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Keywords:

  • sector collapse;
  • debris avalanche;
  • submarine eruptions;
  • explosive volcanism;
  • volcano morphology;
  • landslide

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[1] Repeated multibeam bathymetric surveys at Monowai Cone, a shallow submarine basaltic volcano and part of the Monowai Volcanic Center in the northern Kermadec arc, were conducted in 1998, 2004, and 2007. These surveys document dramatic depth changes at the volcano including negative changes up to −176 m from two sector collapses and positive changes up to +138 m from volcanic reconstruction near the summit and debris avalanche deposits downslope of the slide scars. One sector collapse occurred on the SE slope between 1998 and 2004 with a volume of ∼0.09 km3, and another occurred on the SW slope between 2004 and 2007 with a volume of ∼0.04 km3. The volume of positive depth change due to addition of volcanic material by eruption is of the same order: ∼0.05 km3 between 1998 and 2004 and ∼0.06 km3 between 2004 and 2007. During these time intervals, monitoring by the Polynesian Seismic Network detected frequent T wave swarms at Monowai, indicative of explosive eruptive activity every few months. An unusual T wave swarm on 24 May 2002 was previously interpreted as the collapse event between the 1998 and 2004 surveys, but no similarly anomalous T waves were detected between 2004 and 2007, probably because the Polynesian Seismic Network stations were acoustically shadowed from the second slide event. We interpret that the sector collapses on Monowai are caused by the unstable loading of fragmental erupted material on the summit and steep upper slopes of the volcano (>20°). Moreover, there appears to be a cyclic pattern in which recurrent eruptions oversteepen the cone and periodically lead to collapse events that transport volcaniclastic material downslope to the lower apron of the volcano. Volumetric rate calculations suggest that these two processes may be more or less in equilibrium. The repeated collapses at Monowai are relatively modest in volume (involving only 0.1–0.5% of the edifice volume), have occurred much more frequently than is estimated for larger debris avalanches at subaerial volcanoes, and may be characteristic of how persistently active shallow submarine arc volcanoes grow.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[2] Monowai is a submarine volcanic center in the northern Kermadec arc [Graham et al., 2008], located ∼1500 km NE of New Zealand, between 25°45′S and 25°55′S (Figure 1). The volcanic center includes a large caldera (8 × 11 km wide) and a young basaltic cone built on its southern rim with the base at 1000–1400 m and summit within ∼100 m of the ocean surface (Figure 1). Monowai Cone has been one of the most active submarine volcanoes of the Kermadec arc during the last 30 years (http://www.volcano.si.edu/reports/bulletin/). Its activity has been intermittently visible at the ocean surface as discolored water or vigorous gas emissions [Davey, 1980; http://www.volcano.si.edu/reports/bulletin/], and has been remotely detected as hydroacoustic T waves in New Zealand [Kibblewhite, 1966] and in French Polynesia [Talandier and Okal, 1987].

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Figure 1. (a) Location map of Monowai Volcanic Center at the northern end of the Kermadec arc, about 1500 km northeast of New Zealand, showing its position relative to the stations (red dots) of the Polynesian Seismic Network, at right (after Wright et al. [2008]). (b) Oblique three-dimensional view of Monowai Volcanic Center (2× vertical exaggeration of 2004 data), showing the location of Monowai Cone on the southern rim of Monowai Caldera.

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[3] Recent work by Wright et al. [2008] documented a collapse event and subsequent volcanic reconstruction at Monowai using depth changes between multibeam bathymetric surveys in September 1998 and September 2004. The Polynesian Seismic Network (Réseau Sismique Polynésien, RSP) recorded nine swarms of seismoacoustic T wave events from Monowai over the same period that were interpreted as explosive eruptive activity at the volcano's summit [Wright et al., 2008]. One of those T wave swarms, on 24 May 2002, was distinctive because of its large amplitude (an order of magnitude higher than all other events), long duration, and small number of events. This unusual T wave swarm was interpreted as recording the sector collapse that formed on the SE flank of Monowai between the 1998 and 2004 surveys [Wright et al., 2008].

