The Hadean () is a geologic eon of the Earth predating the Archean. It began with the formation of the Earth about 4.6 billion years ago and ended, as defined by the ICS, 4 billion years ago. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth. W. Brian Harland later coined an almost synonymous term: the "Priscoan period". Other, older texts simply refer to the eon as the Pre-Archean.

"Hadean" (from Hades, the Greek god of the underworld) describes the hellish conditions then prevailing on Earth: the planet had just formed and was still very hot owing to its recent accretion, the abundance of short-lived radioactive elements, and frequent collisions with other Solar System bodies.

Since few geological traces of this eon remain on Earth, there is no official subdivision. However, the Lunar geologic timescale embraces several major divisions relating to the Hadean, so these are sometimes used in an informal sense to refer to the same periods of time on Earth.

The Lunar divisions are:

In 2010, an alternative scale was proposed that includes the addition of the Chaotian and Prenephelean Eons preceding the Hadean, and divides the Hadean into three eras with two periods each. The Paleohadean era consists of the Hephaestean () and the Jacobian periods (). The Mesohadean is divided into the Canadian () and the Procrustean periods (). The Neohadean is divided into the Acastan () and the Promethean periods (). , this has not been adopted by the IUGS.

Now, provide a numbered list of answers to these questions:

1. What eon has very little evidence left on the planet?
2. Is there any official subdivision?
3. What time period did it predate?
4. When did that time period start?
5. With what event?
6. When did it end?
7. Who named the time period?
8. When?
9. What did W. Brian Harland later call the time period?
10. And the earliest name for it?
11. Pertaining to it's Greek meaning, what does Hadean describe?
12. Was part of those hellish conditions due to the planet being super hot?
13. How about lots of temporary radioactive stuff?
14. And getting crashed into by other planetary stuff?
15. When was an alternative scale introduced?
16. Did it add two eons before the Hadean?
17. What were they?
18. How many eras did the new scale apply to Hadean?
19. Each consisting of what?
20. Has the new scale and changes been accepted by the IUGS?

Numbered answers:
1. Hadean
2. no
3. Archean.
4. about 4.6 billion years ago
5. the formation of the Earth
6. 4 billion years ago.
7. Preston Cloud
8. 1972,
9. Priscoan period
10. Pre-Archean.
11. the hellish conditions
12. yes
13. yest
14. no
15. In 2010,
16. yes
17. the Chaotian and Prenephelean
18. Three
19. 2 periods each.
20. no