VolcanoesChapter 9

What is a volcano?What is a volcano?

• A volcano is an opening in the Earth’s surface.

• Ash and lava come out and build up forming a mountain.

• The word, ‘volcano’ comes from the name Vulcan, who was the Roman god of fire.

Activity of VolcanoesActivity of Volcanoes

Volcanoes can be:• Active - has had at least one eruption during

the past 10,000 years. An active volcano might be erupting or dormant.

• Erupting volcano is an active volcano that is having an eruption...

• Dormant volcano is an active volcano that is not erupting, but supposed to erupt again.

• Extinct - has not had an eruption for at least 10,000 years and is not expected to erupt again in a comparable time scale of the future.

Parts of a volcanoParts of a volcano

• Crater – opening of the volcano

• Vent – main tube running through volcano

• Side vent – vent that runs outward from main vent

• Magma Chamber -reservoir of magma in the earth's crust where the magma may reside temporarily on its way from the upper mantle to the earth's surface

How volcanoes formHow volcanoes form

• Heat and pressure inside the Earth cause rocks to melt.

• Magma rises up because it is less dense than the rock around it.

• Magma reaches the surface and flows out of vents – sometimes through cracks.

• Mountains build up as lava and ash flow out.

How volcanoes formHow volcanoes form

Where do we find volcanoes?Where do we find volcanoes?

• Most volcanoes are found along plate boundaries:– Where plates move apart– Where plates move together

• Some volcanoes are found over hot spots.– Hot spots are areas inside the Earth where it

is hotter than nearby areas.

Where do we find Where do we find volcanoes?volcanoes?

Where do we find Where do we find volcanoes?volcanoes?

Plate Boundary TypesPlate Boundary Types

Divergent Boundary – where plates move apart.

Example: Iceland sitson the Mid-Atlantic

Ridge – has many volcanoes

Divergent BoundaryDivergent BoundaryRidges – mountain ranges that form.

New crust is formed

Rift – valleys betweenthe ridges – where volcanic eruptionsoccur

Newer crust

Older crust

Plate Boundary typesPlate Boundary types

Convergent Boundary – where plates move together. Can occur between:

– Ocean and continental plates – most common– Continental and continental plates– Ocean and ocean plates

Ocean – Continental Ocean – Continental convergent boundaryconvergent boundary

•A subduction zone forms– where one plate moves under the other

– Ocean crust is more dense and moves under continental crust.

– Rocks on the subducted plate melt and the magma rises to form volcanoes on the other plate.

•A trench forms – deep valley

•Volcanic arcs form on continental plate.

Convergent BoundaryConvergent Boundary

Convergent BoundaryConvergent Boundary

Ocean – OceanOcean – OceanConvergent BoundaryConvergent Boundary

• Subduction may still occur forming island arcs.

Example:

Japan

trench

Continental – Continental Continental – Continental Convergent ZoneConvergent Zone

• Both plates crumble and pile up forming mountains – but not volcanoes.

• Example:

Himalayas

Hot SpotsHot Spots

• Areas of the mantle that are hotter than other areas.

• They melt rock which gets forced up toward the crust.

• If it occurs under water, it can form volcanic islands, like Hawaii.

• Plates move, but hot spots don’t, so volcanoes can become extinct if they no longer sit over a hot spot.

Hot SpotsHot Spots

Hot SpotsHot Spots

Hot SpotsHot Spots

Extinct VolcanoesExtinct Volcanoes

• The Hawaiian islands that are not on top of the hot spot are extinct. Mauna Loa and Kilauea are on Hawaii – newest island.

• A new island is being formed now – Loihi – it is an underwater volcano.

Why does a volcano erupt?

• A volcano erupts when magma and gases find a way to escape, so they burst to the surface through a vent.

• An eruption can be quite gentle or very violent.

What comes out What comes out of a volcano?of a volcano?