[4] In this paper, we document further changes at Monowai in light of an additional bathymetric survey in May 2007. We compare these new data with the earlier results in the context of T wave monitoring during 2004–2007 recorded by the RSP. Since 2004, continued volcanism at Monowai has largely filled in the 2002 collapse scar, and a new collapse scar has formed on the SW volcano flank.

2. Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[5] Quantitative comparison of repeated bathymetric surveys to document elevation changes associated with submarine volcanism was first developed and used at mid-ocean ridges [Chadwick et al., 1991; Fox et al., 1992; Perfit and Chadwick, 1998], and has also recently been applied successfully in volcanic arcs [Walker et al., 2008; Wright et al., 2008; Le Friant et al., 2008]. In the case of Monowai, the raw multibeam data from the three separate surveys were edited and gridded at 20 m spacing. Subareas of the three grids were compared where no depth changes are assumed to have occurred in order to determine depth biases between the surveys. No horizontal corrections were necessary because all surveys were well-navigated by GPS. After depth bias corrections were applied, grids of raw depth differences were produced by subtracting one grid from another. To distinguish between signal and noise in the raw difference grid, the differences are weighted as a function of slope (which can create false depth differences due to small navigation errors). An empirical threshold is applied to the slope-weighted grids to help distinguish areas of significant depth change. Finally, “cleaned” difference grids, from which areas and volumes are calculated, are made by masking out areas in the raw difference grids where no significant change is interpreted. This process can be somewhat subjective, but in the case of Monowai the magnitudes of the depth changes are so large (>50 m) that the results are relatively unambiguous. The comparison method is described more fully by Fox et al. [1992] and Wright et al. [2008].

[6] The September 1998 survey at Monowai (Figures 2a and 2b) was made by the R/V Sonne using a Hydrosweep multibeam sonar system (15 kHz), the September 2004 survey (Figures 2c and 2d) was collected by the R/V Tangaroa with an EM300 sonar system (30 kHz), and the May 2007 survey (Figures 2e and 2f) was completed by the R/V Sonne with its new EM120 sonar system (12 kHz). Depth biases were determined for the 1998 and 2007 surveys, relative to the 2004 survey. After comparing the bathymetric grids in an area north of Monowai cone, we applied a depth bias of +3 m to the 1998 survey (making it deeper), consistent with Wright et al. [2008], and we applied a depth bias of −3 m to the 2007 survey (making it shallower). Some of the depth change calculations presented here for the 1998–2004 comparison are slightly different (by <10%) than those presented by Wright et al. [2008], because the raw multibeam data from 2004 were reedited and regridded for this analysis to be more consistent with the other surveys. We consider the revised depths and grid differences presented here to be more accurate.

image

Figure 2. Paired (left) bathymetric and (right) slope maps of Monowai Cone from multibeam sonar surveys collected in (a–b) 1998, (c–d) 2004, and (e–f) 2007. New sector collapse scars appear in the 2004 survey on the SE slope and in the 2007 survey on the SW slope. Artificial illumination is from the north and depth contours are at 100 m spacing in the bathymetry. Slope maps show that the source area for the sector collapses has slopes >20°.

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3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[7] The depth changes at Monowai are the largest documented at any submarine volcano by modern multibeam bathymetric surveys. The depth of the summit has changed from 40 m in 1998, to 130 m in 2004, to less than 69 m in 2007 (although the uppermost summit was not surveyed in 2007; Figure 2e). However, the magnitudes of depth changes on the volcano flanks are even larger (up to +138 and −176 m, but mostly between ±100 m). The thicknesses, areas, and volumes of both positive and negative depth changes between surveys are presented in Table 1 and maps of the depth changes are shown in Figure 3 (over the entire cone), Figure 4 (showing details around the summit), and as time-lapse animations in Movies 1 and 2.1

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Figure 3. Depth difference maps between the 1998, 2004, and 2007 bathymetric surveys: (a–b) 1998–2004, (c–d) 2004–2007, (e–f) 1998–2007. Maps at left show raw depth differences, and maps at right are cleaned difference grids (used for area and volume calculations). Contours of depth change are at 10 m spacing.

