According to the film's junior novelisation, the spiral was covered under layers of cooled lava. It wasn't until the climactic fight (which resulted in multiple layers being smashed off, at the cost of Maui's hook) that the spiral was finally revealed.
No longer sure what to do, Moana took a deep breath to calm herself and listened for her voice inside. She turned and saw Te Kā looming over Maui, volcanic lightning flashing all around them. And then she spotted something on Te Kā’s chest. A glowing spiral covered up by cooling lava. Moana looked over her shoulder at the empty crater where Te Fiti should be.
“The spiral…” she said, looking down at the heart in her hand. Its glow brightened with her every step and she looked to the ocean. Moana knew what she had to do.
