Table Of ContentH. S. YODER, JR.
GEOPHYSICAL LABORATORY
Carnegie Institution of Washington
Washington, D.C .
•
i o n
NATIONAL ACADEMY OF SCIENCES
Washington, D.C. 1976
Library of Congress Cataloging in Publication Data
Yoder, Hatten Schuyler, Jr .. 1921-
Generation of basaltic magma.
Based on a lecture series presenced at Dartmouth College. May 5-9. 1975. and at the
Geophysical Laboratory of lhe Carnegie Institution of Washington. June 23-27. 1975.
I. Basalt-Addresses. essays. lectures. 2. Magmatism- Addresses. essays, lectures.
I. Title.
QE462.B3Y6 552'.2 76-29672
International Standard Book Number 0-309·02504-4
Availablt• from
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79 78 76 10 9 8 7 6 5 4 3 2
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Preface
A bequest was made by Dr. Arthur L. Day to the National Academy of
Sciences '"for the purpose of advancing studies of the physics of the
earth." The Arthur L. Day Trust Fund Committee* established the
Arthur L. Day Prize and Lectureship with the expectation that the re
cipient would prepare a publication, preferably a book, giving a "com
prehensive summary and synthesis of current knowledge in his field.''
It was the writer's good fortune to be chosen by the Selection
Committeet as the first recipient of the Arthur L. Day Prize and
Lectureship, and the opportunity to prepare a book on a subject of his
own choosing was indeed a welcome prospect. It had long been his
desire to take the time to think through the details of the process of
magmat generation. Most studies assume the existence of magma and
then consider its transport and crystallization; its actual formation is
rarely treated at length.
The principal problem was finding sufficiently long periods of time in
\Vhich to analyze the data and synthesize the views. The bits and pieces
*Trust Fund Committee: William W. Rubey. chairman; Richard M. Goody; Merle A.
Tuvc; J. Tuzo Wilson.
tSelection Committee: Herbert Friedman. chairman; Jule G. Charney; Allan Cox; Julian
R. Goldsmith; 0 . G. ViUard.
tAs used by geologists. magma can be defined as a naturaUy occurring. mobile liquid
within the earth that may contain suspended crystals or rock fragments as well as
dissolved or exs olved gases and that can form a rock, crystalline to glassy. on intrusion
or extrusion as a lava .
.
..
111
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Preface
1v
were pulled together over a 2-year period and presented as an inte
grated lecture series at Dartmouth College on 5-9 May 1975 and at the
Geophysical Laboratory of the Carnegie Institution of Washington on
23-27 June 1975. The writer is indebted to Professor Robert W.
Decker. Chairman of the Department of Earth Sciences at Dartmouth
College, for organizing the visit and to the students and staff for their
thought-provoking questions-and answers. The staff of the Geophysi
cal Laboratory provided a stimulating and critical audience for what
were intended to be provocative discussions. Their response, substan
tiated by their willingness to undertake some of the suggested research
problems, was most gratifying.
The book is directed to those who would like a comprehensive view
of the generation of basaltic* magma at the site of origin.
The writer's purpose was to bring to a focus the many problems
amenable to theoretical analysis and experimental investigation. It is
evident that direct field study is not possible now. but it cannot be
considered as impossible in the near future. The Magma Tap Program,
being developed by the Energy Research and Development Adminis
tration and the United States Geological Survey in an effort to meet
critical energy needs. may present opportunities for the direct observa
tion of at least auxiliary magma chambers. The large uncertainties in
the deductions presented here will be evident to the reader. and it
would be an understatement to say the points of view are somewhat
controversial. If these views stand for any length of time, then the
attempt to stimulate new researches on the subject will have been for
naught.
The reader will gain an impression of !he tenuous nature of the
evidence on which major concepts about magmas in the earth are
based. The postulated models have become accepted as the reality
instead of the lattice of assumptions they are. Authoritarianism domi
nates the field, and a very critical analysis of each argument is to be
encouraged. The structure of facts is minimal-the reader should
continuously ask, "What is the evidence?" It is recommended that he
withhold judgment on an issue until completing the book. Persuasive
arguments in one section may be countered with equal vigor in another
•Basalt is a mafic. extrusive igneous rock composed chiefly of plagioclase (usually
labradorite) and clinopyroxene. Varieties may contain in addition olivine. ortho
pyroxene, ncphcline. and quartz. individually or in restric1ed combinations. The rock
may be glassy. fine-grained. or porphyritic. Apatite and magnetite are common acces
sories. Basall is in some places inlrusive in the form of dikes; its intrusive equivalent.
however, is generally called diabase (dolerite) or gabbro. The average chemical com·
positions of lhe common varieties of basall have been given by Nockolds ( 1954).
Manson ( 1976), and Chayes ( 1975).
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Preface v
section. Not all arguments can be presented, and some are omitted,
although referenced, because they are based on experiments since
shown to be inaccurate or on assumptions now believed to be unwar
ranted.
The book is organized about the what, where, how, why, and when
of magma production. The lectures included an examination of the
critical parameters and principles involved in the generation of the
most common magma, basaltic in composition.* that reaches the
earth's sutface. Constraints are placed on (a) the nature of the parental
material and the location of melting in the earth; (b) the melting
process, including the heat requirements and sources; (c) the me
chanics of magma accumulation, segregation, and rise; (d) the physical
chemistry of the evolving magma; (e) the tectonophysics of melting;
and (t) the energetics, periodicity, and duration of magmatic events.
The substance of the lectures has been expanded somewhat to meet the
needs of students having only a general course in petrology.
