Volcanism%20and%20volcanic%20rocks

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Volcanism and volcanic rocks

• rocks and sediments produced by volcanic
  processes

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Volcanism plate tectonics

• relationship of volcanism to movements of the
  earths plates (fig 7.1)

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convergent boundaries

• seduction zones often violent eruptions, due to
  high silica content
• pyroclastic sediments thrown from the volcano

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divergent margins rifting
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divergent margins rifting

• under oceans
• basaltic lava pillow lava
• non violent eruptions,
• composition of gabbro,
• low silica content,
• created by partial melting of the low-temperature
  constituents of the mantle

• on land
• plateau basalts
• non violent eruptions of
• basalt flows from fissures (Iceland)
• mantel material

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divergent margins rifting

• under oceans
• basaltic lava pillow lava

• on land
• plateau basalts

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hot spots

• stationary heat plumes in the mantle, hot spots
  in the mantle produce volcanoes in a chain as the
  crust moves over the hot spot (Hawaiian Islands)

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Eruptive Phenomena include

• lava flows
• explosions
• ash falls
• hot-ash flows
• glowing avalanches

• mudflows
• fissures
• earthquakes
• floods
• elevation changes
• gas discharges

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• mudflows
• fissures
• earthquakes
• floods
• elevation changes
• gas discharges

• lava flows
• explosions
• ash falls
• hot-ash flows
• glowing avalanches

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types of lava

• basic lava produce non explosive eruptions or
  effusive eruptions, with lava fountains and lava
  flows, less viscous and thus does not trap gasses
  as much as the lava below
• intermediate and acidic lava produce explosive
  eruptions sudden release of trapped CO2 and SO2
  and steam in the highly viscous lava

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types of lava

• basic lava

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types of lava

• intermediate / acidic lava

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Warnings of an eruptionFig 7.5- 7.7 mt. st.
helens

• begins with upward movement of magma from 50 km
  depth in the crust
• earthquake swarms up to hundreds per day due to
  the rise of magma
• earthquakes at 1 km depth when the eruption is
  nearer at hand
• temperature rise in hot springs and steam in
  volcanic crater
• gas released causes asphyxiation

• snow on the volcano will melt
• bulge of the surface
• explosion casting pyroclastic debris up into
  the atmosphere (known up to 80 km)
• Fall of pyroclastic debris (hot material)
• base surge outward expanding ash-laden cloud
  which sometimes also contains poisonous acid or
  toxins

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On May 18, 1980, Mount St. Helens had a massive
explosion that forever changed the picturesque
alpine landscape, killed almost 60 people and
sent ash for hundreds of miles. The force of the
eruption coated eastern Washington with a thick
layer of light gray ash. When wet the ash became
as dense as cement making it hard to remove from
lawns, roofs and roads. The ash can still be seen
along I-90 and elsewhere in the area. Parts of
Idaho and Montana had deposits as the ash was
caught up in the jetstream winds. The blast
removed 1000 feet off the top of the mountain,
leveled 200 square miles of forest to the north,
moved Spirit Lake and formed new lakes. The sound
of the explosion could be heard as far away as
Canada. Giant mudflows raced down the mountain
into local rivers destroying bridges, vehicles
and houses. The sound of the explosion could be
heard as far away as Canada. Mount St. Helens is
one of the Cascade Volcanoes that reach from
Washington to California.
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events

• glowing avalanches or nueé ardente is a hot
  (700-1000 degree C) ash-laden gas cloud
• moves at extremely fast speed (average of 160
  km/hr) down the volcano slope
• rock formed by this is called ignimbrite or
  welded tuff

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events

• lava flows type of flow depends on viscosity
  which is related to silica content
• stiff, highly viscous silica rich lava flows in
  blocks and forms a blocky surface on the lava
  called aa texture
• fluid, less viscous, lower silica lava flows in
  rope like surface called pahoehoe texture
• Hawaiian names

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events

• Volcanic mudflows (lahars)
• pyroclastic material mixed with water that flows
  rapidly (10 m/s)

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tsunamis

• great sea waves caused by the displacement of
  water due to a sub oceanic volcanic eruption or
  earthquake
• great velocities up to 5 000 km / hr
• as they reach the shore they rise up into giant
  waves that flow in over the land

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tsunami
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tsunami
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tsunami
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tsunami
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tsunami
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Volcanic rocks

• Pyroclastic rocks - molten material is ejected
  and solidifies in the air
• classified as sedimentary rocks

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particles in volcanic rocks

• preexisting rock particles are blocks gt64 mm,
  or lapilli 2-64 mm,
• molten lava which cools are bombs gt 64 mm,
  ash-silt size

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pyroclastic rock names

• Ash tuff - rock predominated by ash sometimes
  simply referred to as tuff.
• Lapilli tuff - rock predominated by lapilli.
• Tuff breccia - rock containing 25 to 75 blocks
  and/or bombs.
• Pyroclastic breccia - rock containing at least
  75 blocks and bombs.
• Agglomerate - rock containing at least 75 bombs.
  
