American Scientist

Beneath the Surface

To the Editors:

When reading Thomas Gernon and Sascha Brune’s article “Mantle Waves” (March–April), I was reminded of an article published last year, “Roaming Rocks” by Marcia Bjornerud (Perspective, March–April 2025). That article described how metamorphic rocks carry evidence of the Earth’s past. I also remembered an earlier infographic about pieces of the protoplanet Theia that are still lodged in the Earth’s mantle (“Hints of an Ancient Protoplanet Inside of Earth,” November–December 2024).

Could these moving bodies from Theia be the source of the waves you identify as mantle waves?

Joe Huber, Jr.
Cuyahoga Falls, OH

Dr. Gernon responds:

The short answer is: yes and no.

The mantle waves themselves arise naturally when there is a sharp edge at the base of a continental plate. That edge creates an instability in the underlying mantle, rather like the ripple that forms when a solid boundary interacts with flowing material beneath it.

We don’t really know what causes a continent to break apart in the first place. However, a leading idea is that breakup can be initiated by hot mantle plumes rising from deep within the Earth (perhaps 2,000 to 3,000 kilometers deep), in the lower mantle. These plumes are thought to originate near vast, anomalous regions known as large low shear velocity provinces (LLSVPs).

Those deep-seated “blobs” are fascinating in their own right. As you pointed out, some scientists have suggested that parts of them may contain very ancient material—potentially even remnants linked to the Theia impact approximately 4.5 billion years ago. If so, it is incredible to think that fragments of Earth’s earliest history may still reside deep in the mantle today.

So, although these deep mantle blobs don’t directly generate the mantle waves, they may play an indirect role. If plumes rising from LLSVP regions help trigger continental breakup, that breakup creates the sharp plate edge needed to initiate the wave. In that sense, the deep structures could set the process in motion (like turning a crank to get an old car moving). The waves themselves may also carry a small amount of chemically very ancient material brought up from those depths. But not too much—if it’s too hot, we can’t form the diamond-rich magmas!

The Physics of Biology

To the Editors:

I read with interest the Sightings piece that reported on the work of Michel Milinkovitch’s group on crocodile scales (March–April, 2025). As a theoretical physicist, I teach my students about the role of physics in biology, especially the ways in which surfaces and volumes scale with length. The shapes of folded organs such as lungs, brains, and intestines are due to that differential scaling with growth, which is an aspect of geometry. As mathematical biologist D’Arcy Wentworth Thompson wrote in 1917, “The form of an object is a ‘diagram of forces.’”

The piece states that crocodile scales are formed “by compression, not tension,” but the two forces act together. The former is related to the animal’s volume, the latter its surface.

A. R. P. Rau
Baton Rouge, LA

Troubleshooting Success

To the Editors:

Tom McGowan’s article “There’s Your Problem” (Technologue, January–February) reminded me of a problem during my early days in field service on wastewater plants.

A paper plant in Louisiana had bought two big, 160-foot-diameter clarifier–thickener machines to treat wastewater from its pulping operations before the water was released into the river. Inorganic fine materials and other debris were supposed to settle to the bottom floor where angled scraper blades mounted on rakes would gradually move the resulting sludge to a center hopper to be pumped out. When one unit was drained, we saw that sticks, shreds, paper chunks, and other debris had formed a beaver-like dam on top of the scraper blade’s 5-inch flanges, preventing smaller debris from settling to the basin floor. Each of the rakes had been furnished with too many scraper blades, which brought the flanges close together. Debris could easily bridge the wide flanges, where it built into a mass that could settle no further. We reduced the number of scraper blades by half, which fixed the problem!

A related story was finding a 30-inch adjustable wrench lying flat on the basin floor. Had that reached the discharge hopper, it would have certainly put a “wrench in the works”!

Barry Straus
Hoschton, GA

How to Write to American Scientist

Brief letters commenting on articles appearing in the magazine are welcomed at editors@amscionline.org. The editors reserve the right to edit submissions for length and clarity.