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Title: Magmatic Explosive Fragmentation: Driven By Juvenile Steam Expansion


1
Magmatic Explosive Fragmentation Driven By
Juvenile Steam Expansion
GEO2750 Lecture
2
Questions
  • What is a magmatic explosion?
  • How are magmatic explosions triggered?
  • What factors influence explosivity?
  • How does the magma get fragmented
  • What do the fragments look like?
  • (Here we are not concerned with emplacement
    mechanisms
  • nor deposit characteristics which we will discuss
    later)

3
What is a volcanic explosion?
  • Not easy to define or even (sometimes) recognize
    from the deposit
  • Magma explosion requires significant fragment
    dispersal (D)
  • (volume dependent though)
  • Magma explosion also requires a significant
    degree of magma fragmentation
  • (F) (but non-explosive fragmentation to a similar
    degree also occurs)
  • Can be defined quantitatively on basis of a
    D-F plot
  • A magmatic (dry) explosion is driven solely by
    expansion of steam bubbles
  • (from water that was dissolved in the magma), ie
    no external water

4
Quantifying explosivity D-F plot
Most explosive
Some non-explosive fragments may have high F
PHREATOPLINIAN
SURTSEYAN
Surtseyan and Phreatoplinian are
phreatomagmatic eruption styes
Names refer to Eruption Styles
  • D-F plot is a convenient way of quantifying
    explosivity. D is a measure
  • of dispersal and F is a measure of fragmentation.
    Hence top right
  • is highest explosivity. Requires unconsolidated,
    uneroded fall deposits.
  • (in order to measure F and D respectively)
  • Note that high F values typically correlate with
    interaction with external
  • water

5
Volatiles provide main driving force of magma
ascent and eruption
Solubilities of volatiles
H2O and CO2 are most important volatiles (on
Earth). Both H2O and CO2 are less soluble at
lower pressures (depths). CO2 will boil off
before H2O
6
Precursors to explosions bubble nucleation,
diffusion and growth
  • Bubble nucleation, diffusion and growth precedes
    all magma explosions.
  • These are complex processes and we wont go into
    any detail in this lecture
  • Bubbles must first nucleate. Nucleation on
    crystals, impurities etc requires less energy
    (lower surface area of bubble-magma interface) to
    reach a critical bubble radius
  • Nucleation can be homogeneous or heterogeneous
  • Bubbles must reach a critical size to nucleate
    and grow (this is when fluid P in bubble equals
    the surface tension of bubble-magma interface)
  • Diffusion of volatile components to the growing
    bubble is strongly dependent
  • on magma viscosity.
  • As bubbles of steam form, surrounding magma
    viscosity increases. Therefore
  • after some max bubble size, bubble fluid pressure
    therefore increases rather than bubbles getting
    bigger (at a constant confining P)

7
How are magmatic explosions triggered?
8
  • Below some critical pressure (depth) explosions
    are suppressed
  • (critical depth depends on density of confining
    material(s) and g)
  • Note that confining pressures influence both
    magmatic (dry)
  • and external water (phreatomagmatic) driven
    explosions

9
Other Forces to Consider
  • There are other components of pressure that we
    need to consider
  • to fully understand explosive fragmentation by
    juvenile volatile expansion
  • Pressure to drive magma through conduit (strongly
    dependent on confining pressure and conduit
    geometry). This is the dominant component when
    gas phase is continuous (ie magma is dispersed
    within gas)
  • Pressure required to overcome surface tension at
    bubble-magma interface (not dependent on
    confining pressure)
  • Pressure required for bubbles to expand against
    viscous resistance of
  • magma (strongly dependent on magma viscosity,
    not significantly
  • dependent on confining pressure)

