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During the last eruption of the Yellowstone super-volcano, approximately half the land area of the contiguous United States was covered by volcanic ash about 1 yard deep (on average). That means the ejected ash amounted to a volume of approximately 4.65 trillion cubic yards (give or take a few billion cubic yards).

[Lokibob]

    From:http://www.space.com/php/multimedia/imagedisplay/img_display.php?pic=yellowtone_volcano_0108_02,0.jpg&cap=Graphic%20shows%20the%20effect%20a%20super%20volcano%20at%20Yellowstone%20would%20have.