Final answer:
The stability of a triple junction is primarily determined by tectonic forces, which include the movement and interactions of tectonic plates at their boundaries.
Step-by-step explanation:
The stability of a triple junction is determined by tectonic forces. A triple junction is a point where the boundaries of three tectonic plates meet. The dynamic nature of plate tectonics means that the stability of these junctions is shaped mainly by the movement and interactions of the plates, which are driven by tectonic forces. While temperature, pressure, and composition can influence the behaviors of rocks and minerals at the boundaries, they are not the primary factors in determining the stability of a triple junction. Instead, the tectonic forces such as the rate of plate movement, the direction of plate movement, and the nature of the interaction (whether convergent, divergent, or transform) are key to understanding the stability of the junction.