Journal of Asian Earth Sciences 38 (2010) 170–172
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Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jseaes
Book Review
Studies in Volcanology: The Legacy of George Walker; edited by
T. Thordarson, S. Self, G. Larsen, S. K. Rowland and Á. Höskuldsson, viii + 413 pp.; published for IAVCEI by the Geological Society,
July 2009, Hardback. List Price £110/$220.
This book is IAVCEI Special Publication No. 2. It honours the
memory and contributions of Professor George Patrick Leonard
Walker, one of the fathers of modern quantitative volcanology,
through a collection of papers inspired by, and building upon,
many of the ideas previously developed by Walker. Following an
introduction on Walker’s contribution to the field of volcanology,
the papers of this volume are logically divided into subsections
on ‘‘Features and emplacement modes of lava flows” (4 papers),
‘‘Processes, styles and hazards of explosive volcanism” (8 papers),
and ‘‘Infrastructures and processes of volcanic provinces and volcanoes” (5 papers, plus 1 pictorial summary). The introduction by
R.S.J. Sparks clearly shows how George Walker, despite his relatively limited mathematical background, revolutionized the science of volcanology through detailed field observations with an
emphasis on quantitative measurements. Therefore, in many ways
the introduction serves as an introduction to modern volcanology
itself.
The book section on Features and emplacement modes of lava
flows fittingly begins with a paper by George Walker (posthumously prepared for publication by Thordarson and Self), on
‘‘The endogenous growth of pahoehoe lava lobes and morphology
of lava-rise edges”. This paper describes in detail the structures observed on lava-rise edges and sutures that form during endogenous
growth of pahoehoe flows due to inflation. The study is based on
observations from the 1859 Mauna Loa flow field, Hawaii, and
the more viscous Xitle basalt flow of the Chichinautzin monogenetic-volcano field, Mexico. It is very well illustrated with field
photographs and drawings, in the style typical for Walker’s contributions. The summary section serves partly as a short discussion
and concludes with the interesting statement that in the two flows
studied, endogenous thickening of lava by sub-crustal injection accounts for over 80% of the total flow volume.
The second paper, by Harris and Rowland, is entitled ‘‘Effusion
rate controls on lava flow length and the role of heat loss: a review”, and focuses on lava flow growth by flow-front advance. It
begins with Walker’s (1973) observation that lava flow length is
broadly controlled by lava effusion rate. The authors show that
some of the observed scatter is due to differences lava viscosity,
emplacement slope, topography, variable flow morphologies, and
degree of insulation provided by cooled lava crust. In terms of
insulation, they discuss in more detail poorly crusted channel-fed
flows, immature and mature tube-fed flows, and heavily crusted
well-insulated flows, and show that Walker’s original contention
that extrusion rates and flow lengths are linked remains true for
each of these cases. Although the effect of variations in initial lava
crystallinity, which may also affect effusive eruption style, are not
discussed, the well-illustrated paper provides a very good quantidoi:10.1016/j.jseaes.2009.12.007
tative review of the existing literature on parameters influencing
flow length and morphology.
The third paper, by Thordarson and Sigmarsson, is entitled
‘‘Effusive activity in the 1963–1967 Surtsey eruption, Iceland: flow
emplacement and growth of small lava shields”. It illustrates the
construction of monogenetic lava shields based on observations
from the subaerial effusive activity on Surtsey. Following an extensive review of the eruption history and the nature of the effusive
eruption phases (including impressive historic photographs), the
authors provide a detailed account of the internal structure of Surtsey lava flow facies based on field observations. In the discussion,
the contemporary observations of the activity are combined with
the field-based results to characterize the growth mechanisms of
the Surtsey lava cones and aprons. Lava cones form by fountaining
and surface flow activity, while aprons mainly grow laterally
through inflating sheet lobes fed by lava tubes. The authors suggest
that these mechanisms may apply to monogenetic lava shields in
general, and hypothesize that the size of lava cones is determined
by the lava discharge rate.
The first section of the book ends with a contribution by Sigmarsson et al., and again deals with Surtsey. Entitled ‘‘Segregations
in Surtsey lavas (Iceland) reveal extreme magma differentiation
during late stage flow emplacement”, this study uses whole-rock
and mineral chemical major oxide compositions of host rocks
and segregations of Surtsey sheet lobe cores to constrain formation
conditions and origin of the segregated material. Although the
mineralogy and mineral chemistry is clearly presented, the paper
would have benefitted from photomicrographs of the segregations.
The authors show that the segregations formed by volatile-induced
liquid displacement following 50–60% fractional crystallization in
the lower crustal boundary layer of the sheet lobes, while interstitial glasses are the final product of extreme closed-system crystallization. The paper concludes with a discourse on FeTi-rich basalts
from Katla volcano (Southern Iceland), which are compositionally
similar to the segregations, and agpaitic lujavrites (such as those
from the Ilimaussaq intrusion in Greenland), which are compositionally similar to the interstitial glasses.
