Glacial flour turns some fjords a vibrant, glowing turquoise.

Glacial flour, also known as rock flour, consists of silt-sized particles of rock generated by glacial erosion. As massive glaciers move slowly over bedrock, they act like giant pieces of sandpaper, grinding the stone into a fine dust with a grain size typically less than 0.002 millimeters. This process, known as glacial abrasion, is powerful enough to pulverize even the hardest granite into microscopic sediment.Because these particles are so incredibly small and light, they do not settle at the bottom like normal sand or pebbles. Instead, they remain suspended in the water column for long periods, creating a suspension called a 'glacial milk.' This phenomenon is frequently observed in places like Kenai Fjords National Park in Alaska and the fjords of Norway and New Zealand.The specific turquoise color is a result of Mie scattering. When sunlight enters the water, the short-wavelength blue and green light hits the suspended rock particles and reflects back to the surface. This creates the characteristic opaque, milky appearance that distinguishes glacial lakes and fjords from clear, deep-blue oceanic waters.Satellite imagery from NASA frequently captures this effect, particularly during the summer melt season when runoff is highest. Researchers from institutions like the University of Oslo have studied how this sediment affects local ecosystems by blocking sunlight, which can limit the growth of phytoplankton. Despite this, the sediment provides essential minerals that can eventually nourish marine life once it settles.