HazBlog-003: Mapping lava flows with HazMapper – the 2002 eruption of Mount Nyiragongo in the Democratic Republic of the Congo

In this post we are going back in time 18 years to highlight the ability of HazMapper to identify lava flow inundation extents. Mount Nyiragongo, one of Africa’s most active stratovolcanoes and part of Virunga National Park, is positioned 20 km north of the city of Goma. In 2002, between January 17 and February 3, Mount Nyiragonga erupted when a 13 km-long fissure opened on the mountain’s south flank. The chemistry of the lava at Mount Nyiragongo is quite silica poor, meaning that the lava is very fluid and is capable of traveling at speeds of up to 60 km/hr down the volcano’s steep flanks.

Lava Lake at Mount Nyriagongo, courtesy Caitjeenk

Lava erupted from three spatter cones that formed at the south end of the fissure. The streams of lava were 200 – 1000 m wide and up to 2 m deep. Within hours of the start of the eruption, lava reached the outskirts of the city of Goma (2019 pop. 2 Million). Two major streams of lava entered the city. The eastern stream covered the northern third of the runway at the Goma International Airport, before eventually continuing through the city and into Lake Kivu, one of the African Great Lakes.

Overview map from HazMapper depicting the 2002 lava flows from the southern flanks of Mount Nyiragongo, through the city of Goma and into Lake Kivu. The lava flows are identifiable as large, coherent decreases in rdNDVI (red color polygons) due to the complete replacement of vegetation.

As a result of the 2002 eruption of Nyiragongo volcano, some 400,000 people evacuated into neighboring Rwanda and around 245 people died from asphyxiation by carbon dioxide and collapse of buildings caused by the lava flows and earthquakes associated with the eruption sequence. The lava flows covered around 15% of the city, destroying 4,500 buildings and leaving 120,000 people homeless.

Detail of the 2002 Mount Nyiragongo lava flows as identified by HazMapper. The Goma International Airport along with ~4,500 buildings in the city were covered (destroyed) by the lava flows. The lava flows are identified in this image as coherent polygons with ~-30% change in rdNDVI using 12-month pre- and post-event observation windows. We used 12 month observation windows with the Landsat 7 multispectral data due to persistent clouds and volcanic haze in the Goma area in the immediate months after the eruption.

For this analysis, we utilized the Landsat-7 data archive available for cloud computing within Google Earth Engine. A key strength of the USGS’ Landsat program is the sheer amount of time the program has been collecting multi-spectral imagery. Started in 1972, eight individual missions have been launched, with the ninth slated for launch in 2021. So although the spatial resolution of the Landsat imagery (30m/pixel) is ~9 times less than that of the Sentinel-2 satellites (10m/pixel), the longevity of the Landsat data archive allows analysis of decades-old natural hazard events, as in this example. Currently, HazMapper is built to ingest Sentinel-2, Landsat-7 and Landsat-8 datasets, bringing analytical capabilities back to ~1999. Because of its coarser resolution, utilization of Landsat data in HazMapper typically results in faster cloud computing times, especially for large spatial extents.

Using a combination of multi-spectral data analysis and repeat satellite imagery, one can observe how Goma developed and evolved in the face of the destructive lava flows. In the following GIF, we can see the extent of lava flows covering the northern third of the runway at the airport and the devastation of the residential areas west of the airport. In the subsequent years, rebuilding occurs and by 2019, conditions look to be very similar to those before the eruption. Note that the HazMapper rdNDVI results in the GIF below are overlain on top of a 2019 satellite image, as a pre-2002 eruption image for Goma is not available in Google Earth.

Animated GIF combining multispectral data analysis using HazMapper and repeat satellite imagery obtained from Google Earth highlight the area that was inundated by lava during the January to February 2002 eruption in the vicinity of the Goma International Airport. Subsequent scenes depict rebuilding of residential and aviation facilities on top of the 2002 lava flow. Note that the HazMapper rdNDVI results are overlain on a 2019 satellite image because a pre-eruption image is not available in Google Earth.

Since the 2002 eruption, homes, businesses, and roads have been built atop the geologically young volcanic rock. Basalt blocks are used as building stones and road materials. A monument in the newly re-developed city center pays homage to the event.

Mount Nyiragongo and its wildlife. A monument in modern day Goma. Photo: Esther Nsapu, GPJ Democratic Republic of Congo

We’ll continue showing examples of what is capable with the HazMapper platform until we can finally release it! Peer-review is underway, stay tuned and thanks for reading.

– Corey Scheip & Karl Wegmann | Geomorphology @ NC State University

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