# Teacher’s Corner: Geologic Stretching

By Sandy Eldredge

### How many inches per year do Salt Lake City, Utah and Reno, Nevada move away from each other?

You may not realize that 15 million years ago, if roads existed, you could have driven from Salt Lake City, Utah to Reno, Nevada in 4 hours instead of 8 hours. OK, understandingly, this tidbit of information is not on everyone’s mind, but you can have fun using math as you geologically sleuth your way to the answer.

First, pull out a relief map that covers Utah, Nevada, and California. You can see that Salt Lake City (and the adjacent Wasatch Range) and Reno (and the adjacent Sierra Nevada Range in California) border an area of the United States where the landscape is dominated by north-south- trending mountain ranges and alternating valleys (basins). Not surprisingly, this area is called the Basin and Range Province.

The ranges and basins have been forming for the past 10 to 20 million years in response to east-west stretching of the earth’s crust. Stretching creates tension that is released by slow continuous movement or sudden movement along a fault (a break in the earth’s crust), which causes earthquakes. During an earthquake, the mountains rise while the valleys drop along the faults. The stretching continues today.

Now back to the original question. The cities are 400 miles apart, yet 15 million years ago they were only 250 miles apart. How many inches per year do the two cities move away from each other?

Answer: Total movement is 150 miles in 15,000,000 years
5,280 feet = 1 mile
5,280 x 150 miles = 792,000 feet
Distance moved in 15,000,000 years is 792,000 feet
792,000 feet/15,000,000 years = 0.0528 feet/one year
1 foot = 12 inches
12 x .0528 feet = 0.6336 inches

The two cities are moving apart at a rate of 0.6 inches (or about ½ inch) per year.

Survey Notes, v. 29 no. 2, February 1997