10
Rapid acceleration and expanding gas bubbles
  • Magmas explode because they are violently torn
    apart
  • by rapidly expanding gas bubbles or pockets
    (typically because of decompression) and/or from
    fluid instabilities associated with rapid
    acceleration (can even be degassed in this case)
  • Gas (H20) is dissolved in magma, exsolved as
    bubbles in magma or is from heating of external
    water (groundwater, sea, lake, ground and surface
    ice etc)
  • (We will consider role of external water in the
    next lecture)
  • Gas/magma volume ratio at near-surface, gas
    bubble expansion rate, bubble fluid pressure and
    rate of bubble rise (compared to rate of magma
    rise) are important factors determining degree of
    explosivity and eruption style

11
Influence of magma viscosity and gas content on
explosivity
  • Both higher magma viscosities and higher gas
    contents individually favour
  • explosions, but do not directly correlate with
    them. But a combination of
  • these does typically correlate with explosivity
  • During any one eruption (or episode) gas content
    tends to decrease, so
  • explosivity tends also to decrease with time, eg
    lava domes commonly
  • form at the end of an explosive eruptive cycle

12
Triggering an Explosion with Juvenile Water
  • Three mechanisms of magma explosion due to
    juvenile
  • water (steam) expansion
  • Explosions can be initiated by decompression
    (first
  • boiling), i.e when confining pressure around
    magma is lowered (often by magma rising buoyantly
    to the near-surface)
  • 2. Explosions can also be initiated by an
    increase in temperature (at a constant P). Water
    solubility is lower at higher temperatures
  • Explosions can also be initiated by anhydrous
    crystallization. This is known as second
    boiling, and arises from supersaturation of
    dissolved water in the melt

13
Explosions due to Juvenile Volatile Expansion by
decompression
  • Decompression of buoyant rising magma and
    exsolution (boiling) of juvenile volatiles
  • Fracturing of rocks around magma chamber also
    lowers confining pressure
  • Confining P may also be lowered by
    collapse/removal
  • of surface rocks, ice, water etc

Montserrat dome
14
Triggering an explosion by increasing T
  • Increasing T may arise from injection of new hot
    magma batch of same
  • composition or especially of batch of basic magma
    into more felsic magma
  • Water is less soluble at higher temperatures
    (constant P) so will boil off

15
Triggering an explosion by anhydrous
crystallization
  • Anhydrous minerals have higher melting points
    than hydrous minerals so always
  • crystallize first
  • Anhydrous minerals that are important include
    olivine, pyroxene, plagioclase,
  • alkali feldspar, quartz
  • Crystallization of anhydrous minerals increases
    proportion of H2O dissolved in
  • melt
  • H2O boils off once its solubility limit (for
    particular P, T, X) is reached

16
Explosive Magma Fragmentation general comments
  • Rate of bubble rise relative to magma rise is
    important. If bubble rise is much
  • faster, then degassing without magma
    fragmentation is likely
  • Faster rates of bubble rise are more common in
    low viscosity magmas (eg basalt)
  • When volume fraction of vesicles exceeds about
    80 then magma typically fragments,
  • BUT explosive fragmentation can occur at much
    lower values (even 0..when
  • nearby bubbles expand)
  • Fragmentation occurs when bubble fluid pressures
    overcome surrounding forces
  • (surface tension, viscous resistance and
    confining P)
  • Fragmentation can be brittle (angular pyroclasts)
    or ductile (fluidal pyroclasts)

17
Pyroclasts basics
Grain size terminology for pyroclasts
  • Proportions of crystals (C), lithics (L) and
    glass (G) varies
  • Vesicularity () varies
  • Shapes of vesicles vary
  • Shapes of pyroclasts vary
  • Surface textures vary

Ash (lt2mm)
Lapilli (2-64mm)
Blocks/Bombs (gt64mm) (Bombs are aerodynamically
moulded)
18
Fragmentation in Hawaiian eruptions
  • Fragmentation by fluid instabilities in rapidly
  • accelerating jet of magma (can be degassed)
  • Jet (fountain) arises due to bubble expansion
  • and nozzle-like constriction
  • Pyroclasts are typically fluidal with
  • morphologies determined by surface tension
  • and aerodynamic moulding
  • Size of pyroclasts determined by jet ejection
  • velocity, degree of instability, viscosity, gas
  • content etc
  • Core of fountain may produce very highly
  • vesicular reticulite
  • Margins of fountain may be spun in wind
  • to form glass fibers (Peles hair)

