The anomaly itself is a large slab of relatively cool and dried out rock. Geologists know this because of the way seismic waves pass through it: slower waves usually means softer, hotter material while faster waves mean stiffer and cooler rocks.
The Isabella Anomaly has been known for a while. But by comparing it with another seismological blob under Baja California, which, for a bunch of reasons, is even more certainly a piece of the Farallon plate, the researchers, led by Donald Forsyth of Brown University and Tun Wang of the University of Alaska, reexamined the seismic data of the West Coast and were able to make an argument that the Isabella anomaly is more of the same. It is the same depth, treats seismic waves the same and, very significantly, the blob is in the right place.
"The geometry was the kicker," Forsyth said in a Brown press release. "The way they line up just makes sense."
Lost Continent Discovered Beneath Indian Ocean
Another kicker is that if Forsyth and his team are correct, it throws a wrench into another theory about how the Sierra Nevada and the Isabella anomaly are related. Oh well, that's science for you: out with the old and in with the new.