The cause of the Columbia River lava floods is still up for debate. See this section of the Wikipedia article and note the location of the Yellowstone hotspot at the time. On the southern edge of the flood zone. Many of the thickest areas are well to the north (over a mile thick!) in Washington. The Yellowstone hotspot location may just be a coincidence.
True … but then that implies the hot spot that cause these basalt floods is now extinct … which provides yet another alternative Yellowstone eruption scenario … “never again” …
I climbed to the caldera’s rim on Mount Etna and looked in. Next thing I knew all I was breathing was SO[sub]2[/sub]. I was starting to feel asphyxiated, when a fresh breeze blew good air into my face. I immediately recognized what gas it was from the smell, because I had used metabisulfite tablets dissolved in water to sterilize winemaking equipment, which produces sulfur dioxide gas. My own mini-anoxic event.
The Ozark uplift is the product of a hot spot that started with explosive volcanoes (of which the remnants are called the St. François Mountains) and then left a trail of magma-stuffed anticline uplift as far as Tahlequa, Oklahoma and finally the hot spot just sort of quit scorching the underside of the North American plate moving across it. It clearly started out with a bang and then trailed off. Perhaps the motion of the continental plate dragged enough stuff along to gradually snuff out the hot? Like shoveling sand onto burning embers? Minus the oxidation, of course.