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Volcanism and Extrusive Rocks
Physical Geology 12/e, Chapter 4
Eruptive Violence and Physical
Characteristics of Lava
•
Violence of eruptions controlled by:
1.
Dissolved gases in the magma
•
2.
•
Water vapor, carbon dioxide, sulfur dioxide, etc.
Ease/difficulty of gases escaping to atmosphere
Viscosity- a fluid’s resistance to flow
1.
Silica content
•
Higher silica contents produce higher viscosities
2. Lava temperature
•
3.
Cooler lavas have higher viscosities
Amount of dissolved gases
•
The more dissolved gases, the more fluid the lava
Volcanism and Earth’s Systems
• Atmosphere originally created from gases released
from volcanic eruption
• Hydrosphere produced by condensation of volcanic
water vapor
• Biosphere both positively and negatively influenced
by volcanism
– Lava flows and ash weather to produce fertile soils
– Violent eruptions can destroy nearly all life in their paths
– Large amounts of ash and volcanic gases in atmosphere can trigger
rapid climate changes and contribute to mass extinctions
Volcanic Eruptions
• Lava is produced when magma
reaches Earth’s surface
• Explosive eruptions can produce
rapidly cooled rock fragments called
pyroclasts
– Size range from dust (ash) to boulders
(blocks and volcanic bombs)
• Calm oozing of magma out of the
ground produces lava flows
• Pyroclastics and lava flows form
extrusive igneous rocks
• Lava flows and pyroclasts pile up to
form volcanoes
Living with Volcanoes
• Mythology, religion and volcanoes
– Hawaii - Pele
– Iceland - Loki
• Growth of volcanic islands (Hawaii)
• Geothermal energy
– Natural steam harnessed as clean
energy resource
• Climatic effects
– Very large eruptions can result in
measurable global cooling
– Resulting crop failures and famines
• Volcanic catastrophes
– Mt. St. Helens, Pompeii, Krakatoa,
Tambora, Crater Lake
Eruptive Violence and Physical
Characteristics of Lava cont.
page 90-91
Igneous
Family
Silicic
(Felsic)
Silica
Content
Viscosity
Violence of
Eruption
Eruptive Violence and Physical
Characteristics of Lava cont.
page 90-91
Igneous
Family
Silica
Content
Viscosity
& Flow
Violence of
Eruption
Silicic
(Felsic)
65%
(Silica
Rich)
35-50%
High &
Sluggish
Violent
Medium
Medium
Less than
50%
Deficient
Low &
Easily
Minimal
More flows
Intermediate
Mafic
Extrusive Rocks and Gases
• Scientific Investigation of Volcanism
– Rocks, gases and events from observed
eruptions compared to similar lavas elsewhere
to infer the nature of past activity
• Rock Composition
– Rhyolite - high silica; light color
– Basalt - low silica; dark color
– Andesite - intermediate silica and color
Extrusive Textures
• Texture - appearance of a rock with respect
to size, shape and arrangement of its grains
• Glassy - glass without mineral crystals
– Obsidian
• Fine-grained - most crystals < 1 mm
– e.g., Basalt
• Porphyritic - larger crystals in a matrix of
much finer crystals (groundmass) or glass
– e.g., Andesite
Extrusive Textures
• Vesicular/Frothy - trapped gas bubbles
– Vesicular basalt
– Pumice
• Fragmental - particles blasted apart by
explosive eruptions
– Dust and ash (<2 mm)
– Cinders (2-64 mm)
– Blocks and bombs (>64 mm)
Volcanic Landforms
• Vent - opening through which lava erupts
• Crater - basin-like depression over the
vent at the summit of the volcano
• Caldera - volcanic depression much
larger than the original crater, having a
diameter of at least 1 km
Volcano Target Notes
Definition
Lifespan &
Eruptive
Material
Type of Volcano
Diagram
Meaningful
Sentence
Types of Volcanoes pages 96-103
Type
Shield
Composite
(Stratovolcano)
Cinder
Dome
Diagram
Define &
any charac.
Lifespan Eruptive
Material
Rock
Type
What am I???
Types of Volcanoes
• Shield volcanoes
– Broad
– Gently sloping
– Composed of solidified lava
flows
– Flows often contain lava tubes
• Cinder cones
– Small
– Steeply sloping
– Composed of a pile of loose
cinders
Types of Volcanoes
• Composite volcanoes
– AKA stratovolcanoes
– Moderately to steeply sloping
– Constructed of alternating layers of
pyroclastic debris and solidified lava flows
– Composed primarily of intermediate
composition volcanic rocks (i.e., andesite)
– Most common type of volcano at
convergent plate boundaries (e.g., Pacific
Ring of Fire)
• Volcanic domes
– Extremely high viscosity, degassed, felsic
lavas (often glassy, e.g., obsidian)
Other Eruption Types
• Flood eruptions
–
–
–
–
Very fluid (basalts)
Extremely large in volume
Create extensive lava plateaus
Eruption times correspond with
largest mass extinction events
Columbia river flood basalts
• Submarine eruptions
– Nearly always basaltic
– Mid-ocean ridge eruptions
– Pillow basalts
Pillow basalts
Volcanism on Other Planets
• Shield volcanoes
– Venus, Mars, Io
Insert Fig. 22.18 here
• Lava domes
– Venus, Moon
Olympus Mons on Mars
• Flood eruptions
– Very fluid (basalts)
– Extremely large in
volume
– Extensive flat lava
plains (Moon, Mars,
Venus, Mercury?)
Insert Fig. 22.27A here
Volcanic eruption on Io
Insert Fig. 22.14A here
Lava domes on Venus
Lava channel in volcanic
plains on Moon
End of Chapter 4