Is Africa splitting in two? Yes… and no.

The Suswa area of Kenya has been in the news over the past few weeks, with striking footage of large fissures up to 20 m across and hundreds of metres long, which have opened up across an important highway. Along with dramatic headlines about Kenya “splitting in two”, much of the coverage has linked the cracks to the East African Rift System (EARS), and speculated they could be due to movement on faults, or even uprising magma.

Other reports have played down these explanations, citing local geologists who suggest the most likely explanation for the fissures is the recent heavy rain washing out soft volcanic ash down pre-existing cracks and faults, leading to the sudden collapse of overlying surface layers.

Matching up the approximate location mentioned in the reports with the topography visible in the footage, the latest crack appears to be around 13 km southeast of Longonot volcano (lat -0.995951°, lon 36.541310°), running approximately north-south. That orientation certainly matches the general structural trend in this part of the rift valley, with this extract from the 1:50k structural layer of NPA’s EARS BasinMap showing many faults and fractures along this trend, including one passing very close to the site of the latest fissures (red star).

But that doesn’t help ascertain whether the latest fissures have been triggered by new displacement along these faults, or simple erosional processes along existing fractures. InSAR uses satellite radar to measure ground motion, and has previously been used to reveal displacements caused by both fault extension and magma injection events elsewhere in the EARS. If these cracks are due to either of these processes, we’d expect to see similar widespread displacement patterns surrounding the fissures. Washout of material down existing cracks would be unlikely to cause any wider displacement away from the fissure itself.

InSAR displacements due to Dabbahu dyke injection, from Wright et al 2006.

NPA’s InSAR team have processed Sentinel-1 data across the area for the period immediately before the fissures appeared. Close to the fissure location, the amount of surface change is too high for InSAR to retain coherence and the measurements are noisy. However the surrounding areas show no evidence of any wider precursor displacement.

Example interferogram 19th Feb – 3rd Mar, showing incoherence across the approximate location of fissures (red oval) and no substantial signals across the wider area.

The lack of precursor deformation makes a dyke injection event very unlikely. No significant local seismic activity has been reported in the area, which would be expected to occur with both faulting and magma injection events. And looking at optical imagery across the area, there is a clear incised drainage channel to the north, which becomes much less prominent across the area of the fissures, before reappearing to the south. Combined with the greener more vegetated appearance of this area, this could well provide additional evidence for erosion by subsurface water flow.

Optical satellite image from Google Earth, showing incised drainage channel to the north and south (red arrows).

So although the rift is certainly tectonically active and that definitely has the potential to cause substantial surface displacements, this time all the evidence supports the opinion of the local geologists on the ground – these particular fissures are triggered by erosion.

Credits – Sources as cited in the text. InSAR processing by David Hamersley of NPA Satellite Mapping, contains modified Copernicus Sentinel data 2018.