Banded iron formation

Banded iron formations are special kinds of rocks made from layers of iron-rich minerals and silica. These rocks can be very thick and stretch for long distances. Scientists believe these formations help tell the story of when Earth's oceans began to contain oxygen.

These rocks formed long ago in the oceans. Tiny water plants called cyanobacteria made oxygen through photosynthesis. This oxygen then combined with iron in the water, creating layers of iron-rich minerals on the ocean floor.

People first found banded iron formations in northern Michigan in 1844. Today, they are very important because they hold a large amount of the world's iron. Many of these formations can be found in places like Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States. They are a major source of iron ore that we mine and use today.

Description

A banded iron formation is a special kind of rock made of thin layers. These layers alternate between silver to black iron oxides, like magnetite or hematite, and iron-poor chert, which is often red. These rocks can be very thick and stretch for many kilometers.

Most banded iron formations are very old, from a time before complex life on Earth. They are made of sedimentary rock with lots of iron—often about 30% of the rock is iron oxides. These formations are very hard and tough, which makes them resist wearing away easily. They show clear layers over large areas, suggesting they formed in deep, calm water. Some banded iron formations, like those near the Great Lakes and in western Australia, have a different appearance with granular iron sediments and are found in shallower water environments.

Occurrence

Banded iron formations are mostly found in very old rocks, called Precambrian rocks, which formed before complex life appeared on Earth. They are found all around the world, in areas where very old continental rocks are exposed. Some of the oldest known banded iron formations are in places like the Isua Greenstone Belt and the Yilgarn and Pilbara cratons.

The largest banded iron formations are in regions known as the Great Gondwana BIFs, such as the Hamersley Range, and they can be very thick, reaching over 900 meters (3,000 feet). These formations tell us about the early conditions of Earth's oceans.

Origins

Banded iron formations are special rocks made of layers of iron oxides and chert. They give clues about when Earth's oceans first got oxygen. Scientists think these rocks formed when deep ocean water, rich in iron, rose up into areas where tiny plants called cyanobacteria could make oxygen through photosynthesis. This oxygen turned the iron into solid iron oxides that settled on the ocean floor.

The patterns in these rocks might come from changes in the number of cyanobacteria over time or from natural cycles in the ocean. Most of these formations are very old, from a time before there was much oxygen in the air. Some later formations might relate to times when Earth was covered in ice.

Formation processes

The tiny layers in these rocks could be from yearly changes in oxygen made by the cyanobacteria. For these rocks to form, the water needed to have lots of iron but no oxygen or hydrogen sulfide, which would stop the iron from settling. The iron might have come from underwater hot springs, dust, rivers, or melting ice.

Source of reduced iron

Iron could come from underwater hot springs, dust blown by wind, rivers, glacial ice, or seepage from land near the ocean. Older formations often show signs that the iron came from hot springs, while younger ones might have come from weathered land.

Absence of oxygen or hydrogen sulfide

The ocean needed to have no oxygen or hydrogen sulfide to let the iron settle. There are two main ideas about why this happened. One suggests the deep ocean slowly got oxygen, stopping iron from reaching the surface. The other suggests the deep ocean had hydrogen sulfide, which would also stop iron from settling.

Banded iron formations in Minnesota sit above a layer from an ancient asteroid impact. This impact might have mixed the ocean and added oxygen, ending the time when these rocks formed.

Oxidation

Scientists debate whether tiny plants or other natural processes caused the iron to turn solid. Some think oxygen from cyanobacteria turned the iron solid. Others suggest different tiny plants or even non-living processes could have done it.

Oxygenic photosynthesis

One idea is that oxygen made by cyanobacteria turned the iron solid. This might have been helped by special bacteria that speed up this process.

Anoxygenic photosynthesis

Some think other tiny plants that don’t make oxygen could have turned the iron solid instead. The chemistry of these rocks supports different ideas about how much oxygen was around when they formed.

Abiogenic mechanisms

Some think natural, non-living processes like sunlight or radioactive decay might have turned the iron solid. However, there is also evidence that tiny plants were present when these rocks formed.

Diagenesis

No matter how it happened, the iron and silica likely turned into solid gels and then became rock through a process called diagenesis. The rocks we see today look very much like the original sediments, suggesting they didn’t change much after forming.

Great Oxidation Event

Main article: Great Oxidation Event

The most time of banded iron formation deposition happened just before a big change called the Great Oxygenation Event. Before this event, the air had almost no oxygen. The disappearance of a special signal in sulfur minerals suggests oxygen started appearing between 2.41 and 2.35 billion years ago. The end of banded iron formation deposition around 1.85 billion years ago is thought to mark when the deep ocean finally got oxygen.

Snowball Earth hypothesis

Main article: Snowball Earth

Until 1992, people thought the rarer, younger banded iron deposits were from unusual local conditions. The Snowball Earth hypothesis suggests that during severe ice ages around 750 to 580 million years ago, oxygen levels dropped a lot. Iron built up in the oxygen-poor oceans. When Earth warmed up, oxygen returned, causing the iron to settle as these rocks. These formations are often linked with evidence of ancient glaciation.

Economic geology

Banded iron formations are very important because they provide most of the iron ore we mine today. Over 60% of the world's iron comes from these rocks, found in countries like Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.

People in different places have different names for these rocks. In Lake Superior, they are called things like "jasper" or "taconite." In Brazil, they are called "itabarite," and in South Africa, they are known as "ironstone." These rocks were first found in Michigan in 1844, and since then, they have been very important for making iron and steel all over the world.

After the Second World War, iron ore became very valuable worldwide. In Australia, huge areas of these rocks were discovered, and they now supply most of the country's iron. In Brazil, large deposits are found in a region called the Iron Quadrangle, helping make Brazil one of the top iron producers.

Mining of these rocks also began long ago in Anshan, China. Even during tough times like wars, the factories there kept producing iron and steel, and today they are still very important for China's supplies.