Volcanoes
What Comes Out of a Volcano?
• Fragments
– Finest sizes called ash or dust (<2 mm)
• Usually a fine sandy or silty powder
• Forms tuff
• Welded tuff if it was hot enough to weld together
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Pea-sized particles called cinders (4-32 mm)
Walnut-sized particles called lapilli
Blocks—large, solid chunks of once-molten material
Bombs—Incandescent blobs of lava ejected from a volcano
What Comes Out of a Volcano?
• Gas
– Dissolved gas held in magma under pressure—Releasing pressure
releases gas, as in the opening of a soda bottle
– Makes up 1 to 5 % of volcanic material
– Gas content of Hawaiian Volcanoes
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Water vapor = 70%
Carbon Dioxide = 15%
Nitrogen = 5%
Sulfur compounds (Sulfur dioxide and H2S) = 5%
Chlorine, hydrogen, and argon = trace
– Makes rocks that are full of holes
What Comes Out of a Volcano?
• Lava
PROPERTY
Silica Content
Viscosity
Tendency to form
lavas
Tendency to form
pyroclastic fragments
Melting Temperature
Rock
MAFIC
Least ~ 45%
Low—Thin, runny
magma
Most likely
INTERMEDIATE
~60%
Intermediate
Intermediate
FELSIC
Most ~ 75%
High—Thick and
pasty
Least likely
Least likely
Intermediate
Most likely
Highest
Basalt—silica poor,
Al2O3, CaO, FeO,
MgO
Intermediate
Andesite
Lowest
Rhyolite—silica
rich,Al2O3, K2O,
Na2O
Lava Flows
• Thin fluid material
• May flow as fast as 30 km/hr
• Thinnest form crust which wrinkles—pahoehoe or ropy
lava
• Thicker, more pasty flows, fragments have dangerous,
sharp edges—aa
– Thick flows
– Flow rates of 5 - 50 meters per hour
Parts of a Volcano
• Vent—single spot from which successive eruptions occur
• Crater—steep-walled depression at the summit of many volcanoes
– Connected by a pipe-like conduit to the magma chamber below
– When the magma chamber empties, the top of the volcano collapses
producing a caldera
– A bulging volcano can fracture the surrounding land producing fractures
which fill with magma to form radiating dikes
Parts of a Volcano
• When the softer debris around a solidified volcanic core
erodes, a volcanic neck, stock, or plug is left
• Fissure eruptions occur along long fractures
• Flank eruptions and the production of parasitic cones
happens on the flanks of many volcanoes
Types of Volcanoes
• Shield Volcano
– Successive lava flows
– Gentle slopes
• A few degrees on flanks
• Maximum of 15° near the summit
• Mean is 2-10°
– Can be spectacularly large
• Mauna Loa is the largest volcano on Earth
– Base is 5000 meters (16,000 feet) below sea level
– Summit is 4170 meters (13,680 feet) above sea level
– It is about 1 million years old
Types of Volcanoes
• Shield volcanoes (continued)
– Quiet eruptions
– Mostly fluid (basaltic) magma
Types of Volcanoes
• Cinder Cone
– Built of a pile of ejected lava fragments
– Unconsolidated cinders
– Steep slopes with high angle of repose
– Very easily eroded
– Short life
– Small—usually less than 300 meters (1000 ft) high
Cinder Cone
Types of Volcanoes
• Composite Cone or Stratovolcano
– Mixture of cinders and lava flows
– Viscous lavas—usually andesitic
– Picturesque
– Most violent type of eruption
• Vesuvious erupted in 79 C.E.
– Buried Pompeii
– Killed more than 2,000 of its 20,000 inhabitants
Types of Volcanoes
• Mt. Pelee on the Caribbean island of Martinque
– Destroyed St. Pierre
» Killed nearly everyone
» Prisoner in a dungeon, shoemaker, and people on ships survived
– Produced nuee ardente
» Glowing avalanche
» Hot gaseous cloud composed of rather large lava framgnets
» Fragments move down slope on frictionless cloud of expanding gas
Types of Volcanoes
• Fissure eruptions
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Occur on long, linear fractures known as fissures
Material squirts out along most of the length of the fracture
Produces the greatest volumes of material
Columbia Plateau
• Northwestern U.S.
• Vast lava flows as much as 50 meters thick covered areas as far away as 150 km (90
miles) from their source
• Called flood basalts
• Pyroclastic debris can also come from fissures
Fissure Eruption
Types of Volcanoes
• Spatter cone
– Lava blobs come out of a vent
– Spatter in a pile around the vent