EPSL ELSEVIER
Earth and Planetary
Science Letters 144
(I 996) E I -E7
Express Letter
Rapid emplacement of a mid-ocean ridge lava flow on the East Pacific Rise at 9’ 46’-5 1’N Tracy K.P. Gregg a**, Daniel J. Fornari a, Michael R. Perfit ‘, Rachel M. Haymon ‘, Jonathan H. Fink d ’ Department of Geology and Geophysics. Woods Hole Oceanographic Institution. Woods Hole, MA 02543, USA b Department of Geology, I 112 Turlington Hall, lJniver.sity of Florida, Gainewille, FL 3261 I USA ’ Department of Geological Sciences, Uniuersity of California, Santa Barbara, Santa Barbara, CA 93106 USA ’ Department of Geology, Box 871404, Artona State Uniclersitv. Tempe. AZ 85287-1404 USA Received 9 July 1996: revised 6 September 1996; accepted 12 September 1996
Abstract In April, 199 1, during a submersible diving expedition to the East Pacific Rise (EPR) crest at 9”46’-5 1’N, a new volcanic eruption on the sea floor was discovered. Here, we report results from numerical modeling of that eruption, which indicate that - 4 X 106-6 X lo6 m3 of lava was emplaced in - l-2 h, with an average eruption rate of - 103-lo6 rn’ SC’ comparable to rates observed in Hawaii at Kilauea’s East Rift Zone. If the rapid emplacement of the 1991 EPR lava and its short eruption duration are typical of volcanic events at fast-spreading mid-ocean ridge crests, these characteristics have broad implications for our ability to detect, monitor and understand the evolution of magmatic and volcanic processes within the axial zone. Keywords:
East Pacific Rise; mid-ocean ridges: eruptions; lava flows; flow mechanism
1. Introduction
and background
Despite the importance of volcanism at mid-ocean ridges (MORS) [I], little is known about the dynamics of individual eruptions because of logistical and technological constraints [2]. Thus, eruption and emplacement parameters must be determined from the resulting lava flows and volcanic morphology. The EPR at 9” 46’-5l’N has an axial summit caldera (ASC), consisting of a 50-80 m wide. 5-8
* Corresponding author. Tel.: + 1 508 289 2615. Fax: + 1 508 457 2183. E-mail:
[email protected] 0012-821X/96/$12.00 Copyright PII SOOl2-821X(96)00179-3
m deep, volcanic collapse zone within which most primary volcanic and hydrothermal processes occur (Fig. 1) [3,4]. S eismic . reflection studies [5-71 indicate that a narrow, partially molten magma region exists at shallow depths ( - I .5- 1.6 km) beneath the ASC here. The 1991 EPR eruption aftermath was observed using DSV Aloin in April, 1991 [3]. ‘10Po-2’0Pb isotope dating [8] of the 1991 flow indicate that the lava was only weeks old when first sampled. It was readily identified by its lustrous surface, fresh appearance, and chemical composition (Table 1). Sonar data [3,9,10] and visual observations [ 1 l- 141, indicate that the primary eruptive fissure was nearly
0 1996 Elsevier Science B.V. All rights reserved.
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T.K.P. Gregg et al./ Earth and Planetary Science Letters 144 (1996) El-E7
9” 51’N
9” SO’N
9” 49’N
9” 48’N
9” 47’N
9” 46’N 104”
2600
18’W
2580
104”
2560 Depth
17’W
2540
2520
2500
(m)
Fig. 1. (a) Location of the 1991 eruption at the EPR between 9” 46’-52’N. Capital letters label transform faults: R = Rivera; 0 = Orozco; C = Clipperton; S = Siqueiros. (b) 5 m gridded contour interval bathymetry for the 1991 eruption area (grid interval is 20 m). The narrow, hydrothermal vent. dark sinuous band (25 15-2520 m) within the bright axial depth contour ( _ 2500 m) is the ASC; 0 = high-temperature
continuous over _ 8.5 km within the ASC, with common, small (_ 3-5 m), en echelon offsets. Fissure width varies from 1 to 4 m. Alvinaltimeter data reveal no large (> N 10 m high) volcanic constructs
on the fissure, suggesting that magma did not localize along the eruptive fissure, in contrast to Hawaii sonar (e.g., [15]>. Based on in situ observations, mapping, and multichannel seismic data, the most
T.K.P. Gregg et al./Earth Table 1 Average chemical composition with 2a from April, 1991, EPR lava flow
for 21 glass
samples
Oxide
wt.%
2u
Element
ppm
2o
SiO, TiO,
49.88 1.29 15.52 9.31 0.17 8.55 12.26 2.56 0.09 0.12 99.84 64.53
0.40 0.07 0.23 0.20 0.02 0.33 0.44 0.12 0.03 0.02 0.31 1.33
co cu Ga Nb Ni Rb Sr Y v Zr
40 75 16 2.5 108 1.2 123 27 265 82
3 4 1 0.7 18 0.9 5 2 9 7
AlzO, Fe0 MnO MgO CaO Na,O K,O P205 Total Mg# Determined elements).
