Gap wants to create a "pop culture moment" for its fall Back to Blue campaign, and it's turning to Tumblr to create it.

On Monday morning, Gap issued a call on Tumblr for creators to share, through an original piece of content, what "blue" means to them — blue being, of course, the color of Gap's logo and of its signature product, the blue jean.

The four winning submissions, as determined by Gap, will be distributed through a mobile ad takeover on Tumblr on Aug. 29. On that day, Gap will purchase every single mobile ad Tumblr runs, the first brand to do so. Tumblr began running mobile ads in late April. The company says its mobile userbase is growing rapidly, and is expected to overtake desktop traffic by early 2014, founder and CEO David Karp has said previously.

By targeting Tumblr's young, share-happy user base, Gap is betting that it can create a new "pop culture moment" on the Internet, Rachel Tipograph, global director of digital and social media at Gap, tells Mashable. "Pop culture doesn't really start on TV anymore," she says. "Pop culture starts on the Internet. When you think about what community is creating pop culture on the Internet, it's Tumblr."

In a way, Gap is using Tumblr as its own creative agency. "We wanted to partner with the best content creators, give them a chance to make the Back to Blue brief, and then take the best pieces of content and turn them into mobile ads," Tipograph explains. Winners will not be given cash compensation for their work; the compensation is in the exposure, Tipograph says. She declined to say how much Gap is paying for the mobile takeover.

In addition to the Tumblr push, Gap is making the usual buys in print, outdoor, direct and in-store marketing, plus a return to TV for the first time in four years. The apparel giant has also partnered with two dozen "influencers," including MTV's Tanisha Long, to create 250 pieces of content to distribute on Facebook, YouTube, Instagram, Vine and Tumblr over the next three months. A spokesperson for the company describes it as Gap's "broadest-reaching campaign in the company's modern history."

Image: Gap