The boundary formed when magma rises to the surface and new crust forms is called a divergent plate boundary.

Here's why:

* Divergent boundaries are areas where tectonic plates move apart from each other.

* As the plates pull away, they create a gap that allows magma from the Earth's mantle to rise.

* This magma cools and solidifies, forming new crust.

* The process of new crust formation is called seafloor spreading when it occurs beneath the ocean, and rifting when it occurs on land.

Examples of divergent boundaries include:

* Mid-ocean ridges: These underwater mountain ranges are where new oceanic crust is constantly being created.

* Rift valleys: These are areas on land where the crust is being pulled apart, often leading to volcanic activity.

Let me know if you have any other questions!