There is no attempt to discuss subsidiary or auxiliary magma cham
bers and the associated differentiation processes taking place there, the
eruptive mechanics, or the morphological expression of volcanic prod
ucts. These subjects have been adequately dealt with in texts on
volcanology and petrology.
No apologies are made for the breadth of subject matter covered in
the book. The writer is obviously not an expert in all the subjects
discussed, but who is? An appreciation and awareness of the many
exciting facets of magma generation are sought.
It is a pleasure to acknowledge the labors of Miss Marjorie Imlay,
who typed drafts and made alterations cheetfully and with patience;
Mr. A. David Singer. who organized the drafting, photography, and
assembly of the many figures; Mr. William C. Hendrix, who drafted all
the figures; and Miss Dolores Thomas, who edited the copy submitted
to the National Academy of Sciences and collated the references. An
early draft was reviewed by Drs. P. M. Bell (Chapters 1-6) and Bjtllm
Mysen and Mr. H. Richard Naslund. More advanced drafts were
reviewed completely or in part by Dr. Nicholas T. Arndt, Dr. Felix
Chayes, Prof. R. W. Decker (Chapters 9 and 10), Prof. John S. Dickey.
Dr. Hugh C. Heard (Chapter 9), Dr. Albrecht Hofmann, Prof. Hans G.
Huckenholz. Dr. T. Neil Irvine. Dr. Ikuo Kushiro, Prof. John B.
Lyons, Prof. Alexander R. McBirney, Prof. S. Anthony Morse, Dr.
*Basalls and related rocks (gabbros. amphibolites, and eclogites) comprise about 42.5
percent of the volume of the crust of the earth, according to Ronov and Yaroshevsky
(1969. p. 49. Table 7). The base of the crust is taken by them to be the M (Mohorovicic)
discontinuity.
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vi Preface
Dean C. Presnall (Chapter 6), Dr. Eugene Robertson, Prof. Jean-Guy
Schilling, Dr. Jan A .. Tullis (Chapter 9), and Dr. Danielle Velde. The
many hours invested by these kind friends have led to a much im
proved manuscript. The debates with these and other specialists and
generalists too numerous to list have been most enjoyable and reward
ing. As every author knows, the writing of a book on a new
subject is a most exhilarating learning experience. All contributors are
thanked for their help and tuition.
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CONTENTS
INTRODl!CTION I
Volcanic Products- A Renewable Resource, I
Systematic Studies Needed- Violent Volcanoes, I
Observed Crystallized Magma Chamber- Transported
Magmas, 2
Models of Magma Sources-A Prologue, 2
Balloon/Soda-Straw Model, 3; Geophysical Model. 4; Crystal-Mush
Model, 4; Benioff-Zone Model, 5; Depth-of-Generdtion Model, 7; Depth
of-Separation Model, 8; Explosive-Diatreme Model. 9; Conclusion, 11
2 12
PARENTAi. MATERIALS
Principal Requirement-Yield Basaltic Magma, 12
Parent- Daughter Test, 13
Assumptions for Test. 15; Basalts Equilibrated to Surface Conditions, 16;
General Types of Source Materials, 18
Basaltic or Gabbroic Source, 19
Eclogitic Source- High-Pressure Equivalent of Basalt, 20
Amphibolitic Source, 22
Olivine-An Important Source-Rock Constituent?, 24
Meteoritic Source, 24
Important Phases in Meteorites. 25; Phase Incompatible wilh Olivine
at High Pressure. 27; Meteorites Related to Peridotite?, 29
Gamet Peridotites from the Upper Mantle, 30
Key Position of Gamet Peridotite. 31; Will Garnet Peridotite Yield
Basaltic Magma?. 32
..
VII
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VIII CONTENTS
Meteorites Do Not Yield Basaltic Magma, 34
Phase Equilibria Support for Gamet Peridotite Source, 36
Source of Potassium?. 37
Seismic Velocities of Garnet Peridotite Suitable?, 38
Monte Carlo Test of Physical Parameters, 41
Composition(s) of the Mantle Not Determined , 42
Gamet Peridotite Accepted as Source- Tentatively, 42
3 DE PT H O F MELTING 44
Origin of Protoearth: Cool Accumulation, Differentiated
Melt, or Continuum of Both?, 44
Crust Formation through Volcanism. 46
Impacting, Deep Surficial Turnover, and Partial Melting, 48
Magma- A Residuum from Earth Formation?, 49
Seismic Evidence-Auxiliary Magma Chambers. 49
Low-Velocity Zone- Evidence· of Partial Melting?, 51
Partially Melted Zone-Experimental Evidence, 52
Attenuation of Seismic Waves, 55
Summary, 55
.
4 MELT I NG PROCE SS ES 57
Multitude of Mechanisms, 57
Melting Behavior of Critical Minerals, 57
The Mechanisms, 60
Stress Relief, 61
Tension, 61; Compression, 61
Thermal Rise to Cusp in Melting Curve, 63
Compositional Cusp. 63; Phase-Change Cusp. 64
Convective Rise, 65
lsentropic Rise. 65; lsenthalpic Rise, 66
Parameter Perturbation, 66
Thermal Conductivity Perturbation. 66; Density Perturbation, 68
Mechanical Energy Conversion to Heat, 71
Thrust Faulting, 72; Subducting Plate. 72; Propagating Crack, 73;
Regenerative Feedback, 74; Tidal Dissipation, 75
Compositional Change, 76
Diffusion, 76; Volatile Fluxing. 78; Other Mechanisms. 86
Phenomenological View of Melting, 86
5 88
TH ERM AL ENERGY REQUIREM ENTS F O R M EL TI NG
Enthalpy of Melting, 88
Measurement. 89; Previous Estimates. 90
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