Case study: volcanic eruption
Nyiragongo is a steep-sided, active volcano [active volcano: A volcano which erupts regularly.] . It is located in the Democratic Republic of Congo. Nyiragongo and the shield volcano [shield volcano: A volcano with gentle slopes that was made by runny, less explosive lava.] of Nyamuragira, 40 km away, are part of the East African Rift Valley [East African Rift Valley: A large scale fracture in the earth in East Africa.] . This is an area of many faults [fault: A fracture in the Earth's crust.] where the plates are being stretched as they move away from each other.
Cause of the eruption
The eruption happened on 17 January 2002. The volcano has a lava lake in its crater. Fissures [fissure: A crack in the land formed by tectonic activity such as earthquakes.] opened up to the south side of the volcano and three streams of lava from the lake drained through the fissures. The lava reached speeds of 60 km/h. There was little warning as the lava reached the city of Goma. The unrest in the country has made it difficult to monitor the volcano and put emergency responses in place.
Social impacts of the eruption (effects on people)
Homes were destroyed by ash and lava.
45 people died in the first 24 hours.
The lava flow made it difficult to travel around Goma as it filled the roads. Aid agencies were unable to access some areas of Goma.
The lava took a long time to cool and it burnt people as they tried to return to their homes.
Cholera [cholera: A disease spread via unclean water.] spread because of lack of sanitation [sanitation: Measures to help cleanliness and hygiene. ] in areas that people fled to.
Around 50 people were killed when fuel exploded while they were trying to siphon it off at a petrol station.
14 nearby villages were destroyed in the lava flow.
Economic impacts of the eruption (effects on money and jobs)
People returned to Goma hoping to find aid. One month after the eruption, 350,000 people were dependant on aid.
People lost their businesses and jobs.
Aviation fuel stores exploded as the lava flow damaged Goma airport.
Environmental impacts of the eruption
Lava covered 15 per cent of the city of Goma and destroyed 30 per cent of the city.
If the lava was to reach Lake Kivu, or seismic activity [seismic activity: Earthquakes that happen as a result of tectonic plate movements.] disrupted the lake, then dangerous gases of carbon dioxide and methane could be released from the floor of the lake.
 Since its discovery in 1894 [Durieux, 2002/2003, and references therein], Nyiragongo volcano is well known for its persistent lava lake activity that has fascinated scientists from all over the world. The volcano is also noted for its foiditic magma composition, which produces extremely fluid lava flows, capable of descending the crater flanks at speeds of up to 100 km h−1 [Tazieff, 1977; Le Bas et al., 1986]. Such deadly lava flows occurred during the only documented historical eruption of Nyiragongo in 1977 [Tazieff, 1977]. Despite the interest scientists have shown in this volcano, very little is still known about its flank or peripheral activity, probably because of the remote location and of the lack of security due to the sociopolitical situation [Smithsonian National Museum of Natural History, 1991].
 The city of Goma, located ∼15 km south of Nyiragongo, was only a small town with 50,000 inhabitants when the 1977 eruption occurred. Following several humanitarian disasters, the Rwandan genocide in 1994 [Tedesco, 1995, 2002/2003], and the subsequent civil war that is still ravaging the country (1996 to present), Goma has expanded to the north toward the volcano, with a major influx of people from the very insecure surrounding countryside. Currently, the population of Goma is 500,000. With Nyiragongo volcano located only 15 km from the city, it is impossible to ignore the multiple volcanic hazards that the inhabitants of Goma are now facing. For this reason, we believe it is crucial to study, in great detail, the most recent eruption that occurred on 17 January 2002. It is important to understand whether the flank fracture system, opened in 1977 and further developed during the 2002 eruption up to the outskirts of Goma, may eventually penetrate the city itself. It is also important to better understand the actual plumbing system below Nyiragongo and to determine if the many peripheral cones existing around the volcano, extending up to and within Lake Kivu (the Goma Volcano Observatory (GVO) is built on one of these peripheral cones, Mount Goma, Figures 1a and 1b), represent products of a previous eruptive style, possibly a past eruptive cycle of the volcano, or are still a viable eruptive style for future activity of the volcano. Because of the current activity of the rift and the geological field constraints, Goma and the several villages located at the foot of the volcano are all at risk of future Nyiragongo lava flows.
 Our aim here is to document what happened before, during and after the very peculiar 2002 eruption of Nyiragongo, using data collected during field work, by the GVO and visual observations of Nyiragongo's crater. We also show how the activity of the volcano evolved chronologically, using a combination of data collected by the GVO and eyewitness accounts provided by local people. Our objective is also to address our understanding how Nyiragongo volcano works, using different data sets, on a local scale and in the broader framework with respect to the activity of the East African Rift system.