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image

Figure 4. Detailed maps of the summit area of Monowai Cone. Bathymetry collected in (a) 1998, (b) 2004, and (c) 2007 (50 m contours), and cleaned depth differences between (d) 1998–2004, (e) 2004–2007, and (f) 1998–2007 (10 m contours).

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Table 1. Thicknesses, Areas, and Volumes of Depth Changes Between Bathymetric Surveys
Surveys Compared (years)Maximum Thickness of Positive Depth Change (m)Area of Positive Depth Change(×106 m2)Volume of Positive Depth Change(×106 m3)Maximum Thickness of Negative Depth Change (m)Area of Negative Depth Change(×106 m2)Volume of Negative Depth Change(×106 m3)
1998–2004511.1614.6−176−1.39−63.0
2004–20071385.55103.−68−1.05−35.2
2004–2007-SE1382.1060.9   
2004–2007-SW513.4542.0   
1998–20077013.1260.−114−1.78−45.4
1998–2007-SE708.90198.   
1998–2007-SW484.1561.5   

[8] As Wright et al. [2008] described, the 1998–2004 comparison is dominated by a large area of negative depth change (up to −176 m) on the SE slope of the volcano (Figures 3a and 3b and 4d) associated with a sector collapse scar that has an area of 1.39 × 106 m2 and a volume of 63.0 × 106 m3. However, it was clear that postslide eruptive activity had partially filled in the collapse scar (and had reduced its volume by perhaps a quarter) because a constructional cone had grown within the slide scar. Thus, the slide itself was probably originally ∼85 × 106 m3 and the subsequent volcanic infilling was estimated to be ∼22 × 106 m3 [Wright et al., 2008]. Little information was available about any deposits downslope of the slide scar due to the lack of complete mapping coverage in the 2004 survey (Figures 2 and 3). In addition to this collapse, positive depth changes up to +51 m were identified west of the summit that were 1.16 × 106 m2 in area and 14.6 × 106 m3 in volume (Figures 3a and 3b and 4d) and were interpreted as due to pyroclastic fallout from eruptions near the summit [Wright et al., 2008]. The total erupted volume during this interval, ∼37 × 106 m3, is the sum of the fallout deposit plus the estimated volume of infilling within the slide scar.

[9] Seismic monitoring by the RSP showed that there were nine distinct T wave swarms at Monowai during the 1998–2004 time period, all interpreted as having been produced by eruptive activity. The unusual swarm on 24 May 2002 that was interpreted as associated with the sector-collapse event [Wright et al., 2008] was the second in that sequence, implying that the volcanic infilling within the collapse scar took place during the subsequent seven swarms over the next 2.3 years.

[10] The new 2007 bathymetric survey (Figures 2e and 2f) documents the subsequent morphologic evolution at Monowai in the following 2.7 years and allows an assessment of net changes since 1998. In the 2004–2007 comparison (Figures 3c and 3d and 4e), the area on the SE slope that had been an area of negative depth change in the 1998–2004 comparison is now an area of positive depth change, showing that by 2007 the collapse scar on the SE slope had almost been completely infilled by continued volcanism. The positive depth change on the SE slope is up to +138 m with an area of 2.09 × 106 m2 and a volume of 60.9 × 106 m3 (Table 1). This area is somewhat larger than the area of collapse in the 1998–2004 comparison because some of the post-2004 deposits have overflowed into a pre-1998 collapse chute to the east of the 1998–2004 collapse scar (Figures 2, 3, and 4). However, the volume of infilling is slightly less than the volume of the 1998–2004 collapse as it has not yet completely infilled (see below).

