Every where I go on the internet everyone claims Zeppelin is metal! Now I'm not saying they aren't hard, they were very hard for their time but anyone who has listened to their albums all the way through, especially the first few, will tell you they are much more blues rock than metal, maybe even more blues rock than hard rock!
Zeppelin = Hard Rock/Blues Rock
Sabbath = Heavy Metal/Doom Metal
Deep Purple = Heavy Metal/Hard Rock

Jimmy Page's guitar tone was incredibly heavy for the time. Most of Sabbath's heaviness came from the combination of Geezer's bass and Iommi's guitar because amps in the late 60's early 70's didn't have a lot of gain. Zeppelin would have a lot of folk songs too. But due to the popularity of songs like "Whole Lotta Love" Achilles Last Stand", "Communication Breakdown", "Heartbreaker", "Immigrant Song", "Black Dog", etc. they get classified as metal although Zeppelin have a much more hard rock type of attitude and feel to their music, as opposed to Purple, Priest and Sabbath
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Ambient Symphonic Depressive Southern Melodic Blackened Death/Doom Metal
They have a number of songs that hit harder than a lot of the rock that was coming out at the time. Sabbath really kicked off the whole metal thing in my mind, but Zep, blue cheer, deep purple, etc, started pushing rock into a heavier, darker direction. I think of them as more proto-metal, in that they were one of the earlier bands showing elements of what would become metal, but weren't themselves metal.

The escalating use of Fuzz and Distortion in hard rock was the precursor to modern hi gain, so the interest in those sounds, or rather the emerging technology to create those intensely saturated guitar tones, led to modern metal more than anything Zep did.

Modern metal has shed much of it's blues influence, but Doom and Stoner and Sludge bands still sound more like Zep than Metallica, like Zep ragged through the mud, but still.
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