chapter 18 volcanism
TRANSCRIPT
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Chapter
18 Volcanism
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18.1: Volcanoes (p. 500)
• Volcanism: describes all the processesassociated with the discharge of magma,hot fluids, and gases
• 20 volcanoes are erupting right now
• Over a year, eruptions occur at 60 places
• Most form at plate boundaries
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Origin of Magma Geologists conclude that magma originates
when essentially solid rock, located in the
crust and upper mantle, partially melts.
18.1 Intrusive Igneous Activity
The most obvious way to generate magmafrom solid rock is to raise the temperatureabove the level at which the rock begins tomelt.
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Origin of Magma Role of Heat
18.1 Intrusive Igneous Activity
• Additional heat is generated by
• The geothermal gradient—Earth’s natural
temperature increases with depth but is notsufficient to melt rock in the lower crustand upper mantle
- friction in subduction zones- crustal rocks heated during subduction
- rising, hot mantle rocks
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Origin of Magma Role of Water
18.1 Intrusive Igneous Activity
• Causes rock to melt at a lower temperature
• Plays an important role in subducting
ocean plates
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Convergent Plate Boundaries Subduction zones: oceanic crust is pushed
down into the mantle
As the crust descends, magma forms Magma moves upward because it is less
dense Usually oceanic continental boundaries Explosive erruptions
18.1 Volcanoes
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Convergent Boundary Volcano
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Convergent Boundary
Volcanoes: 2 Major Belts• See pages 500 and 501
• Circum-Pacific Belt (ring of fire)
• Mediterranean Belt (Mt. Etna, Mt.Vesuvius-Eurasian, African, and Arabianplates)
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Divergent Plate Boundaries
Magma rises through the ridge createdwhen the lithosphere pulls apart.
18.1 Volcanoes
-not explosive
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Hot Spot Volcanoes
occur within a tectonic plate away fromplate boundaries.
Unusually hot regions of Earth’s mantlewhere plumes of magma rise to the surface
Many under the ocean: Hawaiian islands punch holes through crust as the plate
moves, creating chains of volcanoes (seep. 503)
18.1 Volcanoes
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Kilauea, a Hot Spot Volcano
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Types of Volcanoes
18.1 Volcanoes
Anatomy of a Volcano (p. 505) Volcano: mountain formed of solidified
layers of lava Describe each of the following parts of
a volcano and label them on a drawing. Conduit
Vent Crater CalderaAlso, label lava and pyroclastic material
(ejected lava that has hardened)
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Anatomy of a “Typical” Volcano
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18.1 Volcanoes
• 3 Types of Volcanoes (p. 506-507)
– Shield
– Cinder cone
– Composite
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Types of Volcanoes
18.1 Volcanoes
Shield Volcanoes
-broad, gently sloping volcanoes built from fluid
basaltic lavas-non-explosive eruptions create gentle slope
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Shield Volcanoes
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18.1 Volcanoes
• Cinder Cone Volcanoes
– Eruptions eject small pieces of magma intothe air called tephra
– The tephra piles up around the vent
– Steep sides and small
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Cinder Cones
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Types of Volcanoes
10.1 The Nature of Volcanic Eruptions
Composite Volcanoes • Composite cones are volcanoes composed of
both lava flows and pyroclastic material.- Most are adjacent to the Pacific Ocean: Mt.
Rainier, Mt. St. Helens
- Large size
- Interbedded lavas and pyroclastics
- Most violent type of activity: dangerous
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Composite Cones
M S H l B f d
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Mount St. Helens Before andAfter the May 18, 1980, Eruption
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Profiles of Volcanic Landforms
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18.2 Eruptions (p. 508)
• Making Magma
– Read intro on p. 508
– Need high temperature to melt rock
– Magma composition determines explosivity
• As amount of gas increases, explosivity increases
• As viscosity (resistance to flow) increases,
explosivity decreases – High silica=viscous which leads to explosive eruptions
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Types of Magma (p. 510)
• Basaltic: quiet eruptions (least silica)
• Andesitic: middle level (intermediateeruptions-middle amount of silica)
• Rhyolitic: very explosive (most silica)
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Basaltic Magma at the Surface
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Factors Affecting Eruptions
18.2 Volcanoes
Factors that determine the violence of aneruption
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
Viscosity• Viscosity is the measure of a material's
resistance to flow.
