manuscript submitted to
Geophysical Research Letters
Some lava flows may not have been as thick as they
1
appear
2
Jonas Katona
1
,
2
, Xiaojing Fu
1
,
3
, Tushar Mittal
1
,
4
, Michael Manga
1
, and
3
Stephen Self
1
.
4
1
University of California Berkeley, Berkeley, CA, United States
5
2
Yale University, New Haven, CT, United States
6
3
California Institute of Technology, Pasadena, CA, United States
7
4
Massachusetts Institute of Technology, Cambridge, MA, United States
8
Key Points:
9
•
Lava flows can heat and melt underlying flows if the flows are hot enough;
10
•
Superimposed lava flows can merge if erupted in close enough succession;
11
•
Macroscopic structures may not reflect the original flow thicknesses.
12
Corresponding author: Jonas Katona,
jonas.katona@yale.edu
Corresponding author: Xiaojing Fu,
rubyfu@caltech.edu
Corresponding author: Tushar Mittal,
tmittal2@mit.edu
–1–
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Abstract
13
Individual lava flows in flood basalt provinces are composed of sheet p ̄ahoehoe lobes and
14
the 10-100 m thick lobes are thought to form by inflation. Quantifying the emplacement
15
history of these lobes can help infer the magnitude and temporal dynamics of these pre-
16
historic eruptions. Here we use a phase-field model to describe solidification and remelt-
17
ing of sequentially-emplaced lava flows to explore additional processes that may lead to
18
thick flows. We calibrate model parameters using field measurements at Makaopuhi lava
19
lake. We vary the thickness of individual flows and the time interval between eruptions
20
to study the interplay between thermal evolution, flow thickness, and emplacement fre-
21
quency. Our theoretical analysis shows that if the time between emplacement is suffi-
22
ciently short, reheating and remelting may merge sequentially emplaced flows — mak-
23
ing flows appear thicker than they actually were. Our results suggest that fused flows
24
could be another mechanism that creates apparently thick lava flows.
25
Plain Language Summary
26
The observation of thick basaltic lava flows has long been explained by inflation.
27
Here we explore an additional mechanism that could explain the formation of thick lava
28
flows, where a sequence of thinner lobes that are emplaced on top of each other could
29
fuse into one larger flow. Our theoretical analysis suggests the formation of a thick flow
30
by merging can occur if the flows are emplaced relatively close to each other in time.
31
1 Introduction
32
Continental flood basalt (CFB) province eruptions contain the largest (
>
1,000 km
3
,
33
Bryan and Ernst (2008); Self et al. (2014)) and longest (
∼
1000 km; Self et al. (2008))
34
lava flows. Since CFBs are frequently coeval with severe environmental perturbations
35
including mass extinctions, ocean anoxic events, and hyperthermal events (Clapham &
36
Renne, 2019), understanding the physical process and time-scale of flow field emplace-
37
ment would help quantify the release of volcanic gases that have environmental impacts
38
(e.g., CO
2
, SO
2
). Despite decades of work, however, the tempo and style of CFB erup-
39
tions remain poorly quantified.
40
CFB lava flow fields are composed of 5 - 100 m thick dominantly p ̄ahoehoe lobes
41
(Self et al., 1998, 2021). Given the general lack of large lava tubes in CFBs (Kale et al.,
42
2020; Self et al., 1998), the primary process hypothesized for creating thick flows is the
43
formation of p ̄ahoehoe lobes by inflation (Hon et al., 1994). If the quasi-continuous magma
44
flux into individual lava lobes is sufficient, the solidifying surface crust can continuously
45
rise due to increasing pressure (Hon et al., 1994; Hoblitt et al., 2012). If the lateral magma
46
pressure is large enough, the flow can propagate laterally by sporadic breakouts (Hon
47
et al., 1994; Kauahikaua et al., 1998; Hamilton et al., 2020). This process has been ob-
48
served in modern meter-scale Icelandic and Hawaiian lobes (Self et al., 1998). In addi-
49
tion, the lobe structures in CFB flows have similar internal characteristics as Hawaiian
50
inflated lobes (Vye-Brown et al., 2013). The maximal final inflated lobe thickness in Hawai-
51
ian flows, however, is only 10 - 15 m (Kauahikaua et al., 1998), which is smaller than many
52
CFB flows (up to 80-100 m, Puffer et al. (2018); Self et al. (2021)). Furthermore, lava
53
flow inflation has been shown to potentially require pulsating eruptive conditions that
54
may not always be possible (Rader et al., 2017). Thus, a fundamental question remains:
55
how do CFB flows become so thick? This question underlies the broader question: what
56
are the eruptive conditions and fluxes associated with the common
∼
1000 km
3
scale CFB
57
lava flows.
