If you’ve ever seen an outcrop of banded iron, you’ve probably been impressed by the swirling, alternating layers of red and grey. Obviously, banded iron formations contain iron – technically, iron oxide, or rust. If you follow mining, you also know that the banded iron formations are one of the most important sources of iron in the modern world – two of the largest producers, Australia and China, have enormous reserves of iron stored within their banded iron formations.
Outcrops of banded iron are found all over the world. In this post, we’ll take a look at how this distinctive formation come to be.
First, consider the fact that almost all banded iron formation rocks are of the same age – about 2,400 million years. The formation of banded iron, it turns out, occurred in a geologic era when the earth was much different than it is today.
The Archean Era: Earth without Oxygen
In the Achaean era, there were no land plants. In fact, there were no plants at all, or animals for that matter. The only life forms were microscopic algae, which lived in an acidic, iron rich ocean.
The iron rich ocean is key to our story. In the Achaean, the atmosphere didn’t contain any oxygen. No oxygen meant that the formation of iron oxide, or rust, did not occur like it does today. Instead of getting bound up in iron oxide, the iron simply washed into the oceans in ionic form. Over time, iron built up to significant levels.
But then, about 2,400 million years ago, something changed. A new form of life, the cyanobacteria, evolved. The cyanobacteria were the first organisms to photosynthesize, and the ability to directly use the sun’s light to produce energy revolutionized life on earth, allowing for the development of plants, and, thousands of millions of years later, animals.
What does this have to do with banded iron? Consider this: photosynthesis produces oxygen molecules as a byproduct. There are two key facts about oxygen in this situation. First, it can react with iron, forming iron oxide. Second, in high concentrations, oxygen was toxic to early cyanobacteria.
Banded Iron Takes Shape
The stage was set for the formation of banded iron on the seafloor. Here’s how it happened: as cyanobacteria proliferated, they produced more and more oxygen, which reacted with greater and greater amounts of free iron. The resulting iron oxide sank to the seafloor, forming a continuous layer. Eventually, there was so much oxygen that the oxygen-intolerant cyanobacteria suffered a mass die-off. After the die-off, oxygen levels would plummet and iron would no longer be deposited. Instead, sediments from the continents formed a light grey layer. Eventually, the cyanobacteria would begin to proliferate again as oxygen levels dropped. The cycle repeated over and over again, forming the alternating bands we see today, which, in most cases, were subsequently deformed into the swirls and curves we see today.
Eventually, the free iron was all consumed and oxygen began to fill the atmosphere rather than get trapped in iron oxide. The banded iron formations occurred as a one-off event that would never again occur in Earth’s history.
Still curious? Check out this documentary video clip, which covers the chemical processes in a bit more detail.