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Geology 12
Unite # 22
Volcanism and Extrusive Rocks
Chapter #4 (pages 75 - 101)
April, 2006
magma = molten rock made up mostly of silica
lava = when magma reaches earth’s surface
pyroclasts (tephra) = rock fragments when a volcano explodes
lava flows = extruded lava that comes out of fissures
extrusive rock = pyroclastic debris and rock formed by solidification of lava
volcano = a hill or mountain formed by the extrusion of lava from a vent
lava plateau = successive built up layers of accumulated lava
volcanic activity either builds up or destroys landforms
Hawaii = magma plume
- weathered volcanic ash and lava produce excellent soil
- islands are the crests of a series of volcanoes built up from the ocean floor
Geothermal energy = superheated water trapped between layers of rock used for energy
effect on climate = 1980 Mt. St. Helens, 1991 Mount Pinatubo (Philippines) spew ash
into air, blocks solar radiation - lowers global temperature
- 1816 eruption of Tambora in Indonesia spewed 40 sq kms of material
into atmosphere = the year without summer
Mt. St. Helens - side expanded 1.5 meters per day due to magma intruding into vent, side
of mountain displaced over 100 meters = landslide = weakened side = pressure build up =
earthquake loosened side material = landslide = eruption of material beneath. Blast
stipped all vegetation for 10 kms north of blast = gas and lava released for next 30 hours
volcanic mud flows due to snow melt
debris changed stream flows
rained ash for days
Pompeii - 79 AD - Mt. Vesuvius - buried Pompeii under 5-8 meters of ash - continues to
erupt on a regular basis
Krakatoa - Indonesia in western Pacific 1883 - erupted with force of several hydrogen
bombs - Krakatoa island rose 800 feet above sea level - 13 cubic kms of material
exploded out - left 300 meter deep hole in ocean floor where island used to be - blast
heard 5000 kms away - 34,000 people killed mostly by tsunami. One reason why
explosion so violent was the introduction of water into the volcanic system.
Crater Lake in Oregon - estimated that mt was 2000 meters higher before explosion that
occurred 6600 years ago - eruptions occurring over 30-40 million years
(refer to page 80, figure 4.4 how Crater Lake formed)
- series of volcanos in western USA Cascade Mountain Range (page 81, figure 4.5)
fatalities due to volcanic eruptions = (page 82, figure 4.6) increased number of
fatalities = more population , better record keeping with time
pyroclastic flows = hot gases and pyroclstic material - suffocates as it travels down flank
of volcano - famine due to destruction of crops - most deaths occur as roofs collapse and
people being hit by debris
severity of volcanic eruptions depends on: 1) amount of gas in lava or magma
2) ease or difficulty by which gas escapes =
viscosity (resistance to flow dictates how easily gas can escape)
viscosity: depends upon 1) amount of silica (SiO2) content, 2) temperature of lava at
which it solidifies. If lava hotter then its solidification temp - lava less viscous
(700 C for silicic rocks, 1200 C for mafic rocks)
felsic rock = silica content of rock greater then 50% (approximately)
- high silica content and flow slowly
(Rhyolite most abundant silicic volcanic rock)
mafic rock (silica deficient) = silica content of rock less then 50% (approximately)
- low silica content and flow easily
silica induces crystallization even in lava flows (remember silicon tetrahedron bonds
easily with other atoms) silica acts the same way as flour or cornstarch to thicken gravy
silica magmas are most viscous = more violent eruptions when release of gas occurs
Cascade Mts = intermediate type of viscous magma
intermediate rocks = chemical content between felsic and mafic
the more viscous (more difficult for gas to escape) - the greater the volume of gas
trying to escape - more violent the eruption
Extrusive rocks and Gases (p. 82)
volcanic gases = mostly water that condenses into steam, carbon dioxide, sulpher dioxide,
hydrogen sulfide (rotten eggs), hydrochloric acid
- during explosion:
- expanding gases propel material into atmosphere
- reaches troposphere,
- spreads outward
- debris carried by high altitude winds
- blocks solar radiation and ash falls throughout world, red sunsets, etc
pyroclastic flows - (p. 83, figure 4.7)
- hot and suffocating
- travel up to 100 kms per hour
- very dense and often travel down mt slopes
Extrusive rocks:
- amount of silica in magma dictates viscosity of magma and type of rock formed
extrusive silica rocks =
fine grained
light colored (contain lots of feldspar and quartz)
Mafic rocks (less silica) are darker due to other minerals such as iron and magnesium)
Examples of extrusive rocks:
Rhyolite =
Basalt =
Andesite =
usually cream, pink-tan in color due to amount of feldspar
