ICARUS 52, 365-372 (1982)
Magma Vesiculation and Pyroclastic Volcanism on Venus 1
JAMES B. GARVIN*, JAMES W. HEAD*, AND LIONEL WILSON*.t
*Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, and tLunar and
Planetary Unit, Department of Environmental Sciences, University of Lancaster, Lancaster, LAI 4YQ,
United Kingdom
Received January 11, 1982; revised August 10, 1982
Theoretical consideration of the magma vesiculation process under observed and inferred venusian surface conditions suggests that vesicles should form in basaltic melts, especially if CO2 is the
primary magmatic volatile. However, the high surface atmospheric pressure (~90 bars) and density
on Venus retard bubble coalescence and disruption sufficiently to make explosive volcanism unlikely. The products of explosive volcanism (fire fountains, convecting eruption clouds, pyroclastic
flows, and topography-mantling deposits of ash, spatter, and scoria) should be rare on Venus, and
effusive eruptions should dominate. The volume fraction of vesicles in basaltic rocks on Venus are
predicted to be less than in chemically similar rocks on Earth. Detection of pyroclastic landforms
or eruption products on Venus would indicate either abnormally high volatile contents of Venus
magmas (2.5-4 wt%) or different environmental conditions (e.g., lower atmospheric pressure) in
previous geologic history.
INTRODUCTION
The Pioneer/Venus mission global altimetry map of Venus revealed a number of
topographic features for which a volcanic
origin has been proposed (Masursky et al.,
1980). In addition, consideration of mechanisms of lithospheric heat transfer on Venus suggest that volcanism should be an important process (Solomon and Head, 1982;
Phillips et al., 1981). Thus it is important to
consider the suite of possible eruptive processes that might characterize volcanism
on Venus. The first-order effects of the
present Venus environment on eruption
styles are potentially significant, as described qualitatively by Wood (1979). The
high surface temperature (700-800°K), the
wide range of large surface atmospheric
pressures (55-95 bars), and the dense CO2
atmosphere (35 to 70 kg/m 3) are influential
environmental factors. In considering the
nature of volcanism on any planet, it is
meaningful to examine those processes
' Paper presented at "An International Conference
on the Venus Environment," Palo Alto, California,
November 1-6, 1981.
which directly influence the dynamics of
volcanic eruptions (Wilson and Head,
1981). Magma vesiculation influences the
explosiveness of an eruption (Wilson et al.,
1980), and we focus here upon the vesiculation process and its potential role in pyroclastic volcanic processes on Venus (Garvin et al., 1982). Theoretical as well as
observational constraints are placed on the
dynamics of a range of possible eruptions
on Venus, with special emphasis on explosive ones because of the characteristic
landforms and particle transport styles they
can produce (i.e., cinder cones, ash clouds,
and ignimbrite sheets).
ERUPTION STYLES
Eruption style on any planet is governed
by a combination of interrelated factors
such as m a g m a volatile content, which can
be defined as the concentration of dissolved
gas species in a magma and which critically
influences vesiculation and dominates the
eruption velocity in the vent; m a g m a c o m position, which most strongly affects
magma rheology, and which encompasses
the volume fractions of a magma composed
of bubbles, crystals, and melt; tectonic set365
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Copyright© 1982by AcademicPress, Inc.
All rightsof reproductionin any formreserved.
366
GARVIN, HEAD, AND WILSON
ring, which influences the state of stress in
the lithosphere, the depths at which magmas are stored and the widths of the conduits through which they erupt to the surface; and external environment, which
includes such factors as surface atmospheric pressure, temperature, and gravity.
Of these factors, the magma volatile content is the most critical in affecting the explosivity of an eruption (Sparks, 1978;
Wilson et al., 1980; Wilson and Head,
1981). Once the volume fraction of gas bubbles in a magma exceeds about threefourths, there is a strong tendency for the
magma to disrupt (Sparks, 1978). At magma
rise speed, Vr, greater than about 1 m/sec, a
steady discharge of gas and pyroclasts occurs (Wilson and Head, 1981), while at
lower rise rates, such as are typical in basaltic magmas, coalescence of many small
bubbles into a few large bubbles causes intermittent strombolian explosions (Blackburn et al., 1976; Wilson and Head, 1981).