[11] West of the summit there is also a new sector collapse scar in the 2007 survey that is hourglass shaped in plan view (Figures 2e and 2f and 4c). In the 2004–2007 comparison (Figures 3c and 3d and 4e), the depth changes west of the summit comprise an area of negative change (the slide scar) upslope of an area of positive change (the slide deposits). The depth change in the scar is up to −68 m, whereas the maximum depth change in the deposits is +51 m. The area of the deposits is larger than the area of the scar (Table 1), but the volumes are very similar (35.2 × 106 m3 for the scar versus 42.0 × 106 m3 for the deposits). A smaller scar volume is probably due to subsequent eruptive activity that infilled it slightly near the summit; its original volume was probably ∼40 × 106 m3. The thickest deposits and about 70% of their volume are located within 1.5 km downslope of the slide scar, although thinner deposits extend out to at least 5.5 km from the slide headwall (Table 2).

Table 2. Dimensions of Slide Scars and Their Depositsa
Surveys Compared (years)Width of Slide Scar (m)Length of Slide Scar (m)Height of Slide Scar (m)Drop Heightb (m)Runout Distanceb (m)Height/Runout Distance
  • a

    Height of the slide scar is depth at top of headwall minus toe depth, drop height is top of headwall to toe of deposit, and runout distance is top of headwall to toe of deposit.

  • b

    Both the drop height and runout distance are minima since the distal ends of the debris avalanche deposits are beyond the edges of the bathymetric surveys.

1998–200490020007001400>60000.23
2004–2007500–70015005001400>55000.25

[12] A comparison of the original 1998 survey to the most recent 2007 survey clearly shows that the sector collapse on the SE slope has only been partially infilled (Figures 3e and 3f and 4f). Part of the area of the May 2002 slide scar is now shallower than it was in 1998 (near the eruptive vent) and part is still lower (the northeastern half). We can also clearly see that volcaniclastic material has been deposited into the slide chute that existed in 1998 to the east of the collapse (Figures 3e and 3f and 4). For the first time, we can also assess the deposits downslope of the May 2002 sector collapse, which amount to over double the volume of the slide scar (Table 1), because they include both the slide deposits and postslide volcaniclastic material that has been erupted near the summit and funneled downslope through the slide scar. This area of positive depth change tapers upslope, toward the eruptive vent (Figures 3f and 4f). The calculated area and volume of this deposit are probably minima since it appears to extend beyond the mapped area (the outer extents of the 1998 and 2007 surveys lie just beyond the southern limit of Figures 2 and 3).

[13] The new sector collapse west of the summit looks very similar in the 2004–2007 and 1998–2007 comparisons (Figures 3d and 3f), except that in the latter it is evident that the new slide scar is overprinted on the volcaniclastic fallout deposits that were identified west of the summit in the 1998–2004 comparison (Figures 3b and 3f and Figures 4d and 4f). Also, it is clear that the eruptive vent is located within the heads of both slide scars (Figures 3f and 4f).

[14] During the September 2004 to May 2007 time period, 1995 individual T wave events from Monowai were recorded by the RSP, showing that the volcano has remained extremely active. Most of the T wave events occurred within ten distinct swarms, separated by quiet intervals (Figure 5). Each swarm typically lasted a few days to several weeks, with up to 100 detected events per day (Figure 5a), and up to 550 total events (Figure 5b). The amplitudes of the largest T wave events were up to 60 nanometers, as measured at seismic station TVO in Tahiti (Figure 5c). There was a particularly intense swarm in December 2006 to January 2007, which appears as a notable step in the plot of cumulative events (Figure 5b). This eruptive period was actually composed of three major and two minor bursts of events over a 6 week period (Figure 6). Each burst of activity lasted several days and was separated by a week or less of quiescence (Figure 6), which is fairly typical of previous swarms from Monowai [Wright et al., 2008; http://www.volcano.si.edu/reports/bulletin/]. While the R/V Sonne was at the site 1–4 May 2007, discolored water and bubbles at the sea surface were observed, and audible booms were heard and felt on the ship, but interestingly only one relatively low-amplitude (6 nm) T wave event was recorded by RSP during this time (3 May). Wright et al. [2008] noted that the number of T wave events increased markedly after the unusual swarm in May 2002, when the first slide is interpreted to have occurred. This pattern of frequent swarms continued through 2007, although the amplitudes of the largest events have gradually decreased with time (Figure 5c).

