It is considered animal harassment when you approach closer than 25 yards to an elk and 100 yards to a buffalo. Bears, of course, demand even greater distance. Simply described, a caldera is a volcano that has collapsed in part, or wholly. When Yellowstone last erupted, approximately 625,000 years ago, it blew off nearly 100 square miles. By comparison, Mount St. Helen's in Washington blasted about one square mile.
Deer never turn into elk.
They are two different types of hooved animals. The animals are wild and to encourage them to seek food from humans can result in death from inappropriate food or dependency. The maximum speed limit is 45 miles per hour, with many areas posted below that. Some would say, a lifetime. To give yourself a fair overview of Yellowstone, plan to spend at least two days.