Break-up (atomization) of jet
through constricted nozzle (Van Dyke, 1982)
Reticulite (gt90 vesicles)
19
Hawaiian pyroclasts
Heiken and Wohletz, 1979
Coarse spatter can be sticky (agglutinate)
Clasts are often fluidal ash/lapilli but also
include angular ash/lapilli
Peles tears (achneliths)
Peles hair glass fiber
20
Fragmentation in Strombolian eruptions
21
Strombolian eruptions
Kamchatka
Paricutin, Mexico
  • Strombolian eruptions are discrete explosions
    with columns lt few km high (typically)

22
Strombolian pyroclasts
Strombolian bombs
Photos from Sumner, 1996
  • Mostly coarse (lapilli to block) clasts of
  • variable vesicularity, fluidal and angular
  • Variable proportion of (mostly) angular
  • ash
  • Bombs (aerodynamically moulded)
  • are common
  • Agglutinated spatter and agglutinate
  • (clastogenic) lava flows are common

Clastogenic lava flows
23
Fragmentation in Vulcanian eruptions
Vulcanian pyroclasts are typically a mixture
of lithic blocks and finer (lapilli/ash) clasts
(ie bimodal) of variable vesicularity
24
Fragmentation in Plinian eruptions
Eruption column
25
Plinian eruption columns
Umbrella
Convection
Gas thrust
Wind velocity
Gas thrust energy
(Schmincke, 2000)
  • Plinian eruptions are sustained explosions (or
    jets) up to hours long
  • Eruption columns can be up to 30-40km high
  • Rapid decompression of large volumes of gas-rich
    magma

26
Ash dispersal from Plinian columns effects of
wind
  • Ash is spread laterally in umbrella at height of
  • tropopause (altitude varies with latitude)
  • Ash can be spread in different directions at
    different
  • altitudes

Schmincke, 2000
27
Plinian pyroclasts
  • Plinian pyroclasts are typically pumice (glass
    with 30-70 vesicles typically) lapilli
  • and ash (mostly derived from bubble walls). Wide
    range of pyroclast sizes

Cuspate and platy ash are derived from bubble
walls
Morphology of Plinian ash pyroclasts (shards)
Plinian pumice lapilli fallout deposits
28
Pyroclast textures vesicles
Typical Plinian pumice 80 vesicles
Plinian pumice vesicle coalescence etc
Reticulite (gt98 vesicles) polyhedral network
Tube pumice stretched vesicles
Microlites (small xls) inhibiting bubble growth
Collapsed/compressed vesicles
(Cashman et al. 2000)
29
Non-Explosive Fragmentation (During Explosions)
  • Most explosive products also include
  • juvenile fragments generated at same
  • time by non-explosive fragmentation
  • There are two common non-explosive
  • fragmentation processes (during
  • explosive eruptions)
  • Thermal stress granulation
  • Cooling on contraction of glass
  • (especially during interaction with
  • external water)
  • Mechanical stress granulation
  • e.g. abrasion in eruption column, conduit or
  • impact with ground

Thermal stress granulation
Non-explosively generated fragments can resemble
those produced by explosions, but are typically
poorly vesicular
Note that fragmentation of pre-existing clasts
and coherent lava also takes place after
deposition (epiclastic fragmentation)
30
Important Points to Remember
  • Rapidly expanding gas and/or rapid acceleration
    of magma drives all magma explosions
  • Decompression triggers many explosions especially
    of high viscosity and
  • high gas (water) content magmas
  • Many explosive styles involve more than one
    explosive fragmentation process
  • Non-explosive fragmentation usually accompanies
    explosions, and
  • may produce similar fragments
  • High degree of fragmentation does not correlate
    with high explosivity
  • Vesicularity does not correlate with explosivity
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