Black sand beaches have become one of the images most closely associated with Iceland, and for good reason.
You may already know that the answer has to do with volcanic activity. Iceland, after all, is not the only place in the world with such beaches. Hawaii has several notable black sand beaches, for instance, including Punalu’u and Kehena beaches.
The distinctive black sand shared by these volcanic islands comes from the basalt fragments that follow a volcanic eruption. Basalt is by far the most common volcanic rock, accounting for about 90% of all volcanic rocks on earth. Basalt tends to be dark grey or black in colour because of its mineral composition, which includes high levels of augite and pyroxene minerals, which tend to be darkly coloured.
When a lava flow reaches the ocean and comes into contact with water, it cools very quickly and shatters, just like how dishes sometimes shatter in the kitchen if run under cold water directly after heating.
This shattering creates a large amount of fine-grained debris, which is eroded into sand over time.
In many parts of the world, black sand beaches can be formed as a single event and then fade away after the lava flow. But because Iceland is still very volcanically active, the black sand is replenished often, and nearly all of Iceland’s beaches have this distinctive colour.
There are, however, a couple exceptions. Rauðisandur, for example, is a well-known beach in the Westfjords famous for its rusty-red colour.