Metallica is primarily a heavy metal band that helped pioneer the thrash metal subgenre. They started out playing fast, aggressive thrash metal in the 1980s, then shifted towards a more mainstream heavy metal/hard rock sound in the 1990s, before returning to their thrash roots in later albums. The key genres and styles they are associated with are:

Thrash Metal: Fast tempos, complex guitar riffs and arrangements, dark lyrical themes – their early work in the 1980s defined this style
Heavy Metal/Hard Rock: More mainstream and polished sound, slower tempos – evident in their 1990s “Black Album” era
Alternative/Experimental Metal: Incorporation of alternative rock and Southern rock influences in the mid 1990s – showed their willingness to experiment
Garage/Classic Rock Covers: Their versatility shown through covers of older rock songs
Modern Thrash Metal: Return to thrash metal roots with modern production techniques in recent albums

Born to Play Heavy Metal

When Metallica first formed in 1981, they were heavily influenced by New Wave of British Heavy Metal bands like Iron Maiden and Saxon. Their goal was to create the fastest, most intense metal ever. The style they pioneered came to be known as “thrash metal.”

Early Metallica albums like 1983’s Kill ‘Em All and 1984’s Ride the Lightning defined the thrash metal genre. Their music was faster and more aggressive than traditional heavy metal. “Seek and Destroy,” “Fight Fire With Fire,” and “For Whom the Bell Tolls” showed Metallica leading this new extreme sound. They even toured with other thrash pioneers like Slayer and Anthrax – together, these bands were dubbed the “Big Four of Thrash.”

The Black Album Changes the Game

By 1991, Metallica had gotten pretty famous in the metal underground with albums like Master of Puppets and …And Justice for All. But they were still mostly considered an intense, thrashy band. Their self-titled 1991 album, often called The Black Album, totally changed things.

With the help of producer Bob Rock, Metallica trimmed down their thrashy side for a tighter, more mainstream metal sound on The Black Album. Songs like “Enter Sandman,” “Sad But True,” and “Nothing Else Matters” became smash hits on rock radio and MTV. This album introduced Metallica to a wider audience outside of metal fans and thrash kids. They now shared stages with bands like Guns N’ Roses and Aerosmith at huge stadium shows and festivals.

New Sounds and Experimentation

The Black Album’s incredible success gave Metallica the freedom to explore different sounds. On 1996’s Load and 1997’s Reload, they embraced blues, grunge, and Southern rock influences. There were still heavy riffs, but also acoustic ballads and jam sessions. Fans argue about these albums to this day – some loved the experimentation, while metal purists felt betrayed.

In 2003, Metallica took another sharp turn with St. Anger. They ditched guitar solos and incorporated modern techniques from nu-metal bands like Korn or Slipknot. This album remains Metallica’s most divisive – you either think itrules or totally sucks. There’s not much in-between!

Full Circle Back to Thrash

After the poor reception of St. Anger, Metallica knew they needed to reconnect with longtime fans. They brought in superstar producer Rick Rubin for 2008’s Death Magnetic. The band’s energy and tight performances on this album echo their early thrash days. There are lots of fast tempos, shred solos, and even a song over 10 minutes long!

Metallica’s most recent albums, Hardwired…to Self-Destruct (2016) and 72 Seasons (2023), continue showcasing high intensity thrash metal. But they incorporate lessons learned across their long career regarding songwriting and production polish. Basically, this is Metallica’s classic sound but bigger, tighter, and more epic thanks to decades of experience. They still have an adventurous edge though – who knows what they might try next?

No Matter What, It’s Still Metal!

So after 42 years, what genre is Metallica? They can’t be defined by just one subgenre anymore. They helped invent thrash metal in the ‘80s but also achieved mainstream hard rock success in the ‘90s. Lately they’ve returned to thrash but kept growing as musicians.

The bottom line is whatever sound Metallica explores, it’s always rooted in heavy metal. The intensity and attitude at the heart of their music connects with metal fans across different eras and styles. Metallica also inspired later groups like Slipknot, Avenged Sevenfold, and Bullet for My Valentine to push metal forward.

That’s why Metallica remains one of the most popular and influential metal bands ever, even after mixing up their style over the decades. True headbangers respect how they follow their creative spirit rather than conforming to genre rules. So next time someone asks what kind of music Metallica plays, you can school them on all the different metal sounds in the band’s catalog!