Earth scientists separate all rocks into three major groups: igneous, metamorphic and sedimentary. At that point, the process of identifying and classifying a rock has just begun. For each of the three major groups, there is a classification scheme to divide its members into ever-smaller subdivisions based on visual evidence and on the minerals that can be identified from a sample of the rock. Once all the classification questions have been answered, the rock can be identified by a specific name, a name that tells other earth scientists how that particular rock came to be.
Consider a fist-sized sample of the rock bearing the name diorite (DIE-oh-right). A glance at your sample reveals that it is made up of irregularly-shaped, randomly-oriented mineral crystals that form an “interlocking” texture as if it were a 3-D jigsaw puzzle. That observation tells you that first, this is an igneous rock because of its the randomly-oriented interlocking crystals; and second, that it is an intrusive igneous rock, because you can see the coarse crystals without resorting to a magnifying glass. Igneous rocks crystallize as they cool from molten rock (magma), and intrusive rocks have relatively large crystals because they cool slowly, usually deep underground.
There are many intrusive igneous rocks, though, so what else can you determine at a glance? Chances are good that your sample is gray, either light or dark. Coarse-grained igneous rocks range from light granites to dark gabbros, so you might guess that the composition of a diorite is intermediate between the two, a correct guess. Just by looking at a sample you can determine that diorite is an intrusive igneous rock of intermediate composition. For most people, that’s sufficient information.
Of course, igneous petrologists – people who study igneous rocks for a living – will want more information about that “intermediate” composition. Diorites are mostly composed of lighter-colored plagioclase (calcium-sodium) feldspar mixed with black hornblende. Other minerals may be present, such as dark minerals like pyroxene and olivine, and the occasional crystal of quartz or orthoclase (potassium) feldspar. If either quartz or orthoclase exceeds 10% of the volume, the sample is really one of the granite group. If the percentage of crystals of the duller pyroxene exceed those of the shinier hornblende, it’s probably a gabbro.
Like other intrusive igneous rocks, diorite has an extrusive equivalent with the same mineral content, but made up of crystals that cannot be seen with the naked eye. “Extrusive” means that the igneous rock cooled from magma that had reached the Earth’s surface as lava, such as the basaltic lavas of the Hawaiian Island chain. The extrusive equivalent of diorite is named andesite, and gets its name from the Andes Mountains of South America where it is common.
Diorite is less common than either granite or gabbro, the compositional end members of the intrusive igneous rocks. The plate-tectonic explanation for this rock type’s relative rarity is that magmas of granitic composition are generated by melting continental crust and sedimentary rocks, while magmas of gabbroic composition are generated through melting of oceanic crust. Diorite’s composition requires melting a mixture of oceanic and continental crust, such as might occur in subduction zones where an oceanic plate is being consumed beneath a continental plate. A bubble of magma created by melting the oceanic crust deep in the subduction zone is hot enough to melt a portion of the continental crust as it rises, creating a mixture of magma that is of intermediate composition. This scenario might also explain why diorite plutons frequently grade laterally into gabbros, rocks that represent crystallization of a magma derived entirely from oceanic crust.
Although it is not as common as granite, diorite is known from around the world. Deposits are associated with both modern and ancient island arcs, in particular across the western edge of South America and the southern edge of Europe. In North America, diorite plutons have been described in the Sierra Nevada and Arizona Basin and Range, among other locations.