Since the process of bubble formation can
modify the effective viscosity and rise rate
of the magma, it can exert an important
control on the eruption style in the v e n t - - i f
the bubbles are predominantly very small
the magma will have a reduced viscosity
and hence a greater tendency to flow,
whereas larger bubbles, such as those produced by extended periods of growth and
significant coalescence of smaller bubbles,
will tend to increase magma viscosity and
reduce the extent of lava or magma deformation (H. Pinkerton, personal communication, 1982). Vesicles in volcanic rocks
can serve as preserved "snapshots" of preeruption bubble-rich magma, and can provide information on magma volatile content, rise rate, and primary volatile
composition, and possibly the bubble coalescence and deformation history of the
magma. Our approach is to consider typical
Venusian basaltic eruptions and their possible vesiculation histories on the basis of the
theory of bubble growth and dynamics. The
objective is to predict the nature and behavior of volatiles in Venusian magmas in
terms of the composition and abundance of
the volatiles in magma storage regions as
well as in erupting Venusian lavas. A secondary aim involves predicting the plausibility of magma disruption leading to pyroclastic or fire fountain eruptions on Venus
in terms of a range of possible environmental conditions including those typical of the
present.
MAGMA VESICULATION
The process of magma vesiculation involves the exsolution of dissolved volatiles
such as H20 or CO2 from a magma to produce gas bubbles. As magmas containing
dissolved gas species ascend from their
storage regions, gas bubbles nucleate and
grow by both mass transfer (diffusion) and
decompression due to the decrease in local
lithostatic pressure. Vesicles are simply gas
bubbles " f r o z e n " in solidified magma, and
often reflect the pre-eruption bubble distribution and thus volatile content of a
magma. Vesiculation is most strongly controlled by the concentration and type of primary volatile species in the magma
(Sparks, 1978). On Earth, measurements
indicate that water and CO2 are the gas species which predominantly drive gas bubble
formation and growth in basaltic magmas
(Moore, 1979). Very soluble volatiles such
as water produce different vesiculation patterns (i.e., average vesicle size and vesicularity) than relatively insoluble ones such as
CO2 or CO (Sparks, 1978). The presence of
multiple volatile species in magmas (i.e.,
water and CO2), although potentially important, will not be considered in detail in
this paper. However, it is likely that bubble
nucleation commonly occurs on Earth at
depths too great for any water exsolution,
so that COz or some other species must be
responsible for initial bubble formation. In
near-surface regions within a few kilometers of the surface, water would begin to
exsolve, and already-formed CO~ bubbles
could serve as nucleation centers for waterrelated bubble growth and dynamics. On
Venus, volatiles present in magmas stored
MAGMA VESICULATION ON VENUS
in the lower lithosphere could lead to a
small amount of vesiculation in an erupting
magma which would not subsequently be
affected by water-related exsolution in
near-surface regions, because of the apparent paucity of water in Venus and in its
atmosphere (Ahrens, 1981; Goettel et al.,
1981). Thus one might expect to find a relatively uniform distribution of small (submillimeter) vesicles in Venusian lavas.
Information on the concentration of K,
U, and Th from the ~/-ray spectrometers on
the Venera 9 and I0 spacecraft which
landed on the flanks of Beta Regio (>4.5
km relief) in 1975 suggests that basaltic
rocks and their weathering products are the
predominant materials present at the two
localities. In March 1982, the Venera 13
and 14 spacecraft made major-element
chemistry analyses of materials at two new
sites on Venus using an X-ray fluorescence
spectrometer. These measurements are
most consistent with mafic igneous rocks
with variable amounts of K20. Therefore,
there are data which support the existence
of basaltic rocks comparable to those found
on the Earth at four sites on Venus. On this
basis, it is reasonable to use data on terrestrial basalts when modeling the growth and
dynamics of bubbles in Venusian lavas.