• Agglutinate - rock composed of fused, largely
  unrecognizable, basalt spatter fragments.

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pyroclastic rock names

• pumice or scoria has numerous gas holes
• obsidian is volcanic glass which cooled suddenly
• bentonite pure montmorillonite clay formed from
  weathered ash

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volcanic flow rock names

• in order of increasing silica (downwards) and
  increasing explosiveness
• basalt
• andesite
• dacite
• latite
• rhyolite

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volcanic flow rock names

• in order of increasing silica (downwards) and
  increasing explosiveness
• basalt
• andesite
• dacite
• latite
• rhyolite

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volcanic flow rock names

• in order of increasing silica (downwards) and
  increasing explosiveness
• basalt
• andesite
• dacite
• latite
• rhyolite

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volcanic flow rock names

• in order of increasing silica (downwards) and
  increasing explosiveness
• basalt
• andesite
• dacite
• latite
• rhyolite

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Volcanic rock terms

• aphanitic fine grains that are not visible to
  the eye
• phenocrysts large crystals in the aphenitic
  matrix
• traprock light colour aphanitic volcanic rock
• felsite dark colour aphanitic volcanic rock
• vesicles holes in the rock formed by gas
  bubbles
• vesicular rocks with numerous vesicles
• scoriaceous vesicular and extremely porous
• amygdule mineral that fills the vesicle
• amygdaloida a rock with numerous vesicles
  filled with minerals

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Volcanic rock terms

• aphanitic fine grains that are not visible to
  the eye
• phenocrysts large crystals in the aphenitic
  matrix
• traprock light colour aphanitic volcanic rock
• felsite dark colour aphanitic volcanic rock
• vesicles holes in the rock formed by gas
  bubbles
• vesicular rocks with numerous vesicles
• scoriaceous vesicular and extremely porous
• amygdule mineral that fills the vesicle
• amygdaloida a rock with numerous vesicles
  filled with minerals

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Volcanic rock terms

• aphanitic fine grains that are not visible to
  the eye
• phenocrysts large crystals in the aphenitic
  matrix
• traprock light colour aphanitic volcanic rock
• felsite dark colour aphanitic volcanic rock
• vesicles holes in the rock formed by gas
  bubbles
• vesicular rocks with numerous vesicles
• scoriaceous vesicular and extremely porous
• amygdule mineral that fills the vesicle
• amygdaloidal a rock with numerous vesicles
  filled with minerals

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Volcanic rock terms

• aphanitic fine grains that are not visible to
  the eye
• phenocrysts large crystals in the aphenitic
  matrix
• traprock light colour aphanitic volcanic rock
• felsite dark colour aphanitic volcanic rock
• vesicles holes in the rock formed by gas
  bubbles
• vesicular rocks with numerous vesicles
• scoriaceous vesicular and extremely porous
• amygdule mineral that fills the vesicle
• amygdaloidal a rock with numerous vesicles
  filled with minerals

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Volcanic rock terms

• aphanitic fine grains that are not visible to
  the eye
• phenocrysts large crystals in the aphenitic
  matrix
• traprock light colour aphanitic volcanic rock
• felsite dark colour aphanitic volcanic rock
• vesicles holes in the rock formed by gas
  bubbles
• vesicular rocks with numerous vesicles
• scoriaceous vesicular and extremely porous
• amygdule mineral that fills the vesicle
• amygdaloidal a rock with numerous vesicles
  filled with minerals

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Volcanic rock-mass characteristics

• complex in composition, flows, pyroclastic debris
  etc. and interbeds of non volcanics
• flows follow lows in the topography
• resistant to weathering after a long period of
  physical weathering the deposits which once were
  in the bottoms of valleys form tops of mountains,
  table mountains
• irregular lateral extents

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Volcanic rock-mass characteristics

• complex in composition, flows, pyroclastic debris
  etc. and interbeds of non volcanics
• flows follow lows in the topography
• resistant to weathering after a long period of
  physical weathering the deposits which once were
  in the bottoms of valleys form tops of mountains,
  table mountains
• irregular lateral extents

