Peeking under the skin of the Earth

 kw: book reviews, nonfiction, geology, volcanology, volcanoes, scientists, memoirs

My uncle was a professor of geology, and during the summer season he consulted for mining companies. He had a "volcano fund", and when there was a report of a new eruption, if he could get someone to cover his classes, he would go. I don't know whether he went to study the volcano, or if he just wanted to see it. The new book I have in hand is by and about someone who goes, and goes frequently, to study these volcanoes: Adventures in Volcanoland: What Volcanoes Tell Us About the World and Ourselves, by Tamsin Mather. Dr Mather's particular expertise is geochemistry, and much of what she does involves collecting and studying the gases that emanate from volcanic vents; talk about bearding the dragon in its own den!

Fear not for our intrepid scientist. Gathering volcanic emissions can indeed be hazardous, but it is rare these days to be required to carry fragile glassware up a mountainside to the edge of doom. Many gases can be gathered by fly-through drones, and hand-sampling equipment is now longer-reaching and safer to use. Though losing a drone can be costly, it beats losing life or health. In other cases, such as a survey described in Chapter 6, "All in the Balance", the gases of interest were known to be emitted over a wide area at a volcano in Ethiopia named Aluto; the author and another scientist took hundreds of samples laid out in a grid covering one square kilometer. This provided a measure of the point-to-point variability in that square, and a rough statistical value for total gas emission in the whole caldera.

The book surveys the progressive understanding of volcanoes as it developed over the past two-and-a-half millennia (or longer). Current knowledge has two broad branches: present day volcanism and Earth's volcanic history. The total recorded history seems long to us, being something like 80 generations. But the geologic record—written not in documents but in the layout of rocks and their arrangements—can be read with reasonable clarity back to 100 to 200 million years, and with less and less resolution over the rest of Earth's four-plus billion year history. The author relates a powerful illustration: A lecturer held out his arm and said, "Imagine that all the time that Earth has existed is represented by the length of my arm." He went on to say that multicellular life arose about where his hand meets the wrist, and that the demise of the dinosaurs occurred at the base of a fingernail. And how much of this length would all of humanity's existence take up? Just the shavings from a single, quite light, stroke of a fingernail file! So "current knowledge" and "deep time knowledge" have very different scales.

One thing we learn from the deep history that has been discerned is that the absolute extremes of present-day volcanism are a fraction of what has happened in the past. A tool for scaling the magnitude of an eruption has been developed: VEI, the Volcano Explosivity Index, which ranges from zero to eight. The boundary between 0 and 1 is an erupted volume of 10,000 cubic meters. Each step is ten times the size of the one before, except VEI-1 covers a range of 100:1, from 10,000 cubic meters to one million. A few examples will give the idea:

• Kilauea in Hawaii erupts constantly for weeks or months at a time, but at a low daily volume, so it is the prime example of VEI-0. Few are the days that it rises to VEI-1.
• Mt. Etna in Sicily erupts from time to time, and a recent eruption produced more than a million cubic meters, or something over 0.001 cubic km; it is VEI-2.
• The devastating eruption of Mt. Vesuvius in 79BC is considered VE-5. It ejected several cubic km of tephra (the general word for volcanic "stuff"), as did Mt. Saint Helens in 1980.
• The largest eruption in recorded history is probably that of Mt. Tambora in Indonesia in 1815. Estimates of ejecta volume range from 35 cubic km to more than 100, putting it near the VEI-6/VEI-7 boundary. No undisputed VEI-7 or larger eruptions have occurred in recorded history.
• If a "supervolcano" such as that under Yellowstone erupts, it is expected to be VEI-8, yielding more than 1,000 cubic km. There are 20 known supervolcano calderas on Earth, but none has done much more than fuel groups of geysers more recently than a few hundred thousand years ago. Will there be a next time?

There is little element of time in this scale; it refers primarily to single events. If we consider really persistent eruptions that play out over thousands to millions of years, we get a different scale of beast entirely. Total long-term volumes can exceed VEI-8 by a factor of 1,000 or more! Consider this map:

This is a map of the known Large Igneous Provinces on Earth, for the past 330 million years. They number 24. It isn't known where particular eruptive events are on the VEI scale. But they just kept at it for so long that the typical total is between 1 million cubic km and 100 million! So a mid-range one of these could have covered all of the "lower 48" USA to a depth of about a mile, 1.5 km or more.

The "lesser" eruptions of historic times outline an amazing picture. You may have heard of the "ring of fire", that the Pacific Ocean has volcanoes along all its boundaries. There are other chains of volcanism, but this is the largest. This was a mystery before the understanding of Plate Tectonics was developed in the 1960's, just when I began to study geology in college! The knowledge was so new that the only "textbook" was a little (~200pp) paperback titled The New View of the Earth: Moving Continents and Moving Oceans. I don't recall the author.

The crust of the Earth has two varieties: Continental crust is mostly granite and related "light" rocks ranging between 15 and 50 km thick. Oceanic crust is mostly basalt or gabbro and related "dark" rocks ranging between 4 and 10 km thick. Each patch of crust is "riding" on a "lithospheric plate" 100-200 km thick. Beneath that, the hot mantle is in very slow motion, dragging these plates about. Where they collide, one usually slides underneath the other, and the portion going downward gets "cooked", producing upwelling magma, and a line of volcanoes is the result. Where two continental plates collide, they both get squeezed, and one such area is the Himalayas, which are still rising. The Appalachian mountains of North America are the roots of a former Himalaya of 450 million years ago, now worn to just nubbins.

The really big volcanic areas, and the supervolcanoes that they eclipse, come late in the book. The much less powerful volcanoes that abound all over the planet provide plenty of opportunity for scientists to gather data, whether up-close-and-personal (collecting gases or lava samples, etc.) or more remotely, such as by seismic monitoring. There are 38 volcanoes erupting at this moment; this can be monitored here. About 60 others are grumbling and rumbling and "offering to erupt". So, my uncle could actually have found somewhere to go any particular day, if there had been an Internet in the 1950's!

The author has spent half her life visiting volcanoes and studying them, and in particular their chemistry. I think perhaps my uncle longed for such a life, but the life of a college professor was his lot. I also wanted to be a field geologist, but I found that writing software for scientists was more lucrative (and safer! and I didn't have to live in a tent.). I really appreciate a good scientist who is a good writer, who can bring the stories of the Earth to those of us who don't have as much opportunity to "go where nature is". Even more, here and there a youngster could be inspired to embrace a life of natural history.