Q. In 150 years, will the United States become separated physically?

Peabody

A. If you're wondering whether we're cracking up, the answer is yes. Most of us have heard of the crack along the west coast of California. The section of the Southwest called the Basin and Range, which includes Nevada, southeast California, southern Arizona, and a little of southern Utah, is also breaking apart. The cracking is related to plate tectonics.

Floating along on the hot mantle layer 70 miles below, the Pacific plate and the North American plate are grinding past each other in California. The coast is on the edge of the Pacific plate, and the rest of the country is on the North American plate. The Pacific is moving up to the northwest, as the North American plate moves southeast.

For rocks, they're moving pretty fast. If you painted a line across the whole boundary, which includes the famous San Andreas fault and several others, the line would soon tear in half. A year later the space between the sections of line would be 2.4 inches. In 150 years, the gap would be 30 feet wide!

Joann Stock, professor of geology and geophysics at Caltech, says, ``With that much movement, there's an excellent chance of some big earthquake on the San Andreas fault in the next 150 years.'' But ``if such a quake occurs, it will probably create too small a crack to consider that the United States has `separated physically.' ''

But in the way the Earth works, 150 years is nothing. In a mere blink of geologic time, about 10 million years, Los Angeles will be roughly due west of where San Francisco is now. By then, says Stock, there should be a pretty wide ``crack,'' perhaps even covered by the sea, along a line running from the Gulf of California in the south to the border with Oregon in the northwest.

The cracking in the Basin and Range region is less well understood. Most geologists believe that as the two plates grind past each other, the Pacific plate is sort of grabbing onto the western side of the North American plate and pulling it away. That stretches the North American plate, distorting the Earth into the mountains and valleys of the Basin and Range, which includes Death Valley. But the rate of separation in the Basin and Range is only one-third of an inch per year.

Stock says geologists don't know if somewhere in the Southwest the United States will eventually rip apart as dramatically as the West Coast eventually will.