A Two-Stage Geological Saw
Gullfoss is not a single plunge but a two-tiered system cutting through ancient rock. The process began at the end of the last ice age, as meltwater from the Langjökull glacier formed the powerful Hvítá river. The river flows across layers of rock formed during warm interglacial periods. These layers consist of hard dolerite (a type of basalt) and softer, sedimentary layers of conglomerate rock, mudstone, and gravel. The water uses these differences in hardness. It carves through the softer materials more easily, creating a stepped descent.
The upper cascade is a drop of 11 meters (36 feet), followed by a second, more dramatic plunge of 21 meters (69 feet). This second drop turns at a near-right angle into a narrow canyon, the Gullfossgjúfur. This canyon is approximately 2.5 kilometers long and up to 70 meters (230 feet) deep. The constant abrasive action of the water continues to lengthen the canyon by an estimated 25 centimeters each year. The primary cause for this rapid excavation is the presence of loose gravel lenses within the sedimentary layers, which the river easily washes away.
The Force of Glacial Meltwater
The erosive power of Gullfoss is a direct function of the volume of water flowing from the Langjökull glacier, Iceland's second largest. The average flow rate in summer is 140 cubic meters per second (about 4,943 cubic feet per second). This volume decreases to around 80-109 cubic meters per second in winter when much of the glacier is frozen.
The canyon's most significant shaping events are likely catastrophic glacial outburst floods, known as jökulhlaups. During these events, the flow rate can surge dramatically. The highest measured flood recorded a flow of 2,000 cubic meters per second. Such immense volumes of water carry a high sediment load, turning the river into an effective tool for grinding down the basalt canyon walls. While gradual erosion is constant, some geologists believe these periodic, high-energy floods were responsible for the initial, rapid carving of the canyon at the end of the last ice age.