Chapter 3: Volcanoes

Chapter 3.1

• Key Concepts:

–Where are Earth’s volcanoes found?

–How do hot spot volcanoes form?

Definitions:

Volcano: a weak spot in the Earth’s crust where molten material (magma) comes to the surface

Magma: a molten mixture of rock forming substances, gases and water from the earth’s mantle.

Lava: when magma reaches the Earth’s surface.

Volcanoes & Plate Boundaries

• 600 active volcanoes on land, many more under the sea.

• Volcanoes occur in belts that extend across continents and oceans.

• Ring of Fire - major volcanic belt formed by the many volcanoes that rim the Pacific Ocean.

Volcanic belts form along the boundaries of Earth’s plates

• Volcanic belts form along the boundaries of the Earth’s plates

• Diverging ( )and converging () occur.

• Results in fractures in the Earth’s crust

• Magma reaches the surface.

Most volcanoes occur at

• Divergent Boundaries – Mid Ocean Ridge

• Convergent boundary– Points of

subduction

ex. Mount Etna – Eurasian and African plates

Volcanoes on Divergent Boundaries

• Under ocean: point of Mid ocean ridges occurring ( long underwater mountain ranges)

• Many have rift valleys down the center

• Lava pours out of rift valley – creates new mountains.

• Land - Great Rift Valley in East Africa

Volcanoes on Convergent Boundaries

• Volcanoes form when:– Two oceanic plates collide– Oceanic and continental plates collide

• In both situations, oceanic plates sink beneath a trench.

• Rock above the plate melts to form magma.

• Erupts to the surface as lava

Volcanoes on Convergent Boundaries

Volcanoes on Convergent Boundaries

• Magma – less dense than the surrounding rock– Magma rises to the surface and breaks

through the ocean floor creating volcanoes.

– String of islands formed: Island arc

– Examples: Japan, New Zealand, Indonesia, the Philippines

Hot Spot Volcanoes

• Hot Spot – an area where material from deep within the mantle rises and melts forming magma

• Volcanoes form above a hot spot when magma erupts through the crust and reaches the surface.

Hot Spot Volcanoes

• Some hot spots lie in the middle of plates far from plate boundaries.

• Others occur on or near plate boundaries.

Hot Spot Volcanoes

• A hot spot in the ocean floor can gradually form a series of volcanic mountains

• Ex. The Hawaiian islands

Hot Spot Volcanoes

• A hot spot can also form under continents• Ex. Yellowstone National Park, WY

3.2 PROPERTIES OF MAGMA

Vocabulary: • Element-a substance that cannot be broken

down into other substances– Compound-a substance made of two or more

elements that have been chemically combined– Physical Property-a characteristic of a substance

that can be observed or measured without changing the composition of a substance

– Chemical Property-any property that produces a change in the composition of matter.

– Pahoehoe-fast moving, hot lava that has low viscosity

– aa-higher viscosity, slower moving, cooler lava that Pahoehoe

– Viscosity-the resistance of a liquid to flow– Silica—major ingredient in magma made of

silicon and oxygen.

• Magma Composition

• Magma varies in composition and is classified according to the amount of silica it contains.

• The graphs show the average composition of the two types of magma. Use the graphs to answer the questions.

• What materials make up both types of magma?• Silica, oxides, and other solids• Which type of magma has more silica? About how much

silica does this type of magma contain? Rhyolite, 70%• A third type of magma has a silica content that is halfway

between that of the other two types. About how much silica does this type of magma contain? 60%

• What type of magma would have a higher viscosity? Explain. The rhyolite forming would have higher viscosity because it contains more silica

Volcanic Eruptions 3.3• Objectives:

– Explain what happens when a volcano erupts.– Describe 2 types of volcanic eruptions– Identify stages of volcanic activity

• What comes out of volcanic explosions?– Ash, lava, steam, rock, gases, dirt

Volcanic Eruptions 3.3

• Magma forms in the asthenosphere– Convection currents in the mantle bring

magma towards the surface

• Magma tries to flow into any open crack

• When magma reaches a weak spot in the crust, a volcano forms

• Lava is magma that has reached the surface

Magma Reaches Earth’s Surface

• Volcanoes are systems of passage ways through which magma moves.