The next section of the book is on Processes, styles and hazards of
explosive volcanism and begins with a study by Rowland et al., entitled ‘‘Pyroclastic deposits and lava flows from the 1759–1774
eruption of El Jorullo, México: aspects of ‘violent Strombolian’
activity and comparison with Parícutin”. The authors first provide
an interesting review of the eruption chronology and some background information on the general parameters employed in the
characterization of pyroclastic deposits, suitable for uninitiated
readers. Detailed pyroclast isopach and isopleth maps and a revised lava flow map of El Jorullo follow, and the authors show that
the violent Strombolian activity during the El Jorullo eruption produced widely dispersed tephras dominated by fine ash. However,
most tephra deposits lack the characteristics of phreatomagmatic
eruptions, suggesting that alternative fragmentation processes
operated. The authors favour fragmentation within the vent due
Book Review / Journal of Asian Earth Sciences 38 (2010) 170–172
to a combination of overpressure release by rupture of degassed
vent plugs, particularly gas-rich two-phase (gas and magma) flow
enhanced through high juvenile water contents, and pyroclast
recycling. The paper concludes with a comparison of El Jorullo
deposits with those of the 1943–1952 Paricutin eruption.
The second paper, by Sable et al., is entitled ‘‘Eruption mechanisms during the climax of the Tarawera 1886 basaltic Plinian
eruption inferred from microtextural characteristics of the deposits”. The authors begin with an account of previous work on this
eruption, which was sourced from a fissure with closely spaced
vent sites that exhibited a range of intensity from weakly explosive
to Plinian. Through a detailed study of deposit, clast and vesicle
characteristics at several scales, taken from three key field sites,
the authors show that: (i) Plinian intensity was mainly achieved
through a combination of unusually high magma viscosity and ascent rate. (ii) Some source vents failed to achieve Plinian intensity
due to addition of lithics derived from a series of minor vent wall
collapses, decreasing the temperature and increasing the density of
the erupting jet. (iii) Plinian activity eventually ceased because of
bubble coalescence leading to the development of permeable networks that promoted efficient outgassing. The study is well-referenced and provides clear insights into the development of basaltic
Plinian activity.
The third paper, by Clarke et al., is ‘‘An investigation of Vulcanian eruption dynamics using laboratory analogue experiments
and scaling analysis”. The authors studied the evolution of turbulent flows generated by unsteady, final volume injections of liquid-particle mixtures into water, and analyzed the effects of
variable density and momentum on flow front vertical velocity.
They derive a quantitative relationship between buoyancy,
momentum and flow front vertical velocity through scaling analysis, and show that this relationship holds for a variety of Vulcanian
eruptions from Lascar, Soufriere Hills, and Mount St. Helens. Finally, they demonstrate that their analysis may be used for estimating
parameters important to hazard assessment, such as total mass
erupted, and vent mass flux. They conclude with a short but
insightful discussion on uncertainties in vent particle concentrations, deceleration of initially supersonic flows, and the important
distinction between the physical vent and the virtual source at
which an underexpanded jet reaches ambient pressure.
In the fourth paper, entitled ‘‘Patterns of explosive activity deduced from fall deposits in frequently active volcanic regions”,
Wilson and Walker use their simple 1987 ejecta dispersal model
for Plinian eruptions to compare mass fluxes, eruption cloud
heights, and magma volatile content derived from 31 fall deposits
with well-constrained pyroclast grain size distributions. The reader will appreciate the well-written theoretical background section,
in which the authors review their model. The main strength of the
paper, however, is the analysis of uncertainties. The authors show
that mass eruption rates are fairly reliable, with errors of no more
than a factor of 2, corresponding to about 20% uncertainty in clast
dispersal and eruption cloud height. Conversely, accurate magma
volatile content and eruption speed estimates require better constraints on near-vent clast size distributions than are commonly
available. The discussion elucidates inferences that may be made
on wind speed, and concludes with an insightful comparison of
some of the determined eruption cloud heights with those derived
from more recently published ejecta dispersal models.
The fifth paper, by Cassidy et al., is entitled ‘‘GPR-derived facies
architectures: a new perspective on mapping pyroclastic flow
deposits”. The authors begin with a good introduction to the use
of ground-penetrating radar (GPR) for imaging sub-surface sedimentary structures, with an emphasis on volcanogenic materials.
They then provide a detailed and well-illustrated example of a
GPR study on medial and distal deposits of the 1993 Lascar pyroclastic flows. Processed flow-parallel and cross-flow GPR sections
171
are shown, and observations made on basis of these are clearly
presented and interpreted in terms of flow facies architecture.