THEORETICAL MODEL
The critical factors in bubble growth in a
magma on any planet are gravity (g),
magma density (p), magma temperature (T),
surface atmospheric or ambient pressure
(Ps), magma viscosity (-q), magma rise
speed (Vr), magma surface tension (at), and
the weight fraction (n) of a volatile dissolved in the magma as a function of depth
(or lithostatic pressure) in the planet. When
a magma becomes supersaturated in a given
volatile species, bubbles nucleate with
diameters of a few micrometers and grow
initially by diffusion. Later, as the magma
nears the surface, the external pressure decreases and bubbles grow by decompression as well as diffusion. A magma/bubble
foam disrupts when the bubbles occupy
367
about three-fourths of its volume, and the
pressure, Pa, at which disruption occurs
can be determined as a function of the
magma density and temperature. The disruption pressure and the depth at which it is
reached therefore control whether effusive
or explosive eruptions will take place
(Wilson et al., 1980). If Pd < Ps, then bubble growth cannot have produced enough
exsolved gas to disrupt the magma, and quiescent, effusive eruptions of vesicular
magma will occur. We can compute Pd by
tracing the bubble volume fraction (BVF) in
an ascending magma which in turn can be
calculated from the instantaneous pressure
(Pb) in the bubbles. These pressures can all
be determined from a simulation model involving bubble growth and dynamics. The
model enables us to predict the variation
with depth of the bubble diameter and volume fraction in magmas with a specified
volatile content and ascent rate and is an
enhancement of a computer program originally developed by Sparks (1978) and applied to the Moon by Wilson and Head
(1981). The program solves the classical
Rayleigh-Plesset equation for spherical
bubble dynamics in an incompressible fluid.
In this second-order differential equation,
the pressure inside a bubble of radius R (as
a function of time t) is balanced against the
surface tension pressure (2~/R), the inertial
pressure (p[RR + ] R2]), and the dynamic
pressure due to viscous forces tending to
retard bubble expansion (4"q/~/R), as well as
local hydrostatic pressure Ph (Ph = pgz +
Ps). In the basic equation,
2or
4rl/~
Pb = Ph + ~ + p[R/~ + ~/~2] + ---R---' (1)
the pressure in the bubble, Pb, is balanced
against the four other pressure terms to ensure conservation of momentum at the bubble wall. Note that/~ represents dR/dt, and
/~ is d2R/dt 2. In essence, the pressure in a
bubble must exceed the local hydrostatic
pressure of the surrounding magma as the
magma ascends to levels of lower hydrostatic pressure. The other pressure terms
368
GARVIN, HEAD, AND WILSON
cend from nucleation depths less than - 5 0
km and avoid excessive magma cooling
(Wilson and Head, 1981); the upper limit is
the rise speed associated with the highest
effusion rates yet deduced for terrestrial basaltic eruptions (Wilson and Head, 1981).
The following material properties were selected as typical of basaltic magmas after
initial trials showed that large variations in
the values adopted had a negligible effect
on the sizes and volume fractions of vesicles deduced: ~q = 300 Pa sec, p = 3000
kg/m 3, T = 1200°K, ~r = 300 kg/sec 2, a
supersaturation pressure of 2 bars, and
a diffusion coefficient of 1.5 x 10 -1° m2/sec
(Sparks, 1978).
appear in the equation because bubble
growth is governed by the density and viscosity of the enclosing magma as well as the
interfacial tension (Sparks, 1978; Prosperetti, 1982). The simulation model uses a
finite difference code for solving the basic
equation (1) for the bubble radius as a function of time, and includes a scheme devised
by Wilson and Head (1981) for enabling
bubbles to coalesce. Due to limited solubility data as a function of depth or lithostatic
pressure, only water, CO2, and CO can be
considered in the computation. A simulation will continue until the magma disrupts,
that is, BVF exceeds three-fourths, or until
the planetary surface is reached.
The model was run extensively for a wide
variety of possible conditions on Venus:
surface pressures of 55-95 bars, temperatures of 700-800°K, magma rise rates from
0.03 to 1.0 m/sec, and volatile contents
from 0.015 to 10.0 wt% (for H20 and CO2).
