3800 – 3700 million years ago

Year nine
Banded ironstone formations

Banded ironstone formations proliferate

Bacteria start to form banded ironstones.

About 3870 – 3800 million years ago, the first oceans began to develop and the earliest banded ironstone formations began to accumulate. The period of greatest accumulation was about 2500 million years ago when deeper seas had formed.

What are banded ironstone formations?

In Africa, America and some other parts of the world, there are rocks called banded ironstone formations (BIFs). They are red and grey striped rocks made of thin bands of ironstone sandwiched between a kind of flint-like rock called chert. They were formed by bacteria that lived in the seas. During the spring and summer, the bacteria made oxygen by photosynthesis. The oxygen made the iron in the sea water rusty, and this settled on the sea floor to form thin layers of red iron on the sea bed. Sometimes the bacteria made a gas called carbon dioxide instead of oxygen, so that the iron was not turned to rust. These striped rocks show us that there was life by the time Earth was only nine (3700 million years ago), but we don't know what it looked like. There are no fossils of this age.

Bacteria that lived in the sea at this time had a problem: oxygen in the water would kill them, but they made oxygen as a waste product from photosynthesis. There had to be some way to get rid of this oxygen, otherwise they would have sent themselves to extinction. This is why the iron in the ocean waters was so important, because the oxygen created by the bacteria turned it to rust; this is what happens when atoms of oxygen fix themselves onto atoms of iron to form iron oxide, or rust. The rust then fell to the sea floor, taking the oxygen with it, to form the red stripes of the BIFs. With the iron trapped in the sea floor sediment, bacteria could colonise huge areas of the sea floor and their numbers rapidly increased.

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