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Why is an asteroid impact not strongly supported as the main cause of the Permian extinction?

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Permian paleogeography — Permian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins near the end of the Permian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The hypothesis that an asteroid impact caused the Permian extinction is not strongly supported because, unlike the end-Cretaceous extinction event, there is no consistent geological evidence at the Permian-Triassic boundary to substantiate it.1

Specifically, researchers have failed to find the telltale markers typically associated with such a cataclysmic impact, such as an anomalous presence of iridium or the discovery of shocked quartz—quartz grains that have undergone the intense pressures and temperatures characteristic of an impact shock. While some scientists have proposed that a large icy meteorite could have triggered sudden global cooling, this theory remains speculative and lacks supporting physical evidence.12

In contrast to the asteroid hypothesis, current scientific consensus focuses on more gradual or sustained environmental disruptions, such as the prolonged eruption of the Siberian flood basalts, which likely led to severe oceanic anoxia and significant alterations to the global carbon cycle.341