To the surprise of many, New Mexico is sometimes called the Volcano State. It’s not that we have any erupting volcanoes – at present. But the sheer variety of volcanic features here is unrivaled by any other state in the country, including Alaska and Hawaii. We are definitely an igneous state.
Back in fifth grade you probably heard about the three great groups of rocks on Earth: igneous, sedimentary, and metamorphic. Igneous rocks are rocks that crystallize from melts called magma. Magma is a mix of liquid silica-rich melt, suspended crystals, and dissolved gas like water vapor and carbon dioxide. It’s hot: 1300 to 2400 degrees Fahrenheit, hot enough to glow like fire. When it cools, it freezes in complex ways into igneous rocks – “born of fire”.
Most magma remains trapped in the Earth’s crust. But when it gets out, it forms volcanoes, named after Vulcan, the Roman god of fire, with his smoky forge Vulcano in the Mediterranean Sea. Magma extruded at the Earth’s surface is given an older Italian name, lava, which refers to both the flowing melt and the rock into which it cools. Lava flung into the atmosphere by the explosive expansion of dissolved gas forms a variety of fragmented and glassy materials called pyroclasts - “fire fragments”. Lavas, loose pyroclasts, or tephra, and pyroclastic material consolidated into rock, called tuff, collectively form the volcanic rocks.
Since volcanic rocks are quenched at the Earth’s surface, they typically have fine-grained, glassy, or fragmental textures. It is usually easy to recognize a volcanic rock in the field, but assigning them to their specific family – basaltic, andesitic, trachytic, dacitic, or rhyolitic – can be frustratingly difficult. Much depends on finding and identifying small suspended mineral crystals to help out, which is something few of us do on a regular basis. Like algebra. Learning to simply recognize a lava, and to name variations based on texture, like pumice or obsidian, is an easy and rewarding undertaking for natural history buffs.
Magmas trapped deep in the Earth’s crust belong to the plutonic realm, named after a darker god, Pluto, the Roman ruler of the underworld. These magmas are intimately associated with the underworld rocks which they intrude, rocks which have been changed by heat, confining pressure, and shearing stresses into metamorphic rocks.
Plutonic rocks, having cooled slowly deep in the crust, with all their juices sealed in, typically have coarse-grained, visibly crystalline textures – granitic textures. This makes it a little easier to assign them names in the field – granite, granodiorite, tonalite, diorite, gabbro, monzonite, syenite – but since magmatic rocks are mixes, not species, there is always some blurring and overlap. Learning to mentally gauge whether the rock is rich or poor in dark minerals, and rich or poor in visible quartz, helps out here. Light-colored, quartz-rich members of the granite and granodiorite family are much more common than the others.
Simply finding a plutonic or metamorphic rock in the field means something has transported that messenger from the underworld up to the surface. What could it be?
There are any number of rock-identification guides and classification schemes both online and off, but one sweet site you might want to visit has been created by our Australian cousins: Igneous rock types. Go have a look!