I should have noted that the tenure system has ideosyncracies, and can vary from department to department. Tenure in the Humanities is almost completely driven by publication record (with predictable allowances for skin color and gender); in 'hard sciences,' grantsmanship. Social Sciences is somewhere between. Even a fully tenured biologists is up Feces Creek if he/she doesn't bring in the grant dough; the University simply eliminates their reseach centers.
Outside the major research institutions, tenure get even weirder. Many small colleges grant automatic tenure to any faculty with a terminal degree, because they want to look good to the accreditation reviewers.
And then there's the tenure system for teachers at elementary and high schools; quite possibly the biggest education scam of 'em all. A Teamster seniority contract, masquerading as a bulwark against McCarthyites.