are also catastrophic events, which likely contributed to many mass extinctions in the geologic record. Crust formation The extrusion of flood basalts, averaged over time, is comparable with the rate of extrusion of lava at mid-ocean ridges and much higher than the rate of extrusion by hotspots. However, extrusion at mid-ocean ridges is relatively steady, while extrusion of flood basalts is highly episodic. Flood basalts create new continental crust at a rate of per year, while the eruptions that form oceanic plateaus produce of crust per year. Much of the new crust formed during flood basalt episodes takes the form of underplating, with over half the original magma crystallizing out as cumulates in sills at the bast of the crust. Mass extinctions The eruption of flood basalts has been linked with mass extinctions. For example, the Deccan Traps, erupted at the Cretaceous-Paleogene boundary, may have contributed to the extinction of the non-avian dinosaurs. Likewise, mass extinctions at the Permian-Triassic boundary, the Triassic-Jurassic boundary, and in the Toarcian Age of the Jurassic correspond to the ages of large igneous provinces in Siberia, the Central Atlantic Magmatic Province, and the Karoo-Ferrar flood basalt. Some idea of the impact of flood basalts can be given by comparison with historical large eruptions. The 1783 eruption of Lakagigar was the largest in the historical record, killing 75% of the livestock and a quarter of the population of Iceland. However, the eruption produced just of lava, which is tiny compared with the Roza Member of the Columbia River Plateau, erupted in the mid-Miocene, which contained at least of lava. During the eruption of the Siberian Traps, some of magma penetrated the crust, covering an area of , equal to 62% of the area of the contiguous states of the United States. The hot magma contained vast quantities of carbon dioxide and sulfur oxides, and released additional carbon dioxide and methane from deep petroleum reservoirs and younger coal beds in the region. The released gases created over 6400 blast pipes, each typically over in diameter. The pipes emitted up to 160 trillion tons of carbon dioxide and 46 trillion tons of methane. Coal ash from burning coal beds spread toxic chromium, arsenic, mercury, and lead across northern Canada. Evaporite beds heated by the magma released hydrochloric acid, methyl chloride, methyl bromide, which damaged the ozone layer and reduced ultraviolet shielding by as much as 85%. Over 5