58
In this study, we quantitatively analyze a physical process that can, in addition to
59
flow inflation, lead to apparently thick flows: The final flow is an amalgamation of nu-
60
merous smaller lobes, piled on top of each other quickly enough to remelt the interven-
61
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ing solidified crust (Basu et al., 2012, 2013). We are not concerned here with the amal-
62
gamation of small, predominantly non-inflated p ̄ahoehoe lobes such as those in lava piles
63
where hummocky p ̄ahoehoe or “compound” lavas are forming (e.g., S. M. Baloga et al.,
64
2001; Hamilton et al., 2020) but instead are interested in larger CFB flow lobes. We use
65
simplified magma solidification models to constrain how quickly two subsequent flow lobes
66
must be emplaced to fully merge, thus providing a constraint on CFB eruption tempo.
67
In Section 2, we describe a new phase-field model for lava flow cooling. We then simu-
68
late solidification of a single flow lobe and two sequentially emplaced flow lobes using
69
our model in one dimension. In Section 3, we use our results to outline three distinct regimes
70
(fused, in parallel, in sequence) for inter-lobe cooling. We finally compare our results with
71
observations to assess whether remelting can help explain the thick CFB flows as well
72
as analogous thick flows in other planetary settings. Our results are used to put lower
73
bounds on how quickly CFB flow fields were emplaced in order to preserve multiple lobes
74
within a single flow field.
75
2 A phase-field model of lava solidification
76
2.1 Model equations
77
The phase-field framework is a mathematical approach to describe systems out of
78
thermodynamic equilibrium (Anderson et al., 1998). It was first introduced in the con-
79
text of solidification processes and phase transitions of pure or multi-component mate-
80
rials (Cahn & Hilliard, 1958; Boettinger et al., 2002). The framework evolves the solid-
81
ification front as part of the solution to the system of partial differential equations, avoid-
82
ing the need for explicit tracking of the moving interface as is traditionally done in the
83
Stefan problem (Anderson et al., 1998). Here, we consider a simplified model of lava so-
84
lidification where we track the binary solidification of lava through a phase variable, de-
85
noted
φ
(
φ
= 1 for the melt and
φ
= 0 for the solid phase), and the corresponding tem-
86
perature (
T
). In a phase-field framework, the evolution of
φ
and
T
can be described with
87
the following system of coupled, nonlinear partial differential equations:
88
τ
∂φ
∂t
+
∇·
(
−
ω
2
φ
∇
φ
)
=
−
dg
dφ
−
L
H
(
T
−
T
m
)
T
m
dP
dφ
,
(1)
∂T
∂t
+
∇·
(
−
α
∇
T
) =
L
c
p
dP
dφ
∂φ
∂t
,
(2)
where
T
m
is the melting temperature of the lava,
α
=
kρ
−
1
c
−
1
p
is the thermal diffusion
89
coefficient (
k
thermal conductivity,
ρ
density,
c
p
specific heat),
ω
φ
characterizes the length
90
of the interfacial transition zone,
τ
characterizes the time scale of solidification across
91
the interface,
L
is the latent heat of fusion for lava, and
H
is the energy barrier; see Ta-
92
ble S1 for the values of these parameters used in this work.
