Here, Left amount of Uranium = 1/4
So, Number of Half lives gone by = 1/2 / 1/4 = 2
Life after 1 half-life = 1.3 billion years
So, With 2 half-lives, it would be: 1.3 * 2 = 2.6 billion years
In short, The Rock is 2.6 billion Years Old
For second question, "sea floor spreading and plate tectonics indicate that the continents do move or shift" and it is the basic principle of Continental drift theory.
Hope this helps!