annum (ka, Ma, Ga)
Time before present. A point of time in the geological past, measured backwards from now. Often, “now” is defined as the year 1950.
year (kyr, Myr, Gyr)
A measure of duration, not necessarily age and not necessarily measured from the present time.
examples
- The fossil is dated to 110±2 Ma.
- According to zircon dating, the granite crystallised at ~30 Ma.
- The volcano was erupting intermittently for 10 kyr.
- The Jurassic period lasted for 56 Myr, starting at 201 Ma and 145 Ma.
- The Jurassic period lasted for 56 Myr, starting 201 Myr ago and ending 145 Myr ago.