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Chapter 8

Volcanic Activity

Dr. Joao Santos

Chapter 8

Volcanic Activity

Dr. Joao Santos

Essentials of Geology, 3rd edition, by Stephen MarshakEssentials of Geology, 3rd edition, by Stephen Marshak Chapter 5: The Wrath of Vulcan: Volcanic EruptionsChapter 5: The Wrath of Vulcan: Volcanic Eruptions

Volcanic Eruptions � What is a volcano?

� An erupting vent through which molten rock surfaces.

� A mountain built from magmatic eruptions.

� Volcanoes are caused by tectonic activity.

� Volcanoes pose a number of hazards to humans.

� Mexico City.

� Seattle.

� Naples, Italy.

Edited by Joao Santos

� In 79 A.D. Mount Vesuvius erupted violently.

� Pyroclastic flows destroyed Pompeii, killing 20,000.

� A record of Roman life was preserved under ash.

Volcanic Eruptions

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Volcanic Eruptions

� Unpredictable, dangerous.

� 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 effect climate.

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Volcanic Materials � The products of volcanic eruption take three forms:

� Lava flows – Molten rock that moves over the ground.

� Pyroclastic debris – Fragments blown out of a volcano.

� Volcanic gases – Vapor and aerosols that exit a volcano.

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Lava Flows � Lava can be thin and runny or thick and sticky.

� Flow style depends on viscosity, which depends upon...

� Composition, especially silica (SiO2), Fe, and Mg content.

� Temperature.

� Gas content.

� Crystal content.

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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.

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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).

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Basaltic Lava Flows � 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.

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Basaltic Lava Flows � 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.

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Basaltic Lava Flows � 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 later transmit water.

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Basaltic Lava Flows � 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 mid-ocean ridge.

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Andesitic Lava Flows � Higher SiO2 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.

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Rhyolitic Lava Flows � Rhyolite, with the highest SiO2, is the most viscous lava.

� Rhyolitic lava rarely flows.

� Rather, lava plugs the vent as a lava dome.

� Sometimes, lava domes are blown to smithereens.

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Basaltic Pyroclastic Debris � Glass shards and fragmented lava in a range of sizes.

� Basaltic eruptions generate a lot of spatter.

� Lapilli – Pea to plum-sized material.

� Blocks and bombs – Apple to refrigerator-sized.

�Pele’s Hair – Strands of glass created by flying lava droplets.

�Bombs – Streamlined fragments of ejected lava.

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Explosive Pyroclastic Debris � Intermediate and felsic magmas erupt explosively.

� More viscous than basaltic magma (from SiO2).

� Contain more gas.

� Produce large quantities of volcanic ash.

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Pyroclastic Flows � Pyroclastic flows (or, nuee ardentes - French):A

� Avalanches of hot ash (200Co–450oC) that race downslope.

� Moving up to 300 kph, they Incinerate all in their path.

� Immediately deadly; they kill everything quickly.

� Many famous examples: Mt. Vesuvius, Mt. Pelee, and Mt. Augustine.

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Pyroclastic Deposits � Tephra – Deposits of pyroclastic debris of any size.

� Tuff – Lithified ash with or without lapilli.

� Air-fall tuff – Accumulations of ash that fell like snow.

� Ignimbrite (welded tuff) – Tuff that is deposited while hot.

�Hot pyroclastic flow material.

�Fuses together while cooling.

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Volcaniclastic Deposits � Blocks – Preexisting rock fragmented by eruption.

� Blown out of a volcanic vent, blocks pile up nearby.

� Create unstable slopes that easily fail.

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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 per hour).

� They have the consistency of wet cement,.

� A distinct hazard to people living in volcanic valleys.

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Volcanic Gas � Up to 9% of magma may be gas.

� Water (H2O) – Most abundant 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.

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Volcanic Gas � Gases are expelled as magma rises.

� 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 eruption.

