The banded iron formations began to form when
A)carbon dioxide dissolved in seawater.
B)varying oxygen levels allowed the precipitation of alternating layers of iron oxide.
C)oxide minerals were deposited on the ocean bottom.
D)iron from lava flows settled into layered formations.

Question

Quizplus · Accepted Answer

Banded iron formations were formed due to varying oxygen levels in the Earth's atmosphere during the Archean era. Initially, the atmosphere was oxygen-free, and the oceans were rich in dissolved iron. When photosynthetic bacteria evolved, they began to release oxygen as a waste product. The oxygen reacted with the dissolved iron in seawater, causing it to precipitate out of the water and settle on the ocean floor in layers. This cyclic precipitation and sedimentation produced alternating bands of iron oxide and chert, leading to the formation of banded iron formations.

The Banded Iron Formations Began to Form When