Arenado hit the first pitch he saw from All-Star closer Mark Melancon (1-2) into the left-field seats to finish a four-game sweep. The slugger was mobbed at home plate by teammates, with a frenzied sellout crowd chanting “MVP! MVP!”
With the best record in the National League, the surprising Rockies have won five straight overall and nine in a row against the Giants.
Colorado had a 3-2 lead in the top of the ninth but gave up three runs before scoring four in the bottom half.
Pinch-hitter Hunter Pence connected on a go-ahead homer in the ninth for last-place San Francisco, which has dropped six consecutive games and nine of 11.
Melancon retired his first batter, but three singles produced a run and brought up Arenado, who tripled in the first, singled in the fourth and had an RBI double in the sixth. He hit a 91 mph fastball for his 15th homer and first career cycle.
It was his second game-ending homer in the majors and the eighth cycle in club history.
Carlos Estevez (4-0), called up from the minors before the game, struck out one batter in the ninth for the win.
Rockies reliever Jake McGee came on in the ninth but issued a one-out walk to Gorkys Hernandez. Pence hit an 0-1 fastball into the left-center seats for his third career pinch-hit homer, and Crawford added an RBI single later in the inning.
Giants: LHP Madison Bumgarner (left shoulder) is scheduled to throw a second simulated game in Arizona on Wednesday. He threw his first Saturday, the first time he faced hitters since getting injured in a dirt bike accident in Colorado in April.
Rockies: RHP Chad Qualls was placed on the 10-day disabled list with lower back spasms. The move is retroactive to Friday. … Rockies RHP Chad Bettis threw 15-20 fastballs during a bullpen session, continuing his comeback from treatment for testicular cancer. Bettis, who finished chemotherapy this month, had been throwing off the mound to a catcher in front of the plate. Sunday was the first time he threw the full 60 feet, 6 inches.
Rockies second baseman DJ LeMahieu showed why he won the NL Gold Glove in 2014 with a great play to save a run in the first inning. LeMahieu was moving to cover second after Crawford broke from first when Buster Posey hit a grounder toward the vacated spot. LeMahieu reached back to field the ball and tossed it to Story while sitting on the infield. Story stepped on second to start the inning-ending double play. Crawford was initially called safe at second, but the Rockies challenged and the call was overturned.
Giants: RHP Johnny Cueto (5-6, 4.57 ERA) opens a four-game series at Atlanta on Monday.
Rockies: RHP German Marquez (5-3, 4.19 ERA) starts the opener of a key three-game series with Arizona on Tuesday.
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