by microprobe
(oxides) and X-ray fluorescence
(trace
geologically valid dimensions for the primary eruptive conduit are ‘average’ values of 2.5 m wide, 8000 m long, and 1600 m tall. In places, the 1991 flow appears to have overtopped the ASC rim, creating spill-outs of glassy lava. Because the depth of the ASC is 5-8 m between 9” 46’ and 9” 51’N, the flow must have reached a maximum thickness of 8 m locally. We also observe that the tops of lava pillars associated with the 1991 flow within the ASC are 5-8 m tall, suggesting that the flow was largely contained within the ASC and its thickness was between 5 and 8 m. Local on-lapping relations indicate 2-3 m of 1991 lava is left ponded on the ASC floor. Samples from the 1991 EPR flow are aphyric to slightly phyric ( < 5 ~01% phenocrysts). The 1991 lava is easily identified by its high MgO content (m 8.6 wt%) relative to the surrounding older lavas (< 8.0 wt%). It has normal MOR basalt chemistry,
Table 2 Parameters
for 1991 EPR lava used in numeric calculations
Parameter
Unit
Value
Eruption temperature Viscosity Solidification temperature Heat capacity Thermal diffusivity
“C Pa s “C J kg-’ “Cm? s-I
1200 loo 800 1200 1.4x 10-7
After [36,42].
’
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and Planetary Science Letters 144 (1996) El-E7 Table 3 Observed
and estimated
parameters
for 199 1 EPR eruption
Parameter
Estimated range
Value
Fissure width Fissure length Fissure depth Flow depth Flow volume Effusion rate Eruption duration
l-4m _
2.5 m 8000m 1600m
1500-1600 m 5-8 m 3X106-5X106 m3 to)-106 m3 s-’ 3stolOOmin
used
IO3 mJ s-l -l-2h
and its composition varies little between 9” 46’ and 9” 5 IN, demonstrated by the small relative standard deviations of elements measured by microprobe or X-ray fluorescence (Table 1). The chemical heterogeneity of the 1991 EPR flow is less than that of the CoAxial flow [16-181, but is comparable to the ‘Young Sheet Flow’ [19], both on the Juan de Fuca Ridge. It is more homogeneous than lavas along Kilauea’s East Rift Zone [20], but similar to the 1984 Mauna Loa lava [21]. Major-element chemistry and microphenocryst content of the 1991 EPR flow indicate an eruption temperature of _ 12OO”C, density N 2800 kg rnw3, and viscosity of N 100 Pa s $able 2) [22-241. Using the plan-view area of the ASC from 9” 46’ to 9” 51’N, and a flow thickness of 5-8 m, we estimate N 3.6 X lo6 to 5.8 X 10” m3 of basaltic lava was erupted within the ASC during the 1991 event, and was emplaced in N l-2 h (Table 3) - a relatively short-lived eruption in comparison with subaerial volcanoes.
