1. <span>the process started with the collapse of a nebula
The universe almost certainly didn't start from the collapse of a nebula.
2. The mantle is the middle layer.</span>
Answer:
Radioactive isotopes are unstable and will decay. ... The isotopes will decay into a stable isotope over time. Scientists can tell how old the rock was from looking at the radioactive isotope's half-life, which tells them how long it would take for there to be half the radioactive isotope and half the stable isotope.
