A volcán is named for the shape it ends up in: a small tortilla griddled into a stiff, cheese-fused crater that holds a mound of meat at its center like something about to erupt. The defining technique is the cheese. A generous layer, usually Oaxaca or a melting blend, is spread onto a corn tortilla on the plancha and cooked past melting until it browns and crisps into a lace that fuses to the masa, so the tortilla and the cheese become a single rigid disc rather than two soft layers. That disc curls slightly at the rim as it sets, which is where the volcano image comes from, and then it is topped open-faced with hot grilled meat, salsa, onion, and cilantro. The parts need each other precisely: the crisp cheese-tortilla base gives a salty, brittle floor that would be one-note alone, the meat brings juice and char it cannot supply itself, and the salsa and lime cut a richness that would otherwise sit heavy. It is a taco de costra taken to its loud extreme.
The cook is unforgiving because the cheese is doing the structural job. It has to be taken far enough that it actually crisps and bonds, which means holding it on the heat past the comfortable melt and watching it go from white to gold without tipping into burnt and bitter. Too timid and you get a floppy, cheesy soft taco that collapses the moment meat lands on it; too aggressive and the lace turns acrid and shatters into shards. The tortilla underneath should toast in step with the cheese so the whole disc dries and stiffens together. Then the meat, often carne asada, al pastor, or shrimp, goes on hot and chopped so it nestles into the crater without steaming the crisp back to softness. The classic failure is impatience, pulling it before the cheese sets, so the volcán never gets its rigid floor and just slumps. A good one stands on its own as a self-supporting shell, and you eat it fast before the meat's heat and the salsa's moisture finally soften the base, which they eventually will.
Variations follow the protein and how molten the build runs. There are beef volcanes kept relatively spare, al pastor versions with pineapple, seafood ones on the coast, and double-cheese builds where the costra is the whole event and the meat is almost a garnish. Some are finished with guacamole or a slick of bean, some with nothing but a fierce roasted salsa. The constant is the engineered crisp base, cheese cooked hard until it fuses to the tortilla, and the open-faced mound it is built to carry. That broader territory of tacos de costra and crisp-cheese griddle technique, where the cheese itself becomes the structure, deserves its own article rather than being crowded in here.