American beauty: Arizona is home to one of the world's greatest geological marvels. Scientists still don't know when the Grand Canyon was formed. (Geology: rock formation).

IF YOU think summer is one natural wonder, how About the Grand Canyon? In the next three months, several million sightseers, hikers, and river rafters--more than 25,000 visitors a day--will hit one of the most spectacular geological highlights of America's Southwest: a 1,500-meter (1-mile)-deep, 15-kilometer (10 mi)-wide rainbow-hued canyon like none other on Earth. Grand Canyon National Park is home to trees and plants like pinon pines and evening primrose, and an array of animal species from jaguars to the endangered Mexican spotted owl. You can hike, back-pack, or donkey-ride down myriad winding trails to the bottom, where the Colorado River flows and the thrills of Jarring rapids rival any amusement-park scream machine.

When exactly, you might wonder, did the "Big Ditch" form? Surprisingly, says geologist Richard Young at the State University of New York at Geneseo, "we still don't really know."

OLD ROCKS

Geologists do know how the canyon rocks were formed. Almost 2 billion years ago, the rock layers now visible at the canyon bottom formed the base of a mountain range that probably resembled the Rockies. Mountainous rock formations are often made of granite. In the case of the Grand Canyon, the ancient mountain range eroded, or wore away, over millions of years until it became a flat plain.

How do Earth's mountains rise? To find out, scientists studied tectonic plates, continental and oceanic plates that compose the planet's outer skin or crust. The plates shift constantly, slowly ramming against each other or pulling apart. When they meet, they can push up huge slabs of rock, sometimes along with volcanic eruptions, to form mountains. Some of the Grand Canyon's older rock layers are interspersed with ancient volcanic ash and lava. "The geology of the canyon is very dynamic," says Grand Canyon park ranger Tom Pittenger. "Numerous times in the last 2 million years, lava from young volcanoes has spewed into the canyon and dammed the river that ran through it."

While scientists have hashed out the origins of the rock layers, they still don't have conclusive evidence of how and when the canyon itself was carved. Recently geologists convened to hypothesize (form an educated guess) about what might have happened. One theory: The Colorado River overflowed from one ancient lake to another, chiseling away mountains during floods. A second theory: About 5 million years ago, an ancient river carved across a plateau and merged with a second river, forming the single mighty river that cut the Grand Canyon. Scientists now plan to use sophisticated dating methods to figure out when canyon rocks were first exposed to air, pinpointing when a river might have eroded the rest away.

NEW CHANGES

One thing is certain: Modern-day damming by humans has altered the ecosystem of the Colorado River and Grand Canyon. In 1963, the U.S. Bureau of Reclamation completed Glen Canyon Dam to provide electricity for the booming population of the Southwest. The dam also created Lake Powell (now used for recreation)--but flooded extraordinary sites like the "Music Temple," where rippling water melodies once echoed off canyon walls.

The Glen Canyon Dam has been beneficial in some ways, but it also ended the Colorado River's natural spring flooding. "Without the floods, you don't get water scouring the banks," says Pittenger. That scouring used to clear out invasive vegetation at the canyon bottom, leaving behind only the native plant species that thrived in the canyon. Now, without seasonal flooding, only certain native species have adapted (changed habits) to survive the altered conditions, making them vulnerable to extinction. One such spot: Vasey's Paradise, a lush garden growing beneath springs that burst from cliff walls above. The Kenab ambersnail is an endangered species that lives only in this tiny green oasis. Its genetic code (hereditary material) reveals that the ambersnail was isolated from other snail populations 1 million years ago, when it evolved into a separate subspecies. Park service employees have established two new ambersnail populations in similar sites elsewhere in the Grand Canyon to ensure the species' survival should a natural disaster strike Vasey's Paradise.

OTHER PERILS

Damming isn't the only threat facing the Grand Canyon ecosystem: Air, water, and noise pollution also impact the park. One Arizona power plant was nearly shut down for spewing tiny windblown soot particles into the canyon. The region has some of the clearest air in the U.S., but it also has suspiciously high levels of ozone, says John Ray of the National Park Service's Air Resources Division: "We don't know the reason."

