HOW MAGMAS ERUPT :
An Introduction to pyroclastic processes and products
H2/ Geol. -1
H2/ Geol. -2
H2/ Geol. -3
MA NAY YEE LIN AYE
MG LINN THANT MAUNG
MA AYE THANDAR SHANE
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Outline
 Introduction
 The nomenclature of volcanic eruptions and deposits
 Internal structures of pyroclastic deposits
 Microscopic textures
 Calderas
 Summary and conclusion
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1. Introduction

High viscosity and volatile-rich silicic melts erupt explosively

The largest and most damaging eruption of magmas are
pyroclastic eruptions

Major pyroclastic eruptions distributed over a wide geographical
area

Emphasizing the mechanics of pyroclastic eruptions and the
physical character of the deposits
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2. The nomenclature of volcanic eruptions and deposits
2.1 Effusive
volcanism
vs
explosive
 Effusive
and
explosive
activity
ejects
mainly
pyroclastic materials
 Activity at a given vent may
change from one style to the
other during the course of an
eruptive episode
The Augustine volcano in Alaska produce
eruption column and pyroclastic flow
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Effusively or explosively depends on two inter-related properties
 Magma viscosity
 Magma volatile content
(a) Solubility of gas in a
silicate melt as a function of
pressure, melt with a
dissolved gas content of x %
becomes saturated with gas
at y. (b) Progress of melt up a
volcanic conduit; above the
vesiculation horizon
The effect of dissolved gas content in
ascending magma
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 Effusive activity is hot, fluid (low SiO2 ) magma and low dissolved
volatile content
 But eruptive activity being seriously explosive
 The fluidity of basalt lava leads to gently sloping shield
volcanoes
 E.g (Mauna Loa volcano, Hawaii – USGS)
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 More viscous, evolved (higher
SiO2 ) magma is mostly to erupt
explosively
 Explosive eruptions build up
stratovolcanoes, alternations of
lavas and pyroclastic layers
Stratovolcano: aerial view of Mount Hood,
Oregon from the west (US Geological
Survey photo by M. Doukas)
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2.2 Styles of volcanic eruption
(1) Hawaiian

Fire fountains≥ 100 m high,
lava lake & lava tubes

Tiny volumes of pyroclastic
materials (basalt)

Basaltic
shield
summit
caldera,
volcano,
Hawaiian lava fountaining at Pu’ u ‘ O ’o ,
Kilauea, Hawai`i
spatter
cone, lava flows (pahoehoe,
a’a), levees.
Kilauea Iki crater, Hawai`i;
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(2) Strombolian
 Periodic explosions
emitting scoria/ash
clouds
 Well sorted scoria beds
(basalt or basaltic
andesite)
 Scoria cone
Small strombolian eruption column,
Stromboli volcano
Strombolian eruption at night,
Stromboli volcano
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(3) Vulcanian

Discrete
explosions,
short-lived
eruption
column 5-10 km high

Thin beds of ash breadcrust bombs, lithics

Crater (e.g. Fossa)
Vulcanian eruption at the Soufri è re Hills volcano,
Montserrat in October 1997
(photo by kind
permission of the Montserrat Volcano Observatory)
Crater of Fossa volcano, Vulcano; the darker beds
exposed in the upper wall are the products of the 1888
– 90 eruptions
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(4) Plinian
 Sub-plinian
 Sustained eruption column 10-20
km high, PDCs
 Widespread pumice fall blanket
 Plinian and ultraplinian
 Sustained eruption column 20-50
km high and umbrella, PDCs
18 May 1980 plinian eruption of
Mount St Helens, Washington,
 Pumiceous lapilli tuff/ignimbrite
 Stratovolcano and crater; however,
larger plinian eruptions often have
no associated positive edifice, and
may eject enough magma to cause
Plinian eruption column of Pinatubo volcano seen from
caldera subsidence
former Clark Air Base, Philippines in 12 June 1991
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(5) Hydrovolcanic
 Repeated explosions, ‘cook’s tail’
ash cloud, expanding base-surge
cloud
 Thin beds of juvenile ash (often
basalt)
and/or
country-rock
lithics; cross-stratified basesurge deposits
Hydrovolcanic eruptions of an undersea
volcano off the coast of Nuku ’ Alofa, Tonga,
on 18th March 2009
 Tuff (or ash) ring; crater or maar
Schalkenmehrener maar, Eifel,
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Germany, an explosion
2.3 Nomenclature of pyroclasts and pyroclastic deposits
 Pyroclasts - solid fragments of magma formed by fragmentation at
high temperature
 Most pyroclasts form as hot juvenile products
 Vesiculation reaches a maximum in pumice, intermediate to acid in
composition
 Scoria – vesicular pyroclasts
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 Two radically different ways in a plinian eruption column during the
course of such an eruption;
(a) The buoyant - column
stage of a plinian eruption
when pyroclastic fall deposits
form. (b) The collapsing column or ‘fountaining ’stage of
a plinian eruption when the
column is no longer buoyant,
and its contents are carried to
the ground by pyroclastic
density
currents
(PDCs).
(Source: Robin Gill, 2010)
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The distinction
between pyroclastic
fall and pyroclastic
current deposits
(a) A pumice fall deposit
shows mantle bedding
and consists of well
sorted pumice lapilli as
shown in the picture (ruler
10 cm in length). (b) a
pyroclastic
current
deposit (ignimbrite) tends
to be concentrated in
valleys and consists of
ash - rich lapilli - tuff (coin
2.3 cm diameter).(Source:
Robin Gill, 2010)
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3. Internal structures of pyroclastic deposits
3.1 Pyroclastic fall deposits
 Consist of basaltic scoria, pumice and well sorted with the clast size
decreasing at greater distances from the vent
 In strombolian, little fine ash is formed
 In plinian, smaller lapilli and ash, greater D, usually massive,
sometime show stratification
Mantle bedding in a plinian fall deposit, Okareka quarry near Rotorua, New Zealand.
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3.2 Pyroclastic current deposits
 Ignimbrites
 Consist of massive lapilli-tuff
 May show internal stratification
of maximum clast size
 In some, pumice lapilli is lensoid
rather than equant, lie subparallel to each other
Massive lapilli - tuff exposed in a
quarry face, part of the Granadilla
ignimbrite in Tenerife
Taupo ignimbrite, New Zealand the stratified
lower portion and the carbonized tree trunks
that were carried along by the current
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 In pumice lapilli, original equant
& uncompacted shaped as
unwelded and the flattened &
sub-parallel as welded
 Strongly welded are commonly
some lava, e.g. columnar jointing
 A single ignimbrite sheet may
consist
several
layers
of
thickening or thinning along or
across the current direction
 A complete plinian deposit
consists of pumice fall bed at the
base, overlain by pumiceous
lapilli-tuff
Jointing in welded lapilli tuff,Devil’s Post
Pile N. Mon.,California
Jointing in welded Lower Bandelier
ignimbrites in the west wall of San Diego
Canyon, Jemez Mountains, New Mexico
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 Block and ash deposits

