Download `Pyroclastic Rocks` is the first modern comprehensive treatment of

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683
PYROCLASTIC ROCKS By R. V. Fisher and H-U. Schrninscke, Springer-Verlag (1984),
472 pp., 339 Figures. Price: '$ 54.10.
'Pyroclastic Rocks' is the first modern comprehensive treatment of what such
rocks are and how they are formed.
The book is intended to flrovide a guide
through the bewildering array of volcaniclastic rock types and to point out gaps in
our present Knowledge and thereby stimulate further research.
The term' Volcaniclastic Rocks' (introduced by Fisher in 1961) would have
been perhaps a better title for the book series as it includes a]] clastic volcanic
materials formed by any process of fragm~ntation, dispersed by any kind of transporting agent, deposited in any environment or mixed in any significant proportion
with nonvolcanic fragments. In fact the authors have found it convenient to use the
term' Volcaniclastic rocks' at several places as a general term to include all kinds
of fragmental volcanic material.
Pyroclastic Geology is a complex and interdisciplinary subject that includes
aspects of volcanology, igneous petrology, sedimentology, stratigraphy, geochemistry
and metamorphic petrology. The authors approach the problem of learning
pyroclastic geology with a professionalism (which is sadly lacking in many of our
books). Indeed, the text can be termed as a commentary on the published literature
on the subject (there are 1106 references), illuminated by the authors own field
observations and lab studies of over two decades. The authors have dearly defined
all the terms that they have used and what frequently occur in pyroclastic geology.
This is a notable aspect of this book which wiJ) prove to be of immense help especially for beginners.
There are fourteen chapters. The introductory chapter is a brief one, which
gives distribution, chemistry, and types of pyroclastic accumulations-pyroclastic and
hydroclastic. Chapter 2 gives a brief account of volcanoes, volcanic rocks, tbeir
tectonic setting and zonation in magma chambers and Chapter 3 deals with magmatic volatiles-H 20, CO 2 and S, rheological behaviour of materials and answers questions such as, what causes explosive eruption and what caus'es fragmentation of
Java, vesiculation and ash formation. Chapter 4 is devoted to the discussion of
pyroclastic eruptive systems, energy involved, mechanisms and products under the
four main traditional names of eruptions: Plinian, Hawaiian, Strombolian and
Vulcanian. Distinguishing features of Strombolian and Hawaiian type eruptions
and the resulting pyroclastic deposits are provided. Particularly noteworthy is the
description of eruption columns.
Chapter 5 deals with pyroclastic fragments and deposits. Terms such as pyroclastic fragments or 'Pyroclasts', hydroclastic, epic1astic, autoc1astic, alloclastic,
volcaniclastic and tephra are clearly defined; classification of pyroclastic deposits
and mixed rocks are given and the bedform structures, textural and mineralogical
characteristics are discussed, with sufficient number of illustrations. This Chapter
gives the background information necessary for understanding the discussions on
pyroclastic deposits produced by different types and environments of volcanic activity, modes of material transport and depositional environment (Chapters 6 to 11).
Techniques have been described for identification of the environment of deposition and modes of material transport of ancient deposits. The Chapter on pyroclastic flow deposits (Chapter 5) has been dealt with at length.
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Dealing with deposits of hydroclastic eruptions (Chapter 9), the authors proceed with the comment that the importance of external water in explosive eruptions
is underestimated. The name' hydroclastic', used by the authors a1 most in parallel
with pyroclastic, is defined as a term for products of hydroexplosions which are
defined as . explosions due to steam from any kind of water'. A brief review is
provided of the terms involved in the descriptions of hydroclastic deposits, charac~
terstic attributes of deposits and different ways in which hydroc1astic eruptions
can OCCUf.
Alteration of volcanic glass in Chapter 12 presents a comprehensive account of
alteration studies including weathering, diagenesis and hydrothermal alteration of
glassy volcanic and volcaniclastic rocks. Alteration of volcanic glass is receiving
attention for a variety of reasons. Hydrothermal alteration of basaltic glass along
mid·ocean ridges may contribute in a major way to the formation of submarine
massive sulphide ore bodies. Zeolite and bentonite deposits are largely formed by
reaction of pyroclastic material with pore water.
Chapter 13 deals with stratigraphic problems of pyroclastic rocks. Especially
useful to geologists dealing with Precambrian terranes is the information provided
under 'Stratigraphic nomenclature in older volcanic terranes and volcanic facies'.
There is a brief review of Archaean greenstone belt volcanoes in Canada on p. 378
which gives information gathered by detailed studies of volcanostratigraphy carried
out in the last decade.
A rather unusual but welcome part of the book is tbe last Chapter (Chapter 14)
which describes pyroclastic and epiclastic volcanic rocks and how they fit into the
plate tectonic frame work and draws main examples from the Cordilleran system of
America. Only a page and a half is devoted to a very brief discussion of the appli.
cability of plate tectonic models to the Precambrian.
A Chapter dealing with the importance of pyroclastic rocks or generally volcaniclastic rocks as host rocks for economic mineral deposits would have been a very
useful addition.
The authors invite critical comments and suggestions about important papers
they have missed. Reference to the paper on Precambrian Volcanoc1astic rocks in
Southern Sweden-a discussion of their identification and classification by Lars
Persson and Kennert Roshoff (I 975) in Sveriges Geologiska Undersokning is missing.
It is not easy to write a comprehensive book on pyroclastic rocks because of
the complexity of the subject. This book serves the needs of all categories of
researchers in the field of volcaniclastic rocks. This may well be described as an
Encyclopaedia of pyroclastic rocks. The book is printed in the high standards of
the Springer-Verlag Publishing Co. and contains 102 excellent photographs out of a
total number of 339 figures.
Departmelll of Mines & Geology,
Bangafore
V. N. VASUDEV