The Igwisi Hills are the only places in the world where possible kimberlite lava flows have been found, in form of calcite-olivine lavas. Kimberlite tuffs are also found, a rare species which is very susceptible to erosion. True kimberlites are usually very old eruptive rocks, consequently any subsurface volcanic structure has long since been eroded away. These kimberlites were erupted in a fairly nonexplosive fashion. Not all researchers agree that these lavas are kimberlites, however, with the low alkali content being cited as a difference although the Benfontein kimberlites share this property with the Igwisi hills ones. If the Igwisi Hills aren't true kimberlites, the next youngest would be the 32.3 ± 2.2 Ma Kundelungu plateau pipes in the Democratic Republic of Congo.