a very different type
pyroclastic density current
of
 Much of the finest ash component
is aloft to form the familer
billowing ash cloud

In an ashy matrix
Ash cloud from Mont Pele’e
Typical section of a block - and - ash
deposit(Old Horse Springs, New Mexico, USA).
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 Base surge deposits related
to hydrovolcanic eruptions
 A form of pyroclastic density
current, but more dilute than
ignimbrites, owing to water,
cooler and moister
 The maar-forming ascending
basaltic magma and an aquifer
produces alternating beds of
strombolian and base surge
deposits
Alternating juvenile andesitic scoria beds
and laminated base - surge beds at the
Pukeonake scoria cone, Tongariro National
Park, New Zealand
 Accretionary lapilli are another
common
product
of
phreatomagmatic eruption
Accretionary lapilli of
Oruanui rhyolitic ash,
New Zealand; lens
cap diameter 6 cm.
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4. Microscopic Textures
4.1 Pumice
 Most distinctive product of a
plinian eruption is pumice
 Vesicular across a whole range
of scales, from micrometers up
to millimeters, sometimes even
centimeters
 Usually contains vesicle voids
high – 80%
Close - up of a pumice lapillus from
Tenerife, scale bar 1 cm long
 Elongated
indicating
shear
deformation prior to vitrification
 May
contain
phenocrysts
formed at depth and microlites
formed at late stage
Mt St Helens pumice, showing μ m - scale vesicles
(ves) and intervening glass crowded with
plagioclase microlites (pl) formed during ascent
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 Lithified volcanic ash is tuff
 Depending on the dominant constituent, may be refined to
crystal tuff, vitric tuff and lithic tuff
Toba
Tuff
showing
crystal
fragments
of
plagioclase,
sanidine, quartz, biotite and
opaques and cuspate glass
shards (left PPL, right XP). Field
of view 1.3 mm wide.
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4.2 Lapilli-tuff
 Containing a jumble of pumice clasts
 Show no common foliation, poorly sorted character
 The product of a pumice-rich pyroclastic density current from a
plinian eruption column
 More specifically is a pumice lapilli tuff
Unwelded lapilli - tuff (Kneeling Nun
ignimbrite), New Mexico (lower part PPL,
upper XP). Field of view 2.7 mm wide
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4.3 Welded lapilli-tuff
 Consist of highly vesicular glass
 Hot pumice is buried deeper in accumulating pile of ignimbrite, it
readily to form sub-parallel fiamme
 Proximal fall deposits occasionally exhibit welding
Eutaxitic texture in welded lapilli - tuff
(Battleship Rock ignimbrite), Jemez
Mountains, New Mexico (left XP, right
PPL). Field of view is 5.3 mm wide
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5. Calderas
 The largest plinian and ultraplinian eruptions erupt colosal volumes
of evolved magma into the atmosphere
 The topographic depression resulting from magma chamber is
called a caldera
Aerial view of the Crater Lake, Oregon, fills a caldera that was formed
when Mount Mazama erupted catastrophically about 7,600 years ago.
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 Most calderas evolve by a combination of collapse mechanism
(a) climactic ignimbrite - forming stage of a
large - scale plinian eruption
(b) collapse of the unsupported chamber roof
(c) uplift of a resurgent structural ‘ dome ’ owing to the buoyancy and magma
pressure of the magma reservoir beneath
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6. Summary and conclusions
-
Major pyroclastic eruptions occur when volatile-rich magmas rise to
shallows depths
-
Plinian
eruptions may occur whether the magma is silica
oversaturated or silica-undersaturated .
1.
The explosivity of an eruption depends upon the dissolved volatile
content of the magma and its viscosity .
2.
Pyroclastic eruptions is based mainly on clast-size analysis of their
fall deposits.
3.
The deposits leave on the ground lapilli tuff or ignimbrite (if the
PDC is pumiceous), and block and ash deposit.
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4. The ignimbrite may be succeeded by a closing phase in which lava
domes of similar composition to the plinian tephra are erupted.
5.
Hydrovolcanic eruptions – interaction between magma and
external water – produce tephra, fine ash and Lapilli .
6.
Nuees ardentes - consisting mainly of juvenile dense lava clasts
and ash
7.
Base surges are dilute, cool PDCs that migrate radically outward
from a hydro volcanic explosion and deposit laminated ash beds
8.
Large pyroclastic eruptions are accompanied by the formation or
renewed subsidence of a caldera
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