Ashfall by Mike Mullin
Under the bubbling hot springs and geysers of Yellowstone National Park is a supervolcano. Most people don't know it's there. The caldera is so large that it can only be seen from a plane or satellite. It just could be overdue for an eruption, which would change the landscape and climate of our planet.
Ashfall is the story of Alex, a teenage boy left alone for the weekend while his parents visit relatives. When the Yellowstone supervolcano erupts unexpectedly, Alex is determined to reach his parents. He must travel over a hundred miles in a landscape transformed by a foot of ash and the destruction of every modern convenience that he has ever known, and through a new world in which disaster has brought out both the best and worst in people desperate for food, water, and warmth. With a combination of nonstop action, a little romance, and very real science, this is a story that is difficult to stop reading and even more difficult to forget.