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Volcanism and volcanic rocks

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Title: Volcanism and volcanic rocks


1
Volcanism and volcanic rocks
  • rocks and sediments produced by volcanic
    processes

2
Volcanism plate tectonics
  • relationship of volcanism to movements of the
    earths plates (fig 7.1)

3
convergent boundaries
  • seduction zones often violent eruptions, due to
    high silica content
  • pyroclastic sediments thrown from the volcano

4
divergent margins rifting
5
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

6
divergent margins rifting
  • under oceans
  • basaltic lava pillow lava
  • on land
  • plateau basalts

7
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)

8
Eruptive Phenomena include
  • lava flows
  • explosions
  • ash falls
  • hot-ash flows
  • glowing avalanches
  • mudflows
  • fissures
  • earthquakes
  • floods
  • elevation changes
  • gas discharges

9
  • mudflows
  • fissures
  • earthquakes
  • floods
  • elevation changes
  • gas discharges
  • lava flows
  • explosions
  • ash falls
  • hot-ash flows
  • glowing avalanches

10
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11
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

12
types of lava
  • basic lava

13
types of lava
  • intermediate / acidic lava

14
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

15
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.
16
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

17
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

18
events
  • Volcanic mudflows (lahars)
  • pyroclastic material mixed with water that flows
    rapidly (10 m/s)

19
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

20
tsunami
21
tsunami
22
tsunami
23
tsunami
24
tsunami
25
Volcanic rocks
  • Pyroclastic rocks - molten material is ejected
    and solidifies in the air
  • classified as sedimentary rocks

26
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

27
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.

28
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

29
volcanic flow rock names
  • in order of increasing silica (downwards) and
    increasing explosiveness
  • basalt
  • andesite
  • dacite
  • latite
  • rhyolite

30
volcanic flow rock names
  • in order of increasing silica (downwards) and
    increasing explosiveness
  • basalt
  • andesite
  • dacite
  • latite
  • rhyolite

31
volcanic flow rock names
  • in order of increasing silica (downwards) and
    increasing explosiveness
  • basalt
  • andesite
  • dacite
  • latite
  • rhyolite

32
volcanic flow rock names
  • in order of increasing silica (downwards) and
    increasing explosiveness
  • basalt
  • andesite
  • dacite
  • latite
  • rhyolite

33
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

34
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

35
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

36
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

37
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

38
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

39
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

40
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

41
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

42
fractures and permeability
43
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

44
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)

45
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

46
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

47
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

48
Surface excavation
  • excavation often requires blasting
  • blocks often displaced on slopes float in soft
    materials (fig 7.18)

49
Underground excavations
  • Difficulty can be represented by two cases
  • it took 6 years to tunnel 17 km
  • two years to tunnel 263 m

50
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

51
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

52
Underground excavations
  • active areas poisonous gas can occur
  • young areas non cemented rock common
  • warm water flows can occur

53
Dams and canals
  • leakage is a great problem grouting is required
  • compressibility high bearing capacity poor
  • shear strength low slides probable

54
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

55
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

56
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

57
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)

58
Case studies
  • Protection of an Icelandic port from volcanism
    barriers were erected and seawater pumped onto
    the flow to cool it

59
Case studies
  • Round Butte Dam, Oregon

60
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
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