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Actually there is nothing special about this waterfall. Although named Svartifoss which means 'waterfall' black, the color of the water is clear as a waterfall in general.
But what makes it unique is Svartifoss "background cliff". This waterfall flows from the cliff is composed of dark stones with similar shape beam.
Not only are uniquely shaped ridge. River rocks are scattered under the waterfall also has a similar shape. As cube stones scattered on the banks of the river.
According to the website Atlas Obscura, Svartifoss is one of the many phenomena in the world basalt columns. Besides Svartifoss, Giant's Causeway and Fingal's Cave is a phenomenon of the most famous basalt columns.
These basalt columns formed due to volcanic processes that occur in ancient times. The overflow of volcanic material that is sourced from a volcanic eruption met with cold river water, resulting in a sudden temperature change. The volcanic material eventually freeze and form the pillars of stone unique.
This waterfall Iceland inspire architects to create buildings that are similar to Svartifoss. One of them Hallgrimskirkja church in Reykjavik, Iceland's capital.