1,000 years after people: The Yellowstone National Park is overgrowing. But it remains unchanged. But, there is also danger. The Yellowstone Caldera is active after over 640,000 years of being dormant. At the Norris Geyser Basin, a powerful Earthquake triggers an eruption. But the eruption is merely a vent. But the vent is as powerful as the 1980 eruption of Mount St Helens. But the single vent ends up causing a chain reaction that causes dozens of massive eruptions to open up across the park. Which will all eventually merge to form the new caldera. Within a few days, over 12'000 cubic kilometers of ash spew out of the volcano and into the sky. The ash cloud reaches as far east as New York City. But the ash fall is minimal. With only a few centimeters falling in New York. Within weeks, the entire west half of the United States is covered by over 7 meters of ash. With areas closer to Yellowstone buried up to a dozen meters if not more. A few months after, the dust and particles from the eruption will have blocked the sun. Causing global temperatures to cool. The world is eventually plunged into a volcanic winter that will last for decades. Millions of animals will die in the event. And possibly result in the extinction of a species.
2,500 years after people: Even though the eruption has long passed. The scars still remain. In the place where Yellowstone once was, is a new massive valley made by the caldera. But many if not all geysers will never erupt again.