Black And Red Beach Sand?
On many Ontario beaches, you may see bands of sand that are red or black in colour (Photo 1).
Photo 1: A concentration of red sand consisting mostly of heavy red mineral called almandine garnet. The red garnet is accompanied by a small amount of black mineral called magnetite. The red garnet and black magnetite were likely derived from 2 to 3 billion year old Canadian Shield bedrock during the last ice age and trapped in a deposit of glacial till. Erosion, washing, and sorting of the minerals in the glacial till by the action of wind, waves, and water currents removed the fine and less heavy minerals and concentrated the heavy red garnet and black magnetite. Red and black sand is common across Ontario, Canada. Photo composed by Andy Fyon, on the east beach, Rondeau Provincial Park, Lake Erie, Ontario, Canada, Sept 23/22.
The red and black coloured sand stands out against the much more abundant, and common, tan-coloured sand. How did the red and black sand form and why is it red and black in colour?
Origin of Red and Black Sand
The red coloured material consists of grains of almandine garnet (Photo 2). The black coloured material consists of grains of magnetite (Photo 2). Much of the normal, tan-coloured sand comes from the mineral quartz.
Photo 2: Detailed photo of the grains of coloured sand from the beach at Rondeau Provincial Park, Lake Erie, Ontario, Canada. The black mineral grains are magnetite. The red and pink coloured, glassy mineral grains are red almandine garnet. Clear to milky coloured quartz grains and a few other minerals also occur. Photo composed by Libby Fyon, on the east beach, Rondeau Provincial Park, Lake Erie, Ontario, Canada, Sept 23/22.
Where Did The Garnet And Magnetite Come From?
During the last ice age, the ice sheet that covered all of Ontario scraped across the Canadian Shield bedrock and creating rock debris consisting of dust called rock flour, individual mineral grains, and pieces of rock. Garnet and magnetite mineral grains, which were part of that old Canadian Shield bedrock, were liberated at that time. The rock debris is called till and it accumulated at the base of the ice sheet. When the ice sheet melted away about 15,000 years ago, some till deposits were exposed to modern lakes, like Lake Erie, where they were eroded, washed, and sorted by wind, waves and water currents. The sorting separated and removed the fine silts and left behind concentrations of the heavier grains, like red garnet and black magnetite. It is a bit like dumping a shovel of soil into a bucket of water, stirring the material, decanting off the muddy water carrying fine silt, and leaving heavier pebbles on the bottom of the bucket. Additional sorting was carried out by storm waves, which were strong enough to create layers of almost pure garnet and magnetite by removing most of the tan-coloured sand (Photo 1 and 3).
Photo 3: Layers of black magnetite and red garnet on the east beach, Rondeau Provincial Park, Lake Erie, Ontario, Canada. The layers are called beds and they represent the surface where the mineral grains were deposited. This is a thicker layer of magnetite and garnet. It may represent a single storm event when powerful storm waves removed almost all of the tan-coloured sand grains, leaving a thicker concentration of red garnet and black magnetite. Photo composed by Andy Fyon, on the east beach, Rondeau Provincial Park, Lake Erie, Ontario, Canada, Sept 23/22.
So, the next time you walk along a beach, look for red or black sand layers. If you find some, it is likely that you are looking at 2 to 3 billion year old mineral grains of garnet and magnetite that were liberated from Canadian Shield bedrock during the last ice age and concentrated on modern beaches by modern wind, waves and water currents.
Oct 26/22