• Inside a Volcano– Magma collects below the volcano in the

magma chamber– Magma flows upward through a tube (pipe)

that connects the magma chamber to the surface.

Magma Reaches Earth’s Surface

• Inside a Volcano– Gas and magma leave through vents

• Central vent is the opening at the top• Side vent anywhere along the side of the volcano

– Lava pours out of the vents and creates a lava flow

– A bowl shaped area called a crater may form around the central vent

• A Volcanic Eruption– Dissolved gases are trapped in magma under extreme pressure (CO2 in a soda bottle)

– as magma flows , pressure because there is less rock on top of it.

– the dissolved gases expand and form bubbles– When a volcano erupts, the force of the

expanding gas forces magma up the pipe until it explodes out the vent

Magma Reaches Earth’s Surface

Content checkpoint… think/pair share…take

two minutes to answer these questions with a partner nearby…..

• What common everyday occurrence can we relate to a volcanic eruption?

• What happens to the pressure in the magma as it rises toward the surface?

Kinds of Volcanic Eruptions

• Volcanic eruptions can be quiet or explosive.

• Eruptions depend on the properties of magma– Silica content – Viscosity

Kinds of Volcanic Eruptions

• Quiet Eruptions– Magma has low silica content = low viscosity– Magma flows easier so gases bubble out

gently– Lava can flow many kilometers from the vent– Produce both pahoehoe and aa lava– Example: Hawaiian Islands – Mount Kilauea

Kinds of Volcanic Eruptions

• Explosive Eruptions– Magma has high silica content = high

viscosity– Magma doesn’t always flow out of vent and so

it builds up (like a cork in a bottle)– Trapped gases build up pressure until they

explode – Magma is pushed out of the vent with

incredible force

Kinds of Volcanic Eruptions• Explosive Eruptions

– Lava breaks into fragments that cool quickly and harden into different sizes

• Ash – fine, dust sized particles of lava• Cinders – pebble sized particles• Bombs – baseball to car sized chunks of lava

– Pyroclastic flow: when an explosive eruption hurls out gases, ash, cinders and bombs.

– Pumice forms when lava cools quick and traps air bubbles inside

– Obsidian forms when lava cools quick leaving the surface smooth and glass-like

Kinds of Volcanic Eruptions

• Volcano Hazards– Quiet eruptions cause lava to flow far ---

burning and burying everything in its path– Explosive eruptions can bury entire towns in

ash, cause landslides, avalanches, cause damage from gases and cinders/bombs.

Content checkpoint… think-pair-share take two minutes to answer these questions with

a partner nearby.• What is pyroclastic flow?

• How does volcanic ash cause damage?

• What is the main difference between a quiet eruption and an explosive eruption?

Stages of Volcanic Activity

• Life Cycle of a Volcano– Scientists use historical records and monitor

volcanoes to determine what stage of activity a volcano is in

– A volcano can be active, dormant or extinct• An active volcano is erupting or showing signs of

near future eruptions• A dormant (sleeping) volcano can awaken in the

future and become active• An extinct (dead) volcano is unlikely to erupt ever

again

Stages of Volcanic Activity• Monitoring Volcanoes

– Geologists use tiltmeters to detect slight changes in surface elevation cause by magma moving underground

– They monitor gases escaping from a volcano– Increase in temperature might mean magma is

nearing the surface– The changes detected may give a short

warning time BUT• We cannot be certain the type of eruption or how

powerful it will be

Think and Discuss….

• Which is more likely to be dangerous – a volcano that erupts frequently or a volcano that has been inactive for a hundred years? WHY?

