Volcanoes Chapter 9

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 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 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 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 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 volcanoes? Where do we find volcanoes?

Plate Boundary Types Plate Boundary Types Divergent Boundary – where plates move apart. Example: Iceland sits on the Mid-Atlantic Ridge – has many volcanoes

Divergent Boundary Divergent Boundary Ridges – mountain ranges that form. New crust is formed Rift – valleys between the ridges – where volcanic eruptions occur

Newer crust Older crust

Plate Boundary types Plate 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 convergent boundary Ocean – Continental convergent 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 Boundary Convergent Boundary

Ocean – Ocean Convergent Boundary Subduction may still occur forming island arcs. Example: Japan trench

Continental – Continental Convergent Zone Both plates crumble and pile up forming mountains – but not volcanoes. Example: Himalayas

Hot 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 Spots

Extinct 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.eruptsvent An eruption can be quite gentle or very violent.eruption

What comes out of a volcano? Gases Tephra Lava Pyroclastic Flow

Gases 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)

Tephra 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 is melted rock flowing on the Earth’s surface.

Pyroclastic 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 Eruptions

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

Shield Volcano Made from quiet lava flows – the lava spreads out without building up tall. Volcano is flat and wide. Angle of repose is under 15 o. Mountain made of lava only Ex: Hawaiian Islands

Shield Volcano

Cinder 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 30 o.

Cinder Cone Volcano

Composite Volcano or Stratovolcano Alternates between quite lava flows and violent eruptions. Made of alternating layers of lava and tephra. Angle of repose is between o

Composite Volcano or Stratovolcano

Igneous 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 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 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 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 Features

Volcanic 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.

Caldera 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)