as an alluvial plain. The "Yirrigué" caldera is part of a large, rhyolitic, shield-shaped volcano that developed atop a tectonic horst, which in turn may have been formed by intrusion of magmas. The ignimbrites have buried older terrain and filled valleys.

Southeast of Toussidé lies the 8 x 6 km caldera Trou au Natron (also known as Doon or Doon Orei). Its 700-1000 m rim is cut into sequences