Why do scientists believe that igneous rocks were common during the archean age?

Because igneous rock is rock that formed by magna rising to the surface and cooling, creating rock.

Indegeneous rocks were believed to be common during the Archean age because at that time based on scientific studies these were mostly the rocks on Earth. Also, volcanic activity was frequent and ARE evident in the atmosphere during that period which further supported this claim

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Old telephone. is prehistoric object