I read this in (most likely) the 1980s or 1990s. The protagonists found what they believed were alien spacecraft; over time, they "morphed" into Mesoamerican-style pyramids (stepped, not smooth like Egyptian pyramids). Each time they "grew", an area where electrical equipment either failed or was unreliable grew; the pyramid itself became a new magnetic pole. It turned out that there were a dozen of these objects, spaced to cover the entire world from the corners of an icosahedron; the final morph of the pyramids caused the fields of electrical "disablement" to cover the entire world.