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).
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- generally slow moving (fluid) and confined to low areas
Geometry depends on viscosity and preexisting topography. Thin= widespread or viscous=lobate (distinct margins).
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 causes 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).
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. (Read 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.
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).
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.