A canyon's catastrophic birth
Fjaðrárgljúfur presents a deceptive timeline. While the bedrock itself is ancient—primarily palagonite tuff around two million years old, formed during cold periods of the Ice Age—the canyon as we see it was created with incredible speed. Its formation dates to the end of the last Ice Age, approximately 9,000 years ago.
The dramatic, winding gorge was not slowly etched by the small Fjaðrá river that flows through it today. Instead, it was violently excavated by a glacial lake outburst flood, a phenomenon known in Iceland as a jökulhlaup. As the massive glaciers of the Ice Age retreated, a large lake formed in a valley, dammed by a resilient segment of rock. Runoff from the melting glacier poured into this proglacial lake, heavy with abrasive sediment. Eventually, the natural dam failed. A torrent of water and sediment burst through, carving its way through the softer palagonite rock with immense force. This single, catastrophic event gouged out the 2-kilometer-long and up to 100-meter-deep canyon in a geologically short span.
From torrent to trickle
Once the proglacial lake had emptied and eventually filled with sediment, the river's power diminished significantly. The Fjaðrá river, which now occupies the canyon floor, is smaller than the flood that created it. It originates in the Geirlandshraun mountain and continues to modify the canyon, but its flow is minor compared to the ancient flood.
The canyon's sheer walls record this geologic history. The layers of palagonite, a type of volcanic rock created when lava cools rapidly in water or ice, give the walls their unique, rugged texture. Today, these dark cliffs are often covered in a fragile, bright green moss, which can take decades to recover from damage. The serpentine shape of the canyon, with its steep, narrow passages, resulted from the powerful floodwaters finding the path of least resistance through the two-million-year-old bedrock.