As a Narrator, I'd let an Ernaldan try to warm her fields as that logically stems from Earth. But if she said she wanted to spurt red-hot burning magma from the earth, I'd say that fails the credibility test.
The Veskarthan cult writeup should make that clear that they have a cult special ability to use the Earth Rune to have limited power over Fire. Just like Barntar subcultists have a cult special ability to use the Air Rune to have limited power over the Earth.
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