What prevents complex molecules from spontaneously and rapidly degrading into atoms?

Bonding.

Explanation:

Bonding prevents complex or macro-molecules from spontaneously and rapidly degrading into atoms because force of attraction present between atoms. These bonds are very strong which is only breakable when there is high amount energy is supplied so we can conclude that bonding between atoms of complex molecules are responsible for their intact structure and prevent rapid degrading into atoms.

it cools slower in the earth meaning the crystals have more time to form than lava on the surface