mod duty covers weeding out spam adverts and eliminating spam bots at moments notice. Other than that, there's a lot of behind-the-scene things that normal members would not know existed. But I'll leave that to your imagination.
Members can probably volunteer themselves and be voted on by others. However, I think another way is that Admin and mods together can nominate a few members or choose to nominate the volunteers and have the nominee share about their thoughts and opinions about being nominated, and what they can offer as a potential mod. Then SC and us mods will cast our votes.
If you want to be a mod, you still have to post responsibly and can't spam on a regular basis or lead spammish conversations in various threads. I'm not trying to be a party-pooper, just stating the technical aspect of it.