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Figure 5. Monitoring data from the Polynesian Seismic Network showing T wave swarms at Monowai throughout 2002–2007. The times of the September 2004 and May 2007 bathymetric surveys (dashed vertical lines), and the time of the anomalous 24 May 2002 swarm (arrows) are shown. (a) Number of T wave events per day. (b) Cumulative number of T wave events. (c) Amplitudes of T wave events, in nanometers as recorded at station TVO in Tahiti. Note unusually high amplitude of the 24 May 2002 event, interpreted as the sector collapse between the 1998 and 2004 surveys.

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Figure 6. Monitoring data from the Polynesian Seismic Network showing that the T wave swarm at Monowai in December 2006 to January 2007 consisted of multiple bursts over a 6 week period. (a) Number of events per day, (b) cumulative number of events, and (c) amplitude at station TVO in nanometers.

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[15] In contrast to the 1998–2004 time interval [Wright et al., 2008], there were no seismoacoustic signals during 2004–2007 with unusually large amplitudes and long durations that could be candidate records for the second sector collapse event (Figure 5c). This might be because the 2004–2007 collapse is located on the western flank of Monowai Cone, whereas all RSP stations are located east of the volcano (Figure 1), and so were probably acoustically shadowed. To our knowledge, no unusual seismic or seismoacoustic signals were recorded by any other station in the region (including the closest seismic station at Raoul Island, located 400 km to the SSW on the Kermadec Ridge (S. Bannister, personal communication, 2008)). Likewise, no tsunami waves have been reported in the region that might have originated from Monowai over this time period. Empirical calculations by Wright et al. [2008] suggest that collapse events on the scale of those at Monowai would produce tsunami waves with a maximum height of a few meters. Therefore, we cannot establish exactly when the western sector collapse failed within the September 2004 to May 2007 time period. However, it may have occurred late in this period since the collapse scar has not been significantly infilled by subsequent eruptive deposits.

4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[16] It is clear that the two competing processes of collapse and volcanic reconstruction documented by Wright et al. [2008] at Monowai during 1998–2004 continued through 2007. Changes since 2004, and the identification of older slide scars in the 1998 and 2004 bathymetry (Figures 2 and 4), suggest that these processes are likely repetitive and perhaps cyclical. The morphology of both sector collapse scars suggests that they are single, short-lived catastrophic events, which appear to occur every few years. In contrast, based on the T wave data, volcanism (at least in this eruptive cycle) is more frequent (every few months) and more continuous. How might these two processes be interrelated?

[17] There are at least three possibilities for what may cause the shallow and repetitive sector collapses at Monowai: (1) eruptions that deposit tens of meters of volcaniclastic material near the summit lead to oversteepening and loading of the volcano's upper slopes [McGuire, 1996; Morgan and McGovern, 2005], (2) intrusion in, or around, the upper conduit leads to deformation and oversteepening of the upper flanks [Voight and Elsworth, 1997; Tibaldi, 2001], or (3) the slides are caused by failure of weak or hydrothermally altered areas within, or under the volcano [van Wyk de Vries and Francis, 1997; Reid et al., 2001]. Seismic shaking or eruptive activity could be the actual trigger for any of these underlying causes of instability. All three of these candidate processes may contribute, but the first possibility (volcanic loading) seems the most likely underlying cause of the observed collapses at Monowai for the following reasons. As shown above, the slide scars have headwalls that include the eruptive vent, where the maximum volcanic loading is expected. Furthermore, the 2004–2007 collapse scar bisects the area of volcaniclastic fallout that accumulated between 1998 and 2004 (up to +51 m) on the western flank of the summit (Figures 3f and 4f), which is consistent with the eruptive deposits having oversteepened and destabilized the upper slopes of the volcano.

