Captain Marvel Jr #92 (1950)

Rufus Jones, an elderly desert prospector, sees what he believes to be a meteor, but when he approaches, he finds is a spaceship containing aliens that resemble sand-colored bipedal rhinoceroses with drill-shaped horns and sharp tusk-like teeth. Even more confusingly, they begin eating from a giant pile of sand, eating an entire dune before moving on. Rufus rides back to town where they are due to announce three face carved in a nearby mountain… only to ignore him, reveal the faces… and find they've been eaten into crude, disturbing skulls. Soon enough, Freddy Freeman reads about it in the paper and calls CAPTAIN MARVEL to investigate as Captain Marvel, Jr. He finds the eaten faces and finds that the marks of them show these creatures have long tusk-like teeth to eat the sandstone with and soon hears the Desert Hill Hotel nearby collapsing! He quickly props it back up from falling over and piles huge boulders in front of it to keep it in place as the owner says they built it on a solid sandstone foundation. He soon finds out about what Rufus saw from the local Sheriff and figures out that these creatures apparently eat silicon, a key element found in the Earth's crust in both sand and sandstone. He soon flies out to go see Rufus about this and finds a long furrow of eaten sand in the desert and then Rufus taking potshots at the ravenous sand-eaters. It has little effect other than making them announce they intend to kill him. Junior punches them over and they are balked for a moment, but eat more sand, becoming twice as strong (though Junior is still far stronger than them.) One of them sucks up a ton of sand to try to go toe-to-toe with the World's Mightiest Boy, while the others escape to their ship, taking Rufus as a hostage. Junior soon finds the stronger monster harder to fight, until he just flies it up away from the sand and then punches him out in mid-air until he refuses. Junior decides to sneak in as Freddy so they won't try to kill Rufus, only for the defeated Sand-Eater to turn on him when he sees that Freddy is weaker than Junior. Freddy and Rufus are soon bound and gagged in a corner as they take off quickly (thinking the Boy in Blue is nearby) and their leader, Bexxx, decides to just wipe out the human race so they can eat as much sand as they want, dumping their hostages out of a bomb-bay door as they decide to blast the Earth with “Snurx” Bombs. Falling through a rain-cloud on their way down, Junior manages to slip his bonds from them getting wet and pulls off his gag in time to call CAPTAIN MARVEL enough to save them both as Junior once more. Junior flies back to find the Snurx Bombs are dropping already and punches them as soon as they're dropped, keeping them from reaching the Earth and soon breaks into their ship to start cleaning house. They land on a tiny island and try to team up against him. However, Bexxx has found that he's eating too much of the island they're on now… leaving them stranded in the ocean where they fall in and disintegrate, like sand in water, revealing that water was their sole weakness all along. Later, Freddy finds the Evening Star reporting that a certain atoll has disappeared and scientists are baffled… though Freddy is sure they'd never believe the truth!