∇
is a partial differential op-
93
erator defined in Text S2.
g
and
P
are auxiliary functions of the phase-field model (see
94
Text S1). We impose convective and radiative boundary conditions at the surface, while
95
we impose a fixed temperature at the bottom of the domain. Moreover, we integrate our
96
phase-field equations over a large enough domain such that the lower boundary does not
97
influence the temperature and phase during solidification (see Text S2). Because the hor-
98
izontal dimensions (kilometers) are much larger than the vertical scale (meters) of the
99
flows of interest here, we perform all the simulations in a 1D vertical dimension. Con-
100
sequently, the conductive heat transfer will be primarily in the vertical direction. We pro-
101
vide additional details regarding the numerical schemes in Text S3.
102
2.2 Model validation and limitations
103
To obtain parameter values of the model that accurately characterize solidification
104
of basaltic lava, we adopt typical thermal properties of basaltic melt (Patrick et al., 2004;
105
Audunsson & Levi, 1988; Peck et al., 1977; Wright & Marsh, 2016; Cooper & Kohlst-
106
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edt, 1982; Worster et al., 1993). The phase-field modeling parameters
τ
and
ω
2
φ
are de-
107
rived in terms of measurable quantities in Text S1 using the approach in Kim and Kim
108
(2005), and then calibrated based on field data collected from Makaopuhi lava Lake (Wright
109
& Okamura, 1977; Wright et al., 1972; Wright & Marsh, 2016). The calibration results
110
(Figure S1) show that the model agrees well with lava lake data for a range of param-
111
eters (see Table S1). As an additional test of the model, we use the calibrated param-
112
eters from Makaopuhi lava lake to simulate measurements of inflating p ̄ahoehoe lava flows
113
on the K ̄ılauea Volcano in Hawaii, taken from Figures 8-10 in Hon et al. (1994). The re-
114
sults (Figures S2-S3) show decent agreement at depths deeper than
∼
10 cm below the
115
surface, although there is noticeable disagreement near the cooling surface.
116
These validation efforts demonstrate that, while our model robustly captures macro-
117
scopic cooling of lava across multiple data sets, it lacks accuracy in describing the tem-
118
perature evolution in the uppermost section of cooling lava (
∼
10 cm). One explanation
119
is that the bubbles that accumulate near the lava’s surface (Cashman & Kauahikaua,
120
1997) decrease
α
near the surface (Keszthelyi, 1994). We also neglect the temperature
121
dependence of
α
(Jaupart & Mareschal, 2010). While it is possible to include these ef-
122
fects in the model, it would also introduce additional parameters that are challenging
123
to calibrate. The model presented here, although simplified, still captures the first-order
124
effects of latent heat and thermal diffusion that dominate lava cooling while allowing us
125
to simulate cooling processes spanning from seconds to years.
126
2.3 Setup for lava cooling simulations
127
We use the model to perform two types of simulations. We first simulate solidifi-
128
cation of a single lava lobe of thickness
h
to obtain the total time
t
h
it takes to reach com-
129
plete solidification for a single lobe. The results are used to design the second set of sim-
130
ulations, where we simulate sequential emplacement of two lava lobes of equal thickness
131
h
, separated by a time period of
t
emp
. We consider 17 different lobe thickness
h
, from
132
0.1m to 20m, to explore the behaviors of both thin p ̄ahoehoe lobes (
<
1 m), as seen in
133
recent K ̄ılauea eruptions (Lundgren et al., 2019; USGS, 2019), and thick lobes (
1 m),
134
as seen in Columbia River Basalt Group (CRBG) and other CFBs (Self et al., 2021). For
135
the sequential emplacement simulations, we scale
t
emp
relative to
t
h
and explore nine dif-
136
ferent emplacement intervals for each thickness:
t
emp
= 2
−
4
t
h
,
2
−
3
t
h
,...,
2
4
t
h
. Here,
137
t
emp
is sampled along multiple orders of magnitude in order to capture a wide range of
138
cooling times. The lower bound for the emplacement intervals is based on typical p ̄ahoehoe-
139
type flows (Hon et al., 1994; Anderson et al., 1999) while the upper bound is provided
140
by examples from flood basalt provinces (Thordarson & Self, 1998; Self et al., 1996).