3.4 Volcanic Landforms

• Objectives:– List the landforms that lava and ash create– Explain how magma that hardens beneath

earth’s crust creates landforms– Identify other distinct features that occur in

volcanic areas

• Volcanic activity on and beneath the surface has built up Earth’s land areas.

Landforms from Lava and Ash

• Volcanic eruptions create landforms made of lava, ash and other materials– Shield volcanoes– Cinder cone volcanoes– Composite volcanoes– Lava plateaus

• Another landform results from the collapse of a volcanic mountain– caldera

• Shield volcanoes– Gently sloping mountains– Thin layers of low viscosity lava build up over time– Ex) the Hawaiian Islands– CSAV Hawaii: Pele's Hair (Volcanic Glass) - YouTube

Kilauea Lava Flows on May 6, 2012 - YouTube

Shield Volcano in Iceland

• Cinder cone volcanoes– High viscosity lava produces ash, cinders and

bombs– Those materials build up around the vent in a

steep cone shaped hill/mountain.– Ex) Sunset Crater in Arizona

• Composite Volcanoes– Form when volcanoes alternate between quiet

lava flows and explosive eruptions of ash, cinders and bombs

– Tall, cone shaped mountain with alternating layers of ash and lava

– Ex) Mt St. Helens, Washington and Mt. Fuji, Japan

• Composite Volcanoes• Form when volcanoes alternate between quiet lava flows

and explosive eruptions of ash, cinders and bombs• Tall, cone shaped mountain with alternating layers of ash

and lava• Ex) Mt St. Helens, Washington and Mt. Fuji, Japan

• Lava Plateaus– Low viscosity lava flows out of several long

cracks and travels far before cooling– After millions of years a high plateau forms– Ex) Columbia Plateau in Washington, Oregon

and Idaho

• Calderas– Huge hole left by the collapse of composite

volcanic mountains– Filled with fallen pieces of the volcano, ash

and lava– Enormous eruptions may empty the pipe and

the magma chamber– there is only hollow space left that can’t

support the weight and it collapses on itself– Overtime water from rain and snow may fill

the caldera– Ex) Crater Lake, Oregon

• Soil from lava and ash– People settle near volcanoes because of the

rich, fertile soil– At first, after volcanic eruptions the land is

empty and barren– Overtime the ash breaks down and provides:

• Potassium• Phosphorus• Other substances that plants need

Landforms from Magma

• Sometimes magma forces its way through cracks in the crust but never reaches the surface

• The magma cools and hardens within the rock layers

• Overtime ice, wind, or rain erode away the surrounding rock and expose the hardened magma

• Volcanic Necks– Forms when magma hardens in the pipe– Overtime the outer rock is weathered away– Looks like a giant tooth– Ex) Ship Rock, New Mexico

• Dikes and Sills– Dike: when magma forces its way across

multiple rock layers and hardens– Sill: when magma squeezes between

horizontal layers of rock and hardens– Ex) Palisades, NY & NJ

dike sill

• Batholiths– Large masses of rock that are formed when a

huge mass of magma cools inside the Earth’s crust

– The outer layers of rock erode away leaving the hardened magma exposed

– Ex) Sierra Nevada Batholith

• Dome Mountains– Created by smaller bodies of magma– The hardened magma uplifts the surrounding

rock and causes it to bend into a dome shape– Ex) Black Hills, South Dakota

Geothermal Activity

• Geo = Earth, therme = heat

• Geothermal activity – when magma a few km beneath the surface heats underground water.– Ex) hot springs and geysers found near past

and present volcanic activity

• Hot Springs– Forms when underground water is heated by

magma or hot rock– The heated water rises and collects in pools

• Geysers– Fountains of water and steam that erupt from

the ground– The heated water and steam build up

pressure until finally it erupts– Ex) Old Faithful, Yellowstone National Park

• Geothermal Energy– Heated water can provide an energy source– Heated water can be used to power turbines

and create electricity

A panoramic view of the Geysers geothermal power plant in Geysers, Calif. The site, located above Santa Rosa, is the largest geothermal development in the world.