[18] The volcanic loading interpretation is also consistent with the results from analog experiments by Acocella [2005], who investigated modes of collapse in cones of dry, unconsolidated, granular material. In those experiments, slide scars produced by volcanic loading were relatively narrow and shallow and always originated at the summit (Figure 7). The mean width of the collapses in the experiments was a 10°–40° wide sector (in plan view), and their depth was less than 10% of the cone height, consistent with the Monowai collapses (Figures 2 and 3). Moreover, in the analog experiments, the locations of the initial collapses were clustered in the same sector, showing that subsequent slides were influenced by the existence of preexisting slide scars. However, that influence decreased with time and the slide locations became more scattered and distributed around the edifice as the cumulative number of collapses increased [Acocella, 2005].

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Figure 7. Photograph from the analog experiments of Acocella [2005] showing a sector collapse caused by volcanic loading at the summit (by addition of the darker granular material). The narrow and shallow morphology of the resulting collapse is similar to those at Monowai (after Acocella [2005]).

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[19] At Monowai, the location of the post-1998 failures has clearly changed with time, but there is also evidence for pre-existing slide scars on the W, SW, and SE slopes (Figures 2 and 4). These similarities between the analog experiments and Monowai Cone imply that at least the surficial cover of Monowai (the uppermost 100–200 m) consists of unconsolidated volcaniclastic deposits and that the addition of fragmental eruptive material near the summit is the underlying cause of the sector collapses we observe. The alternative causes of sector collapse (intrusion, hydrothermal alteration, or volcanic spreading) would be expected to produce both wider and deeper collapses with larger volumes [Acocella, 2005] because the driving force or structural weakness is more deep-seated. It would also be expected that such causes of failure would be both less frequent and repetitive than what we observe at Monowai. Nevertheless, acidic hydrothermal alteration is well documented at other submarine arc volcanoes [de Ronde et al., 2005; Resing et al., 2007; D. A. Butterfield et al., Magma degassing, acid alteration, and metal volatility at the actively erupting NW Rota-1 submarine volcano, Mariana arc, submitted to Geochemistry, Geophysics, Geosystems, 2008] and is almost certainly a contributor to structural weakness at Monowai, based on the high sulfur content of hydrothermal emissions there [de Ronde et al., 2008; Graham et al., 2008]. Other volcanoes where volcanic loading has been called upon as a contributor to sector collapse include Stromboli [Tibaldi, 2001], Soufrière Hills [Voight et al., 2002], Mombacho [van Wyk de Vries and Francis, 1997], Socompa [van Wyk de Vries et al., 2001], Llullaillaco [Richards and Villeneuve, 2001], as well as volcanoes in the Lesser Antilles [Deplus et al., 2001; Le Friant et al., 2003; Boudon et al., 2007], and Canary Islands [Palomo et al., 1997; Carracedo, 1999; Masson et al., 2002]. Many of these examples are volcanic islands with submarine flanks, but none are completely submarine like Monowai. However, the lack of previous examples is most likely simply due to the relative difficulty in documenting such events underwater.

[20] Sector collapses create debris avalanche deposits, which are often hummocky in their upper reaches due to large intact blocks from the source area, and have natural levees at their margins [Ui, 1983; Siebert, 1984; Glicken, 1996; Ui et al., 2000]. This is also true at volcanic islands where most of the slide deposits are underwater [Moore et al., 1989; Holcomb and Searle, 1991; Mitchell et al., 2002; Oehler et al., 2004]. However, at Monowai, the deposits downslope of the collapses are relatively smooth and lack hummocks or levees, indicating that they probably consist of mostly fragmental pyroclastic material. In addition, they are relatively small in volume compared to other volcanic debris avalanches (Figure 8), which may exceed 1 km3 in size [Ui, 1983]. Nevertheless, the collapse heights (H, ∼1.4 km) and runout distances (L, >5.5 km) at Monowai are comparable to other volcanic debris avalanches documented on land (Table 2 and Figure 8). The ratio of the two (H/L, ∼0.2) is slightly higher than at other volcanoes (Figure 8), but this may simply reflect that Monowai runout estimates are only minima. The frequency of these slides at Monowai (two in 9 years) also far exceeds the frequency of large debris avalanches at other volcanoes, which is estimated to be only once every 102–104 years [Siebert, 1984; Ui et al., 2000]. Undoubtedly, this is due to the eruption frequency at Monowai, the dominantly explosive nature of eruptive activity, the small volume of the failures, the unconsolidated and seawater-saturated state of the volcaniclastic deposits, and the steep slopes near the summit of the volcano. The slopes at Monowai are ∼10° on the lower flanks, ∼20° on the upper flanks above 500 m, and up to ∼30° (near the angle of repose) on the constructional cones that have grown within the heads of the collapse scars (Figure 2).