2. 1991 Eruption volume and effusion rate models Using the properties of the 1991 lava (Table 2) and the geometry of its eruptive conduit (Table 3), effusion rates can be constrained by models based on: (1) magmatic pressure [25]; (2) cooling rates within the dike 126,271; and (3) flow morphology [28]. Although multiple pulses of magma could have emplaced the 1991 lava, detailed analyses of glasses collected throughout the eruption region are required to confirm this. Based on the small erupted volume
T.K.P. Gregg et al./Earth
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and Planetary Science Letters 144 (1996) El-E7
and a lack of evidence suggesting multiple pulses, we consider here a single eruption. For the 1991 EPR magma to erupt on the surface, it must have exceeded: (1) the pressure of the overlying rock (P, ); (2) pressure created by forcing viscous fluid through a narrow conduit (P,); and (3) the downward force acting on the magma as it attempted to rise through less dense host rock (Pa). These parameters are given by [25,26]: P, = p,gh, + p,gh,
(la)
P, = 12vqh/w2
(lb)
Pa = Apgh
(lc)
PTOT
(14
= ‘I_ + ‘F + ‘Ei
in which p, is the average density of Layer 2 (2400 kg mm3 [29-311); g is gravity (9.78 m s-l); h, is depth from the sea floor to the dike source region; pW and h, are water density (1000 kg rnm3> and depth (2500 m), respectively; v is magma rise velocity within a dike of width w; and Ap is given by p - ph. Using h, = 1600 m, w = 2.5 m, and varying v from 1 to 100 m s- I shows that P, dominates, by 2 orders of magnitude, over P, and Pa. Increasing v by 10’ increases PToT by less than a factor of 2. Similarly, increasing dike width to 4 m decreases P TOT by a factor of 0.99. The excess pressure (P,) available to drive the magma to the sea floor is given by [32]: Pa =
in which D is thermal diffusivity (1.4 X 1O-7 m* s-’ I and b is a dimensionless number equal to 0.65. Using the conduit geometry in Table 3 yields a minimum rise velocity of 1.2 X 10m4 m s-‘, equal to minimum effusion rates of 2.4 m3 s-’ (3.0 X 10u4 m2 s-’ m-l>. Effusion rate can be estimated from lava flow morphology by associating flow type with a range of p values, where q is a dimensionless number incorporating the physical parameters of an extrusion and its environment [28]. Using the lobate flows created where the 1991 flow locally overtops the ASC rim (rather than sheeted flows within the ASC that reflect drainback) gives an effusion rate of - 3 X lo3 to 9 X 10’ m3 s- ’ (- 25-700 m* s-’ m-l). (These values are determined using a revised 1E’ value of 0.65, which marks the transition between pillowed and lobate flows; see [33].) By examining results obtained from these models, noting that cooling and pressure models provide minimum constraints, and using values in Tables 2 and 3, we find that effusion rates range from - 10” to lo6 m3 s-’ (- 1-125 m2 SK’ m-‘I. Calculated linear eruption rates for the 1991 EPR flow are similar to those observed during basaltic a’a eruptions (lo-100 m’ s-’ rn- ‘) along Hawaiian rift zones (e.g., [34]); volumetric rates are comparable to
0.1
PTOT -
PII, t&r
(le>
in which pm is the density of the magma. The pressure (P,) required to erupt the 1991 EPR lava is - 1.8 X lo7 Pa. Effusion rate is constrained by 1251: Paw3L Q=_
2
0.01
3 .u SE si $
hh,
in which L is fissure length. Using the fissure dimensions in Table 3 gives a minimum effusion rate of 1.2 X lo6 m3 SK’ (150 m2 s-l m-l>. Because the eruption viscosity for the 199 1 lava is - 100 Pa s, the basalt could not have cooled sufficiently while rising through the dike to increase its viscosity above this value. Cooling provides a minimum rise rate, given by 1261: 8Db2 h, Vmin = W2
(3)
0.001
0.0001~ 0.01
0.1 Time
1
10
after emplacement (hours)
Fig. 2. Rate of crust growth on submarine basalt, using the depth from the surface to the basalt glass transition temperature isotherm ( - 730°C) as crust thickness, assuming that crust grows by thermal diffusion and neglecting latent heat. The shaded box marks the solidification time for a basaltic crust 5-7 cm thick, as observed on the 1991 EPR flow.
T.K.P. Gregg et al./ Earth and Planetary Science Letters 144 (1996) El-E7
-*
t = -1 min. b--J=-
a
E5
1991 EPR eruption was short-lived. At an effusion rate of IO3 m3 s-‘, the 1991 lava would have filled the ASC to a depth of 5-8 m in N 50-100 min; at lo6 m3 s-‘, the AX would have filled in N 3-6 s. Although the latter value is unreasonable, these results show that the 199 1 EPR eruption was emplaced in minutes to hours, instead of days to weeks. The actual effusion rate may have been closer to 103- lo4 m3 s-l, similar to Hawaiian rates [15]. The _ 5-7 cm thickness of lava crusts where the 1991 flow overtopped the rim, and on the tops of lava pillars created during the 1991 eruption, indicate that the ponded lava remained at its maximum level for N 4 h, based on models of submarine basalt crust growth [28,36,37] (Fig. 2). The primary surface crust of the ponded lava then foundered, and was replaced by a crust formed during lava drainback and ‘drainout’ into hollow lava tubes that pierce the ASC walls [3] (Fig. 3). Ponded 1991 lava N 2-3 m thick remains in the ASC.
