04/08/2026
Here is my final installment of Iceland geology. I’m closing it out with a little bit of geomorphology: knickpoints and waterfalls.
A knickpoint is basically a sudden change in slope. A waterfall is the most dramatic expression of a knickpoint. A river normally has a smooth, concave-up profile. When something disrupts that equilibrium, a step forms in the profile. That step becomes a knickpoint, and if steep enough, it appears as a waterfall. Waterfalls, or knickpoints, slowly erode upstream.
Iceland is almost a perfect natural laboratory because multiple processes constantly create and maintain knickpoints:
1. Glacial retreat (major driver)
• During the last ice age, glaciers carved deep valleys. When glaciers retreated, rivers were left “hanging” above deeper main valleys. This creates sudden drops → waterfalls.
2. Basalt layering & differential erosion
• Iceland is built from stacked basalt lava flows. Hard basalt layers sit over weaker materials. Rivers erode softer rock faster, leaving a step or ledge.
3. Tectonic and volcanic activity
• Iceland sits on the Mid-Atlantic Ridge. Uplift, faulting, and lava flows continually reset river profiles. Each disturbance can generate new knickpoints.
4. Base-level change (sea level + land uplift)
• Changes in sea level or land elevation alter a river’s base level. Rivers respond by cutting downward, forming migrating knickpoints.
