Magical Mondays: Earth Has Magic Now? Source??

I’ve been a reading fiend lately—my new library card is beginning to wear down under the abuse. I’m trying to catch up on several years’ worth of not-having-consistent-time-to-read, and have especially been trying to reacquaint myself with both what’s new and what classics I haven’t read from the genres I love. Because of this, in the last two weeks I’ve found myself reading two different books that are set on an Earth so post-apocalyptic as to be unrecognizable. These books are both ostensibly fantasy—they include magic items, fantastical locales, and creatures of legend—but the fact that they’re supposedly set on our Earth gives me pause. If there wasn’t magic before the apocalypse, where did this magic come from?

A map of Shannara. Not pictured: anything that looks like any part of the current planet Earth. Also not pictured: topography that makes logical sense.