The Antarctic plate finds the side abutting the Pacific under pressure, as the plates due to fold or crumble or subduct in the Pacific begin to do this. This in effect is forcing the Antarctic plate to fold itself, like a fan, and it resists this pressure to fold. The tendency is rather for the Antarctic plate to break away from these bordering plates, so it then becomes free to move. Likewise, along the Atlantic, the spreading plates in the Atlantic are asking the Antarctic plate to rip apart, which it resists, being a very solid plate. The point that gives, the weak point, is the border between the Antarctic plate and those in the Atlantic, as these are already weakened points. Thus, these weak points separate, the Atlantic plates going their way, the Antarctic staying in place.
The Antarctic plate is now free on all sides! The overlapping and compressing plates on the Pacific side represent pressure and push. The separating and spreading plates on the Atlantic side represent an opening, so the Antarctic plate moves in that direction. Freed of tight borders, it is free to float or sink in the sea of lava that all plates ride upon, and as a land plate, being lighter than those that have sunk under the deep seas, it floats - new land! There are contours on plates, below water, just as there are contours above water. The portion of the Antarctic plate that emerges above the waves is higher than the rest of the Antarctic plate . Thus, the new land and existing Antarctica are not joined.