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The Wrath of Vulcan: Volcanic Eruptions

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1 The Wrath of Vulcan: Volcanic Eruptions
Chapter 9 The Wrath of Vulcan: Volcanic Eruptions LECTURE OUTLINE earth Portrait of a Planet Third Edition ©2008 W. W. Norton & Company, Inc.

2 The Wrath of Vulcan: Volcanic Eruptions
Prepared by Ronald Parker Earlham College Department of Geosciences Richmond, Indiana

3 Volcanic Eruptions What is a volcano?
An erupting vent through which molten rock surfaces. A mountain built from magmatic eruptions. Volcanoes are a clear result of tectonic activity. Volcanoes pose a number of hazards to humans. Mexico City. Seattle, U.S. Naples, Italy.

4 Volcanic Eruptions In 79 C.E. Mt. Vesuvius erupted violently.
Pyroclastic flows destroyed Pompeii, killing 20,000. A record of Roman life was preserved under ash.

5 Volcanic Eruptions Unpredictable, dangerous. Eruptions can…
Build large mountains. Blow mountains to bits. Eruptions can… Provide highly productive soils to feed a civilization. Can extinguish a civilization in a matter of minutes. Eruptions affect climate.

6 Volcanic Materials The products of volcanic eruption take three forms:
Lava flows – Molten rock that move over the ground. Pyroclastic debris – Fragments blown out of a volcano. Volcanic gases – Vapor and aerosols that exit a volcano.

7 Lava Flows Lava can be thin and runny or thick and sticky.
Flow type depends on viscosity via composition. Composition depends on silica (SiO2), Fe, and Mg.

8 Lava Compositions Lavas with high silica / low Fe and Mg are called...
Silicic, felsic, or rhyolitic. Lavas with low silica / high Fe and Mg are called… Mafic or basaltic. Lavas with moderate silica, Fe and Mg are called… Intermediate or andesitic.

9 Basaltic Lava Flows Mafic lava – Very hot, low silica, and low viscosity. Basalt flows are often thin and fluid. They can flow rapidly (up to 100 km/hr). They can flow for long distances (up to several 100 km).

10 Pahoehoe Pahoehoe (pa-hoy-hoy) - a Hawaiian word describing basalt with a glassy, ropy texture. Pahoehoe forms when extremely hot basalt forms a skin. With flow, the skin is rolled into ropy ridges and furrows.

11 A’a’ A’a’ (ah-ah) is a Hawaiian word describing basalt that solidifies with a jagged, sharp, angular texture. A’a’ forms when hot flowing basalt cools and thickens. With flow, lava crumbles into shards and fragments.

12 Lava Tubes A cooled crust forms on top of a basalt flow.
A conduit – a lava tube – develops in the flow. Tubes prevent cooling, facilitating flow for miles. Lava tubes become caves that can transmit water.

13 Columnar Jointing Solidified flows may contract with vertical fractures that are hexagonal in cross-section. Columnar jointing indicates lava.

14 Pillow Basalts Underwater, basalt cools instantly; it cannot flow.
It cools to form a rounded blob called a pillow. The pillow surface is cracked, quenched glass. Lava pressure ruptures a pillow to form the next blob. The process repeats to form a mound of pillow basalts. Common on the MOR.

15 Andesitic Lava Flows Higher SiO2 content makes andesitic lavas viscous. Unlike basalt, they do not flow rapidly. Instead, they mound around the vent and flow slowly. The outer crust fractures, creating rubble. Andesitic lava flows remain close to the vent.

16 Rhyolitic Lava Flows Rhyolite; the highest SiO2 – the most viscous lava. Rhyolitic lava rarely flows. Rather, lava plugs the vent as a lava dome. Sometimes, lava domes are blown to smithereens.

17 Pyroclastic Debris Fragmental material ejected from a volcano.
Glass shards, fragmented lava in a range of sizes. Ash – Powdery glass shards. Lapilli – Pea- to plum-sized material. Blocks and bombs – Apple- to refrigerator-sized. Blocks – Pre-existing rock torn from the volcano. Bombs – Streamlined fragments of ejected lava.

