The P*T extinction was rapid, probably taking place in less than a million years. Although it was much more severe in the ocean, it affected terrestrial ecosystems too. A prolific swamp flora in the Southern Hemisphere had been producing enough organic debris to form coals in Australia, but the coal beds stop abruptly at the P-T boundary. No coal was laid down anywhere in the world for at least 6 m.y. afterward. A large change in carbon isotopes occurred across the P*T boundary, which signifies an important and global drop in photosynthesis that lasted a long time.
There is some evidence for an impact at the P*T boundary. The continental collisions that formed Pangea in the Permian would account for a major drop in diversity but not for a sudden, enormous mass extinction. Perhaps most important of all, the Permian extinction coincides with the largest known volcanic eruption in Earth history.
In addition to plate tectonics, the Earth also has plume tectonics. Occasionally, an event at the boundary between the Earth's core and mantle sets a giant pulse of heat rising toward the surface as a plume. As it approaches the surface, the plume melts the crust to develop a flat head of basalt magma that can be 1000 km across and 100 km thick. Penetrating the crust, the plume generates enormous volcanic eruptions that pour hundreds of thousands of cubic kilometers of basalt‹flood basalts‹out over the surface. If a plume erupts through a continent, it blasts material into the atmosphere as well. After the head of the plume has erupted, the much narrower tail will continue to erupt for 100 m.y. or more, but now its effects are more local, affecting only 100 km or so of terrain as it forms a long-lasting hot spot of volcanic activity.
Plume events are rare: there have been only eight enormous plume eruptions in the last 250 m.y. The most recent is the Yellowstone plume: at about 17 Ma it burned through the crust to form enormous lava fields that are now known as the Columbia Plateau basalts of Oregon and Washington, best seen in the Columbia River gorge. North America drifted westward over this "hot spot," which continued to erupt to form the volcanic rocks of the Snake River plain in Idaho (Valley of the Moon and so on), and it now sits under Yellowstone National Park. The hot spot is in a quiet period now, with geyser activity rather than active eruption, but it produced enormous volcanic exposions about 500,000 years ago that blasted ash over most of the mountain states and into Canada.
A massive plume eruption took place exactly at the P*T boundary. A new plume burned through the crust in what is now western Siberia to form the "Siberian Traps," gigantic flood basalts that cover 2.5 million sq km in area and are perhaps 3 million cu km in volume. The eruptions coincided exactly with the P*T boundary, at 253 Ma, and lasted at full intensity for only about a million years‹the largest known, most intense eruptions in the history of the Earth. They lie across the P*T boundary and were formed in what was obviously a major event in Earth history.