We have assumed that CO2 or H20 are the
primary magmatic volatiles on Venus, on
the basis of terrestrial observations (Moore,
1979) and theoretical constraints (Ahrens,
1981). The range of plausible rise rates for
basaltic magmas on Venus is 0.03 to - 3 . 0
m/sec: the lower limit is set by the lowest
speed at which a basaltic magma could as-
MODEL
RESULTS
Table I shows the depths at which bubble
nucleation and magma fragmentation will
occur in erupting magmas on Venus and
Earth for three magma/volatile combinations. CO2, being relatively insoluble, nucleates at much greater depths than H20 on
both planets. On Earth, magma fragmentation must occur, at or below the surface, if
the total volatile content in a basalt exceeds
- 0 . 1 wt%. H o w e v e r , on Venus the volatile
weight fraction must exceed - 2 . 5 wt% for
H20 and - 4 wt% for CO2 if true explosive
TABLE
I
VALUES OF THE DEPTHS (IN METERS) AT W H I C H BUBBLES NUCLEATE AND ERUPTING MAGMAS DISRUPT
INTO PYROCLASTS ON EARTH AND VENUS FOR THREE MAGMA/VOLATILE COMBINATIONS AND A RANGE OF
MAGMA VOLATILE CONTENTS
EaCh
Total volatile content (%)
H20 in
basalt
H20 in
rhyolite
COs in
any magma
1
Venus
5
3
0.3
0. I
5
3
Nucleation
depth (m)
Fragmentation
depth (m)
8,200
3,400
815
140
27
9,000
4,500
678
387
116
28
6
--
460
105
--
Nucleation
depth (m)
Fragmentation
depth (m)
5,000
1,800
200
15
--
--
5,800
1,800
--
546
287
65
7
--
--
299
Nucleation
depth (m)
Fragmentation
74,000
44,000
15,000
4,400
1,500
440
336
196
62
23
3
depth (m)
Note. A null entry indicates that the appropriate event does not take place.
0.03
2
--
90,000
42
1
630
--
--
54,000
18,000
--
--
MAGMA VESICULATION ON VENUS
eruptions are to occur; furthermore,
magma must ascend from a depth of at least
- 4 km for H20 or - 7 0 km for CO2 if the
magma is to contain the required amount of
the volatile. Thus explosive eruptions producing steady fire fountains are not likely to
occur in Venusian basaltic magmas containing what would be regarded on Earth as
plausible amounts of CO2 and H20. However, there is still considerable uncertainty
about the chemical composition of Venusian rocks; even if H20 is not present in
large amounts in the Venusian magma
source areas (Ahrens, 1981; Goettel et al.,
1981), other relatively soluble volatiles
might play an equivalent role. For example,
a significant proportion of high-K basalts on
Earth with compositions similar to the
rocks at the Venera 13 landing site contain
at least 1 wt% total of the halogens C1 and F
(R. Macdonald, personal communication,
1982) as well as showing evidence of having
been in contact with a CO2 phase during
their ascent to the surface (Scott, 1982).
Figure 1 shows the maximum size of CO2
bubbles in magmas on Venus and Earth as a
function of total COz content for three
magma rise speeds through the crust: 0.03,
0.3, and 3 m/sec. Reference to Table I
shows that, for the range of COz contents
used in Fig. 1, all of the terrestrial eruptions
depicted would be explosive, so that the
bubble sizes refer to the vesicle sizes in
ejected pyroclasts. In contrast, n o n e of the
Venusian eruptions would be explosive
(apart from certain cases where mild strombolian activity could o c c u r - - s e e below)
and the bubble sizes refer to vesicles in effusive lava flows. Figure 2 shows the vesicle volume fraction in such lavas on Venus
as a function of total CO2 content and it is
clear that a wide range of values is possible.
