Previous studies on the alternating lava-hyaloclastite units of the Siða Formation, SE Iceland have provided contrasting interpretations on the eruptive environment, lava- hyaloclastite emplacement mechanisms and depositional setting. This study combines a review of previous literature with new fieldwork, to refine previous models and determine hyaloclastite emplacement mechanisms. Sedimentological and geometric hyaloclastite analysis using photo-mosaics, ground-truthing and sample analysis revealed ‘sheet-like’ and ‘lensoid’ hyaloclastite architectures, in addition to a variety of sedimentary structures (i.e.cross-beds). The two-dimensional geometry of the sheet-like units suggests the emplacing flows ‘infilled’ paleo-depressions and ‘buried’ topographical highs. This coupled with their large lateral extents (100s meters) and unit thicknesses (≥75m) implies the hyaloclastite was deposited by unconfined large-volume floods. Horizontal upper-plane bedding, large-scale (>2m) cross-beds, clinoforms (>5m), “floating clasts” and antidunes in the sheet-like units also infers the occurrence of glacier outburst floods since these features are documented in flood sequences on nearby glacier outwash plains. In contrast, lensoid hyaloclastite units are characterised by large u-/v- shaped bodies, which thin towards the margins. Large-scale clinoforms on the margin of a hyaloclastite lens at Location 2 resembles point bar formation in a braided river channel, while small-scale cross- and trough-cross bedding indicates the development braided river bars. The geometries of the lensoid hyaloclastite units are comparable to those of fluvial channels documented in the literature and their size is analogous to the dimensions of braided rivers found on the surface of Mýrdalssandur and Skeiðaràrsandur outwash plains.This study argues that the Siða Formation represents a new glaciovolcanic “type sequence”, formed by large-volume (flood-basalt) eruptions in an ice-marginal (possibly ice-free) vent setting. During the event, lava emplacement down subaerial river channels on outwash plains was followed by deposition of large volumes of volcanogenic meltwater flood debris on top of still-active lava flows. If this interpretation is correct, such processes could have dominated sequences triggered by deglaciation across much of the active rift zones of Iceland.