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.
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.
Plate Boundary Types Divergent Boundary – where plates move apart. Example: Iceland sitson the Mid-Atlantic Ridge – has many volcanoes
Divergent Boundary Ridges – mountain ranges that form. New crust is formed Rift – valleys betweenthe ridges – where volcanic eruptionsoccur
Newer crust Older crust
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 • 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.
Ocean – OceanConvergent 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.
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. • An eruption can be quite gentle or very violent.
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
Types of Eruptions • The way a volcano erupts is dependent upon: • Amount of trapped gases • Amount of silica in magma • Amount of water in magma
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 15o. • Mountain made of lava only • Ex: Hawaiian Islands
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 30o.
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 15-30o
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
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)