low silica, silica bonded to iron or magnesium minerals ,therefore darker
in color, e.g. olivine (dark green in color), no quartze since not enough
silicic material left over to form quartz
intermediate rock, usually gray -green in color
texture of extrusive rocks: (texture = refers to rocks appearance with respect to the size,
shape and arrangements of its grains or other constituents)
- grain size most important texture characteristic
1) fine grain = crystals less then 1 mm in size
factors that dictate crystal size = rate of cooling and viscosity
2) porphyritic rock = larger crystals enclosed in a finer crystal
material (matrix)
- indicates two stages of cooling; slow while magma
underground, then quick when magma expelled
phenocrysts = larger crystals
3) texture due to trapped gases - gases in magma, due to pressures, are
in a dissolved state similar to gases in a pop bottle
- when pressure released gases are released, same as opening a pop
- when magma solidified quickly - gas bubbles trapped in material
leaving holes = vesicles = cavities in extrusive rocks due to trapped
gases when material cooled
pumice = very viscous lava, lava churned into a ‘froth’
(like froth on top of soft drink when poured), cooled quickly, lots of air
space in rock material, floats on water
volcanic ‘bombs’ and ‘blocks’ = when magma ejected, angular (no round
edges) material expelled = ‘blocks’
= when molten blob becomes streamlined
‘bomb’ (page 86, fig. 4.13)
tuff = rock composed of fine grained pyroclastic material
volcanic breccia = rock that includes larger pieces of volcanic rock
(blocks and/or bombs)
TYPES OF VOLCANOES - terms to know
vent = opening through which magma flows and eruption takes place
crater = basin like depression over a vent
flank eruption = vents on the side of volcano
caldera = volcanic depression much larger then crater (diameter at least 1 km)
- created when top blown off
- created when top collapses
Three type of volcanoes: 1) Shield, 2) composite, 3) Cone
(review info on page 87, fig. 4.2)
1) Shield volcano: broad, gently slopping
- constructed from lava flows
- central vent not built up
Islands of Hawaii are a series of shield volcanoes
- eruptions not spectacular lava less viscous and is fluid
types of lava associated with Shield volcanoes:
- pahoehoe = ripply, billowy lava, quick cooling surface
(page 88, fig. 4.17)
- aa lava = partially solidified mass and front shoved forward
- spatter cone = small vent developed in a flow that releases gas
2) Cinder cone volcano (Pyroclastic cone): built up by the accumulation
of pyroclastic material (not lava build up), most material when ejected
lands near vent, building up the cone with steeper side then shield volcano
and have slope of approx. 30 degrees
- commonly found on sides of shield volcanoes
- associated with mafic or intermediate lava
- life span short, eroded easily, don’t last long in geologic time
3) Composite Volcanoes (Stratavolcano) = build by combination of
alternating lava and pyroclastic deposits, slopes = intermediate slope,
- flanks built up more then the summit
- lava flows cover pyroclastic material = less erosion of flanks compared
to Cinder volcano
- eruptions intermittent,
- built up over long periods of time
- surface erosion and weathering changes slopes of Composite volcanoes
- long development times result in very large Composite volcanoes
(page 91, fig. 4.21)
- Aconcagua in Andes = composite volcano = highest peak in western
hemisphere = 6,960 meters (22,835 feet)
- material extruded mostly intermediate type - andesite rock most
associated with composite volcanoes
Distribution of Composite volcanoes:
- most larger volcanoes in world are Composite volcanoes
1) circum-Pacific belt (Ring of Fire) (page 92, fig. 4.22)
2) Mediterranean Belt
Volcanic Domes: = steep sided, spine shaped masses of rock formed from viscous lava
solidified in or immediately above a volcanic vent.
- because lava so viscous, it solidifies and acts as a cork/plug to the volcano
- violent eruptions occur with domes
- to measure possible eruptions use = tilt meters, gas monitoring instruments,
plateau basalts = very non-viscous lava, spreads evenly over a large area of land
- build up over time, up to 3000 meters in some places
columnar structure = cooling of lava after solidifying, lava contracts , tension builds
and fractures form = hexagonal patters (Devils Peak, North Mountain basalt)
lava tubes = tunnel like features within lava flows during stages of cooling and
(page 97, figure 4-29)
Submarine Eruptions:
- almost always mafic (flow easily)
- pillow basalts (blobs squeezed out like toothpaste from a tube)
Assignment questions page 100
Page 100:
(group #1) #1, #2, #5
(group #2) #6, #7
(group #3) #12, #13, #15, #16, #17, #18, #19, #20, #21, #22, #23
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