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Factors Affecting Eruptions
18.2 The Nature of Volcanic Eruptions
Viscosity
• Factors affecting viscosity
- Temperature (hotter magmas are less viscous)
- Composition (silica content)
1. High silica—high viscosity
(e.g., rhyolitic lava-explosive)2. Low silica—more fluid, less viscous (e.g.,basaltic lava)
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Factors Affecting Eruptions
18.2 The Nature of Volcanic Eruptions
Dissolved gases• Mainly water vapor and carbon dioxide
• Gases expand near the surface
• A vent is an opening in the surface of Earththrough which molten rock and gases arereleased.
• Provide the force to extrude lava
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Factors Affecting Eruptions
18.2 The Nature of Volcanic Eruptions
Dissolved gases• Violence of an eruption is related to how easily
gases escape from magma- Gases escape easily from fluid magma.
- Viscous magma produces a more violenteruption.
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Magma Composition
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Volcanic Material
18.2 The Nature of Volcanic Eruptions
Lava Flows• Basaltic lavas are more fluid.
- Pahoehoe lava (resembles braids in ropes)
- Aa lava (rough, jagged blocks)
• Types of lava
Gases• One to 5 percent of magma by weight
• Mainly water vapor and carbon dioxide
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Pahoehoe (Ropy) Lava Flow
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Slow-Moving Lava Flow
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Volcanic Material
18.2 The Nature of Volcanic Eruptions
Pyroclastic Materials • Pyroclastic materials is the name given to
particles produced in volcanic eruptions.• The fragments ejected during eruptions range in
size from very fine duct and volcanic ash (lessthan 2 millimeters) to pieces that weigh several
tons.
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Volcanic Material
18.2 The Nature of Volcanic Eruptions
Pyroclastic Materials • Types of pyroclastic material
- Ash and dust—fine, glassy fragments- Pumice—frothy, air-filled lava
- Lapilli—walnut-sized particles
- Cinders—pea-sized particles
• Particles larger than lapilli - Blocks—hardened lava
- Bombs—ejected as hot lava
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Other Volcanic Landforms
18.2 The Nature of Volcanic Eruptions
Calderas
• Calderas are large depressions in volcanoes.
• Formed by collapse
• Nearly circular
• Size exceeds one kilometer in diameter
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Plutons (p. 514) Most volcanism is beneath Earth’s
surface
Plutons are intrusive igneous structuresthat result from the cooling and hardeningof magma beneath the surface of Earth.
18.3 Intrusive Igneous Activity
• Intrusive igneous bodies, or plutons, are
classified according to their shape, size, andrelationship to the surrounding rock layers.
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Plutons Sills and Laccoliths (see p. 515)
18.3 Intrusive Igneous Activity
• Sills and laccoliths are plutons that form when
magma is intruded close to the surface.- Sills resemble buried lava flows and may
exhibit columnar joints. Parallel to layers ofrock.
- Laccoliths are lens-shaped masses that archoverlying strata upward. Rounded top and flatbottom.
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Plutons Dikes (see p. 516)
18.3 Intrusive Igneous Activity
• Many dikes form when magma from a largemagma chamber enters fractures and cracks inthe surrounding rocks.
• Dikes are intrusive igneous features that cut
across preexisting rock layers.
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Plutons Batholiths
18.3 Intrusive Igneous Activity
• Batholiths largest, masses of igneous rock that
formed when magma deep in Earth becamecrystallized and eventually was exposed byerosion.
• An intrusive igneous body must have a surface
exposure greater than 100 square kilometers tobe considered a batholith.