The stress force that pulls on the crust and thins the rock in the middle is called tensile stress.

Here's why:

* Tensile stress occurs when a force pulls on an object, stretching it and making it thinner.

* In the context of the Earth's crust, this happens when tectonic plates move apart, a process known as divergent plate boundaries.

* The pulling force creates a rift valley in the middle, where the crust is thinnest and often stretched apart, creating new oceanic crust.

Here's a simple analogy: Imagine pulling on a rubber band. The rubber band stretches, becoming thinner in the middle. This is similar to what happens with the Earth's crust under tensile stress.