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Essentials of Geology, 9e
Volcanic Activity
Chapter 4
InstructorJennifer BarsonSpokane Falls Community College
Geology 101
The Nature of Volcanic Eruptions
Factors determining the violence orexplosiveness of a volcanic eruption
Composition of the magma Temperature of the magma
Dissolved gases in the magma
The above three factors actually controlthe viscosity of a given magma which inturn controls the nature of an eruption
The Nature of Volcanic Eruptions
Viscosity is a measure of a materials resistance toflow (e.g., Higher viscosity materials flow with greatdifficulty)
3 Factors affecting viscosity
Temperature - Hotter magmas are less viscous
Composition - Silica (SiO2) content Higher silica content = higher viscosity
(e.g., felsic lava such as rhyolite)
Lower silica content = lower viscosity or more fluid-like
behavior (e.g., mafic lava such as basalt)
The Nature of Volcanic Eruptions
Factors affecting viscosity, continued Dissolved Gases
Gas content affects magma mobility
Gases expand within a magma as it nears theEarths surface due to decreasing pressure
The violence of an eruption is related to howeasily gases escape from magma
The Nature of Volcanic Eruptions
Factors affecting viscosity, continued
In Summary
Fluid basaltic lavas generally produce
quiet eruptions
Highly viscous lavas (rhyolite or andesite)produce more explosive eruptions
Materials extruded froma volcano
Lava Flows Basaltic lavas are much more fluid
Types of basaltic flows Pahoehoe lava (resembles a twisted or ropey
texture)
Aa lava (rough, jagged blocky texture)
Dissolved Gases One to six percent of a magma by weight
Mainly water vapor and carbon dioxide
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aa flow
Figure 4.7 A & B
pahoehoeMaterials extruded from
a volcanoPyroclastic materials Fire fragments
Types of pyroclastic debris
Ash and dust - fine, glassy fragments Pumice - porous rock from frothy lava
Lapilli - walnut-sized material
Cinders - pea-sized material
Particles larger than lapilli
Blocks - hardened or cooled lava
Bombs - ejected as hot lava
Figure 4.9
Volcanoes
General Features
Opening at the summit of a volcano
Crater - steep-walled depression at thesummit, generally less than 1 km in diameter
Caldera - a summit depression typicallygreater than 1 km in diameter, produced bycollapse following a massive eruption
Vent opening connected to the magma
chamber via a pipe
Volcanoes
3 Types of Volcanoes
Shield volcano
Broad, slightly domed-shaped
Composed primarily of basaltic lava
Generally cover large areas
Produced by mild eruptions of large volumesof lava
Mauna Loa on Hawaii is a good example
Mauna Loa a shield volcano
Figure 4.12
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Figure 4.1a
A nueardente
on Mt. St.Helens
Figure 4.20
Figure 4.1b Figure 4.2
Figure 4.22
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Formation
of Crater
Lake,
Oregon
Caldera
Figure 4.22
A Size Comparison
Copyright 2006 Pearson Prentice Hall, Inc.
Other Volcanic Landforms
Fissure eruptions and lava plateaus
Fluid basaltic lava extruded from crustalfractures called fissures
e.g., Columbia River Plateau
Lava Domes
Bulbous mass of congealed lava
Most are associated with explosive
eruptions of gas-rich magma
The Columbia River basalts
Figure 4.23
The ColumbiaRiver basalts
Figure 4.24
A lava dome on Mt. St. Helens
Copyright 2006 Pearson Prentice Hall, Inc.
Figure 4.25
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Other Volcanic Landforms
Volcanic pipes and necks
Pipes are short conduits that connect amagma chamber to the surface
Volcanic necks (e.g., Ship Rock, NewMexico) are resistant vents left standingafter erosion has removed the volcaniccone
Formation of a volcanic neck
Figure 4.27
Plutonic Igneous Activity
Most magma is emplaced at depth withinthe Earth
An underground igneous body, oncecooled and solidified, is called a pluton
Classification of plutons Shape
Tabular (sheetlike)
Massive
Orientation with respect to the host(surrounding) rock Discordant cuts across sedimentary rock
units
Concordant parallel to sedimentary rockunits
Plutonic Igneous Activity
Plutonic Igneous Activity
Types of intrusive igneous features
Dike a tabular, discordant pluton
Sill a tabular, concordant pluton (e.g.,
Palisades Sill in New York)
Laccolith
Similar to a sill
Lens or mushroom-shaped mass
Arches overlying strata upward
Batholith Largest intrusive body
Intrusive Igneous Structures
Figure 4.28 A
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Intrusive igneous structuresexposed by erosion
Figure 4.23 B
Sill - Salt River Canyon, Arizona
Figure4.30
Plutonic Igneous Activity
Intrusive igneous features continued
Batholith
Largest intrusive body
Surface exposure of 100+ square kilometers(smaller bodies are termed stocks)
Frequently form the cores of mountains
Figure 4.28
A batholithexposed by
erosion
Plate Tectonics and Igneous Activity
Global distribution of igneous activity isnot random
Most volcanoes are located within or nearocean basins
Basaltic rocks are common in bothoceanic and continental settings, whereasgranitic rocks are rarely found in theoceans
Active volcanoes are often associatedwith plate boundaries
Plate Tectonics and Igneous Activity
Subduction zone (convergent)
Rising magma can form either An island arc if in the ocean
A volcanic arc if on a continental margin
Spreading centers (divergent) The greatest volume of volcanic rock is produced
along the oceanic ridge system
Intraplate volcanism Activity within a tectonic plate, associated with
plumes of heat in the mantle
Form localized volcanic regions in the overridingplate called a hot spot
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Distribution of some of theworlds major volcanoes
Figure 4.33Figure4.18
Volcanism on a tectonicplate moving over a hot spot
Figure 4.35
Figure4.34
Figure4.34
End of Chapter 4