Coastal Processes FAQ – Why is there black sand on the beach occasionally?
A layer of black sand is often seen on the beach after storms. Although it’s surprising to see black sand beaches along Delaware’s coast, it’s not too unusual. This black beach covering is an accumulation of dark sand grains which are more difficult for winds and waves to move than the lighter colored quartz sand. Along Delaware’s shoreline, the dark sands typically consist of heavy iron-rich minerals such as magnetite and illmenite; garnet, tourmaline and hornblende are another minerals commonly found in the dark sand mix. These heavy minerals have a smaller grain size or diameter, but a greater specific gravity or density when compared to the white quartz sand. These characteristics result in a different hydraulic behavior from quartz grains and therefore these heavy minerals are often concentrated by waves and currents to form dark layers within the beach sand. Black sand deposits are often “left behind” as a lag layer during high wave conditions of winter, when the quartz-rich sand
