4. VOLCANISM

I.    Volcanism- magma and gases extruded onto Earth surface and/or into atmosphere.

   About 550 are currently active (Mount St. Helens, Hawaii, Mount Pinatubo).

   Many are dormant (Mount Vesuvius).

   Large number are extinct or inactive.

 

 

-"explosive type"- (Mt. St. Helens) pyroclastic debris (hot rock fragments [ash]) explosively blown out of volcano (fig. 4.2c).

 

-"nonexplosive type"- (Hawaii) streams of lava flowing down volcano (figs. 4.2a,b).

 

Volcanic Gasses

 

Pressure build in viscous magma -> explosion

gas in lava or magma mostly water vapor, some carbon dioxide, nitrogen, and sulfur gases.

 

How easily gas can escape from magma depends on viscosity- high viscosity makes it difficult for gases to escape; get explosive eruption.

 

 

-viscosity controlled by:

1) temperature of magma (high temperature [mafic], low viscosity).

2) composition (silica content) of magma (high silica content, high viscosity).

 

Lava Flows and Pyroclastic Matericals

 

lava flows- generally slow moving and confined to low areas (fluid)

Geometry depends on viscosity and preexisting topography. Thin= widespread or viscous=lobate (distinct margins).

 

Two types of Lava Flows

1- pahoehoe- ropy surface (Fig 4.4a).

2- aa- jagged blocky surface ("a painful surface on which to walk") (fig. 4.4b).

 

Features associated with lava flows:

-lava tube-a conduit of lava flow when all margins solidify.

-pressure ridge-pressure on solidified crust cases the surface to buckle (Fig 4.6a)

-spatter cones- small volcano-like features formed on lava flows due to escaping gasses (Fig. 4.6b).

-columnar joints- polygonal fractures formed due to contraction of cooling magma (Fig. 4.7; Perspective 4.1).

 

-pillow lavas- pillow-shaped blobs of basalt resulting from rapid cooling of lava in water (Fig. 4.8).

-pyroclastic material- fragmental material explosively ejected from a volcano; includes ash (<2mm), lapilli (2-64mm), and blocks (angular) / bombs (streamlined) (>64mm) (Fig. 4.9).

 

II.           Volcanoes-

Conical mountains formed around a vent where lava and/or pyroclastic materials are erupted; have a circular depression at the summit (crater if <1km, caldera if >1km [fig. 4.10]).

 

-3 main types of volcanic landforms:

-shield volcano-(Hawaiian type)-form the largest volcanoes in world; broad, gently sloping dome shape (fluid [low viscosity] mafic magma forms basalt) (Figs. 4.11).

-cinder cone- made up entirely of pyroclastic material; rock fragments ejected from a vent (ash, cinders, blocks, bombs) and piles up around the vent (Fig. 4.12); small ~400 m high max; form steep slopes of unconsolidated material.

-composite volcano- (Mt. St. Helens) combination of shield and cinder type -alternating layers of pyroclastic and solidified lava flows; slopes gentle on flank and steep at summit (intermediate to felsic lavas form andesite and rhyolite) (Fig. 4.13); Cascade range of NW U.S. (Perspective 4.2; guest essay p. 120).

 

-other volcanic features:

-lava dome- forms when pressure slowly forces a very viscous (felsic) magma through a volcanic vent (composite) (Fig. 4.15); can be very explosive if pressure builds up enough (nuee ardente) (Fig. 4.16).

-fissure eruption- lava ejected along narrow fissures; very fluid so no cone develops (Fig. 4-17).

-form large flat areas called basalt plateaus (Fig. 4.17).

-pyroclastic sheet deposit- thick, sheet-like deposits covered by felsic volcanic rocks.

-pyroclastic flows exploded from a caldera (fig. 4.10).

 

III. Predicting volcanic eruptions-~35-40 eruptions a year but only the most dangerous few are monitored (measure changes in physical and chemical attributes of a volcano). Methods include:

1) history of eruption; 2) tiltmeter or lasers to measure "inflation"; 3) changes in earthquake activity; 4) changes in gas emissions; 5) changes in local magnetic / electrical fields; 6) changes in temperature; 7) changes in groundwater level; etc. (Fig. 4.19).

 

 

IV.       Eruption Seize

Volcanic Explosive Index (VEI):

Express the size of volcanic eruption- semi-quantitative; based on subjective criteria. Range from 0 8.

 

V. Distribution of volcanoes-not random; at convergent (subduction zones) and divergent (mid-ocean ridge) plate boundaries (Fig. 4.20).

-Circum-Pacific belt (60%) and Mediterranean belt (20%) volcanoes (most composite) erupt intermediate to felsic lavas (andesite -rhyolite)

-oceanic ridges erupt mafic lavas (basalt) that make up the entire ocean floor.

 

VI. Plate Tectonics and Igneous Activity (volcanic and plutonic) (Fig. 4.21).

-observations: almost all igneous activity concentrated in two areas:

1) divergent plate boundaries (mid-ocean [spreading] ridges)-ocean floor made ~entirely of basalt noneplosive.

2) convergent plate boundaries (subduction zone) -volcanic rocks mostly andesite; plutonic rocks mostly diorite and granite possibly explosive.

 

-Igneous activity at divergent plate boundaries (spreading ridges/oceanic ridges) - partially melt peridotite to get basalt; high temp. at low pressure and water allows melting to occur.

-Igneous activity at convergent plate boundaries (subduction zones)- partial melting of mafic rock (basalt) to form more felsic magma (andesite/diorite and granite); assimilation and melting silica-rich sediments also a factor.

 

-Igneous activity not at plate boundaries (intraplate volcanism)- rare; mantle plume (stationary column of hot magma that originates deep in Earth's mantle) creates hot spots (localized zone of melting) at the Earth's surface (Hawaiian islands). Figure 4.22.