The North American continent is splitting open.

A geological rift running north and south through Colorado and New Mexico is widening slowly but surely -- by about one millimeter a year.

Geologists watching the process say that millions of years from now a new ocean could form there, as the forces of continental drift drive the eastern two-thirds of what is now the United States away from the western third.

The geological feature, which follows the course of the Rio Grande, has long been known to geologists as a "depression." Only in recent years did they realize it was a rift, caused by a weakening of the Earth's crust, rather than simple erosion in the river bed.

Now a team of geologists has produced evidence that the Earth's crust is thinning under the river as molten rock rises from below.

"Someday, if this process continues long enough, the molten rock will break through and become the floor of a new ocean," Paul M. Davis, a geologist at the University of California at Los Angeles, said in an interview. Davis is one of the leaders of a team of geologists that detected the thinning of the crust by studying the ways in which shock waves from distant earthquakes travel through the crust.

Davis said it is not clear what is causing the crustal thinning. He said it could be a plume of hot, semi-molten rock rising and melting through the crust. Or, he said, it could be that the North American continental plate is being pulled apart.

The plate is one of many that fit together like a jigsaw puzzle to make up the Earth's crust. Geologists know that the crustal plates move slowly, sliding against one another as does California's San Andreas fault, or colliding ponderously to thrust up mountains such as the Himalayas. In some cases, the forces cause plates to crack in the middle.

Whatever the cause, the crust under the Rio Grande rift is thinning. The crust on either side of the rift is about 30 miles thick, Davis said, but along the rift it has been reduced to about 22 miles.

The rift runs for nearly 600 miles between Leadville, Colo., and the Mexican border, where it fans out into a less well-defined feature. It varies in width, but averages about 30 miles. It is not known whether the rift continues southward, hidden under a normal-looking surface.

The rift is the third-largest known in the world. The largest by far is the East African rift that runs for thousands of miles from the Red Sea southward through Ethiopia, Kenya and Tanzania. Second largest is the Baikal rift in the Soviet Union just north of Mongolia.

As the Rio Grande rift continues to widen, it is likely to lengthen as well. Kenneth H. Olsen, another leader of the research group, said it could turn eastward around El Paso and follow the Rio Grande to the Gulf of Mexico.

Olsen, a geologist at the Los Alamos National Laboratory in New Mexico, said that if the rift does follow the river, its widening could someday allow gulf waters to flood the rift and start a new ocean.

If this happens, the upwelling of molten rock would form a feature similar to the mid-Atlantic ridge, an oceanic rift that is causing the Atlantic Ocean to widen by about one centimeter a year -- 10 times faster than the Rio Grande rift.

Once the crust becomes thin enough, the molten rock will erupt, forming a chain of volcanoes. The nature of the rock that forms in the rift valley as lava cools dictates that the valley will become an ocean.

The cooling lava will form basalt, a denser rock than the granite that makes up the bedrock of continents. Both types of rock exist as crustal plates that float on a subterranean sea of molten rock. Because granite is less dense, it floats higher than basalt.

Once basalt starts welling up into the rift valley, its weight will cause the valley floor to sink below the level of the granite on either side. Eventually this process will push the valley floor below sea level, preparing a deep basin that could be flooded either by the Gulf of Mexico or by local rivers.

The new interpretation of the Rio Grande rift emerged from studies using 20 seismographs arrayed along a 600-mile line that crosses the rift to form a rough X.

The seismographs recorded shock waves from earthquakes in other parts of the world.

Because shock waves bounce off subterranean layers where the rock density changes abruptly, analysis of the waves that reach the surface can yield a profile of layers miles below.

Shock wave patterns from 40 distant earthquakes revealed the thinning of the crust and the presence of an unusually hot -- and, therefore, less dense -- rock rising under the rift.

The rift itself is unusually prone to small earthquakes. Davis said quakes measuring about 3 on the Richter scale occur weekly. Stronger quakes, measuring 4, happen about once a year.