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Figure 8. Comparison of the Monowai collapses (red stars) to other volcanic debris avalanches (filled circles) and nonvolcanic landslides (open circles). (a) Plot of drop height (H) versus runout distance (L), showing that the Monowai slides plot slightly above the diagonal H/L ratio of 0.2. Note that since the runout distances at Monowai are minima they may actually plot further to the right below the 0.2 line. (b) Plot of H/L ratio versus volume (V), showing that the Monowai slides are at the lower end of volumes and at the upper end of H/L ratios for volcanic debris avalanches (after Ui [1983]).

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[21] The closest on-land analog to Monowai's behavior may be Augustine volcano, Alaska, where Begét and Kienle [1992] noted repeated cycles of collapse and lava dome regrowth with a recurrence interval of 150–200 years during the last 2000 years. They attributed this behavior to a relatively high and sustained effusion rate during this time.

[22] Using the quantitative depth changes at Monowai over a 9-year time period, we can make first-order estimates of the volumetric rates of eruption and collapse. To derive total volumes of collapse and eruption, we have estimated the slide scar volumes before infilling and the volumes of partial infilling due to eruption (Table 3). Such adjustments are necessary because the measured volumes of negative depth change are only minima, since they include a component of erupted material that has partially infilled the slide scar after the collapse. Also, the measured volumes of positive depth change interpreted as due to eruption can be either minima or maxima. They are minima when they do not include erupted material located within slide scars (areas of net negative depth change) and/or areas beyond the bathymetric surveys. They are maxima where the deposits downslope of the vent include both erupted material and debris avalanche deposits in an unknown proportion. With these considerations in mind and making appropriate corrections for missing volumes, the estimated rate of positive volume change from eruption is 6–24 × 106 m3/a, and the estimated rate of negative volume change from collapse is 14–15 × 106 m3/a (Table 3). At first order, these volumetric rates are comparable and suggest that at Monowai, at least during phases of persistent and frequent eruption, the rates of construction and destruction are in a quasi-state of equilibrium.

Table 3. Estimated Rates of Growth and Collapse at Monowai
Surveys ComparedaVolume of Positive Depth Change Due to Eruptionb (× 106 m3)Rate of Positive Volume Change From Eruption (×106 m3/a)Volume of Negative Depth Changec (×106 m3)Rate of Negative Volume Change From Collapse (×106 m3/a)
  • a

    Number of years in interval are shown in brackets.

  • b

    Estimated volume including infill within collapse scar is shown in brackets.

  • c

    Estimated collapse volume without erupted infill material is shown in brackets.

  • d

    Minimum because some erupted material was located within a collapse scar (an area of net negative depth change) and/or erupted material could extend beyond the area of bathymetric coverage.

  • e

    The range of rates of positive volume change depend on what assumptions are made (all values are × 106 m3/a). The value of 2.4 reflects only the volume outside of the collapse scar (14.6) over the full 6-year period. The estimate of 6 assumes a volume change of 37 (14.6 + 22 within the collapse scar) over 6 years. The rate of 16 assumes that all the positive depth change (37) occurred in the 2.3 years after the 24 May 2002 T wave swarm. Wright et al. [2008] estimated a rate of 13, based on their volume calculations.

  • f
  • g

    Maximum because they include both erupted material and debris avalanche deposits in unknown proportion.

