Erupts Some Volcanic Materials • Low-water content, lowviscosity magma lava flows • High-water content, high-viscosity magma pyroclastic debris

How a Volcano Erupts Nonexplosive eruptions • Pahoehoe: smooth ropy rock from highly liquid lava • Aa: rough blocky rock from more viscous lava Figure 8. 15 Figure 8. 16

How a Volcano Erupts Explosive eruptions • Pyroclastic debris: broken up fragments of magma and rock from violent gaseous explosions, classified by size • May be deposited as: – Air-fall layers (settled from ash cloud) – High-speed, gas-charged pyroclastic flow Figure 8. 17 a Figure 8. 18

How a Volcano Erupts Explosive eruptions • Very quick cooling: – Obsidian: volcanic glass forms when magma cools very fast – Pumice: porous rock from cooled froth of magma and bubbles Figure 8. 19

Side Note: How a Geyser Erupts • Geyser: eruption of water superheated by magma • Can only exist in areas of high heat flow underground • Water boils (becomes gas) at 100 o. C unless it is under pressure – no room for expansion to gas state – Water can be heated to higher than boiling temperature superheated – When superheated water reaches point of lower pressure, it flashes to steam violently, and erupts out of the ground Figure 8. 20

The Three V’s of Volcanology: Viscosity, Volatiles, Volume • Viscosity may be low or high – Controls whether magma flows easily or piles up • Volatile abundance may be low, medium or high – May ooze out harmlessly or explode • Volume may be small, medium or large – Greater volume more intense eruption

The Three V’s of Volcanology: Viscosity, Volatiles, Volume • By mixing different values for the three V’s, can forecast different eruptive styles for volcanoes

The Three V’s of Volcanology: Viscosity, Volatiles, Volume • By mixing different values for the three V’s, can define different volcanic landforms

The Three V’s of Volcanology: Viscosity, Volatiles, Volume Shield Volcanoes: Low Viscosity, Low Volatiles, Large Volume • Basaltic lava with low viscosity and low volatiles flows to form gently dipping, thin layers • Thousands of layers on top of each other form very broad, gently sloping volcano like Mauna Loa in Hawaii • Great width compared to height Figure 8. 22

The Three V’s of Volcanology: Viscosity, Volatiles, Volume Hawaiian-type Eruptions • “Curtain of fire”: lines of lava fountains up to 300 m high • Low cone with high fountains of magma – Floods of lava spill out and flow in rivers down slope – Eruptions last days or years, usually not life-threatening but destroy buildings and roads Figure 8. 24