18 Pyroclastic Debris Tephra – Deposits of pyroclastic debris.
Volcanic agglomerate – Lapilli and bombs near the vent. Tuff – Lithified ash with or without lapilli. Air-fall tuff – Accumulations of ash that fall like snow. Welded tuff (ignimbrite) – Tuff that is deposited while hot. Hot pyroclastic flow material that fuses before cooling.

19 Pyroclastic Flows Pyroclastic flows (or, nuée ardentes - French):
200oC - 450oC avalanches of hot ash that race downslope. Moving up to 300 kph, they incinerate all in their path. Immediately deadly; they kill everything quickly. Many famous examples: Vesuvius, Mt. Pelée, Augustine.

20 Lahars Tephra is readily moved by water as a debris flow.
Known as lahars, these flows are often deadly. Lahars move rapidly (up to 50 km/hr). They have the consistency of wet cement,. A distinct hazard to people living in volcanic valleys.

21 Volcanic Gas 1 to 9% of magma may be gas.
Water (H2O) – Most abundant dissolved gas. Carbon dioxide (CO2) – Second in abundance. Sulfur dioxide (SO2) – Rotten egg smell. Magma composition controls gas content. Felsic magmas are gas-rich; Mafic magmas are less so.

22 Volcanic Gas Gases are expelled as magma rises (P drops).
SO2 reacts with water to form aerosol sulfuric acid. Style of gas escape controls eruption violence. Low viscosity (basalt) – Easy escape; mellow eruption. High viscosity (rhyolite) – Difficult escape; violent. Gas bubbles in rock are called vesicles.

23 Volcanic Architecture
Volcanoes have characteristic features. A magma chamber. Fissures and vents. Craters. Calderas. Distinctive profile.

24 Magma Chamber Located in the upper crust.
Usually an open cavity or area of highly fractured rock. Contains a large quantity of magma. Some magma cools here to form intrusive rock.

25 Magma Chamber Some magma rises via a conduit to the surface.
Magma may also erupt along a linear tear, a fissure. Fissure eruptions may display a “curtain of fire.” Fissures evolve into discrete vents.

26 Vents A vent is a lava outlet on a volcano.
Vents can form anywhere on the volcano. Summit vent – Located at the top of the volcano. Flank vent – Located on the side of a volcano.

27 Craters Crater – A bowl-shaped depression atop a volcano.
Craters are up to 500 m across; 200 m deep. Form as erupted lava piles up around the vent. Accentuated by summit collapse into conduit.

28 Caldera A gigantic volcanic depression. Magma chamber empties.
Much larger than a crater. 1-10s of kilometers across. Magma chamber empties. Volcano collapses into it. Crater Lake, Oregon. Yellowstone National Park.

29 Crater Lake Caldera, Oregon

30 Shapes and Sizes Magma type governs volcano shape and size.
Categories: Shield volcanoes – Largest. Cinder cones – Smallest. Stratovolcanoes – Intermediate in size.

31 Volcano Types Shield volcanoes.
Broad, slightly domed-shaped (like an inverted shield). Lateral flow of low-viscosity basaltic lava. Have a low slope and cover large geographic areas. Mauna Loa on Hawaii is a good example.

32 Volcano Types Cinder cone – Conical piles of tephra.
The smallest type of volcano. Built of ejected lapilli-sized fragments piled up at a vent. Slopes are at the angle of repose. Often symmetrical with a deep summit crater.

33 Volcano Types Stratovolcanoes (composite volcanoes).
Large, cone-shaped volcano. Composed of alternating layers of lava and tephra. Often symmetric; can be odd shapes from landslides, etc. Examples include Mt. Fuji, Mt. Rainier, Mt. Vesuvius.

34 Eruptive Style Will it flow, or will it blow? Two dominant styles.
Effusive eruptions – Produce lava flows. Explosive eruptions – Blow up.

35 Eruptive Style Effusive eruptions – Produce lava flows.
Lava flows stream away from vents. Lava lakes can form around the vent. Can produce huge lava fountains. Commonly basaltic, these eruptions create shields.

36 Eruptive Style Explosive eruptions – Produce pyroclastic flows.
Caused by gas pressure in the more viscous magma. Pressure is released explosively. Create stratovolcanoes. May create calderas. Blanket the landscape with tephra. Andesitic and rhyolitic compositions.