Although an untrained eye may in some cases have difficulties to
make out the structural complexities imaged, the authors provide
a convincing comparison of one of the GPR sections with a deposit
cross-section from a trench excavated in the field. The paper concludes with a discussion of how the findings corroborate and complement existing emplacement models of pyroclastic flows.
The sixth paper, by Wadge, is entitled ‘‘Assessing the pyroclastic
flow hazards from dome collapse at Soufriere Hills Volcano, Montserrat”. Because the physical mechanisms of dome collapses are
not well understood, the author uses a semi-analytical simulation
code (PYROFLOW) to model the probability distribution of future
dome collapse pyroclastic flows at Montserrat, based on the statistics of flow directions and runout distances observed during past
episodes of dome growth. The results generally mimic the areal
extent of dome collapse pyroclastic flow inundation observed to
date. This is not necessarily expected, given the ongoing changes
in topography due to repeated flow deposition. Therefore, the
model constitutes an important tool in hazard assessment.
However, the need for very conservative hazard mitigation strategies is also apparent, given that hydrovolcanic surge and lateral
blast surge deposits, which are not considered here, now also cover
large areas of southern Montserrat.
The seventh paper, by Wilson et al., is entitled ‘‘Volcanism in
the central Taupo Volcanic Zone, New Zealand: tempo, styles and
controls”. Following an introduction on the factors that lead to
incomplete preservation of eruptive deposits in the Taupo Volcanic
Zone (TVZ), the authors provide an account of the dominantly
rhyolitic eruptive activity since 1.6 Ma in terms of numbers of
eruptions and their cumulative volume, with an emphasis on the
well-documented deposits from the last 61 ka. From this record,
they show that timing and size of eruptions is unrelated to repose
periods, making effective eruption forecasting impossible. The
paper then compares and contrasts the eruptive histories of the
TVZ and two Cordilleran volcanic provinces (Yellowstone and the
Southern Rocky Mountain volcanic field). A well-referenced but
partly speculative discussion on the complex relationships between volcanism, magmatism and tectonism in the area, and a
lengthy summary, conclude the paper.
The book section on explosive volcanism concludes with ‘‘A genetic classification of collapse calderas based on field studies, and
analogue and theoretical modelling”, where Martí et al. first discuss the literature on collapse caldera structures, sizes, and deposits, and then provide an overview of the variety of previously
employed classification schemes based on field studies. The
authors show that these schemes are not linked to the stress conditions that result in caldera collapse, which can be better understood through experimental and mathematical modelling. Based
on findings from these approaches, the authors suggest a new
and relatively simple distinction between overpressure and underpressure calderas. Overpressure calderas form when ring faulting
is the direct result of regional doming and occurs at the onset of
eruption, generating exclusively syn-collapse deposits. Underpressure calderas form when ring faulting is triggered after the onset of
eruption, when magmatic pressure drops significantly below lithostatic pressure, resulting in failure of the roof. In this case, pre-collapse deposits should be observed in the field. The study provides
some interesting insights into the parameters involved in eruption
triggering and ring fault development during the formation of collapse calderas.
The last section of the book deals with Infrastructures and processes of volcanic provinces and volcanoes and begins with CañonTapia’s study of ‘‘Hydrostatic principles of volcanic systems”. The
author investigates the relative effects of depth of melting region,
magma column height, and strength of overlying rocks in
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Book Review / Journal of Asian Earth Sciences 38 (2010) 170–172
determining the eruptive potential of magmas, using a simple
hydrostatic melt extraction model analogous to an upside down
version of the exploding barrel experiment of Pascal. This concept
provides some new insights into the conditions necessary for eruption of magma at the surface once it has formed at depth. The
lengthy introductory sections on the lack of direct causal connections between rapidly occurring processes within volcanic systems
on one side and slowly occurring processes such as mantle convection and plate tectonics may be of less interest. In fact, the significant influence of local and regional (tectonic) forces on the
location, orientation and chemistry of volcanism are outlined by
the author in the second part of the paper, although detailed constraints are not discussed.
In the next paper, on ‘‘Physical volcanology of continental large
igneous provinces: update and review”, White et al. first use examples from the Ferrar and Karoo provinces to elucidate the geometry
and emplacement processes of intrusive rocks, which make up the
bulk volume of continental large igneous provinces. The authors
then review the structures of flood basalt lava flows and vents, before turning to a description and detailed listing of a number of
typical mafic and silicic volcaniclastic deposit types, and the implications that can be drawn from these for the fragmentation processes, vent types and environmental conditions during explosive
volcanic activity. Given the diversity of deposit types, this review
paper cannot and does not provide detailed observational constraints. However, it may serve as an extensive and well-illustrated
overview with about 200 references, and excels in summarizing
the key points in a succinct conclusion section.
