Where does much of our history of Precambrian rocks come from? fossils on the rock surface ores mined from shields ash taken from volcanoes trenches on the sea floor

Ores mined from shields.

Explanation:

Ores mined from shields are the source from which we get the history of Precambrian rocks. The Ores from shield experience the Precambrian era so that's why its rocks provides information about that era. The Precambrian supereon is the division of the geological time scale which is the first and longest stage in the history of the Earth which also includes the Hádic, Archaic and Proterozoic eons.

they are both synthetic elements

explanation:

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