Roldan, from nearby Pico Rivera, California, opened the scoring in the 12th minute, getting on the receiving end of Morris' cross. Morris sped unmarked up the left side with the ball and remained onside after a short video review, allowing the goal to stand.
The Galaxy had a chance soon after, with Julian Araujo shooting too high and over the goal in the 14th minute,
Morris did more damage in the 38th minute, with Nico Lodeiro sending a long ball from one side of the pitch to the other to find an open Morris, who scored his seventh goal of the season.
The Galaxy had more chances. A header from Sebastian Lletget was on target in the 50th minute but was saved. But the Sounders took control in the 61st minute with Roldan's chip over the head of Galaxy goalkeeper David Bingham for a 3-0 lead.
Seattle (7-3-3, 24 points) maintained a share of first place in the Western Conference, with a game in hand over the co-leading Portland Timbers. The Sounders have won three of their last four matches.
The Galaxy (4-6-3, 15 points) have lost three straight and are winless in their last four games. They avoided being shut out with Lletget's 88th-minute goal, his fifth of the season, and if not for the goal would have gone scoreless in four consecutive matches.
Araujo, the Galaxy's 19-year-old right back, drew a yellow card in the first half and will miss the team's next match due to yellow card accumulation. Araujo has appeared in eight of the Galaxy's games this season.
Morris just missed a second goal when his shot caromed off the right post two minutes before the final whistle.