Question

1. what is the longest and oldest unit of geologic time? why dont scientists have much information about the time period?

Explanation:

The longest and oldest unit of geologic time is the eon. Scientists don't have much information about the earliest eons (like the Hadean) because rocks from that time are scarce or altered. Early Earth had intense geological activity (e.g., volcanic eruptions, asteroid impacts) that destroyed or metamorphosed rocks. Also, life was simple (if present) and left few fossil traces.

Answer:

The longest and oldest unit of geologic time is the eon. Scientists lack much information about the earliest time - periods (e.g., Hadean eon) because of scarce/altered rocks (due to early Earth's intense geological activity) and a lack of fossil evidence (as life, if present, was simple).