支持小行星撞击说的科学家们推断，这次撞击相当于人类历史上发生过最强烈地震的100万倍，爆炸的能量相当于地球上核武器总量爆炸的1万倍，导致了2.1万立方公里的物质进入大气中。由于大气中大量高密度的尘埃，太阳光不能照射到地球上，导致地球表面温度迅速降低。没有阳光，植物逐渐枯萎死亡;没有植物，植食性的恐龙饥饿而死;没有植食性动物，肉食性的恐龙失去食物来源，在绝望和相互残杀中缓慢消亡。几乎所有的大型陆生动物都未能幸免于难。小型的陆生动物，像一些哺乳动物依靠残余的食物勉强为生，终于熬过了最艰难的时日，等到了古近纪陆生脊椎动物的再次大繁荣。

　　撞击假说的支持者发现了许多有力的证据，来证明他们的观点。最有力的证据来自在K/T(白垩纪和古近纪)地质界线上发现的铱异常和冲击石英。科学家推测，这种高含量的铱元素就是撞击地球的小行星带来的，冲击石英在撞击过程中形成，但同时撞击所形成的撞击坑却未被找到，多数的陨石坑被认为其大小与推测不相符合。

　　美国人查特吉约提出了一种类似的假说。他认为在白垩纪末期撞击地球的凶手不是一颗小行星或者陨石，而是彗星雨。大量的彗星雨撞击到地球上，形成一个环绕地球一周的撞击带，其中有2块巨大的彗星体成为了恐龙大灭绝的“主犯”：一块形成了墨西哥湾附近的巨大的陨石坑，另外一块撞击到印度大陆上，形成的陨石坑比墨西哥湾附近的陨石坑还大。

Explanatory theories

　　Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, and the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.

　　Plate tectonics. At the time of the Permian extinction, all the continents had recently joined to form the super-continent Pangaea and the super-ocean Panthalassa. This configuration radically decreased the extent and range of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. As the planet's epicontinental systems coalesced, many marine ecosystems, especially ones that evolved in isolation, would not have survived those changes. Pangaea's formation would have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons. Pangaea seems to have formed millions of years before the great extinction, however, and very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.

　　Impact event. When large bolides (asteroids or comets) impact Earth, the aftermath weakens or kills much of the life that thrived previously. Release of debris and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Evidence of increased levels of atmospheric carbon dioxide exists in the fossil record. Material from the Earth's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. At present, there is only limited and disputed evidence of iridium and shocked quartz occurring with the Permian event, though such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event. If an extraterrestrial impact triggered the Permian extinction event, scientists ask, where is the impact crater? Part of the answer may lie in the fact that there is no Permian-age oceanic crust remaining; all of it has been subducted, so plate tectonics during the last 252 million years have erased any possible P-T seafloor crater. Others have claimed evidence of a possible impact site off the coast of present-day Australia.

　　Supernova. A supernova occurring within ten parsecs (33 light years) of Earth would produce enough gamma radiation to destroy the ozone layer for several years. The resulting direct ultraviolet radiation from the sun would weaken or kill nearly all existing species. Only those deep in the oceans would be unaffected. Statistical frequency of supernovas suggests that one at the P-T boundary would not be unlikely. A gamma ray burst (the most energetic explosions in the universe, believed to be caused by a very massive supernova or two objects as dense as neutron stars colliding) that occurred within approximately 6,000 light years would produce the same effect.

　　Volcanism. The P-T boundary was marked with many volcanic eruptions. In the Siberian Traps, now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic effect on Earth, lasted for millions of years. The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life. The theory is debated whether volcanic activity, over such a long time, could alter the climate enough to kill off 95 percent of life on Earth. There is evidence for this theory though. Fluctuations in air and water temperature are evident in the fossil record, and the uranium/thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the Siberian Traps—the largest known eruption in the history of Earth—occurred at this time, while no direct evidence of bolide impact has been located.

　　Atmospheric hydrogen sulfide buildup. In 2005, geoscientist Dr. Lee R. Kump published a theory explaining a cascade of events leading to the Great Extinction. Several massive volcanic eruptions in Siberian Traps, described above, started a warming of the atmosphere. The warming itself did not seem to be large enough to cause such a massive extinction event. However, it could have interfered with the ocean flow. Cold water at the poles dissolves atmospheric oxygen, cools even more, and sinks to the bottom, slowly moving to the equator, carrying the dissolved oxygen. The warmer the water is, the less oxygen it can dissolve and the slower it circulates. The resulting lack of supply of dissolved oxygen would lead to depletion of aerobic marine life. The oceans would then become a realm of bacteria metabolizing sulfates, and producing hydrogen sulfide, which would then get released into the water and the atmosphere, killing oceanic plants and terrestrial life. Once such process gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide. Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. Increase of concentration of carbon dioxide would not cause extinction of plants, but according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages the ozone layer, and fossil spores from the end-Permian era show deformities that could have been caused by ultraviolet radiation.

　　Methane hydrate gasification. In 2002, a documentary, The Day the Earth Nearly Died, summarized some recent findings and speculation concerning the Permian extinction event. Paul Wignall examined Permian strata in Greenland, where the rock layers devoid of marine life are tens of meters thick. With such an expanded scale, he could judge the timing of deposition more accurately and ascertained that the entire extinction lasted merely 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost total extinction of marine life. Such a process seemed too long, however, to be accounted for by a meteorite strike. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation for such a spike—rotting vegetation—seemed insufficient. Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released by the upwelling of frozen methane hydrate from the seabed. Experiments to assess how large a rise in deep sea temperature would be required to sublimate solid methane hydrate suggested that a rise of 5°C would be sufficient. Released from the pressures of the ocean depths, methane hydrate expands to create huge volumes of methane gas, one of the most powerful of the greenhouse gases. The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth.

　　A combination. The Permian extinction is unequaled; it is obviously not easy to destroy almost all life on Earth. The difficulty in imagining a single cause of such an event has led to an explanation humorously termed the "Murder on the Orient Express" theory: they all did it. A combination involving some or all of the following is postulated: Continental drift created a non-fatal but precariously balanced global environment, a supernova weakened the ozone layer, and then a large meteor impact triggered the eruption of the Siberian Traps. The resultant global warming eventually was enough to melt the methane hydrate deposits on continental shelves of the world-ocean.

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