141
3 Results
142
We perform a total of 153 simulations of the sequential emplacement of two lava
143
lobes and identify three distinct regimes of inter-lobe solidification. These regimes can
144
be delineated based on the ratio between
t
emp
and the conductive time scale (more pre-
145
cisely
t
h
, but approximated by
h
2
/α
). Below, we describe each regime in detail with ex-
146
amples for the case of
h
= 10 m lava flows in Figure 1.
147
In sequence (
t
emp
>
0
.
06
h
2
/α
):
The first lava lobe completely solidifies before the sec-
148
ond lobe is emplaced (Figure 1, right). The cooling times of both flows are sim-
149
ilar and the bottom flow does not remelt.
150
In parallel (
0
.
01
h
2
/α < t
emp
<
0
.
06
h
2
/α
):
As indicated by the narrowing of both black
151
contours in the top plot and the decreasing melt thickness in the lower plot with
152
time, both lava lobes solidify for overlapping time, but the interface between them
153
does not remelt (Figure 1, middle). Because the bottom flow is hot, the collective
154
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cooling of both flows is slower than
in sequence
flows, as indicated by the decrease
155
in slope in Figure 1 (bottom middle).
156
Fused flow (
0
< t
emp
<
0
.
01
h
2
/α
):
After emplacement, the solidified portion of the
157
lower lava lobe eventually remelts completely, and then both lobes combine to form
158
one large lobe which solidifies as one. For early times, there are four solid-melt
159
interfaces that correspond to the simultaneous solidification of two independent
160
lobes. However, the two interior interfaces disappear at some point, marking the
161
melting and merging of the two lobes. The remelting event is also evident when
162
we track the total melt thickness over time (Figure 1 bottom). After the arrival
163
of the second lobe (indicated by red dot), the total melt thickness increases slightly
164
at some point, corresponding to the remelting that caused a reduction in the solid
165
fraction. Despite a monotonic loss of entropy over time after the second flow ar-
166
rives, the remelting can occur as some sensible heat is converted into latent heat.
167
In the other two regimes, the melt thickness never increases after the arrival of the
168
second lobe.
169
Figure 1.
Emplacement of two 10m-thick lava slabs where the second slab is emplaced after
8.5 days (left), 2 months (middle), and 3 years (right). Top: Evolution of the temperature field
over time. The white line marks the ground and the dark line marks the solid-liquid boundary
defined by
φ
= 0
.
5. The ground portion extends between 0-40 meters (only half of the ground is
shown here). Bottom: the corresponding solidified fraction of the total emplaced lava over time.
The red dot marks the arrival of the second slab.
We compile the results from all the simulations into a regime diagram in Figure
170
2, which shows the combined control of individual flow thickness and emplacement in-
171
tervals on the inter-lobe solidification during sequential emplacement. We map the three
172
regions of inter-lobe solidification, separated by two boundaries extrapolated from our
173
results:
t
emp
= 0
.
01
h
2
/α
and
t
emp
= 0
.
06
h
2
/α
. These regimes and the boundaries that
174
define them are universal for both thin and thick lobes.
175
The bottom four panels in Figure 2 also illustrate real-life examples of lava flows
176
of various thicknesses that appear to have been emplaced
in parallel
or
in sequence
as
177
suggested by their distinct inter-lobe boundaries. These examples are also marked in the
178
regime diagrams, where the vertical position of the marker corresponds to the minimum
179
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emplacement interval predicted by our model (e.g.,
t
emp
= 0
.
