Final answer:
The composition of magma evolves as it cools, starting with the early crystallization of high temperature minerals like feldspar, followed by others like quartz. The rate of cooling affects the texture, with slow cooling leading to large crystals in intrusive rocks like granite, and rapid cooling producing fine-grained extrusive rocks like basalt. The original magma composition reflects the tectonic setting, influencing the composition of the resulting igneous rock.
Step-by-step explanation:
As magma cools, its composition typically evolves in a few notable ways. Early in the cooling process, high temperature minerals like feldspar begin to crystallize, giving them defined shapes. These minerals are usually lighter in color and, as they form at higher temperatures, they tend to be the first to crystallize out of the magma. As cooling continues, minerals like quartz start to crystallize later and occupy the spaces between earlier formed minerals. This slower cooling allows the formation of large, identifiable crystals.
In situations where the magma cools more rapidly, such as at or near the surface, mineral crystals will have less time to grow, which results in fine-grained textures. The environment where magma cools greatly influences the final texture and mineralogy of the igneous rock formed; this can be either intrusive or extrusive. Intrusive rocks like granite form from magma that cools slowly beneath the Earth's surface, fostering the development of large crystals and a coarse-grained texture. Conversely, extrusive rocks such as basalt are the result of rapid cooling at or near the Earth's surface, leading to a fine-grained or even glassy texture.
The original composition of the magma can also reflect the tectonic setting it comes from, such as mafic magma produced at mid-ocean ridges contributing to the oceanic crust, and more silicic magma generated at subduction zones or hotspots contributing to continental crust. Ultimately, the magma composition, rate of cooling, and location where cooling takes place, all play crucial roles in determining the characteristics of the igneous rock that is eventually formed.