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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)



One to 5 percent of magma by weight

Mainly water vapor and carbon-dioxide



" 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



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



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


Tabular (sheetlike)


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)


Forms in same way as sill

Lens shaped mass

Arches overlying strata upward


Largest intrusive body

Surface exposure 100 + square kilometers (smaller bodies are termed stocks)

Frequently form the cores of mountains


Igneous activity

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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