The Library of Congress' National Recording Registry is essentially a giant collection of influential music throughout the ages, or as the registry itself puts it, "culturally, historically or aesthetically significant." The album has to be at least 10 years old and has to has some cultural significance… so doesn't it make sense that the first metal album that's been inducted to the registry be Metallica's 1986 classic Master of Puppets?
The Library of Congress entry for Master of Puppets describes the seminal release as:
The third release by the band Metallica shows the group moving away from its thrash metal history and reputation and exploring new ideas. Thrash, a reaction against the pop metal of the early 1980s, aimed to renew metal by emphasizing speed and aggression. For example, the song "Battery" on this album—with rhythm guitarist James Hetfield's galloping power chords, Lars Ulrich's machine-gun drumming, and lead guitarist Kirk Hammet's blinding tapped leads—is as rousing an example of the sub-genre as one could find and the technical proficiency is astonishing. However, other songs on the record break free of thrash orthodoxy. Cliff Burton's clean bass lines, volume swells, and careful harmonies, for example, on "Orion," set that song apart from the standard metal song. The title track starts unsurprisingly enough with a crisp power chord and catchy riff, but halfway through, the tempo slows and a clean arpeggiated progression, accompanied by cello-like tones, introduces Hetfield's mid-tempo lead which eschews tapping, sweep picking and, other metal guitar techniques. Black Sabbath bassist and lyricist Geezer Butler has commented that Metallica's 1980s output brought the music "back to the spirit of [Black] Sabbath" and, he further emphasizes, "If we started it, then [Metallica] reinvented it."
So there you have it – the government is recognizing Metallica alongside artists like Santana and Gloria Gaynor, and Mark Riddick is doing merchandise for Justin Bieber. 2016 truly is the year that metal makes it into the mainstream!