Here's why:

* The mantle is primarily composed of peridotite, a rock rich in silicate minerals like olivine and pyroxene.

* The mantle is subjected to immense pressure and high temperatures, which vary with depth.

* As rocks rise towards the Earth's surface, the pressure decreases and the melting point lowers. This leads to partial melting, where only a portion of the rock melts.

* The melt is typically composed of minerals with lower melting points, such as silica, aluminum, and sodium, which are less dense than the surrounding solid rock. This allows the magma to rise towards the surface.

Other factors that influence magma formation include:

* Water content: The presence of water lowers the melting point of rocks, making them more likely to melt.

* Plate tectonics: Plate movement and collisions generate heat and pressure, leading to magma generation in subduction zones and mid-ocean ridges.

It's important to note that while the mantle is the primary source of magma, some magmas can also form from the melting of pre-existing crustal rocks.