01
h
2
/α
). In particular,
180
the polygonal marker corresponds to
∼
10 cm thin lobes as seen in the Kupaianaha flow
181
field that are predicted to be emplaced at least
∼
4 minutes apart. The square marker
182
corresponds to
∼
0.5m thin lobes as seen in Elephanta Caves (Deccan Traps), and are
183
predicted to be emplaced at least
∼
2 hours apart based on our results. The circular marker
184
corresponds to
∼
8 m thick lobes as seen in Rajahmundry Traps (Fendley et al., 2020a),
185
that are predicted to be emplaced at least
∼
20 days apart; the star-shaped marker cor-
186
responds to
∼
20m thick lobes as seen in CRBG that are predicted to be emplaced at least
187
∼
4 months apart.
188
Figure 2.
Regime diagram of two-lobe emplacement for different flow thickness and emplace-
ment intervals, focusing on thin lobes (left) and thick lobes (right). The black dots mark the
parameters we have simulated using our model. The four red outlining markers in the top two
panels correspond to the observed examples in the bottom four panels, as explained in the main
text. In the two scatter plots (which are scaled quadratically along the vertical axis), we omit
the top two lines of points corresponding to
t
emp
= 2
3
t
h
,
2
4
t
h
for sake of visual clarity (all points
above the line
t
emp
= 0
.
06
h
2
/α
corresponds to in sequence cases).
4 Discussion
189
There is a body of literature that commonly assumes that even the thickest (
>
40
190
m) CFB flows were formed by flow inflation (e.g., Self et al., 1996, 1998; Anderson et al.,
191
1999; Rader et al., 2017), inspired by the observations of Hawaiian lava flows (Hon et
192
al., 1994). However, our theoretical analysis suggests that thick (30-40 m total height)
193
flows could also arise by fusing of flows if eruption intervals are shorter than a month
194
or two. Even thicker flows can form by fusing more than two flow lobes with a similar
195
time delays (i.e.,
∼
months between each set of lobes). One practical challenge to test
196
our proposed mechanism is the ability to identify fused flow boundaries in the field since
197
fusing would remove structures that correspond to the crusts of the two lobes. However,
198
some relics of the originally distinct flow may remain, such as compositional differences
199
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(Vye-Brown et al., 2013; Reidel, 2005) and possibly structures indicative of fused flow
200
crusts such as vesicle-rich horizons and multiple entablature zones (Figure 3).
201
4.1 Potential example of a fused CFB flow
202
One potential example of a fused CFB flow is the
∼
70 m thick Cohassett Flow from
203
the CRFB. The flow is part of the Grande Ronde Basalt Group and is a member of the
204
Sentinel Bluffs Member lava flows in the Pascoe Basin (e.g., McMillan et al., 1989; Rei-
205
del, 2005, see Figure 3A for a map of outcrops and drill core data). As shown in the an-
206
notated picture in Figure 3B, the Cohassett has a multi-tiered structure with alternat-
207
ing entablatures and colonnades, as well as a 6.5 m thick internal vesicular zone (IVZ,
208
∼
20 m from the flow top, Figure 3B,C,D) with many
∼
1 cm diameter vesicles (McMillan
209
et al., 1989; Tomkeieff, 1940). The Cohassett flow exhibits one of the most striking geo-
210
chemical variations amongst the Grande Ronde flows. The flow has an approximate ver-
211
tical bilateral symmetry geochemically centered just under the IVZ, as seen from the data
212
across sections more than 50 km apart (Figure 3). Using characteristic patterns in TiO
2
,
213
P
2
O
5
(and other major and trace elements), Reidel (2005) defined four distinct compo-
214
sitional types within the flow - California Creek, Airway Heights, Stember Creek, and
215
Spokane Falls. Typically, these compositional types are separated by a vesicular hori-
216
zon. For example, a horizon
∼
13-15 m from flow top separates massive basalt of the Cal-
217
ifornia Creek composition from the Airway Heights composition. Similarly, the Airway
218
Heights and Stember Creek transition is characterized physically by a series of large vugs.
