Random Events - just how complex should they be?

The PCs are on a mission for the King and as they go about there business SUDDENLY
a Volcano erupts in the middle of the path the PCs are walking down
or the Capulets & the Montagues meet in a town square and suddenly draw arms
or they stumble across a Coatl is locked in battle with a Fire Giant
or they walk into a dark alley and see Jack the Ripper killing another victim
or a hungry Grey Render burst out of the undergrowth

which of these are viable 'random encounters' and which are too complicated and likely to lead to new adventures?

Should Random events be hooks to new adventures or just stuff that happens?

Depends entirely on how the GM handles them. All of these could be adventure hooks, or just glimpses of life in your millue that give the PC's verisimilitude.

By definition (personally, again), a random event should be 'just stuff that happens'.

I would agree with the above that a random event should be 'just stuff that happens' However, random events could lead to larger storylines.

In general, however, I'd say you don't want to have random events interfere too much with the main story. A fine balance is necessary, I think.

All of the events listed in the first post could be 'random events' that don't ultimately confuse the main storyline if DMed properly, IMO, though the Vulcano erupting right in the PCs path seems a little bizarre.

I think that it's based on the players' response. Sometimes they will investigate the event further and cause it to become a whole side adventure. Other times, they may deal with whatever happens and then move on.

Also it depends on how "complete" the event is. A pack of worgs attacking a party on the road is a fairly complete random event. The party is in the wilderness, a group of worgs have been attracted to their scent, and come looking for prey. Worgs attack the party. Party slays them, and really, that's the end of that. However, a mysterious Jack the Ripper figure attacking a woman in an alleyway can be the basis of a whole adventure, especially if he escapes. Who is he? Why is attacking these women? Is he a madman or is it part of a greater storyline?

I'm not big on random encounters, so what I've done in the past is stat out little scenarios on index cards and use them whenever the game lulls for a while, or if I want to do something other than say "You travel from Waterdeep to the town of Corm Orp. The journey is uneventful and takes about 5 days".