A Brief Introduction to Volcanoes

Before we get started I gotta come clean: The year was 1957, I was six years old in southeastern Nebraska, and Pop had called a family meeting to inform my sister and I that he had accepted a job at the university in Long Beach, California and we were moving. Real soon.

All I knew about California at the time was that they had volcanoes, and I was absolutely petrified. There’s no reason to delve too deeply into the nightmares that followed (and how the heavenly hook that descended from the sky saved me from the lava), but suffice it to say that my preoccupation with volcanoes had been fully launched within two sleeps.

Well, I worked through it and now I just love volcanoes: they’re so damn photogenic, and I’ve been chasing them as fate and fortune have allowed. We’ll circle back to them many times as my blog matures, and you can always expect oodles and scads of images to support the narrative.

With that being said…

For the most part, volcanoes come in two flavors: they either kaBoom or ooze. While we will surely explore both in greater detail in future blogs, it would seem like a brief overview and a bit of background would be helpful here at the beginning:

In these days of unrestrained polarization very little is accepted by one and all, but I’m pretty sure that everyone who cares about such things would agree that the earth gets hotter with depth, and that volcanic activity at the surface is a clear indication that magma (liquid rock) is being produced deep within the earth’s crust and upper mantle.

It takes two things for rock to change from the solid into the liquid phase: heat, and a reduction of pressure so it has room to expand. The heat is obvious, but since the liquid phase of any substance expands in volume and therefore requires more space, a lowering of the confining (a.k.a. lithostatic) pressure is necessary. (This physical reality exists for all natural substances except water, but more on that later.) Click here for more information on the phases of matter.

Lava is nothing special; it’s just magma that has reached the surface.

Most volcanoes are directly related to Plate Tectonics, and are associated with either the oceans (these are nearly always mafic volcanoes), the continents (usually felsic eruptions), or the coastlines (often intermediate magmas, but anything can erupt: expect what you see!).

Click here for more information on the compositional terms mafic, intermediate, and felsic.

Click here for an earlier post regarding the terms mafic and felsic.

Click here for more on cycles of volcanism.

Unlike earthquakes, volcanoes generally give some warning, although they can still create massive challenges for the biosphere. These warnings can take many forms, but — assuming the warnings are understood, and respected — will give the locals days to weeks to months to get ready.

But respected and heeded or not, volcanoes are the earth’s essential thermal relief valves (much like the pressure cap on your car’s radiator, or that funny vent of top of your hot water heater). They help keep our planet from exploding in a cosmically insignificant kaBoom — that would be bad — so we should probably like them, even when they seem angry.

Volcanoes that ooze: These are generally mafic in composition and are commonly associated with the seafloor and zones of crustal divergence. These are the hottest lavas with the lowest viscosity and volatile content; which are, in part, why they ooze and don’t kaBoom. (Another reason has to do with Bowen’s Reaction Series and the atomic structures of silicate minerals, but we’ll sneak past that complexity for now.) The most common mafic volcanic rock is basalt.

Common examples of mafic volcanoes include Iceland (smack-dab on the Mid-Atlantic spreading ridge), and Hawai’i — not actually on a plate boundary, but riding on the Pacific Plate across a deep-seated mantle plume (a.k.a. hot spot).

Other examples include the fields of monogenetic (“one and done”) cinder cones littering the earth, and continental flood basalt — such as the Modoc/Columbia River basalt flows in the western United States, and the Deccan Traps in India.

Mafic volcanoes — like the ones in Hawai’i, for example — are often called “shield volcanoes” due to the gentle slopes of their sides. And for good reason: basaltic lavas are usually so hot and runny that they can flow just like water.

Volcanoes that kaBoom: These are either intermediate or felsic in composition, with the felsic varieties being the most explosive (kaBoom²). True felsic volcanoes are generally found on the continental landmasses, and include such favorites as the Long Valley Caldera in California, and the Yellowstone Supervolcano. These guys are truly ginormous, and can leave behind calderas measuring tens of miles across — sometimes many tens of miles. As much as I love the explosive volcanoes, even I am not crazy enough to want Yellowstone to erupt any time soon!

Most intermediate volcanoes (with felsic and/or mafic overtones) are found along the coastlines, and as such are generally related to zones of crustal convergence. The classic example is along the west coast of South America, where the eastward moving Nazca tectonic plate (one of the oceanic plates that makes up the floor of the Pacific) is running into the westward drifting South American continental plate along the Peru-Chile Trench, a deep furrow that parallels the coastline. This heroic defile, thirty-six hundred miles long and plunging to depths greater than twenty-six thousand feet, marks the top of the subduction zone.

The descending basaltic slab angles down and under the continent from there, taking with it sediment and goo (there can be a lot of sediment and goo), and water (a very important addition). The explosive volcanic peaks of the Andes are the gas and thermal relief valves above much deeper portions of this major plate boundary, where — with increasing depth and heat — the waterlogged basalt and debris is ultimately re-melted. New magmas are formed: volatile, lower in density, and buoyantly fighting for the surface.

When they get there, guess what happens? Just like a bottle of carbonated soda that is shaken and then the cap removed, they explode; in this case forming the Andes Mountains. The most common intermediate volcanic rock produced in one of these is called andesite — cleverly named after the Andes (you just gotta love GeoSpeak).

Other zones of convergence (also hosting kaBoom volcanoes) include the Cascades in the Pacific Northwest, Japan, the Aleutians, and many, many others around the “Pacific Ring of Fire.”

Intermediate volcanoes are also called “composite cones” due to the cycling of pyroclastic kaBooms followed by andesitic flows that struggle to even reach the base of the cone.

Subduction zone volcanoes are also referred to as “stratovolcanoes” because, since neither the big pieces of the pyroclastics nor the andesitic flows get very far, most of the ejecta piles up around the vent. These puppies can get really tall.

So yeah, kaBoom or Ooze. I promise more details on both in future blog posts, but this should at least get us started…