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Volcanic rock-mass characteristics

• complex in composition, flows, pyroclastic debris
  etc. and interbeds of non volcanics
• flows follow lows in the topography
• resistant to weathering after a long period of
  physical weathering the deposits which once were
  in the bottoms of valleys form tops of mountains,
  table mountains
• irregular lateral extents

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Volcanic rock-mass characteristics

• complex in composition, flows, pyroclastic debris
  etc. and interbeds of non volcanics
• flows follow lows in the topography
• resistant to weathering after a long period of
  physical weathering the deposits which once were
  in the bottoms of valleys form tops of mountains,
  table mountains
• irregular lateral extents

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fractures and permeability
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Weathering products

• contrasting potential for weathering - basalt is
  more basic than granite and thus more inclined to
  decay due to chemical weathering
• on one hand the rocks are often impermeable in
  themselves which would deter chemical weathering
• on the other there are often numerous joint which
  make the rock mass on a whole very permeable,
  enhancing chemical weathering
• young basalt often is not weathered, but old
  basalt is deeply decomposed to a clay soil,
  expansive montmorillonite

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Engineering problems with volcanism and volcanic
rocks

• Enormous damage potential!!

• ash fall risks
• abrasive
• clogs drains
• poisonous
• causes fires
• weight can damage structures (like water logged
  snow)

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Engineering problems with volcanism and volcanic
rocks

• Enormous damage potential!!

• lava flow risks
• flow relative slow
• diversion possible trenches, barriers and
  spraying with cold water can be used to deter the
  flow
• predict flow path possible

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Engineering problems with volcanism and volcanic
rocks

• Enormous damage potential!!

• mudflow risks
• huge quantity
• high velocities
• path predictable far in advance but the velocity
  and size of the flow makes it difficult to
  contain, dams are easily broken, barriers jumped
• preparatory measures can be taken, lower the
  level of water reservoirs

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Exploration and volcanic rocks

• complexity of the deposits makes it difficult to
  predict their vertical and lateral extent
• stratigraphy can vary greatly laterally
• marker layers are needed
• need to find the extent of the material with
  lower strength and high permeability

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Surface excavation

• excavation often requires blasting
• blocks often displaced on slopes float in soft
  materials (fig 7.18)

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Underground excavations

• Difficulty can be represented by two cases
• it took 6 years to tunnel 17 km
• two years to tunnel 263 m

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Underground excavations

• large water inflow is common due to the
• open fractures and joint
• highly permeable layers
• permeable interbeds
• folded beds can entrap water in compartments

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Underground excavations

• hard and dense if unweathered too hard for a
  tunnel machine
• large extent of jointing results in high
  potential for rock fall shortcrete required
• horizontal stress ? zero, vertical stress
  weight of the overlying rocks thus there is a
  high potential for the roof to collapse

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Underground excavations

• active areas poisonous gas can occur
• young areas non cemented rock common
• warm water flows can occur

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Dams and canals

• leakage is a great problem grouting is required
• compressibility high bearing capacity poor
• shear strength low slides probable

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Dams and canals

• Hoover dam Fig 7.22, height 222 m
• founded on volcanic breccia
• grout curtain depth originally to be 40 m ended
  up 130 m deep and horizontally 90 m

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Dam in Sardinia

• rock fill dam
• concrete face
• founded on a series of lava flows with columnar
  jointing and tuff beds
• serious differential settling
• fault under dam

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Engineering materials

• volcanic rocks are used in all aspects of
  engineering
• aggregates
• concrete
• asphalt
• rock fill
• dams
• breakwaters
• coarse grade
• dimension stone
• Note there are some things to look out for

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Problems as engineering material

• volcanic glass reacts with alkalies in Portland
  cement cracking of the structure
• amygdules that are often filled with the
  following minerals opal, zeolite, gypsum these
  are not good in concrete, reactive
• pillow lava has often an unstable rind which is
  reactive with Portland cement
• weathering can be rapid for some rocks X ten
  years sand and gravel
• disintegration tests should be made to test the
  life expectancy of the rock (top p.286)

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Case studies

• Protection of an Icelandic port from volcanism
  barriers were erected and seawater pumped onto
  the flow to cool it

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Case studies

• Round Butte Dam, Oregon

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Case studies

• Round Butte Dam, Oregon
• 133m high dam in lava flows with interbeds of
  non-volcanics. Fig 7.25
• Several layers required grouting
• In all 42 km of grout holes were filled with 4000
  m3 of Portland cement grout and took two years to
  carryout