Here's why:
* Subduction Zone Magma: Magma generated in subduction zones is typically andesitic to basaltic in composition. This means it's rich in silica, sodium, and potassium.
* Cooling and Crystallization: As magma rises towards the surface, it cools and starts to crystallize. The minerals that crystallize first are generally denser and sink to the bottom of the magma chamber.
* Gabbro Formation: If this magma cools and solidifies at depth, the resulting igneous rock will be gabbro. Gabbro is a coarse-grained, dark-colored intrusive igneous rock with a composition similar to basalt.
Other possibilities:
While gabbro is the most likely outcome, other rocks could potentially form depending on the specific composition of the magma and the depth of solidification. These could include:
* Diorite: If the magma is more andesitic in composition, a rock called diorite, which is a bit lighter in color than gabbro, could form.
* Tonalite: If the magma is even more silica-rich, a rock called tonalite, which is light-colored and has a higher silica content, could form.
In summary: The specific rock formed from magma at depth depends on the composition of the magma and the pressure and temperature conditions during crystallization. However, gabbro is the most likely outcome for magma solidifying at considerable depth beneath volcanoes above subduction zones.
