Here's why:

* Banded iron formations are sedimentary rocks that are characterized by alternating layers of iron oxides (like hematite and magnetite) and silica-rich minerals (like chert and jasper).

* These layers form due to the precipitation of iron and silica from water in ancient oceans, often under conditions of low oxygen levels.

* The "light" bands are typically made of silica minerals, while the "dark" bands are composed of iron oxides.

Let me know if you have any other questions!