• Gases

• Tephra

• Lava

• Pyroclastic Flow

GasesGases

• Gases include:Water vapor, carbon dioxide, hydrogen sulfide, hydrochloric acid, sulfur dioxide, methane, ammonia, carbon monoxide, hydrogen sulfide

• Gases are toxic: – can cause respiratory problems– burn eyes (hydrochloric acid)– cause acid rain (hydrochloric acid, sulfur dioxide)– Suffocates people (carbon dioxide, carbon monoxide)

TephraTephra

• Pieces of rock and dried lava– Vary in size – ash is smallest, then cinders,

bombs, and blocks.– Ash can harm lungs – wear masks to keep out

of lungs.– Large pieces of rock can hurt people and are

hot.

Lava Lava

• Lava is melted rock flowing on the Earth’s surface.

Pyroclastic FlowPyroclastic Flow

• A fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the side of a volcano

• The way a volcano erupts is dependent upon:– Amount of trapped gases– Amount of silica in magma– Amount of water in magma

Types of EruptionsTypes of Eruptions

Trapped GasesTrapped Gases• Gases dissolve in liquids better if they are

under pressure (soda cans).• Deep inside the Earth there is a lot of

pressure – allowing gases to remain dissolved.

• As the magma gets closer to the surface, pressure decreases allowing gases to escape.

• More dissolved gases – more explosive

Silica in magmaSilica in magma

• Magma contains substance called silica – like sand.– more silica – thicker lava – traps gases.– less silica – thinner lava – lets gases out

• Gases must get through the lava to escape– More silica –more explosive– Less silica – less explosive

Water content of magmaWater content of magma

• Lots of water – forms lots of water vapor – a gas.

• Lots of water is the same as lots of gases.

• More water – more explosive

Forms of volcanoesForms of volcanoes

• The eruptions creates the volcano.

• Different types of eruptions form different types of volcanoes.– Less explosive volcano – is flat– More explosive volcano – is tall

Forms of VolcanoesForms of Volcanoes

• Volcanoes are classified by their – Angle of repose– Composition– Eruption type– Lava type

Shield VolcanoShield Volcano

• Made from quiet lava flows – the lava spreads out without building up tall.

• Volcano is flat and wide.

• Angle of repose is under 15o.

• Mountain made of lava only

• Ex: Hawaiian Islands

Shield VolcanoShield Volcano

Cinder Cone VolcanoCinder Cone Volcano

• Very explosive eruptions

• Tephra (Rock and solidified lava) drop from the air as ash, cinders, bombs, and blocks.

• Loosely packed tephra builds up mountain.

• Angle of repose – over 30o.

Cinder Cone VolcanoCinder Cone Volcano

Composite Volcano or Composite Volcano or StratovolcanoStratovolcano

• Alternates between quite lava flows and violent eruptions.

• Made of alternating layers of lava and tephra.

• Angle of repose is between 15-30o

Composite Volcano or Composite Volcano or StratovolcanoStratovolcano

Igneous Rock FormationIgneous Rock Formation

• Lava and magma cool and harden into igneous rock.– Lava – turns into extrusive igneous rock.– Magma – turns in to intrusive igneous rock.

Extrusive Igneous RockExtrusive Igneous Rock

• Rock that forms on the surface of the Earth from lava.

• Cools quickly – there is no time to form nice, large crystals

• May contain holes as gases escape

• Examples: basalt, pumice, obsidian

Intrusive Igneous RockIntrusive Igneous Rock

• Rock that forms inside the Earth from magma.

• Magma is not exposed to air so it cools slowly and has time to form nice large crystals.

• Examples: Granite, gabbro, diorite

Intrusive Igneous FeaturesIntrusive Igneous Features

• Not all the magma exits a volcano.• Some gets stuck underground, forming

intrusive igneous features.– Batholiths – large areas of intrusive rock– Lacoliths – area that forms a dome shape– Dikes – vertical sections that cross through

rock layers– Sills – horizontal sections that rucn between

rock layers

Intrusive Igneous FeaturesIntrusive Igneous Features

Volcanic neckVolcanic neck

• When a volcano stops erupting, the magma hardens inside the vent.

• If the volcano erodes away only the solidified vent remains – it is called a volcanic neck.

CalderaCaldera

• Sometimes the top of a volcano collapses and produces a large opening called a caldera.

• Sometimes the caldera fills with water.

• Example: Crater Lake Active volcano inside the lake (Wizard Island)