219
The IVZ acts as the contact between the Spokane Falls and the Stember Creek compo-
220
sitional types (Figure 3B,C,D). Finally, a vesicular horizon
∼
40 m from flow top defines
221
the transition from the Spokane Falls back to the Stember Creek compositional types.
222
Interestingly, the subsequent compositional type changes from Stember Creek to Cal-
223
ifornia Creek/Airway Heights lack clear vesicular horizons (Figure 3).
224
Corresponding spatially with these geochemical changes, the Cohassett flow also
225
exhibits systematic changes in plagioclase abundance and fine-grained fraction (groundmass,
226
Figure 3C based on data from Reidel, 2006). In particular, the flow part comprising the
227
IVZ and the Spokane Falls composition member has a fine fraction much more indica-
228
tive of a flow top rather than the flow interior. Thus, this flow interior was potentially
229
emplaced rapidly and cooled faster than a continuously inflating flow lobe interior (McMillan
230
et al., 1989; Philpotts & Philpotts, 2005). The IVZ-entablature-colonnade sequence in
231
the Spokane Falls lava further supports the conclusion that the cooling rates in this part
232
of the flow were more akin to a flow top (DeGraff et al., 1989; Forbes et al., 2014). Even
233
on an overall flow scale, the textural data for Cohassett flow are inconsistent with the
234
slow cooling expected for a
∼
70 m flow. The plagioclase crystal size does not signifi-
235
cantly change throughout the flow, unlike the case for a slowly cooling ponded lava lake
236
(Philpotts & Philpotts, 2005; Cashman & Marsh, 1988).
237
Previously, Reidel (2005) proposed that the Cohassett flow formed by the combi-
238
nation of different sheet flows (for each compositional type), each sourced from a differ-
239
ent magma reservoir and eruptive vent. These individual flows sequentially intruded into
240
the Cohassett flow as flow lobes and inflated it to its final height. However, the Reidel
241
(2005) model does not explain the abrupt shift to distinct compositional types along with
242
sharp vesicle horizons (Figure 3B, B1-B2) without any signs of magma mixing or shear
243
instabilities despite intrusion and transport within the Cohassett flow for 10s of km. Al-
244
ternatively, Thor Thordarson (personal communication, see also Vye-Brown et al. (2013))
245
proposed that the Cohassett flow was formed by semi-continuous inflation with chang-
246
ing magma compositions in the magmatic system feeding the eruption. Philpotts and
247
Philpotts (2005) proposed that crystal-mush compaction in an inflated sheet lobe can
248
also partially explain the observed geochemical variation. Here, we put forward a third
249
alternative, building upon the original idea proposed by Reidel (2005). We posit that
250
the Cohassett flow is an example of a
fused
flow with multiple flow lobes having differ-
251
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ent compositions. Suppose the Cohassett was close to the boundary between the fused
252
and in-parallel flow types (Figure 2). In that case, the presence of separating vesicle hori-
253
zons as well as high fine-grained size fraction, especially for the Spokane Falls type, can
254
be explained. Within this scenario, each constituent
∼
10-20 m lobe would have to be
255
emplaced within a few months of the previous lobe. However, more detailed modeling
256
work specifically focused on the Cohassett as well as textural analysis, e.g., stratigraphic
257
crystal size distributions to estimate cooling rates (Cashman & Marsh, 1988; Giuliani
258
et al., 2020), would be needed to ascertain which of the proposed models is correct and
259
if Cohassett is indeed a
fused
flow.
260
It is similarly challenging to distinguish between
in parallel
and
in sequence
flows
261
based on field volcanological observations alone without detailed textural analysis. One
262
potential distinguishing feature may be the 2D shape of the bottom flow lobe in a
in par-
263
allel
flow since it will be visco-elastically deformed by the load from the overlying flow
264
lobe (Abbott & Richards, 2020). One consequence of this would be the formation of squeeze-
265
up structures at flow lobe edges seen in some CFB flow edges, e.g., for the Western Ghats
266
and the Rajahmundry Trap flows in the Deccan CFB (Dole et al., 2020; Fendley et al.,
267
2020b).
