The outwash plain originated primarily as alluvial land, i.e. as an accumulation of sediments from the rivers. The sediments deposited here in the Holocene alone range from 100 to 200 km³. However, volcanic eruptions of the volcanic systems of Grímsvötn and Öræfajökull have added to these sediment deposits. The sediments are carried during glacial floods with the glacier water, which thaws during volcanic eruptions, but also through ash deposits.