Here's why:

* Normal faults occur when the Earth's crust is pulled apart (extensional forces). This causes the hanging wall (the block of rock above the fault) to move down relative to the footwall (the block of rock below the fault).

* The Rio Grande Rift is a classic example of an extensional tectonic environment. As the crust stretched and thinned, the rocks fractured along normal faults. This process resulted in the sinking of large blocks of land, creating the valley.

Additional details:

* The Rio Grande Rift is a series of interconnected valleys that extend from Colorado to Texas.

* The process of rifting is still ongoing in this area, albeit at a much slower rate than in the past.

* The normal faults within the rift zone are often associated with volcanic activity, as magma can rise up through the weakened crust.

Note: While other types of faults exist, like reverse faults and strike-slip faults, they are not responsible for the formation of the Rio Grande Rift Valley.