The very large values of bubble diameter
shown in Fig. 1 for basalts rising at low
speeds are the consequence of bubble coalescence (Wilson and Head, 1981). The operation of the coalescence process is associated on Earth with the intermittent
strombolian eruption style (Blackburn et
i
369
/
//
//
/I
I
~_~
Imrn
~
o lmm
0.05
~ '~
0.1
TOTAL
I
0.5
0.2
W E I G H T % CO 2 IN
I
I
I
2
MAGMA
FIG. 1. The maximum sizes of gas bubbles and,
hence, vesicles, in magmas reaching the surface on
Venus (9) and Earth (~)) are given as a function of
total magma CO2 content and magma rise velocity
through the crustal conduit system. All of the combinations of parameters illustrated lead to explosive
eruptions on Earth, whereas none of the combinations
lead to steady explosive eruptions on Venus. Thus the
bubble sizes refer to pyroclasts on Earth and lava
flows on Venus. The range of magma rise speeds given
spans the entire range which is geologically plausible
(see text).
1976). Mild strombolian explosions
could occur in Venusian basalts over a wide
range of conditions: (1) in low-rise speed,
high-volatile content magmas as a result of
the large rise velocity through the liquid of
large bubbles produced by coalescence, or
(2) in high-rise speed, low-volatile content
al.,
IOO
i
0.01
0.03
TOTAL
0.1
0.3
I
W E I G H T % CO 2 IN M A G M A
FIG. 2. The vesicle volume fraction of an erupting
lava on Venus is given as a function of the total magma
CO2 content. An explosive eruption would occur for a
CO2 content greater than 4 wt% as the vesicle volume
fraction exceeded about three-fourths.
370
GARVIN, HEAD, AND WILSON
magmas when small bubbles disrupt
through the surface of the lava in the vent
due to the presence of internal pressures
significantly in excess of the local atmospheric pressure. Our simulation program
predicts these excess pressures and we calculated ejecta velocities from the pressures
using the methods described in Blackburn
et al. (1976), Wilson (1980), and Wilson and
Head (1981). Figure 3 shows the maximum
ejecta velocities as a function of total
magma CO2 content and magma rise speed
for Venusian eruptions. It is important to
note that the large ejecta velocities from
high-volatile content magmas (to the right
in Fig. 3) apply to bubbles which are at least
0.1 to 1 m in size and so the sizes of the
pyroclasts produced will be in this range or
somewhat smaller (Wilson and Head,
1981). Even in the absence of atmosphere,
clasts will not be expelled to ranges larger
than a few tens of meters. H o w e v e r , the
large aerodynamic drag forces exerted by
the present Venus atmosphere on the much
I 00
3O
q
E
10
0
3
0.3
03
O.Ol
I
I
I
I
0.03
0.1
o.3
I
TOTAL WEIGHT % CO2 IN MAGMA
FIG. 3. Maximum ejecta velocities in strombolianstyle explosions from basaltic magmas on Venus,
given as a function of total magma CO2 content and
magma rise speed through the crustal conduit. At low
magma volatile contents, the sizes of ejected fragments would be so small that atmospheric drag would
restrict their ranges to a few meters (see text). At high
magma volatile contents and low magma rise speeds,
the ejected fragments would have sizes up to the order
of a meter and could be ejected to a few tens of meters
range.
smaller pyroclasts produced by the bursting
of the millimeter-size bubbles generated in
low-volatile content magmas (to the left in
Fig. 3) will ensure that these pyroclasts
have only very small ranges on Venus (the
order of a few meters).
At sufficiently high magma rise speeds
such that bubble coalescence is unimportant, Fig. 1 shows that bubbles in Venusian
magmas will be about an order of magnitude smaller than bubbles in terrestrial magmas with the same volatile content if the
volatile content is small, but will be nearly
the same size when the volatile content is
large. Thus, whereas vesicles in terrestrial
basalts commonly have sizes in the range
1-100 mm (Sparks, 1978), the range for Venusian basalts may commonly lie between
0.1 and 100 mm. Some of the features seen
on plate-like rocks of probable basaltic
composition photographed on Venus at the
Venera 10 landing site have the appearance
of dark spots or depressions and one possible interpretation of these features is that
they are vesicles (Garvin et al., 1982). If so,
their sizes (which average about 15 mm) lie
comfortably within the range calculated for
Venus. If the identification of the features
as volcanic vesicles were correct, then additional information about flow geometry
and effusion rate could provide insight into
the volatile content involved in the eruption.
