In Death Valley, the ground can get hot enough to boil water, but in general, to get steam temperatures, you need insulation and something to cut off airflow. Concentrated sunlight doesn't hurt, either.
That's a clever idea, using asphalt that's going to get hot, anyway. It might make an economical water heater in a place like Hawaii or Florida. The downside in other places is that the asphalt won't get very hot all year round, whereas a water heating panel with glass could function even with freezing air temperatures.