Earth Science

Andrexa Peluso

07-Volcanoes.pptx

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Volcanoes and Other Igneous Activity

Chapter 7 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Mount St. Helens

Largest historic eruption in North America

Lowered peak by more than 400 m

Destroyed all trees in a 400 km2 area

Mudflows 29 km down Toutle River

Ejected 1 km3 ash more than 18 km into stratosphere

Mount St. Helens Versus Kilauea

Kilauea

Quiet eruption of fluid basaltic lava

Occasional lava sprays

Eruption began in 1983 and has been ongoing for more than 20 years

Mount St. Helens Versus Kilauea

Viscosity

Resistance to flow

How to decrease magma viscosity

Increase temperature

Decrease silica content

Rhyolitic magma (>70% Si) forms short, thick flows

Basaltic magma (~50% Si) is fluid

Gas content also dictates nature of eruption

Directly related to composition

Most common gas is water vapor

Quiescent Versus Explosive Eruptions

The Nature of Volcanic Eruptions

Triggered by addition of magma to near-surface magma chamber

Inflation and fracture of volcano summit

Fluid basaltic lava

Ongoing eruption of Kilauea since 1983

Quiescent Hawaiian-Type Eruptions

Pressure decreases as magma rises

Dissolved gas forms expanding bubbles

Viscous magma expels fragmented lava and gas

Buoyant plumes of material (eruption columns)

Rapid ejection of magma

Reduces pressure in magma chamber

Causes further expansion and eruption

How Explosive Eruptions Are Triggered

90% of lava is basaltic

Most is erupted on seafloor (submarine volcanism)

Flows in thin, broad sheets or ribbons

Flow rate ~10 to 300 m/hr

Up to 30 km/hr downhill

~9% is andesitic/intermediate

<1% is rhyolitic

Thick flows move imperceptibly slow

Don’t flow beyond a few km from vents

Lava Flows

Two types of basaltic lava flows:

Rough, jagged blocks with sharp edges

Cooler, more viscous basaltic flows

Pahoehoe

Smooth, ropy surfaces

Hotter, less viscous basaltic flows

Lava Flows

Lava tubes

Insulated pathways of a lava flow

Pillow lavas

Numerous tube-like structures stacked atop each other

Form on the ocean floor

Lava Flows

Gases in magma are volatiles

Dissolved into magma because of confining pressure

~1–8% of total magma volume

Most abundant gases (in decreasing order): water vapor, carbon dioxide, sulfur dioxide, lesser amounts of hydrogen sulfide, carbon monoxide, and helium

Contribute to atmosphere

Significant quantities can alter global climate

Gases

Pyroclastic materials or tephra

Particles erupted from a volcano (ash and dust)

Hot ash fuses to form welded tuff

Lapilli and cinders are the size of small beads to walnuts

Blocks are larger than 64 mm

Bombs are streamlined blocks ejected while still molten

Scoria is vesicular ejecta

Pumice is felsic equivalent

Pyroclastic Materials

Materials Extruded During an Eruption

Volcanic landforms are the result of many generations of volcanic activity

Eruptions start at a fissure

Flow is localized into a circular conduit

Conduit terminates at a vent

Successive eruptions form a volcanic cone

Anatomy of a Volcano

Crater

Funnel-shaped depression at the summit

Form by erosion during, or collapse following, eruptions

Calderas are craters >1 km

Flank eruptions generate parasitic cones

Secondary vents that emit gas only are called fumaroles

Anatomy of a Volcano

Broad domed structures built by accumulation of basaltic lava

Most begin as seamounts (submarine volcano)

e.g., Canary Islands, Hawaiian Islands, Galapagos, Easter Island, Newberry Volcano in Oregon

Shield Volcanoes

9 km high

Low angle slopes

Well-developed caldera from collapse of magma chamber following eruption

Mauna Loa: Earth’s Largest Shield Volcano

Kilauea is most active and studied volcano

50 eruptions since 1823

Most recent began in 1983

Magma chamber inflates and earthquake swarms indicate an impending eruption

Kilauea: Hawaii’s Most Active Volcano

Cinder cones (scoria cones)

Symmetrical

Steep-sided

Loose accumulations of ejected scoria

Commonly pea- to walnut-sized fragments

Basaltic composition, reddish-brown color

Some produce lava flows

Craters are relatively large and deep

Cinder Cones

Cinder cones form quickly

Many in less than one month

Generally in a single eruptive event

Small size (30–300 m)

Cinder Cones

Composite cones or stratovolcanoes

Located around the Ring of Fire

Large, symmetrical cones

Built by layers of cinder and ash alternating with lava flows

Primarily silica-rich andesitic magma

Associated with explosive eruptions and abundant pyroclastic material

Steep summit and gradually sloping flanks

Composite Volcanoes

70 volcanic eruptions expected each year

One large-volume eruption each decade

500 million people live near active volcanoes

Volcanic hazards include:

Pyroclastic flows

Lahars

Lava flows

Ash and volcanic gasses

Volcanic Hazards

Pyroclastic flow (nuee ardente)

Hot volcanic gas infused in incandescent ash and lava fragments

Gravity driven, can move up to 100 km/hr

Low-density cloud of hot gases and fine ash on top of layer of vesicular pyroclastic material

Caused by collapse of eruption columns

Pyroclastic Flow: A Deadly Force of Nature

Volcanic Hazards

Lahars

Fluid mudflows

Water-saturated volcanic debris move down steep volcanic slopes

Can occur on dormant/extinct volcanoes

Lahars: Mudflows on Active and Inactive Cones

Tsunamis

Caused by collapse of volcano flanks into the ocean

Ash

Can damage buildings, living things, aircraft engines

Sulfur dioxide

Affects air quality and creates acid rain

Atmospheric cooling

Ash and aerosols reflect solar energy

Other Volcanic Hazards

Volcanic Hazards

Caldera

Steep-sided crater less than 1 km in diameter

Formed by summit collapse following draining of the magma chamber

Other Volcanic Landforms

Fissure eruptions

Emit basaltic lavas from fissures (fractures)

Basalt plateaus

Flat, broad accumulations of basalt emitted from fissures

Flood basalts

Molten lava having flowed long distances within a basalt plateau

Other Volcanic Landforms

Volcanic necks (plugs)

Eroded volcanic cones expose the solidified magma inside the conduit

Other Volcanic Landforms

Magma that crystallizes in Earth’s crust displacing host or country rock forms intrusions or plutons

Exposed by uplift and erosion

Classified according to shape

Tabular or massive

May cut across existing structures

Discordant

Or inject parallel to features

Concordant

Intrusive Igneous Activity

Tabular intrusive bodies

Magma is injected into a fracture or other zone of weakness

Dikes are discordant

Sills are concordant

Columnar jointing occurs as a result of shrinkage fractures that develop when igneous rocks cool

Intrusive Igneous Activity

Large intrusive bodies include:

Batholiths

Linear masses of felsic rocks hundreds of km long

Stocks

Surface exposure <100 km2

Laccoliths

Lift the sedimentary strata that they penetrate

Intrusive Igneous Activity

Most volcanoes are found near:

Ring of Fire around the Pacific Ocean

Mid-ocean ridges

Few are randomly distributed

Plate Tectonics and Volcanism

Intraplate volcanism

A mantle plume of hot material ascends to the surface

Plate Tectonics and Volcanic Activity