 Because of the difficult human and social situation in North Kivu particularly in Goma, and also because of the complications faced during our fieldwork after the eruption, not all the data we planned to gather were obtained. Despite these challenges, the experience we gained, the data collected and local witnesses' stories allowed us to start understanding what happened in the weeks and days that preceded the eruption, and also contributed to monitoring and forecasting the activity of the volcano. The available data also permit us to develop a model of how the future activity at Nyiragongo volcano may evolve.
2. The 2002 Eruption of Nyiragongo
2.1. Precursory Signals
 The five-station seismic network maintained by the GVO was vandalized during a civil war lasting from 1996 to present. Only two of the original five seismic stations survived (Figure 1a). Beginning in December 2000, volcanic tremor was recorded. On 6 February 2001, Nyamulagira volcano, located about 15 km NW of Nyiragongo, erupted. Following the Nyamulagura eruption, seismic activity, volcanic tremor and long-period earthquakes continued to be recorded by the two surviving stations (Bulengo and Katale; Figure 1a, operational since 1994). The recorded activity was considered by GVO as an indication of high fluid and/or lava pressure within the system [Global Volcanism Network Bulletin, 2001]. However, using data from only two seismic stations did not allow understanding of whether the activity was related to Nyamulagira or to Nyiragongo.
 On 7 October 2001, an earthquake of magnitude 3.5–4.0, located in the Nyiragongo area, followed by high-amplitude volcanic tremor, was recorded by the seismic network and strongly felt by the local population. During two visits to Nyiragongo in late 2001 (29 October to 1 November and 8–10 December), GVO researchers witnessed several changes/anomalies: (1) a small, dark ash plume was emitted from the small 1995 spatter cone within the crater (Figure 2a); (2) emissions of water vapor from the 1977 eruptive fracture, located above the Shaheru crater (Figure 1a) 2 km south of the summit crater at 2700 m above sea level (asl), and (3) similar water vapor emissions from concentric cracks on the floor of the 1995 lava lake in the central crater of Nyiragongo (Figure 2b). Variations in ground temperatures, typically 5–9°C, up to 28°C were recorded, and anomalous temperatures of ∼50°C were measured in cracks upslope of the Shaheru crater.
 On 4 January 2002, a seismic event, with characteristics similar to that of 7 October 2001, occurred. Witnesses from villages on the eastern flank of Nyiragongo volcano reported that the earthquake was accompanied by emissions of a dark plume and by rumbling sounds from the central part of Nyiragongo. Volcanic tremor remained at a high level until 16 January 2002 but the eruption itself was preceded by almost 8 hours of perfect calm without tremors or long-period earthquakes. On 16 January 2002, a few hours before the eruption commenced, a strong smell of sulphur was reported ∼1000 feet above the volcano by the pilot of a TMK local airline flying north of Nyiragongo (T. Hoaru, personal communication, 2002).
2.2. Chronology of the Eruption
 The eruption started at 0825 local time (LT) (0625 UT) on 17 January 2002 with the reopening of the 1977 eruptive fracture system [Tazieff, 1977]. The eruptive fracture started at an altitude of 2800 m, between the central cone of Nyiragongo and the Shaheru crater (Figure 1a).
 The drainage of still-molten lava stored in the summit crater after the 1995 lava lake activity opened the eruptive sequence. The presence of lava “nests” chilled in tree branches at a height up to 5 m, located up to 30 m from the eruptive fracture above Shaheru, suggests a lava fountaining activity during the initial phase. Very fluid lava flows ran across the forested SE flanks of Nyiragongo and rapidly cut the road north from Goma to Rutshuru. The low viscosity of the lava is inferred from the ≤1.5 m highstand marks left by the lava flow on trees, compared with the thickness of the solidified flow, which is only 5–15 cm.
 Within the next hours the fracture system and associated eruptive activity propagated down to the base of the volcano. Two sets of parallel eruptive fractures, about 300 m apart, first opened through the southern flank of Shaheru cone (Figure 1a) and then extended downslope forming a series of grabens (∼5–10 m wide) across banana fields, villages, and older peripheral volcanic structures. Between 1000 and 1100 LT, lava flows erupted from a series of vents at an elevation between ∼2300 and 1800 m, devastating several villages. Between 1400 and 1620 LT the propagation of magma along a dike radial to the volcano continued simultaneously with the southward propagation of fractures toward Goma, down to 1580 m asl, forming a line of spatter cones SE of Munigi village and only 1.5 km NE of Goma airport. These vents produced intense lava fountains and a voluminous lava flow that ran through the airport and central Goma and finally reached Lake Kivu late in the evening.
