According to the United States Geological Survey, the largest flood basalt in the United States is the Columbia River flood basalt, which is an area covering over 63,000 square miles on the Pacific Northwest, including parts of Washington state and Oregon. Scientists believe that the volcano that caused the Columbia River flood basalt erupted around 12 million years ago, and the flow of basalt lava may have lasted as long as 10 million years. Affects of this flood basalt include forcing the Columbia River into its modern day direction, as well as the creation of the Columbia Basin.
Another large flood basalt is located in Russia and, according to the Massachusetts Institute of Technology, a volcano erupted around 250 million years ago with enough lava to cover the entire continental United States. Some scientists believe that this may be the cause of the Permian era extinctions, which was a period of time before the dinosaurs in which around 90 percent of marine species and 70 percent of land species went extinct. A large eruption like this could have caused this high level of extinction because the resultant ash may have blocked out the sun for an extended period of time.
The Deccan Traps are a huge flood basalt located in modern-day India. According to Rochester University, the Deccan Traps were formed around 65 million years ago, which coincides with the extinction of the dinosaurs at the end of the Cretaceous period. The Deccan Traps were likely formed from lava that erupted after incubating for over 3 million years in the Earth's core. Scientists were able to determine the date and initial location of the lava by using isotropic dating on basalt rocks found in India.
Flood basalts are not unique to Earth, as they also have been found on the moon and Mars. According to a study published in the Oxford Journal of Petrology, flood basalts are one likely cause of the maria, which look like seas to the naked eye, that is seen on the moon. While the moon has not been volcanically active for billions of years, tests ran on moon rocks from the Apollo mission confirm that they are basalt.