What does it say about Joanna Newsom that she mounts the stage in heels that could kebab a small dog? That she seems fearless? That will have come as no great revelation. Singing harpist, after all, is no one’s idea of a safe career choice.

And yet, in the weeks since its release, the 25-year-old Californian’s Ys album has trounced preconceptions, not just about the Marmite-like qualities of Newsom’s voice, but about what can be done within the parameters of popular song.

With the London Symphony Orchestra sharing this extraordinary concert, it seemed a given that the album — described by its arranger, Van Dyke Parks, as “a contained rapture” — would be aired. Where she used to make a point of accentuating her vocal extremities, Newsom yielded to restraint from the off. In doing so, she gave the words of Emily — a feverish sung letter to her younger sister — every chance to tell their story.

Quite how a half-remembered account of the difference between meteors, meteorites and meteoroids can make for such a moving melodic passage was a mystery merely intensified by this plaintive telling. But Newsom is adept at wringing emotional resonance from unpromising set-ups. And here she was, plucking for dear life on behalf of the protagonists in Monkey & Bear, making us forget that we were listening to a song whose plot wasn’t so very different from that of Madagascar.

During Sawdust & Diamonds it took minutes to realise that the orchestra was wholly at rest for the song. As bass notes and countermelodies resounded from Newsom’s harp, the muscles on the right side of her face rose like the string-operated wings of the bird in the story. Lest we forget that the romantic exhilaration detailed on much of Ys came from a real place, the Smog frontman (and Newsom’s boyfriend) Bill Callahan dispensed two warm, meaningful nods at her after their duet at the end of Only Skin.

Before she took to the stage, many of Newsom’s fans would have been wondering how she might follow Ys. After an interval she returned — flanked by a drummer and bouzouki player — with an answer that, in light of her dazzling versatility, now seems obvious. Six minutes of hallucinogenic marching music. The ovation that followed merely confirmed what many were too speechless to articulate. If there’s one thing better than a rapture contained, it’s a rapture unleashed.

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