2The Wrath of Vulcan: Volcanic Eruptions Prepared byRonald ParkerEarlham College Department of GeosciencesRichmond, Indiana
3Volcanic 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.
4Volcanic 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.
5Volcanic 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.
6Volcanic 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.
7Lava 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.
8Lava 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.
9Basaltic Lava FlowsMafic 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).
10PahoehoePahoehoe (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.
11A’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.
12Lava 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.
13Columnar JointingSolidified flows may contract with vertical fractures that are hexagonal in cross-section.Columnar jointing indicates lava.
14Pillow 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.
15Andesitic Lava FlowsHigher 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.
16Rhyolitic Lava FlowsRhyolite; 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.
17Pyroclastic 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.
18Pyroclastic 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.
19Pyroclastic 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.
20Lahars 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.
21Volcanic 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.
22Volcanic 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 arecalled vesicles.
23Volcanic Architecture Volcanoes have characteristic features.A magma chamber.Fissures and vents.Craters.Calderas.Distinctive profile.
24Magma 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.
25Magma 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.
26Vents 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.
27Craters 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.
28Caldera 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.
30Shapes and Sizes Magma type governs volcano shape and size. Categories:Shield volcanoes – Largest.Cinder cones – Smallest.Stratovolcanoes – Intermediate in size.
31Volcano 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.
32Volcano 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.
33Volcano 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.
34Eruptive Style Will it flow, or will it blow? Two dominant styles. Effusive eruptions – Produce lava flows.Explosive eruptions – Blow up.
35Eruptive 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.
36Eruptive 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.
37Eruptive 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.
38Controls 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.
39Eruptions 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.
40Eruptions 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.
41Tectonic Settings Plate motion is a dominant control on volcanism. Volcanic types are linked to tectonic settingsHot 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.
43Hot 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.
44Hot 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.
45Hot 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.
46Hot 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.
47MOR 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.
48Iceland 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.
49Convergent 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.
50Continental 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.
51Volcanic 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 volcanicbehavior is the best defense.
52Volcanic 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.
53Volcanic 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.
54Volcanic 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.
55Volcanic 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.
56Volcanic 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.
57Volcanic 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.
58Volcanic 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.
59Volcanic 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.
60Active 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.
61Predicting 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.
63Mitigating 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.
64Volcanoes and ClimateVolcanic aerosols and fine-debris block sunlight.Examples of volcanism affecting climateMt. Tambora, Indonesia – 1815Pinatubo, Philippines – 1991Makes for moreintense sunsets.
65Volcanoes 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
66Volcanoes 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.