One of the joys of teaching is, oddly, learning. Teaching a basic class in physical geology to a wide range of students at the local community college brings me weekly opportunities to hone my communication skills – to try to find concise ways to summarize complex topics in an accurate, yet simple, manner. The struggle to do this teaches me far more, I suspect, than I teach my students.
Volcanism is a case in point. It’s a big topic. Is there a way to capture its essence in a blog-sized post? Here’s a try:
As I’ve mentioned before, New Mexico is the Volcano State. The sheer variety of volcanic features here is unrivaled by any other state in the country, including Alaska and Hawaii. And yet all of this volcanic activity can be encompassed between two poles: effusive eruptions and explosive eruptions.
Effusive behavior refers to relatively quiet outpourings of molten lava from a volcanic vent. Effusive eruptions of fluid basaltic lavas are exceedingly common, forming stacks of thin flows that pile gently into broad shield volcanoes, or spread out across the countryside in lava plateaus.
Effusive extrusions of viscous silicic lavas, like dacite or rhyolite, are much less common. These eruptions typically form short, thick, glassy flows, piling into rubble-covered lava domes that usually plug their own vent.
Explosive eruptions occur when violently expanding gases fragment molten lava into clouds of pumice, scoria and volcanic ash. The first phase of many otherwise effusive eruptions of basalt is a gassy discharge that builds a one-shot, steep-sided cone of scoria, commonly called a cinder cone.
Eruptions of somewhat more silicic lavas, like andesite, dacite, and trachyte often alternate between explosive and effusive phases, over tens of thousands of years, building “composite” cones of lava and ash that can grow into large – but unstable – mountains.
Although thankfully uncommon, very large eruptions of silicic rhyolite lava can be catastrophically explosive, burying thousands of square miles of countryside under incandescent blankets of ash flow tuffs up to a thousand feet thick. Events of this magnitude probably alter the global climate. These eruptions leave circular zones of collapsed crust, miles across, called calderas, so large that they are difficult to recognize from the ground.
What lies below
Magmas trapped and crystallized as shallow intrusives in the feeder systems beneath volcanoes – dikes, sills, and shallow stocks – are often grouped together with the other volcanic rocks. Where erosion exposes these formerly buried structures, fascinating landforms result, complementing the flows, cones, craters, and calderas of modern New Mexico.