268
4.2 Relevance for fused flows for planetary geology
269
Our results also have implications for inferring eruption conditions on other plan-
270
etary bodies (Moon, Mars, Venus, Mercury) where we can observe only the final lava flow
271
thickness from remote sensing observations. On each of these bodies, multiple thick lava
272
flows have been observed.
273
•
Venus
: the most striking example of a thick lava flow (
∼
100-200 m) on Venus
274
is the Ovda Flactus flow which has a surface area (
∼
60,000 km
2
) comparable to
275
many CFB flows (Wroblewski et al., 2019; Moore et al., 1992). Although initially
276
classified as a potentially high silica or rhyolitic flow, morphological analysis by
277
Wroblewski et al. (2019) shows that Ovda Flactus has an emplacement rheology
278
most consistent with basalt. Several other basaltic flows mapped across Venus have
279
thicknesses ranging from 30 m to 100 m (Zimbelman, 2003; Moore et al., 1992;
280
MacLellan et al., 2021; Lancaster et al., 1995; Guest et al., 1992). We envision that
281
our proposed process of fused flows can help explain these large flow thicknesses,
282
especially since cooling rate during solidification on Venus will be smaller (
∼
30-
283
40 % compared to Earth) due to higher surface temperatures (Snyder, 2002). Cor-
284
respondingly, individual flow lobes can be separated by longer times and still fuse
285
together.
286
•
Mars
: There is a wide range of estimated flow thickness for Martian lava flows
287
with values ranging from a few meters to
∼
100-125 m (Peters, 2020; Zimbelman,
288
1998; P. Mouginis-Mark & Yoshioka, 1998; P. J. Mouginis-Mark & Rowland, 2008;
289
Hiesinger et al., 2007). In particular, both the Tharsis volcanic province and the
290
Elysium Planitia region on Mars have a number of flows with typical thickness greater
291
than 40 m (Peters, 2020). As discussed earlier, these flow thicknesses are challeng-
292
ing to explain with inflation, but may be explained by fusing. Since the mean Mar-
293
tian surface temperature (-60
◦
C) is colder than Earth, the lava cooling time to
294
complete solidification is about 5% shorter than on Earth (based on results from
295
a model with Martian surface temperature). Since Martian surface gravity is
∼
296
38% of Earth’s, flow lobes may inflate to greater thickness before overcoming basalt
297
yield strength to form new breakouts.
298
•
Mercury
: Du et al. (2020) used observations of partially and completely buried
299
impact craters on Mercury to estimate lava flow thicknesses and found values from
300
23 to 536 m with a median of 228 m (with potentially even thicker lava flows), con-
301
sistent with some other estimates, e.g., 180 m (Wilson & Head, 2008). Even if we
302
account for the difference in surface gravity, 38% of Earth’s, the median thickness
303
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manuscript submitted to
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of 228 m translates to 86 m Earth equivalent (in terms of flow dynamics, the cool-
304
ing times are independent of gravity).
305
•
Moon
: Lunar mare basalts have a range of flow thicknesses, from thin (
<
1 m)
306
to thick (
>
100 m). These estimates come from a combination of in situ obser-
307
vations by Apollo Astronauts, remote sensing, and lunar penetrating radar on rovers
308
(Gifford & El-Baz, 1981; Spudis & Guest, 1988; Hiesinger et al., 2011; Chen et al.,
309
2018; Rumpf et al., 2020). In particular, there are many mare basalt flows with
310
an average thickness of 25-40 m (Chen et al., 2018; Morota et al., 2011; Hiesinger
311
et al., 2002). However, flow thickness is typically
∼
10 m (Yuan et al., 2020; Chen
312
et al., 2018; Enns & Robinson, 2013). Thus, similar to Earth’s CFBs (Self et al.,
313
2021), there may be a population of thick mare basalt flows that are the product
314
of fusing. Since the lunar surface gravity and mean temperature are much lower
315
than on Earth, lava flows can inflate to greater thickness before breakout and also
316
cool faster.