 Another eruptive fissure opened at 1530 LT at a higher elevation (2250–2000) about 1.5 km west of the main fracture system (2 km west of Kibati; see Figure 1a). Eyewitnesses reported that this fissure initially produced passive effusive activity feeding pahoehoe lava flows. However, the presence of a scoria deposit around the northernmost portion of the vent indicates that the activity did include a phase of lava fountaining that eventually produced a line of hornitos and spatter cones. Effusive activity then resumed to produce aa-type lava flows, 1–2 m thick, that flowed down the southern flanks of the volcano and formed the second main flow that reached western Goma, stopping on the main Goma-Sake road (Figure 1b).
 The death toll during the eruption was reportedly 170, and as many as 350,000 people fled the advancing lava, mainly eastward to nearby Rwanda. Lava emission appears to have stopped during the night of 17 January 2002, hence the eruption lasted ≤12 hours. However, molten lava continued to flow toward (and then into) Lake Kivu for a few more days. This lava created a delta approximately 800 m wide and 120 m long (Figure 1b) which, according to submersible investigations [Halbwachs et al., 2002], extended into the lake up to a depth of 60 m. Lava flows destroyed one third of the airport runway, Goma's main business and commercial center, and the homes of ∼120,000 people. Between 60 and 100 people died on 21 January as a result of an explosion at a gas station surrounded by hot lava and about 470 were reported injured with burns, fractures and/or gas intoxication.
2.3. Sulfur Dioxide Emissions
 Nyamulagira is well known for degassing significant quantities of sulfur dioxide (SO2) during its frequent effusive eruptions, but prior to the January 2002 eruption, there had been very few quantitative observations of gas emissions from Nyiragongo. Emissions during each of Nyamulagira's 14 eruptions since 1980 have been measured by the NASA Total Ozone Mapping Spectrometer (TOMS) instruments [Carn et al., 2003; Carn and Bluth, 2003]. Phases of elevated degassing have occurred in the past at Nyiragongo, most notably in the 1970s [e.g., Le Guern, 1987; Carn, 2002/2003], but the most vigorous episodes preceded the launch of the first TOMS mission in late 1978. Any subsequent degassing (e.g., during reactivation of the lava lake in 1994) did not produce SO2 concentrations large enough to be measured by TOMS, and the first ground-based or airborne measurements of SO2 emissions from Nyiragongo were not attempted until 2004.
 SO2 emissions during the 17 January 2002 eruption were measured from space by the TOMS instrument on the Earth Probe (EP) satellite [Carn, 2002/2003]. A cloud containing ∼9 ± 3 kilotons (kt) of SO2 was detected extending WNW from Nyiragongo at 1108 LT on 17 January. Infrared (IR) cloud top temperatures derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite indicate that the eruption cloud rose close to the tropopause (∼14–17 km), though SO2 had dispersed to below EP TOMS detection limits by the following day. The eruption, and most likely the SO2 emissions, continued after the EP TOMS over passed, and extrapolation yields an estimated total SO2 discharge during the eruption of 15–31 kt. Interpretation of several satellite data sets in conjunction with the eruption chronology indicates that the eruption plume emerged ∼25–45 min after the onset of flank lava flows, which argues against significant preeruptive volatile overpressure and supports a regional tectonic trigger for the eruption [Carn, 2002/2003]. Inferred SO2 emission rates during the first ∼2 hours of the 17 January 2002 eruption (∼850–1700 kg s−1) are significantly lower than peak rates measured during recent eruptions of Nyamulagira (as high as ∼10,000 kg s−1) [Carn and Bluth, 2003]. This is most probably a consequence of lower lava effusion rates at Nyiragongo and/or substantial preeruptive degassing of the erupted magma batch in Nyiragongo's long-lived summit lava lake, but could also be partly attributed to scrubbing of SO2 in the eruption plume, co-emission of hydrogen sulfide (H2S), and/or lower sulfur concentrations in Nyiragongo magmas.