The National Park Service is now surveying water quality in areas where motorized recreational watercraft are used. Last fall, the U.S. Environmental Protection Agency measured watercraft emissions in the Glen Canyon National Recreation Area to see what pollutants might occur in the canyon downstream. The results will be released this year.

The National Park Service also wants to limit the number of motor vehicles allowed into the vicinity in order to slash car emissions. But the biggest pollution threat, says Pittenger, is air-traffic noise: "You're trying to have a wilderness experience, and you may have an aircraft flying overhead every two minutes." Park management is working with the U.S. Federal Aviation Administration to reroute flights from the canyon and curtail the number of flights over the area.

Despite these environmental troubles, a trek to the Grand Canyon still beats a backyard camping trip!

Did You Know?

* The Grand Canyon's north and south rims are only 16 km (10 miles) apart, but travel between them requires 34 km (21 miles) by trail or 343 km (214 miles) by road.

* The Grand Canyon was one of the last unmapped places in the U.S. The U.S. Geological Survey produced the first maps in 1902.

* From around 700 to 1150 A.D., the Anasazi Indians lived in the canyon, where the dry climate has preserved their baskets of grain in ancient storerooms built into rock walls.

Cross-Curricular Connection

Language Arts: Pueblo Indians lived in the Grand Canyon around 1200 A.D. Havasupai Indians dwelled there in the 1700s. What was it like to live in the canyon? Research and then use your imagination to write a story or a poem about canyon life.

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CHECK FOR UNDERSTANDING

Directions: Answer the following questions in complete sentences.

1. What is granite and how does it form?

2. What can happen when the plates that compose Earth's crust shift?

3. Which zones of the Grand Canyori's have desert climate conditions?

4. What dam has altered the Colorado River and the Grand Canyon ecosystem?

ANSWERS

1. Granite is a coarse-grained rock forged from hot liquid called magma. Pressure and high temperatures slowly solidify granite deep in Earth.

2. When Earth's plates meet, they can push up huge slabs of rock, cause volcanoes, and create mountains.

3. The lower Sonoran zone and the canyon foot are desertlike.

4. The Glen Canyon Dam has altered the Colorado River and the Grand Canyon ecosystem.

National Science Education Standards 

Grades 5-8: evidence, models, and explanation * structure and function in living systems * populations and ecosystems * structure of the Earth system * Earth's history * populations, resources, and environments * science as a human endeavor

Grades 9-12: interdependence of organisms * energy in the Earth system * origin and evolution of the Earth system * natural resources * environmental quality * natural and human-induced hazards * science as a human endeavor

Resources

Down the Great Unknown: John Wesley Powell's 1869 Journey of Discovery and Tragedy Through the Grand Canyon, by Edward Dolnick, Harper Collins, 2002 "Making Sense of Grand Canyon's Puzzles," by Sandra Biakeslee, The New York Times, June 6, 2000 Learn about Grand Canyon geology at these Web sites: Grand Canyon Explorer www.kaibab.org/gc/geology/gc_geol.htm

National Park Service www.nps.gov/grca/

RELATED ARTICLE: Inside the Canyon.

The GRAND CANYON carves into Earth a massive gash, 15 kilometers (10 miles) wide, A journey from the forested canyon rim to the rocky desert 1.5 km (1 mi) below is an amazing trek back in geologic time: The rocks at the very bottom formed 2 billion years ago! Just as amazing: Diverse plant and animal life teem in the canyon's every nook and cranny.

Forest Zone At 2,480 meters (8,140 feet) above sea level, the remote North Rim is the canyon's highest point. Here dense forests of aspen, fir, and spruce trees shelter deer, mountain lions, and endangered Mexican spotted owls from winter's heavy snows.

Upper Sonoran Zone This is the upper side of the canyon. Juniper and pinon pines thrive here, and many species of birds nest among their branches.

Lower Sonoran Zone As you near the bottom of the canyon (especially at the western end), you enter a desert landscape where scorpions and rattlesnakes lurk and owls perch atop tall Saguaro cacti.

Canyon Foot The wildlife here have adapted to desert climate, Many spend sweltering days burrowed underground and surface only in the cool of night. In abrupt contrast to the desert, the banks of the Colorado River abound with trees and plants.