Google Adsense introduced new ad unit 970 x 90 also known as super leaderboard for every bloggers and webmasters to earn more. The new ad unit will fit your entire site width and now it shows text ads only even you have enabled rich media ads.
Google says "The new super leaderboard ad unit can show up to four text ads organized in a column layout but, as with other ad sizes, the number of text ads per unit may vary per impression. When enabling image and rich media ads for this unit, display ads in the size of a 728x90 unit will be displayed initially. Over time, we expect that the inventory of image and rich media ads for the 970x90 will grow and you’ll see more full-size display ads being served."