3. Posteruptive Events
 Arguably the most important feature of the January 2002 eruption is the intense, and unusual, seismic activity recorded mainly after the effusive activity. About 100 tectonic earthquakes (M > 3.5) located between Goma (the lake shore) and Nyiragongo were recorded during the 5 days following the eruption. The strongest earthquake (magnitude 5) struck at 0014 (UT) on 20 January 2002 [Tedesco et al., 2002]. The seismicity was locally recorded by two GVO seismic stations (Bulengo and Katale, Figure 1a), complemented after 24 January 2002 by one additional seismometer brought from France [Komorowski et al., 2002/2003]. This seismic activity impacted Goma and Gisenyi but was also clearly felt as far away as Bukavu (60 km south in DRC), Kigali (Rwanda, 120 km east) and Kampala (Uganda, 150 km NE). The number of earthquakes gradually declined with time but remained at abnormally high levels [Tedesco et al., 2002].
 The seismic network in operation during the eruption and until 30 January 2002 allowed neither an accurate assessment of the location nor the depth of earthquakes. However, the small difference in the arrival times of P and S waves indicated epicenters close to seismic stations. Seismic events showed a significant range of magnitude, frequency contents, and S-P times, suggesting several different origins. Seismic signals showed a typical sequence of tectonic events followed within minutes to hours by long-period events and volcanic tremor often lasting more than 10 hours (Figure 3). Such a sequence, which is similar to that recorded during the year preceding the eruption, suggests fracturing of rocks followed by intrusion of fresh magma and movement of juvenile fluids.
 During the first days of February 2002, the estimated location of epicenters seemed to cluster in two different areas. One cluster was located along the system of N-S fractures running between the crater and Lake Kivu, and the second was located near the town of Sake (Figure 1a), close to the western border of the rift [Tedesco et al., 2002].
3.2. Crater Collapse and Explosive Activity
 The solidified lava lake surface inside Nyiragongo's summit crater, lying 280 m below the rim since 1995 [Tedesco, 1995, 2002/2003], was still present on 21 January 2002, 3 days after the eruption, although it was cut by a series of annular rings of fumaroles (Figure 2b). The crater collapsed during the night of 22–23 January 2002. A detailed report by eyewitnesses in Rusayo (8 km SW of the summit) suggests that collapse started at about 2051 LT on 22 January, in connection with a series of felt earthquakes. It was accompanied and followed by roaring sounds and flames above the crater and, soon after, by emission of ashes, which fell over Rusayo and the southern flank of Nyiragongo. Intense and continuous seismic tremor over the next 4 hours suggested a postcollapse episode of phreatomagmatic explosive activity (which most likely involved only magmatic gases as the juvenile component) and ash emissions. Light ash falls also occurred over the Goma-Sake road during the same night and, in the following morning, in Goma. On 24 January, during a helicopter survey, an ash deposit was observed to mantle the forested SW flank of the volcano. Meanwhile, an assessment of the extent of collapse in the summit crater (Figure 2c) was also carried out. The depth of the inner crater had increased from 280 m before the eruption to ∼900 m, with an inverted cone geometry (Figure 2c). Sporadic explosive activity occurred after the main collapse. A dense cloud rose above the volcano at 0910 LT on 24 January 2002 shortly after a helicopter overflight. On 27 January 2002, fresh impacts and complete destruction of trees were observed in the forest on the upper northern flank of the volcano.
3.3. Gas and Fluid Emanations
 Noticeable hydrocarbon odors occurred days after the eruption in several parts of Goma. Measurements with a portable IR spectrometer and with more precise preevacuated flasks, later analyzed in the laboratory, revealed that these odors were accompanied by CH4 and CO2 (both odorless) in areas up to 800 m from lava flows [Tedesco et al., 2002; Allard et al., 2002; Vaselli et al., 2002/2003]. The emanations therefore had no clear or direct relationship with organic matter burned or heated by the flows. In several cases, orange-blue flames were witnessed directly on or beside the lava flows. Methane concentrations of a few per cent and sometimes approaching the 5% flammability threshold in air were found both in open air (discharged through pavements, in gardens and close to the airport runway) and indoor sites (garages, hotels) [Tedesco et al., 2002; Allard et al., 2002]. In some cases, the gas discharge occurred very violently with gas explosions, destroying concrete pavements several centimeters thick. The threat posed by toxic gas emissions has been subsequently highlighted by the discovery of a long fissure under the Kanisa LaMungu church in central Goma [Tedesco et al., 2002]. Carbon dioxide emissions from church floor cracks were strong enough to cause two cleaning ladies to faint. The church was subsequently sealed off.
 It is noteworthy that most of the CO2 emissions and gas bursts occurred in areas that are broadly aligned with the N-S fracture system cutting the volcano (Figure 1a