April 2007 collapse of Piton de la Fournaise: A new example of caldera formation

2.1. General View

[4] PdF is the active volcano of La Réunion Island (Figure 1). The eruptive centre is located in a large 8 km across caldera, the Enclos Fouqué, where most of the eruptions occur since 4.5 ky [Bachèlery, 1981]. The present day summit shows two collapsed structures named the “Bory crater”, which is currently inactive, and the “Dolomieu crater”, which is the locus of numerous summit eruptions. Carter et al. [2007] recently showed that the pre-2007 elongated geometry of Dolomieu results from recurrent pit crater collapses.

[5] The activity of PdF is characterized by fissure eruptions fed by a magma reservoir located at about sea level [Fukushima et al., 2005; Peltier et al., 2007]. Considering the location of the eruption site, three types of eruptions can be distinguished. 1- Summit eruptions start and remain in Dolomieu. 2- Proximal eruptions, which may start in the summit but progress to the flanks of the central cone and usually propagate downslope to the Enclos caldera floor. 3- Distal eruptions develop away from the central cone, starting in the Enclos caldera floor. During the last century, the distal eruptions were in some cases associated with the development of summit collapses [Bachèlery, 1981; Carter et al., 2007]. Among them, the most voluminous eruption of 1931 with 130 Mm³ was related to a large collapse corresponding to the eastern half of Dolomieu [Lacroix, 1938, Figure 1]. The lack of any continuous observation and monitoring during the largest events does not allow the determination of the precise relationship between these eruptions and the associated summit collapses.

2.2. Summary of the March 1998–February 2007 Evolution

[6] Since 1998, PdF has been characterized by an intense eruptive activity with 2–4 eruptions per year. Most of the eruptions correspond to summit and proximal events. Some of them, the summit eruptions, contributed to the progressive filling of Dolomieu by the accumulation of pahoehoe lava flows. Total filling of Dolomieu was attained with the August 2006–January 2007 summit eruption during which a pile of 20–30 m of lava flows accumulated on the crater floor. Five distal eruptions occurred during the 1998–2007 period. They were mainly concentrated in the Plaine des Osmondes depression, along the NE rift zones (Figure 1).

[7] Before 2000, the eruptions of PdF were showing a similar evolution with a progressive disappearance of the tremor [Battaglia et al., 2005]. Volcano-tectonic (VT) events seldom occurred during the eruptive phases (Figure 2). The eruption evolution progressively changed with an increase of both tremor amplitude and seismicity before a rapid end of the eruption [Longpré et al., 2007]. Since 2000, the seismicity was especially abundant during voluminous eruptions, most of them being distal. During the November 2002 eruption, the seismicity intensified 5 days before the end of the eruption and continued at shallower levels before the collapse of a small pit crater on December 23 in Dolomieu [Longpré et al., 2007]. Such a surface phenomena did not occur during the other distal eruptions despite a strong increase of the VT seismicity in both frequency and magnitude below the summit. However, the coeval intensification of the emission rate and the VT events suggests that the overlying rock column beneath Dolomieu experienced recurrent destabilizations.

