Pilbara Craton in Western Australia is home to the oldest chert on Earth, which is thought to have formed around 3.45 billion years ago. These fine-grained sedimentary rocks contain small amounts of zircon, which is a silicate mineral that is stable over time and helps geologists track the age of the oldest rocks.

The reddish colors of this formation reveal the presence of iron-rich minerals, which indicate that the oceans of the early Earth were rich in dissolved iron and probably twice as salty as they are today. Because iron is reactive to oxygen, the Marble Bar cherts help scientists track the evolution of atmospheric oxygen during Earth’s history.

Photo by Sanjoy Som and Lena Vincent