Simplify the following text for a Grade 9 native English-speaking person.
Original: Three deep canyons are in the park , cut through the volcanic tuff of the Yellowstone Plateau by rivers over the last 640,000 years ( since the last huge explosion ) .
Simplified: Three deep canyons are located in the park , cut through the volcanic tuff of the Yellowstone Plateau by rivers over the last 640,000 years .