317
In aggregate, there are a number of very thick planetary basaltic flows that are poten-
318
tially emplaced by the same process as we are proposing for thick terrestrial CFB flows.
319
4.3 Implications for eruption rates
320
In combination with viscous flow models for channelized lava flows (e.g., Jeffrey’s
321
equation), measurements of lava flow thickness have been used to estimate the mean flow
322
velocity and viscosity (e.g., Glaze et al., 2003; S. Baloga et al., 2003; S. M. Baloga et
323
al., 2001; Harris & Rowland, 2001; Chevrel et al., 2018). Frequently, the estimated flow
324
velocities are used in combination with constraints on the total flow volume to calculate
325
an eruption duration. However, these calculations are predicated on the assumption that
326
the final flow thickness is representative of the molten channelized flow thickness at the
327
time of the eruption. Since the velocity depends strongly on the lava flow thickness, an
328
incorrect estimate can strongly impact the estimates of eruption rate. Suppose that the
329
observed lava flow is a consequence of merger of two (or more) flow lobes. In that case,
330
the models can overestimate instantaneous effusion rates significantly if they use the fi-
331
nal flow thickness. This issue is further exacerbated by the fact that during the emplace-
332
ment of p ̄ahoehoe flows by inflation, the “active” molten part of the flow is smaller than
333
the thickness of each lobe.
334
In the other end-member wherein the total observed flow thickness is assumed to
335
be only a consequence of flow lobe inflation, most models require a relatively continu-
336
ous, long lived effusion rate that gradually thickens the flow lobe (Self et al., 1998; Hamil-
337
ton et al., 2020; Hon et al., 1994; Kauahikaua et al., 1998). However, if the CFB flow
338
is a fused flow, each constituent flow lobe has to inflate to a smaller thickness. Conse-
339
quently, since the two lobes can be sequentially emplaced days to months apart, the to-
340
tal eruptive duration can be smaller and/or allow for more effusion rate variations but
341
will still form a single fused flow in the end. In conclusion, the possibility that different
342
flow lobes merged into a single flow has significant implications for inferences of effusion
343
rate, especially its steadiness.
344
5 Conclusion
345
We provide the theoretical lower bound on emplacement interval that distinguishes
346
a
fused flow
from non-merged flows. For instance, a distinct boundary between two lobes
347
of 10 cm each suggests that they were emplaced at least 4 minutes apart (
t
emp
>
0
.
01
h
2
/α
≈
348
4 minutes). The same calculation for two 20 m thick lobes suggests that the emplace-
349
ment interval is at least 4 months if a distinct boundary exists between the two lobes.
350
Furthermore, while it is often assumed that the 10
−
100 m thick lobes found in flood
351
basalt provinces are primarily formed from inflation, our results suggest that these large
352
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manuscript submitted to
Geophysical Research Letters
lobes could also have been formed by smaller lobes emplaced in quick succession. Re-
353
latedly, some volcanological studies could be overestimating eruption rates and flow ve-
354
locities by assuming that solidified flows belong to one flow rather than a series of smaller
355
flows that merged with little to no trace of their original separation.
356
While noting some limitations, we also demonstrate the effectiveness of using phase-
357
field models in simulating observed lava solidification over a range of timescales faith-
358
fully (Wright et al., 1972; Wright & Okamura, 1977; Wright & Marsh, 2016; Hon et al.,
359
1994), with some local deviation
∼
10 cm near the top surface. The phase-field model
360
can be generalized to arbitrary domains in higher dimensions and account for additional
361
complexities in thermal diffusivity, flow, and nonlinear rheology.
362
–10–
ESSOAr | https://doi.org/10.1002/essoar.10507565.1 | CC_BY_4.0 | First posted online: Tue, 20 Jul 2021 09:00:27 | This content has not been peer reviewed.