Eruptions That Shook the World

Don L. Anderson, Reviewer

California Institute of Technology, Pasadena, California 

Physics Today, May 2012, page 55

http://www.physicstoday.org/resource/1/phtoad/v65/i5/p55_s1?bypassSSO=1

Eruptions That Shook the World, Clive Oppenheimer, 

Cambridge U. Press, New York, 2011. 

$30.00 (408 pp.). ISBN 978-0-521-64112-8

Geological and biological evolution and epic myth are punctuated by such catastrophic events as massive volcanic eruptions that shake Earth, change the climate, cover a continent with ash, and reset evolution. The term super volcano, popularized by the Discovery Channel, does not begin to describe the phenomenon, which leaves some evidence in geological, archaeological, and DNA records, but blows most of it away. Civilization, certainly, and humankind, probably, will not survive the next one. Clive Oppenheimer’s Eruptions That Shook the World makes the case that we should understand these events. It also inadvertently makes the case that contemporary theories about the ultimate causes of world-class eruptions are as myth-driven as those involving Hades and Poseidon.

Oppenheimer, a reader at the University of Cambridge, argues that volcanoes and life have been intertwined throughout time. Did volcanic eruptions extinguish dinosaurs, change evolution, help humans evolve, decimate human populations 73 000 years ago, and contribute to the French Revolution? Oppenheimer uses all sorts of evidence to unravel the stories behind some of the greatest and most significant volcanic cataclysms. The book is well illustrated, including many examples of magnificent mushroom clouds—volcanic plumes—that bear startling resemblances to the cartoon in chapter 1 showing “killer plumes” in Earth’s interior. Each chapter starts with a well-selected quote and ends with a useful summary.

Eruptions That Shook the World opens with the profound statement, “The Earth is cooling down!” The implications of that cooling are far-reaching and are even now not fully appreciated, 150 years after Lord Kelvin’s spat with geologists. The mantle still retains enormous quantities of original heat. Volcanoes do not require heating or the importation of heat and matter from Earth’s remote, deep interior. They occur because the mantle melts as it rises, in response to tectonic forces, or because it already contains magma that waits to be tapped. The relatively trivial effects of glaciers are enough to trigger eruptions by changing the load on or the orientation of the least compressive axis in the crust. Cooling, stretching, and breaking of large insulating plates allow the underlying magma to erupt: Hawaii and Yellowstone are prominent examples. Gas pressure and changing stress—not deep hot jets—trigger the release of magma.

Oppenheimer argues that recycled crust covers the core and converts core heat into killer plumes. However, that mechanism for raising or maintaining temperatures is 10 times more efficient at the top of the mantle. Moreover, the largest of the million-year-long eruption episodes signaling the breakup of plates empties out only a fraction of the magma that is stored in the shallow mantle beneath the plates. The large volume of available shallow magma was recently confirmed by Scott Bryan, Edgardo Cañón Tapia, and the late Paul Silver.

What Oppenheimer does not mention is that the understanding of the origin of large volcanic provinces is undergoing a classic paradigm shift—a shift back to the theories that favor top-down, stress-guide, low-pressure, and athermal shear-driven plate tectonic–related processes. This book presents the bottom-up, pot-on-the-stove analogy as noncontroversial conventional wisdom for the formation of killer plumes. That analogy is motivated by the shapes of thunderheads and volcanic plumes in the atmosphere, with no regard for scale or physics.

Mantle cooling drives convection and affects the geotherm in nonintuitive ways. Heat generated by radioactive decay modulates that cooling (see the Letters discussion, PHYSICS TODAY, November 2010, page 8). The competing processes of conductive cooling, radioactive heating, and thermal convection and advection, acting in the upper mantle, create a thermal bump—the cause of the asthenosphere and the source of most magma. J. Tuzo Wilson, one of the fathers of the plate-tectonic and hot-spot hypotheses, proposed that volcanic chains such as the Hawaiian Islands arose from the depths just below the rapidly moving plates.

I recommend Eruptions That Shook the World as motivational reading for physics students looking for a thesis topic in Earth or environmental sciences. The book may encourage physicists to take up the fascinating but challenging mission of understanding the workings of deep Earth and the claims that are made for it. The deep-Earth sections need to be read in parallel with Gillian Folger’s Plates vs Plumes: A Geological Controversy (Wiley-Blackwell, 2010) to get a balanced view of the issues (see also http://www.mantleplumes.org).

Ironically, the early views of Walter Elsasser and others on plate tectonics and mantle convection as top-down processes with volcanoes as by-products can be understood without invoking deep-Earth physics. However, appreciation of the effects of secular cooling, self-compression and scale, and the classical physics of Elsasser, Francis Birch, Peter Debye, and Eduard Grüneisen is required if scientists want to avoid the fundamental errors that occur in existing canonical models of mantle dynamics and geochemistry. Birch noted that words such as “dubious” and “vague suggestion” become “undoubtedly” and “positive proof” when applied to deep-Earth theories. By extension, “killer plumes” is merely a “high-pressure” name for volcanoes or a thick series of lava flows.