The geology of Iceland is comparatively young — it owes its existence to the Mid-Atlantic Ridge that splits the island in half. Volcanoes along the ridge, such as Katla, erupt with some regularity continuing to add surface area and mass to the “land of ice and fire” and to augment the black sand beaches (visible on this image). Glaciers cover approximately 11% of Iceland. The streaks heading to the sea visible in this image of the South-Eastern Coast of Iceland are composed primarily of run-water from the Vatnajökull glacier, the largest glacier in Iceland (not pictured - North).
As the melt water from a glacier starts to flow in the spring time it carries with it glacier silt or "rock flour". The silt is created when rocks underneath the surface of the ice are grinding from the movement of the glacier. The rock flour is very light and stays suspended in the water for a long time. The sunlight that reflects off this rock flour is what gives the water this spectacular turquoise blue or green colour.
The image was taken by Sentinel 2 (Copernicus program) in August 2017.