In instances where a website has only a desktop version and an AMP version, meaning no responsive or mobile site, Google will index the desktop version by default.
Google’s Maile Ohye revealed that information after being asked a question by Jennifer Slegg at the State of Search conference this week.
— Jennifer Slegg (@jenstar) November 14, 2016
With Google having stated time after time that we’re living in a mobile-first world, it’s a bit of a mystery that its search index is designed to index a desktop site over an AMP site when no other mobile-friendly version exists.
It would be reasonable to have assumed Google would default to indexing the most mobile-friendly version available, which would be the AMP version. On the other hand, it’s possible Google considers the desktop variation a more complete version of the site, thus indexing it over the AMP version because it provides more content to users.
In cases where only an AMP version of a site is available Google will still index it. There’s no rule that states both a traditional desktop and mobile/responsive site need to exist in order for AMPs to get indexed.
If you’re in the unique situation where you have a desktop site, but the only mobile version that exists is written in AMP HTML, there is a workaround for getting your AMP site indexed. It’s called the rel=alternate tag.
Even if you’re not great at coding, it’s super simple to implement with a WordPress plugin. The rel=alternate tag can be used as a direction for Google’s crawlers letting them know where they can find the mobile version of your site.
So you can use that tag to define the AMP version of your site as the “mobile” version, and hopefully there should be no issues when it comes to getting AMP pages indexed in mobile search.