What is this roof constructed from?
It looks like it could be a foam roofing material to me. That's a relatively new development though, as in less than 20 years old. It's used a lot on older buildings however because it's just sprayed on as a liquid that expands and foams up to get into all of the crevices in old tar paper and gravel or rolled roofs, without having to remove the old roofing. It also adds quite a bit of thermal insulation, typically lacking in old roofs and buildings. I've heard that now 90% of buildings in Arizona are getting foam roofing done now.