I own a 98 Pontiac grand prix with a 3.1 the camshaft as broken in half I removed it and was wondering what causes it to crack??

Was any major engine work, previously done to engine?

Engine abused, over revved?

Poor lubrication causing cam bearing failure?

Rare for a cast cam to fail on it'* own.....

You will have other problems......if one part of the cam moved and the rest didn't, could have bent valve, bent push rod, piston damage, head damage, etc.

Need to check timing chain and sprockets.....

Have check heads no bent values but cam has some groves in one Bering

Grooves in a bearing could mean failure somewhere else. With it being a 98, I'd recommend a full rebuild. Gaskets and new bearings and rings.

Plenty of 3.1'* available in junkyards....

That too.