A subduction mélange is a mixture of rocks that have been carried deep into the Earth at the boundaries of tectonic plates, but how they later returned to the surface is unclear. By studying the garnet crystals in the mélange from Cedros Island, Baja California, Mexico, we show that these rocks were exhumed surprisingly fast. These garnet crystals show complex, sharply defined compositional zones like tree rings, which suggest that they grew in quick bursts during rapid and extreme pressure changes. After reaching their deepest point, the rocks cooled down incredibly fast—at a rate of about 10°C every 1,000 years, much quicker than what existing geological models predict.