The equivalent calculations to those
shown in Fig. 1 were performed for H20 as
the volatile phase on Venus. It was found
that, for a given magma rise speed, 2.5
times more of the volatile is needed to produce gas bubbles of a given size if 1-120 is
used instead of CO2. The equivalent factor
for eruptions on Earth is about 4. As a
result, bubble sizes in Venusian magmas in
which H20 is the dominant volatile would
most commonly lie in the range 0.1 to I0
mm.
It has been suggested (Wood, 1979) that
the properties of lavas erupted on the ocean
floor on Earth at depths between 500 and
1000 m may be good analogs of those of
MAGMA VESICULATION ON VENUS
lavas on Venus since the ambient pressures
(50 to 100 bars) are similar in both cases.
Data on vesicle sizes, vesicle volume fractions, and volatile contents in submarine
basalts have been obtained by Moore et al.
(1977) and Moore (1979), who found that
total COz content commonly exceeds 900
ppm (0.09 wt%) and that lavas contain a
few percent by volume of vesicles with
sizes near 1 mm. We simulated the eruption
at 715 m depth on the ocean floor (pressure
69 bars) on Earth of a Hawaiian tholeiite
containing 900 ppm CO2 and rising at 0.07
m/sec through the crust. Such an eruption
yields a lava with 6 vol% vesicles having a
maximum size of 1 mm, in good agreement
with the observed lava properties. There is,
of course, a wide range of combinations of
plausible magma rise speeds and volatile
contents near the values given above which
can produce the observed vesicularities. If
the same values of total volatile content and
rise velocity are used in a Venus eruption
simulation, the vesicle content is found to
be 4.6 vol% and the maximum vesicle size
to be 1.7 mm, thus confirming that the differences in ambient temperature and planetary gravity between Earth and Venus do
not have a major effect on vesiculation conditions and that terrestrial ocean floor lavas
are good analogs of Venusian lavas, at least
at the moment of eruption. We note, however, that the subsequent rate of cooling of
subaqueous lavas on Earth will be much
greater than that of Venusian lavas and that
this may significantly influence the migration of gas bubbles within a lava flow moving away from the vent (Head and Wilson,
1982).
CONCLUSIONS
Our theoretical bubble growth and dynamics studies show that gas bubbles (and
hence vesicles) with sizes between 0.1 mm
and 0.1 m should be c o m m o n in Venusian
basaltic lavas, provided there are at least
trace amounts of volatiles. There is still a
need for more detailed study of such factors
as heterogeneous bubble nucleation of mul-
371
tiple volatile species in magmas and bubble
interactions in addition to those that lead to
coalescence, but it is unlikely that these
factors will significantly change our conclusions.
To trigger substantial magma disruption
and explosive volcanism under present atmospheric conditions on Venus requires
large (2.5 to 4 wt%) concentrations of magmatic v o l a t i l e s - - m u c h larger than those observed in typical terrestrial basaltic magmas, including those that disrupt. The only
exception to this may be the possibility of
weak strombolian activity producing ejecta
deposits up to a few tens of meters in
diameter around vents. Therefore, if larger
landforms that formed as a result of explosive volcanic eruptions are detected on Venus, then either (i) the landforms predate
the present hot, dense, high-pressure CO2
atmosphere, implying that conditions at the
surface have changed substantially over geologic time, or (ii) volatiles are (or were at
some time) much more abundant in the
source regions of Venusian magmas than
they are at present on Earth.
ACKNOWLEDGMENTS
The authors are grateful to Sue Sharpton for manuscript preparation. This research was made possible
by NASA Grant NGR-40-002-088, of which the authors are most appreciative. Helpful comments by C.
A. Wood and an anonymous reviewer were useful in
revising an earlier version of the manuscript. Constructive criticism by P. J. Mouginis--Mark and M.
Zuber improved the revised manuscript. Special
thanks must also go to H. Hseih for her drafting of the
figures.
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