Here's why:
* Subduction: This process involves one tectonic plate (the subducting plate) being forced beneath another (the overriding plate). The subducting plate is typically denser and older than the overriding plate.
* Pulling Force: The primary force driving subduction is the pull of the subducting plate's own weight. As the denser plate sinks, it pulls the rest of the plate along, creating a tensile force at the leading edge.
* Slab Pull: This pulling force is known as slab pull and is a major driving force in plate tectonics.
* Other Factors: While slab pull is dominant, other forces contribute, including:
* Ridge Push: The upward pressure of magma at mid-ocean ridges can also push the plates apart, contributing to the movement of the subducting plate.
* Friction: Friction between the plates can generate heat and resistance, but it doesn't directly create the tensile force.
To summarize:
* The leading edge of a subducting plate experiences tensile stress primarily due to slab pull.
* This force is responsible for the downward movement of the subducting plate and the resulting geological features like trenches and volcanic arcs.