37 Eruptive Style Phreatomagmatic eruptions – Less common style.
Volcanic eruption where magma interacts with water. Some phreatomagmatic eruptions can be cataclysmic. A magma chamber breaches and admits water. Water flashes to steam and blows the entire volcano apart. Examples: Santorini, Krakatau.

38 Controls on Eruptive Style
Viscosity – Controls the ease of lava flow. Basalt – Low-viscosity lava flows away from vent. Felsic – High-viscosity lava builds-up at the vent. Gas Pressure – Greater P favors explosive style. Basalt – Low-viscosity allows gas release. Felsic – High-viscosity prevents gas release. Environment – Where eruption occurs important. Subaerial lava flowing on land cools slower than… Submarine lava, which is quickly quenched.

39 Eruptions to Remember Mt. St. Helens – Erupted May 18, 1980, 8:32 A.M.
Earthquake-triggered landslide released pressure. Initial vertical blast followed by a much stronger lateral blast that tore off the entire north side. 396 m was blasted away.

40 Eruptions to Remember Mt. St. Helens – Erupted May 18, 1980, 8:32 A.M.
The blast devastated 600 km2 and killed 61 people. Lahars plugged the Toutle River; closed the Columbia. Ash fell in North Dakota; highways and rail lines stopped. Destroyed timber valued at several $100 million.

41 Tectonic Settings Plate motion is a dominant control on volcanism.
Volcanic types are linked to tectonic settings Hot spots - Where mantle plumes cut the lithosphere. Oceanic hot spots. Continental hot spots and flood basalts. Mid-ocean ridges – Spreading axes. Convergent boundaries – Subduction zones. Continental rifts – Incipient ocean basins.

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43 Hot Spot Eruptions Oceanic hot spot – Plume under an oceanic plate.
Basalt erupts at the seafloor and forms a growing mound. A volcano builds above sea level to form an island. Then basalt will not quench and can flow long distances. Lava builds upward and outward and the island grows. Submarine slumps remove large masses of the volcano.

44 Hot Spot Eruptions Continental hot spot – Cuts a continental plate.
Often erupts both basaltic and rhyolitic material. Basaltic – Makes up the mantle plume. Rhyolitic – Basalt melts the granitic crust it passes through.

45 Hot Spot Eruptions Continental hot spots.
Yellowstone – Eruption 640 ka created a 100 km caldera. 1000 times more powerful than Mt. St. Helens. Deposited 10s of m of ignimbrites. Magma beneath the caldera still fuels geysers. Heads a hot spot track.

46 Hot Spot Eruptions Continental hot spots.
Flood basalts – Voluminous lava eruption above a plume. Thinned lithosphere erupts magma from long fissures. Lava spreads over large areas; great thicknesses stack up. With time, a shrinking plume creates “normal” volcanoes.

47 MOR Eruptions Most lava erupts along the mid-ocean ridge (MOR).
MOR-produced oceanic crust covers 70% of Earth. Basalt erupted from fissures quenches as pillows. Pillow mounds are pulled apart with plate motion.

48 Iceland Iceland is a hot spot beneath the MOR.
Lava has built the hot spot/ridge above sea level. The island is being torn apart by plate motion. Volcanoes trace the MOR rift.

49 Convergent Boundaries
Most volcanoes form at convergent boundaries. Volatiles from subducting plate initiate melting. Arc volcanoes develop on overriding plate. May cut through oceanic or continental crust. The “Ring of Fire” dominates Pacific margins.

50 Continental Rifts Yield an array of volcano types reflecting...
Partial melting of the mantle (mafic magmas). Partial melting of the crust (felsic magmas). Examples: East African Rift. Basin and Range. Mid-continent Rift.

51 Volcanic Hazards Volcanic eruptions cause great harm to humans.
Eruptions have profoundly influenced human history. In the past 2,000 years: an estimated 250,000 deaths. Many populated areas ring active volcanoes. More humans live in volcano hazard areas than ever. Understanding volcanic behavior is the best defense.