� Gas bubbles in rock are

called vesicles.

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Volcanic Architecture and Landforms � Volcanoes have characteristic features.

� Magma chamber.

� Fissures and vents.

� Craters.

� Calderas.

� Distinctive profiles.

�Shield volcanoes.

�Cinder cones.

�Stratovolcanoes.

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Magma Chamber � Located in the upper crust.

� Usually an open cavity or area of highly fractured rock.

� May contain a large quantity of magma.

� May inflate and deflate.

� Some magma cools here to form intrusive rock.

� Some magma may rise to the surface to form a volcano.

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Magma Release � 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.

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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.

� Summit eruptions – Located within the summit crater.

� Flank eruption – Located along the side of a volcano.

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Caldera

Figure 4.1a

� A gigantic volcanic depression.

� One to ten kilometers across, larger than

a crater.

� Steep sidewalls and flat floors.

� Form from massive eruptions.

� The volcano collapses.

� Crater Lake, Oregon.

� Yellowstone National Park.

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Crater Lake Caldera, OregonCrater Lake Caldera, Oregon

CalderaCrater

Volcano Types � Shield volcanoes.

� Broad, slightly domed-shaped (like an inverted shield).

� Made by lateral flow of low-viscosity basaltic lava.

� Have a low slope and cover large geographic areas.

� Mauna Loa on Hawaii is a good example.

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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.

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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.

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Eruptive Style � Will it flow, or will it blow? Two dominant styles.

� Effusive eruptions – Produce lava flows.

� Explosive eruptions – Blow up.

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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 create shield volcanoes.

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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.

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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 is important.

� Subaerial lava flowing on land cools slower than…

� Submarine lava which is quickly quenched.

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Tectonic Settings � Plate motion is a dominant control on volcanism.

� Volcanic types are linked to tectonic boundaries.

� mid-ocean ridges – Spreading axes.

� Convergent boundaries – Subduction zones.

� Continental rifts – Incipient ocean basins.

� Oceanic and continental hot spots – Mantle plumes.

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Mid-Ocean Ridge Eruptions � Most lava erupts along the mid-ocean ridge (MOR).

� MOR-generated oceanic crust covers 70% of Earth.

� Basalt erupted from fissures quenches as pillows.

� Pillow mounds are pulled apart with plate motion.

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Convergent Boundaries � Most volcanoes form at convergent boundaries.

� Volatiles from subducting plate initiate melting.

� Arc volcanoes develop on the overriding plate.

�May cut through either oceanic or continental crust.

� The “Ring of Fire” dominates Pacific margins.

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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 Province.

� Mid-continent Rift.

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Oceanic Hot Spot Volcanoes � 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.

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Continental Hot-Spot Volcanoes � Continental Hot Spot – Cuts a continental plate.

� Yellowstone – Eruption ~ 640 Ka created a 100 km caldera.

�1,000 times more powerful than Mt. St. Helens.

�Deposited vast ignimbrite deposits.

�Magma beneath the caldera continues to fuel geysers

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Flood-Basalt Eruptions � Voluminous lava eruptions above a plume.

� Thinned lithosphere erupts magma from long fissures.

� Lava spreads over large areas; great thicknesses stack up.

� Create plateaus called Large Igneous Provinces (LIPs).

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Iceland � Iceland is a hot spot beneath a mid-ocean ridge.

� Lava has built the hot spot/ridge above sea level.

� The island is being torn apart by plate motion.

� Volcanoes trace the mid-ocean ridge rift valley.

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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 volcanoes to inflate.

� Emission increases – Changes in gas mix and volume.

� These signs indicate that an eruption is imminent.

� They cannot predict eruption timing or style.

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Volcanoes and Climate � Volcanic eruptions can be large enough to alter climate.

� Ash and aerosols high in the atmosphere block sunlight.

� This causes atmospheric cooling.

�1815 was the “year without a summer” due to Tambora.

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End of Chapter 8