Abstract A composite welded tuff layer surrounding the Krafla volcanic center in northern Iceland was erupted when the caldera was formed during the last interglacial period in Iceland, 70 to 100 thousand years ago. The stratigraphic units of the up to 50 m thick layer are: silicic ash, basaltic scoria, intermediate vitrophyre, intermediate welded and agglutinated spatter, and intermediate densely welded tuff. Stratigraphic and petrochemical study of the layer suggests that it was formed by an initial explosive eruption of silicic magma triggered and immediately followed by basaltic magma, both erupting in Plinian activity. The eruption continued with Strombolian activity producing incompletely mixed silicic/basaltic pyroclastics. Most of the layer is made up of this, apparently intermediate, heterogeneous welded tuff. Welding within the layer is the result of the different solidus temperatures of basalt and dacite; after deposition, the silicic component is being kept above its softening point by the hotter but solid basaltic component. Isopleth and isopach maps of the layer indicate a source area in the southeastern part of the caldera. Estimated DRE volume of the layer is 2.4 km 3 which accounts for a 50–55-m subsidence of the 45-km 2 caldera floor. Drill hole data from within the southern margin of the caldera reveals the layer at a depth of 290 m, which indicates accumulation of about 13.5 km 3 of volcanic rocks since its formation.