3. Discussion
t = +lOO min.
d(
t=+lOOhr. Fig. 3. Cartoon depicting emplacement of 1991 lava at ASC. Views are cross-sections with approximate times shown. (a) The AX before the eruption. (b) At the eruption onset magma intrudes the primary fissure. (c) Lava begins to fill the ASC. (d) Maximum lava depth is attained in - l-2 h. (e) After vent activity ceases, some lava drains back into the primary fissure and out of the ASC. leaving ponded lava and lava pillars.
the highest rates observed at Hawaii (103-lo4 m3 s-‘) [15,35]. The rates calculated here are order of magnitude estimates at best; however, they demonstrate that the
The 1991 event on the EPR has an eruptive volume, rate, and initial fissure length similar to those of individual episodes of Hawaiian rift zone eruptions. Although Hawaiian episodes generally last for several hours or more, durations < 4 h have been observed (e.g., episode 12A, 1969-1971 Mauna Ulu eruption [35]; opening phases of the August 1968 eruption at Kilauea’s east rift zone [38]). Local emplacement rates (at flow margins and at the advancing flow front, for example) for the 1991 EPR lava were probably much lower (- I m3 s-r) than those calculated to have occurred at the vent. We estimate N lo6 m3 of lava was removed from the ASC after the 1991 EPR eruption ceased, based on the depth of ponded lava remaining in the ASC (N 2-3 m) and the estimated maximum flow thickness (8 m>. Although lava primarily drained back into the eruptive fissure, some flowed outside the ASC, as demonstrated by tongues of 1991 lava on the crestal plateau trending away from the ASC rim, and on the floors of collapse pits in older lobate flows outside the ASC, several hundred meters from the ASC margin [3].
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T.K.P. Gregg et al/Earth
and Planetary Science Letters 144 (1996) El-E7
An important goal for future MOR studies is quantitative documentation of volcanic terrain associated with specific lava flows so that similar methods can be applied to other seafloor eruptions. For example, in 1993 an eruption was detected along the CoAxial segment of the Juan de Fuca Ridge (JFR) [39]. The ‘CoAxial Event’ generated a 2.5 km long lobate flow with a volume of N 5 X IO6 m3 [17,39,40]. Flow morphology suggests that the CoAxial flow was emplaced in N 10 days, with half of the volume emplaced in the first N 2 h [28]. The longer eruption duration for the CoAxial Event is attributed to higher lava viscosity (N 103-lo4 Pa s [28]).
4. Conclusions Volcanic eruptions occur quickly at MORs, although they may be longer lived on slower spreading ridges [41]. Results from modeling the emplacement of N 4-6 X IO6 m3 of lava between 9” 46’ and 9” 5 1’N on the EPR crest indicate that eruptive activity ceased N l-2 h after eruption onset. The speed with which lava is emplaced at a MOR axis has important implications for studies which seek to detect and monitor submarine volcanic events along the ridge crest. These include studies of eruption frequency, the architecture and evolution of oceanic crust, and the evolution of hydrothermal vents. To characterize MOR eruptions fully, it is necessary to develop high-resolution techniques and equipment for detection and monitoring of rapid magmatic events at a ridge crest, including: ground deformation equipment (e.g., tilt meters and extensiometers). photographic arrays (both real-time and time-lapse), seismic and teleseismic detectors, autonomous underwater vehicles (AUVs), and acoustic telemetry systems that can provide real-time interaction with instruments and AUVs on the sea floor.
Acknowledgements We thank our colleagues who participated on Alvin dive programs to the EPR, the Alvin at-sea operational team and shore-based support group, and the officers and crew of the R/V Atlantis-II. D. Smith, S. Humphris, M. Tivey, C. Lanmuir and four
anonymous reviewers helped to improve this manuscript. This work was supported by the National Science Foundation under the following grants: OCE9505384 (TKPG and DJF), OCE9100503 (DJF and MRP), OCE9020111 (RMH), and OCE9202251 (JHF). IczJ
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