VolcanoesVolcanoes

Earth’s Creators and Earth’s Creators and DestroyersDestroyers

Structure of the EarthStructure of the Earth

• The earth is composed of layers:The earth is composed of layers:• Inner CoreInner Core – solid – solid• Outer CoreOuter Core – liquid – liquid• MantleMantle – solid – solid• Upper MantleUpper Mantle – plastic – plastic• CrustCrust – solid – solid

Diagram of Earth’s Diagram of Earth’s LayersLayers

Tectonic PlatesTectonic Plates

• The lithospherelithosphere is the Earth’s hard, outermost shell that is divided into a mosaic of 16 major slabs, or tectonic plates.

• These plates float on the upper mantle.• As the plates move about they spread

apart, collide, or slide past each other

Why Plates Move: Convection Why Plates Move: Convection CellsCells

Earth’s Tectonic PlatesEarth’s Tectonic Plates

Where Volcanoes OccurWhere Volcanoes Occur• Volcanoes occur most frequently at plate

boundaries.• Some volcanoes, like those that form the

Hawaiian Islands, occur in the interior of plates at areas called hot spotshot spots.

• The greatest number of volcanoes occur on the ocean floor along spreading ridgesspreading ridges.

• Over 80% of those on land occur at edges of continents, or subduction zonessubduction zones, where one plate dives, or subducts, under another plate.

Why Volcanoes OccurWhy Volcanoes Occur• Temperatures in the mantle are hot

enough to melt rock into magmamagma.• Less dense than the solid rock around it,

magma rises and some of it collects in magma chambersmagma chambers.

• As the magma rises, pressure decreases allowing trapped gasses to expand and propel the magma through openings in the Earth’s surface causing an eruption.

• Erupted magma is called lavalava.

How Volcanoes EruptHow Volcanoes Erupt• Eruptions are described as explosiveexplosive

or effusiveeffusive (loosely flowing).• How explosive an eruption is depends

on the magma’s chemical composition and gas content, which affect the magma’s stickiness, or viscosity.

• If magma is fluid, gases can escape rapidly and lava flows; if magma is viscous the gases can not escape and pressure builds inside the magma until the gases escape, sometimes violently.

Magma vs. LavaMagma vs. Lava

• MagmaMagma is molten rock beneath the surface.

• LavaLava is erupted magma. There are 2 types of lava:– A aA a (ah ah) is largely solidified

rock that gets pushed forward.– Pahoe hoePahoe hoe (pah hoy hoy) is

flowing “liquid” with a ropy, billowy surface.

Lava – Pahoe Lava – Pahoe hoehoe

Lava - AaLava - Aa

Types of Lava FlowsTypes of Lava Flows

• Lava flows are superheated streams of molten rock that flow at 1 – 50 mph.

• Pyroclastic flows are avalanches of hot ash, rock fragments, and gases that flow at speeds greater than 100 mph.

• Landslides are avalanches of rock, snow and ice on slopes of volcanoes (loosened and tumbling due to seismic activity).

• Lahars (mud flows) are a mixture of volcanic ash and water (like wet concrete)

Volcano TermsVolcano Terms

• A ventvent is an opening through which eruptions take place.

• A cratercrater is a basin like depression over a vent, at the summit of a volcano

• A calderacaldera is a depression larger than the original crater (>1km. Diameter) that forms when the summit is blown off, or when the volcano collapses into the empty magma magma chamberchamber.– ExampleExample: Crater Lake atop Mt. St. Helens.

Types of VolcanoesTypes of Volcanoes

• Repeated volcanic eruptions build volcanic mountains of three basic three basic typestypes, or shapes, depending on the composition of the materials deposited by the eruption.

• Shield volcanoesShield volcanoes• StratovolcanoesStratovolcanoes• Cinder conesCinder cones

Shield VolcanoesShield Volcanoes

• Shield volcanoesShield volcanoes are broadbroad gently sloping volcanic mountains slowly formed slowly formed by layerby layer over layer of solidified lava.over layer of solidified lava.

• Shield volcanoesShield volcanoes are formed by effusive formed by effusive eruptions of fluid lavaeruptions of fluid lava.

• These can become very large as the low viscosity lava spreads widely and thickly.

• ExamplesExamples: Kilauea, Hawaii and Mt. Etna, Italy

Kilauea, HIKilauea, HI

KilaueaKilauea

Mt. Etna, ItalyMt. Etna, Italy

Mt. EtnaMt. Etna

Mt. EtnaMt. Etna

Stratovolcanoes Stratovolcanoes (Composite)(Composite)

• StratovolcanoesStratovolcanoes are formed from formed from bothboth explosive and effusive eruptions. explosive and effusive eruptions.

• Layers of tephratephra alternating with layers of viscous lava flowsviscous lava flows create steep-steep-sided, often symetrical conessided, often symetrical cones that can be very large.

• Formed over long spans of time as periods of 100,000+ yrs. separate periods of a few years of intense activity.

• ExamplesExamples: Aconcagua, Andes (22,825’) and Mt. St. Helens

Aconcagua, AndesAconcagua, Andes

AconcaguaAconcagua

Mt. St. HelensMt. St. Helens

Mt. St. HelensMt. St. Helens

Cinder Cone VolcanoesCinder Cone Volcanoes• Cinder conesCinder cones are the smallestsmallest volcanoes

(< 500’), formed by explosive eruptions formed by explosive eruptions of lavaof lava. Blown violently into the air, the erupting lava breaks apart into fragments called cinderscinders that fall and accumulate around the vent.

• Cinder conesCinder cones are temporary geologic temporary geologic featuresfeatures as they are easily erodedeasily eroded. They have short life spansshort life spans as gas causing violent eruptions is quickly depleted.

• ExampleExample: Paricutin, Mexico

Paricutin, MexicoParicutin, Mexico

1966

1944

1946 1948

1950

Evolution of ParicutinEvolution of Paricutin

Monitoring and Predicting Monitoring and Predicting EruptionsEruptions

• Volcanic activity is monitored using several observations:

• Land deformationLand deformation• Ash cloudsAsh clouds• Tremors- measured by seismic Tremors- measured by seismic

datadata– Volcanic Tremors (VT)Volcanic Tremors (VT)– Rockfall (RF)Rockfall (RF)

Active,or Recently Active, Active,or Recently Active, VolcanoesVolcanoes

NameName TypeType Last EruptedLast Erupted

Bezymianmy Stratovolcano 1993

Erebus Stratovolcano 1980

Kilauea Shield 1995

La Palma Stratovolcano 1954

Mt. Etna Shield 1993

Mt. St. Helens Stratovolcano 1986

Nevada del Ruiz

Stratovolcano 1991

Ol Doinyo Lengai

Stratovolcano 1993

Paricutin Cinder cone 1952

Pinatubo Stratovolcano 1992