Rock Talk By Larry Larason

Zircon and Navajo Sandstone

A diamond is a girl's best friend. But if she's a geologist she might prefer a zircon. These small crystals have played a key role in some recent discoveries that tell us much about the world millions, even billions, of years ago.

Zircon is not well known, but you've probably heard of cubic zirconia. Zircon is found in many igneous and metamorphic rocks, but zirconia, while it does occur occasionally in nature, is a man made gem stone, often considered as a fake diamond. Zirconia is harder than zircon, but neither is as hard as diamond, although they both have diamond-like luster. Their crystalline structures differ even though the chemical composition of both is zirconium silicate.

Zircon crystals form, usually as small grains, in granite and other igneous and metamorphic rocks. While the granite may be eroded and broken down, the nearly indestructible zircons survive to be encased in sedimentary rocks that form later. Small amounts of uranium and thorium are often incorporated in the crystals and this means that zircons are datable due to the radioactive breakdown of uranium to lead. By comparing the relative amounts of the radioactive elements and stable lead geochemists can determine when the zircon crystal was formed.

Here is one example of what zircons can tell us. Until a few decades ago paleontologists believed that life on our planet began in the Cambrian Period between 540 and 490 million years ago. The Earth is considered to be four and a half billion years old, but the rocks known to predate the Cambrian were mostly metamorphic. That is, they had been altered in ways that left few clues to their origins. Because of this scientists considered that geological events in the Precambrian, which comprises eighty-eight percent of the planet's history, were essentially unlearnable. But geologists have developed more sophisticated techniques of teasing information out of rocks, and have shown that life began in the Precambrian, which they now divide into three eras. The earliest of these eras is the Hadean, which lasted from the formation of the planet until 3.8 billion years ago. It was assumed that because of volcanic eruptions and constant bombardment by meteors, the Hadean would have been so hellish that the crust of the Earth would have been partially molten. However, zircons found recently in Australia have been dated to 4.3 billion years, demonstrating that granite was forming at that time, thus, the planet had cooled rapidly and probably had water on the surface. So our world was ready to support life a billion years before anyone had thought possible before zircons showed the way.

Also, zircons have given us new information about the geology of the Four Corners. Navajo Sandstone is a major feature in the scenery of the Region. Its dramatic crossbedding makes it memorable. It formed in dunes as much as fifty feet high during the early Jurassic Period. The Navajo dune field was one of the largest known in the geologic record, extending from Idaho to Southern Arizona, an area about the size of Texas. In places it is nearly 2500 feet thick. In other states it has different names: in Nevada it is called the Aztec Sandstone; in parts of Utah it is named the Nugget Sandstone.

Probably the best known views of Navajo Sandstone are in Zion National Park. It can also be seen west of Kayenta, or north of Rock Point, Arizona. In the latter instance, Navajo 12 cuts through some Wingate Sandstone as it climbs up from the flood plain of Chinle Wash, then rises onto Navajo Sandstone, which it traverses for about 10 miles. The Navajo Sandstone changes colors for unknown reasons. Near Rock Point it is the same red as the underlying Wingate, while only a few miles beyond it appears nearly white, as it is where it forms the eastern slopes of Comb Ridge west of Bluff, Utah. This is near the eastern limit of this formation; it is not found east of the Defiance Plateau.

A couple of years ago several teams of geologists began studying the tiny zircons found in the Navajo Sandstone, hoping to determine where all that sand came from. Results are still being debated, but it appears that the bulk of the zircons, probably at least half, came from the Appalachian Mountains and central Canada. Only one fourth may have been derived from the nearby Ancestral Rockies. This implies a continent spanning river system, rivaling the Amazon River in Brazil, to transport the sediment. So, the Appalachian Mountains may have been the Continental Divide for at least the northern part of North America during the early Mesozoic Era.

We may not be able, as William Blake wrote, "To see a world in a grain of sand," but we can learn a lot from a tiny zircon.