Volcaniclastic sediments of the active Kolbeinsey Ridge, north of Iceland, reflect changing fragmentation mechanisms due to increasing sea level during the last deglaciation. Glass shards deposited on the western flank of the mid-oceanic ridge before 13.4 ka are vesicular (20%-60%), whereas shards deposited since 13.4 ka are mostly vesicle free and of hyaloclastic origin. The vesicle-rich shards display morphology thought to be atypical for submarine eruptions at this depth (400-500 m). The environmental conditions during the deposition of these layers (regional ice cover, more than 200 km from the nearest subaerial source) preclude a subaerial origin for these shards. We propose an eruption mechanism in which hot vesiculating bombs from a low-energy eruption are transported upward through the water column by convection. These particles reach their volcanic fragmentation depth (VFD) and undergo secondary fragmentation, creating the vesicle-rich shards recovered near the ridge. Rising sea level since the end of the last glaciation was sufficient to prevent the erupted particles from reaching the VFD, thus ending the formation of vesicle-rich shards. This model explains the presence of vesicular shards in sediments of mid-oceanic ridges where no subaerial source can be inferred.