Black Sabbath's ‘Iron Man' takes on Motorhead’s ‘Ace of Spades' in round one of Ultimate Classic Rock's March Riff Madness competition.

It took a while -- and yes, threats of violence were made -- but we selected 32 of classic rock's most iconic riffs and paired them off in 16 first-round duels. Over the next five weeks the field will be repeatedly cut in half by your votes, until we theoretically arrive at the most awesome riff in all of rock history.

This Global Metal region's battle features colossal riffs from opposite ends of heavy metal’s first decade: the ‘70s. ‘Iron Man' was one of several all-time classics (see also ‘War Pigs,’ ‘Paranoid,’ etc.) introduced by Black Sabbath’s sophomore album, in 1970, but its leaden riff played on slightly detuned strings literally epitomized the primal heavy metal blueprint as first devised by the Birmingham quartet.

Motorhead, by comparison, took that original, primarily slothful template and accelerated it to the limits of human tolerance by decade’s end, culminating in perhaps their best known song and riff in the shape of the relentless ‘Ace of Spades. If speed metal begins anywhere, it’s here.

So who wins this race of polar opposites? The tortoise or the hare? You can vote once per hour until round one of our March Riff Madness closes on Sunday, March 9 at 11:59PM ET. The big winner will be revealed on April 7.

Watch Black Sabbath Perform 'Iron Man'

Watch Motorhead Perform 'Ace of Spades'