Question

why are time periods on the geologic time scale not even in length? (1 point)
○ they are based on a different type of calendar used to track earth’s history.
○ they are based on the thickness of the earth’s rock layers.
○ they are based on important events that happened in earth’s history.
○ they are based on the type of fossils found in rock layers of the earth.

Explanation:

To determine the correct answer, we analyze each option:

• Option 1: The geologic time scale is not based on a different calendar type; it's about Earth's history events, so this is incorrect.
• Option 2: Time periods aren't defined by rock layer thickness, as thickness can vary due to other factors, so this is wrong.
• Option 3: The geologic time scale divides time based on major geological or biological events (like mass extinctions, formation of key rock types), so time periods' lengths vary with these events. This is correct.
• Option 4: While fossils help identify periods, the time periods' lengths are based on events (like when those fossils' organisms appeared/ went extinct), not just fossil types. So this is incorrect.

Answer:

C. They are based on important events that happened in Earth’s history.