chapter 9 volcanoes. 9.1 what causes volcanoes? volcano: an opening in the earth’s surface which...
TRANSCRIPT
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Chapter 9
Volcanoes
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9.1 What causes volcanoes?
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• Volcano: an opening in the Earth’s surface which forms a mountain when layers of ash & lava build up
• More than 600 active volcanoes
• Most volcanoes are inactive
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Two Examples of Volcanoes
• May 18, 1980– Mount St. Helens in Washington– One of North America’s most damaging
eruptions– 59 people were killed—most by landslides &
flooding due to snow on the mountain melting
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Two Examples of Volcanoes
• Mount Kilauea in Hawaii– World’s most active volcano– It has been erupting continuously since 1983
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Volcano Terms
• Magma: melted rock INSIDE THE EARTH
• Melts due to high levels of heat & pressure
• Composed of a mixture of molten rock, minerals, & gases
• Melting occurs in the mantle– Which layer of the Earth is this?
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Volcano Terms• Density: m/v (which means?)• Mass divided by volume• This means how much something weighs
(m), in relation to how much space it takes up (v)
• Magma is less dense than the rock which surrounds it…so…
• Magma rises to the surface of the Earth
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How do volcanoes form?• After long periods of time magma may
reach the surface of the Earth• Then it will flow out of an opening called a
vent• Lava will flow out & cool continuously• This can make large mountains made of
lava & ash packed together• Crater: steep-walled depression around a
volcano’s vent
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Anatomy of a Volcano
• Magma Chamber: area underneath the Earth where magma comes from
• Lava: magma which reaches the surface of the Earth
• Vent: opening in the crust of the Earth where magma is released
• Crater: top of a volcano (bowl-shaped) which is connected to the magma chamber by the vent
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Where do volcanoes occur?
• Most volcanoes occur along plate boundaries—areas where plates interact with one another
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• 1. Divergent Boundary: where plates move apart
• Plates move apart and magma is forced upward through faults (?)
• Rifts: openings in plates caused by divergent boundaries
• Ex. Iceland—Mid-Atlantic Ridge
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• 2. Convergent Boundary: plates come together
• Plates move together and the plate which is more dense will sink and melt under the other one.
• Then the magma will rise up.
• 80% of volcanoes are convergent
• Ex. Pacific Ring of Fire, Mt. St. Helens
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• 3. Hot Spots: areas in the mantle which are hotter than others
• Plates move over a hot spot and this causes the plate to melt—then magma rises to the surface
• Ex: Hawaiian Islands
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• Hot spots also form…
• Seamounts: underwater islands
• Once an island is past a hot spot, its volcanoes will become dormant (meaning they will no longer erupt)
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9.2 Types of Volcanoes
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Styles of Eruptions
• Some volcanic eruptions are explosive & some are quiet.
• There are 3 factors of magma that affect the type of eruption.
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• 1.Trapped Gases
• Gases such as water vapor & CO2 are trapped in magma by the pressure of the surrounding magma & rock
• Gas escapes easily from some magma during quiet eruptions.
• Other eruptions are explosive because the gas builds up to a very high pressure
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• 2. Magma Composition:
• Magma low in silica (an explosive compound found in magma that traps gas) is fluid (flows easily) & produces quiet eruptions
• Basaltic magma
• Lava pours from vents (Hawaii)
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• Magma with a lot of silica produces very explosive eruptions (Mt. St. Helens)
• Silica-rich magma is thick & gas gets trapped causing pressure to build up
• Granitic magma
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• 3. Magma Water Content: High water content will cause explosive eruptions
• Magma at some converging zones contains a lot of water vapor
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Types of Volcanoes
• The appearance of a volcano depends on the material that forms it and the type of eruption.
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• 1. Shield Volcano: mountain with gently sloping sides and a circular base
• Forms when layer upon layer of lava accumulates after non-explosive eruptions
• Ex: Kilauea (Hawaiian Islands)
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• 2. Cinder-Cone Volcano: generally small volcanoes with steep sides
• These volcanoes throw rock fragments called tephra high into the air
• The tephra lands around the vent and begins to pile up creating a steeply-sided, loosely packed pile.
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• 3. Composite Volcano: very large, steep-sided mountains
• Begin with extremely explosive eruptions (magma with high silica content)
• Tephra is thrown into the air
• Next there is a quiet period with lava flows
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• This repeating cycle of tephra-lava, tephra-lava forms the mountain
• Ex: Mt. St. Helens
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9.3 Igneous Rock Features
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Intrusive Rock Activity• Because magma rises but only sometimes
reaches the surface, it will intrude on rock layers which already exist
• These igneous (“born of fire”) structures are called plutons
• Can range in size from 1km to 100 km• These areas can be seen in places that have
been eroded or uplifted
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• Batholith: largest intrusive igneous rock bodies
• Form when magma cools underground before reaching the surface
• One that can be seen is in Yosemite National Park
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• Dikes and Sills:
• Dikes: magma that is squeezed into a vertical crack which cuts across other rock layers
• Sills: magma squeezed into a horizontal crack between rock layers and hardens
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• Volcanic neck:
• Forms when a volcano stops erupting
• Magma hardens inside the vent
• Soft cone erodes away first leaving behind a solid igneous core—volcanic neck
• Ex: Ship Rock, New Mexico
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• Many times the tops of volcanoes will sink into magma chambers.
• The hole left behind is called a Caldera.
• The caldera usually fills with water
• Example: Crater Lake in Oregon
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Damage From Volcanoes
• Many snow-capped volcanoes will melt during eruptions.
• This melting snow causes severe landslides, mudslides, and flooding.
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• Many homes are destroyed due to fire caused by hot ashes or lava coming into contact with homes.