Chapter 8 Igneous Activity
Igneous Activity
Volcanic eruptions
Factors that determine the violence of an eruption
Composition of the magma
Temperature of the magma
Dissolved gases in the magma
Viscosity of magma
Viscosity is a measure of a material's resistance to flow
Factors affecting viscosity
Temperature (hotter magmas are less viscous)
Composition (silica content)
High silica - high viscosity (e.g. granitic lava)
Low silica - more fluid (e.g. basaltic lava)
Viscosity of Magma
Dissolved gases
Mainly water vapor and carbon dioxide
Gases expand near the surface
Provide the force to extrude lava
Violence of an eruption is related to how easily gases escape from magma
Easy escape from fluid magma
Viscous magma produces a more violent eruption
Materials associated with volcanic eruptions
Lava flows
Basaltic lavas are more fluid
Types of lava
Pahoehoe lava ( resembles braids of rope)
Aa lava (rough, jagged blocks)
Gases
One to 5 percent of magma by weight
Mainly water vapor and carbon-dioxide
Pyroclastics
" Fire fragments "
Types of pyroclastics material
Ash - fine, glassy fragments
Pumice - from " frothy " lava
Lapilli - " walnut size
Cinders - " pea-sized" with voids
Particles larger than lapilli
Blocks - hardened lava
Bombs - ejected as hot lava
Volcanoes
General features
Opening at summit
Crater (steep-walled depression at summit)
Caldera (a summit depression greater than 1 km diameter)
Vent (the conduit that connects the crater to the magma chamber)
Types of Volcanoes
Shield
Cinder
Composite cone or Stratovolcano
Shield Volcano
Broad, slightly domed
Primarily made of basaltic (fluid) lava
Generally large
Generally produce a large volume of lava
e.g., Mauna Loa in Hawaii
Cinder cone
Built from ejected lava fragments
Steep slope angle
Rather small size
Frequently occur in groups
Composite cone or Stratovolcano
Most are adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. Shasta)
Larger size
Interbedded lavas and pyroclastics
Most violent type of activity (e.g., Vesuvius)
Often produce nuee ardente
Fiery cloud
Hot gases infused with ash
Flows down sides of a volcano
Speeds up to 200 km per hour
May produce a lahar, a type of mudflow
Volcanic Landforms
Crater or caldera
Volcanic neck
Fissure eruption and lava plateau
Pyroclastic flow
Crater or caldera
Steep walled depression at summit
Calder - a crater that exceeds one kilometer in diameter
Volcanic neck
Resistant vent left standing after erosion
e.g., Ship Rock, New Mexico
Fissure eruption and lava plateau
Volcanic material extruded from fractures
e.g., Columbia Plateau
Pyroclastic flow
From silica-rich magma
Consists of ash and pumice fragments
Material is propelled from the vent at
high speed
e.g., Yellowstone plateau
Magma emplaced at depth
Underground igneous body is called a pluton
Plutons are classified according to
Shape
Tabular (sheetlike)
Massive
Orientation with respect to the host (surrounding) rock
Discordant-cuts across sedimentary beds
Concordant-parallel to sedimentary beds
Types of igneous intrusive features
Dike, a tabular, discordant pluton
Sill, a tabular, concordant pluton
(e.g., Palisades Sill, NY)Laccolith
Forms in same way as sill
Lens shaped mass
Arches overlying strata upward
Batholith
Largest intrusive body
Surface exposure 100 + square kilometers (smaller bodies are termed stocks)
Frequently form the cores of mountains
Igneous activity
and
Plate tectonics
Origin of magma
Temperature and magma generation
Magma must originate from solid rock
Temperature melts solid rock
Temperature increases with depth (one possible heat source is radioactive decay)
Role of pressure
Increase in pressure causes an increase in melting temperature
Drop in confining pressure
Lowers the melting temperature
Occurs when rock ascends
Partial melting
Igneous rocks are mixtures of minerals
Melting over a range of temperatures
Forms a melt with a higher silica content
Distribution of igneous activity
Igneous activity along plate margins
Oceanic ridge spreading center
Lithosphere pulls apart
Less pressure on underlying rocks
Partial melting occurs
Large quantities of basaltic magma are produced
Convergent plate margin
Subduction zone (trench)
Descending plate partially melts
Magma slowly rises upward
Rising magma can form
Island arc in an ocean
Andesitic-granitic volcanoes on a continent
Associated with the Pacific Basin
Called "Ring of Fire"
Explosive - high gas volcanoes
Intraplate volcanism
Activity within a rigid plate
Basaltic magma source
Partial melting of mantle rock
Plumes of hot mantle material
Form hot spots on the surface
A plume may be located below Hawaii
Granitic magma source when the continental crust is remelted over a mantle plume
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