52 Volcanic Hazards Lava flows – Lava threats are mostly from basalt.
Lava may completely destroy immovable objects. It is rare for lava flows to kill people. Usually there is enough notice. Lava rarely moves faster than people. Sometimes people watching lava flows are killed.

53 Volcanic Hazards Tephra – Ash and lapilli fall around the volcano.
Can completely bury landscapes, killing plants and crops. Tephra is heavy; it causes roof collapses. Tephra is gritty; it abrades car and airplane engines. Floodwaters easily move tephra as deadly lahars.

54 Volcanic Hazards Pyroclastic flows – aka nuée ardente
Clouds of roiling ash and gas that race downslope. Ride over a carpet of superheated air. Eases passage of the flow. Permits high velocities. Immediately deadly to anything living.

55 Volcanic Hazards Blast – Rarely, explosions are ejected sideways.
Mt. St. Helens – Lateral blast tore off north side. Destroyed over 600 km2 of forest. Killed 61 people. Blast hazard is uncommon; most eruption is vertical.

56 Volcanic Hazards Landslides – Eruption-related slope failures.
Eruptions can trigger landslides. Large masses of material are deposited rapidly near vent. Earthquakes initiate failure of unstable slopes. Mt. St. Helens. The eruption immediately followed a 3 km3 slope failure. Slide material moved more than 20 km from the peak.

57 Volcanic Hazards Lahars – Mudflows result when water moves ash.
Like concrete, this material is more dense than water. It can carry away everything (people, houses, bridges). Nevada del Ruiz, Colombia, buried Armero +25,000 people.

58 Volcanic Hazards Earthquakes – Moving magma causes earthquakes.
Although usually small in magnitude, they are frequent. Can cause slope failures and damage to structures. Tsunamis – Water explosions create giant waves. Tsunamis from Krakatau (1883) killed 36,000.

59 Volcanic Hazards Gas – Volcanic gases can be poisonous (H2S, CO2).
Lake Nyos, Cameroon, 1986. Magmatic CO2 built up in the lake located in a crater. The lake overturned (burped) and the CO2… Moved down the valleys as a heavier-than-air underflow. Killed 1,742 people; 6,000 cattle.

60 Active vs. Extinct Recurrence interval – Average between eruptions.
Active – Erupting, recently erupted or likely to erupt. Dormant – Volcano that has not erupted in 100s to 1000s of years (but could still do so). Extinct – No longer capable of erupting. Tectonic changes can shut off the magma “fuel.” After extinction, erosion takes over.

61 Predicting Eruptions Warning signs precede most eruptions.
Earthquake activity – Magma flow increases seismicity. Heat flow – Magma causes volcanoes to “heat-up.” Changes in shape – Magma causes expansion. Emission increases – Changes in gas mix and volume. These signs indicate that an eruption is imminent. They cannot predict the exact timing or the style.

62 Mitigating Hazards Danger assessment maps. Delineate danger areas.
Pyroclastic flows. Lahars. Landslides. Used for planning, zoning.

63 Mitigating Hazards Evacuation – Moving those at high risk saves lives.
Mt. St. Helens – Timely evacuation saved 100s. Sometimes eruptions don’t occur, large expenses. Diverting flows – Flowing lava can be diverted. Explosives. Heavy equipment. Seawater.

64 Volcanoes and Climate Volcanic aerosols and fine-debris block sunlight. Examples of volcanism affecting climate Mt. Tambora, Indonesia – 1815 Pinatubo, Philippines – 1991 Makes for more intense sunsets.

65 Volcanoes and Civilization
Humans and volcanoes have coexisted for millenia. This coexistence has both bad and good facets. Volcanic soils are highly fertile; excellent for crops. Civilizations have prospered and been erased. Minoans - Santorini

66 Volcanoes on Other Planets
Volcanic activity evident on the Moon and planets. Lunar maria (dark “seas”) are regions of flood basalts. Olympus Mons – Extinct Martian shield volcano. The Jovian moon Io has active volcanoes.

67 The Wrath of Vulcan: Volcanic Eruptions
This concludes the Chapter 9 The Wrath of Vulcan: Volcanic Eruptions LECTURE OUTLINE earth Portrait of a Planet Third Edition ©2008 W. W. Norton & Company, Inc.


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