3.1. Development of the Eruption

[8] On February 26 the seismic activity began below the summit zone. It progressively increased and reached the value of more than 100 daily events on March 28 to 30. A seismic crisis started on March 30 at 16:25 UTM and the magma emission begun at 18:50 from an eruptive fissure located at 1900 m asl SE of the central cone (Figure 1). The tremor ceased on March 31 at 05:15, after a first eruptive phase of less than 10 hours during which only a small volume of magma was emitted. The summit seismicity continued until the April 2 new eruptive phase in the Grand Brûlé at ∼600 m above sea level and 7 km away from the summit (Figures 1 and 3) . The eruption site was characterized by ∼50 m high continuous lava fountains feeding voluminous lava flows. After a classical period of decreasing activity, the number of VT events gradually increased from April 3 (period 1 in Figure 3). On April 5, the VT events disappeared while both a general low frequency seismic signal and the tremor intensified (period 2 in Figure 3). The permanent GPS located ∼200 m north of the northwestern rim (see Figure 1 for location) started to show an inward displacement of the summit zone at ∼12:00 (Figure 3e), which was coeval with the increase of the seismic signal at the eruption site. The activity changed at 20:48 after a M_d 3.2 earthquake occurred below the summit crater (C₁ in Figures 3c and 3e). This event was contemporaneous with a sudden outward displacement of ∼15 cm of the GPS station (Figure 3e). An increase of the seismic signal of ∼50% was recorded at Tkr seismic station after the large earthquake. Then, the seismic signal was organized in cycles, the frequency of which gradually increased from one cycle every two hours to one cycle every 30 minutes (period 3 in Figure 3b). The seismic cycles were coeval with a step-by-step increase of the tremor until April 06 08:00 (Figure 3d). GPS data indicate that each cycle was characterized by progressive inward displacements and ended by a sharp outward motion. April 6 corresponds to a paroxysmal phase during which 200 m high lava fountains were observed in the Grand Brûlé. The first observations of the summit zone made the afternoon of the 6th revealed that the intensification of both the seismicity and the tremor was coeval with a collapse of Dolomieu (Figure 4b). The tremor progressively went down to its initial level, i.e., before the paroxysmal phase, whereas the cyclic seismic signal remained until the 7th of April 01:00. The eruption continued until the 1st of May with a fluctuating tremor. The total volume of magma emitted during the eruption is hard to assess since a large amount of lava flowed down to the sea where it formed a large platform. However, given the topography and the bathymetry before the event, a volume of ∼100–140 × 10⁶ m³ has been inferred. This makes this eruption one of the most voluminous of PdF during the XXth and XXIth centuries.

3.2. Collapsed Structure

[9] Prior to the April 2007 eruption the summit zone of the central cone was occupied by a main collapsed structure, the Dolomieu crater. Historical reports reveal that it results from the coalescence of several pit craters aligned in the E-W direction [Lénat and Bachèlery, 1990; Carter et al., 2007]. Before April 2007, the ∼74 × 10⁴ m² structure (800 m wide and 1100 m long), was completely filled in by lava flows (Figure 4a).

[10] The April 2007 eruption led to the largest historical collapse of PdF, i.e., since 1760. Although the previous coalescent collapse structures were interpreted as pit craters [Carter et al., 2007], we propose that the new structure, which is at least twice larger than the previous collapses is most akin to calderas. Indeed, the collaspe that is of the size of small calderas [Geshi et al., 2002] directly influenced the magma chamber dynamics. The caldera was first recognized in the afternoon of April 6, about 16 hours after the beginning of the seismic cycles. Observations revealed that the collapse affected first the northern part of the pre-existing Dolomieu (Figure 4b). The new structure was elongated along an E-W direction and bounded by sub-vertical scarps in the E, N and W. Its geometry was about 200–300 m deep, ∼1 km long and ∼600 m wide. Two annular plateaus corresponding to the pre-existing floor of Dolomieu were remaining in the E and the S. On April 10 the caldera was enlarged to about the size of the pre-existing Dolomieu structure (i.e., 800 m wide and 1100 m long; Figure 4c), and deepened to ∼330 m. Only few perched terraces remained from the collapse eastern plateau.

[11] The geometry of the collapsed caldera did not significantly change after April 10. The size of the caldera increased by a few tens of meters with the lateral collapses of the western and north-western scarps along pre-existing concentric fractures already described by Lénat and Bachèlery [1990] and Carter et al. [2007]. The total area of the April 2007 caldera (82 × 10⁴ m²) is roughly similar to that of the pre-existing Dolomieu (74 × 10⁴ m²). This might be due to the fact that the collapse was mainly controlled by pre-existing concentric faults and/or that the magma chamber, the size and the location of which were recently determined from inversion of GPS data (diameters of 1.4 and 1 km in the E-W and N-S directions, and 0.3 km a.s.l. [Peltier et al., 2007]) did not change since the XX^th century. A maximum caldera depth of ∼320–340 m was determined from triangulation, leading to a final volume of the caldera of 100–120 × 10⁶ m³. These values are confirmed by ASTER stereo images (M. Urai et al., Depression volume at Piton de la Fournaise volcano estimated by ASTER stereo imaging function, submitted to Geophysical Research Letters, 2007). Note that ∼80% of this volume results from the early stage of the collapse, before the first observations on April 6.