1998–2004 [6.0]14.6d [∼37]2.4d [∼6–16e]−63.0d [∼−85f]−10.5d [∼−14]
2004–2007 [2.7]60.9d [∼66]22.5d [∼24]−35.2d [∼−40]−13.0d [∼−15]
1998–2007 [8.7]198.g22.8g−45.4d [∼−125]−5.22d [∼−14]

[23] Interestingly, at Augustine volcano, where there is also a high recurrence of cyclic growth and collapse, the processes of construction and destruction also seem to be roughly balanced [Begét and Kienle, 1992; Siebert et al., 1995]. The long-term effusion rate at Augustine volcano is estimated at 2.5 × 106 m3/a, and the rate of avalanching is similar [Begét and Kienle, 1992]. Although this effusion rate is 2–10 times less than the rate we estimate for Monowai, the growth at Augustine is more episodic, so individual eruptions and avalanches are larger at Augustine, commonly about 5–10% of the edifice volume [Siebert et al., 1995]. At Monowai, where the edifice volume above 1200 m depth is 23 km3 (based on the 1998 bathymetry in Figure 2a), the collapse volumes (Table 3) are only 0.1–0.5% of the edifice.

[24] We interpret the sector collapses at Monowai to be a cyclic process that distributes volcaniclastic material from the summit to the lower slopes of the volcano at about the same volumetric rate that they are being erupted. Davidson and De Silva [2000] distinguished “youthful” triangular profiles on composite volcanoes from “equilibrium” profiles that are concave-upward. Mass wasting is the process primarily responsible for this transformation from one shape to the other, as broad aprons of clastic material create gentle distal slopes as volcanoes grow. At Monowai, a consequence of having constructive and destructive processes nearly in balance is that the volcano has a rather “mature” concave-upward morphologic profile (Figure 9), even though it is relatively young. Again, we see that the processes of aggradation and degradation are operating at an unusually fast pace at Monowai.

image

Figure 9. Profile of Monowai volcano showing concave upwards morphology of the slopes (4× vertical exaggeration, 1998 data). Endpoints for the profile are SW and NE of the summit at −25°57′, −177°15′ and −25°50′,−177°08′.

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[25] This style of volcano growth is controlled in part by water depth. In relatively shallow water, reduced hydrostatic pressure is conducive to explosive fragmentation and concomitant production of dominantly clastic deposits [Wright et al., 2003; Chadwick et al., 2008]. These volcaniclastic deposits inherently have relatively high pore space that is saturated with seawater, high angles of repose, low cohesion, and accordingly are likely to fail frequently. Other examples of active seamounts that rise to <600 m water-depth with evidence that recent fragmental volcaniclastic deposits have been subsequently redistributed downslope include: Rumble III [Wright et al., 2002], Myojinsho [Fiske et al., 1998], Kavachi [Baker et al., 2002], Macdonald [Cheminée et al., 1991], and NW Rota-1 [Chadwick et al., 2008; Walker et al., 2008]. This style of frequent and repeated small sector collapses followed by infilling and volcanic loading by eruption documented at Monowai may be characteristic of shallow, persistently active submarine arc volcanoes with frequent, dominantly explosive activity. Systematic resurveys of active submarine arc volcanoes should be continued to expand our understanding of how intraoceanic arc volcanoes grow and to better evaluate the implications of these results for tsunami hazards.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

[26] Thanks are due to the officers and crews of the R/V Sonne and R/V Tangaroa for the bathymetric surveys conducted in 1998, 2004, and 2007. This work was funded by the New Zealand Foundation for Research Science and Technology (contracts C01X0203 and C01X0702), the German Bundesministerium für Bildung und Forschung (BMBF project 03G0192), and the U.S. NOAA Vents program. The manuscript was improved by helpful reviews by Steven Carey and Lee Siebert. Susan Merle assisted with processing multibeam data and creating figures. NOAA/PMEL contribution 3221.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Methods
  5. 3. Results
  6. 4. Discussion
  7. Acknowledgments
  8. References
  9. Supporting Information

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