The short third paper, by Hjartarson, on ‘‘Central volcanoes as
indicators for the spreading rate in Iceland” begins with Walker’s
(1975) controversial statement that ‘the spreading rate is several
times greater in part of Iceland than elsewhere on the Mid-Atlantic
ridge’. The author uses the age of old central volcanoes and their
distance from respective spreading centers in an attempt to show
that there is indeed a 70% greater mean rift in Iceland than on the
surrounding ocean floor. While intriguing, the paper neglects to
address the significant range (18–137%) in calculated excess drift
for individual central volcanoes. Further, the author assumes
spreading rate is proportioned symmetrically to both sides of the
active rift axis. Looking closely at the age distribution of central
volcanoes in Fjordland and easternmost Iceland, it becomes apparent that at least in the mid Miocene, spreading was asymmetric
and mostly taken up by the North American plate. Walker’s original contention of excess spreading rate across part of Iceland could
be consistent with the Northern Atlantic spreading rate if most of
the new crust is formed west of the northern and eastern rift axes.
On a positive note, therefore, the data compiled in this chapter
may resolve a longstanding issue of apparent inconsistencies between spreading rates in Iceland compared to the surrounding
Mid-Atlantic ridge.
In the forth paper, Gudmundsson et al. use numerical modelling
to show the ‘‘Effects of dyke emplacement and plate pull on
mechanical interaction between volcanic systems and central volcanoes in Iceland”. They begin with a review of the concept of Iceland’s volcanic systems. After describing the nature of Icelandic
dyke swarms, they discuss the theoretical background of the buildup of overpressure during dyke emplacement, and then show
numerically that mechanical interactions between closely spaced
volcanic systems due to dyke emplacement and plate pull may
be common, using the Reykjanes Peninsula as an example. Further
fieldwork on fault orientations and slip directions in the area will
be required to substantiate these findings. The second part of the
paper considers interactions between central volcanoes in Central
Iceland on basis of plate pull, and shows that simultaneous
seismogenic faulting and shared dykes, as observed there, are
predicted from the regional stress field. Unfortunately, the second
part of the paper is essentially a slightly simplified copy of work
published by some of the same authors in 2007, and does not
add any new insights.
The fifth paper, by Burchardt and Gudmundsson, focuses on
‘‘The infrastructure of the Geitafell Volcano, Southeast Iceland”,
which represents a deeply eroded Tertiary central volcano. The
upper parts of the gabbroic magma chamber are presently exposed. The study impresses with a large number of field measurements of cone sheets, joints and mineral veins, from which the
evolution of the local stress field through time is inferred. Again,
a numerical modelling approach is taken to explain the findings.
A brief discussion compares and contrasts Geitafell with other deeply eroded volcanoes in Iceland and elsewhere. The study provides
some insights into magma transport in the upper subvolcanic
plumbing system of Iceland’s central volcanoes, and into the development of geothermal systems during the final stages of
magmatism.
The book concludes with ‘‘A pictorial summary of the life and
work by George Patrick Leonard Walker”, prepared by Rowland
and Sparks based on a series of posters that were displayed during
the Walker symposium on Advances in Volcanology in June 2006
in Iceland to commemorate Walker’s life and work. Many of these
illustrations are photos of George Walker at work in the field, but
there are also some stunning examples of Walker’s hand-drawn
field sketches, geological maps, diagrams, and classroom posters,
which are well worth a close look.
In summary, although some minor typographical and grammatical errors could probably have been avoided, the editors of this
book have done an excellent job in compiling a number of insightful papers on volcanological phenomena. In terms of research
themes addressed, the section on lava flows is dominated by the
two very detailed studies from Surtsey, but these do have some
wider implications. The section on explosive volcanism provides
a very balanced overview of different eruption styles and methods
of investigation, with examples taken from many parts of the
world. The section on volcanic provinces begins with a theoretical
study, followed by a global perspective, but then is again dominated by three papers dealing with Iceland. The relatively large
number of studies on Icelandic topics clearly reflects George Walker’s lifelong interest in Icelandic geology. However, most papers
are based on very detailed observations and do not require
advanced mathematical or numerical skills to be useful to the
reader. Therefore, the volume should be of interest to advanced
undergraduates and graduate students of volcanology. Intriguingly, ‘‘Studies in Volcanology” shows that a detailed understanding of many processes can be gained by employing purely
volcanological methods, without much use of data derived through
advanced geochemical or geophysical analytical methods. As such,
this volume will also be excellent reading material for professional
geochemists and geophysicists with interests in volcanic systems,
because it will provide them with another dimension of relevant
observations. My high recommendations for this book are probably
best articulated by stating that there is no doubt George Walker
himself would have enjoyed reading the papers in this comprehensive and beautifully illustrated volume.
Georg F. Zellmer
Institute of Earth Sciences,
Academia Sinica, 128 Academia Road Sec. 2,
Nankang, Taipei 11529,
Taiwan
E-mail address: [email protected]