Geology of the National Parks

Textbook 10.1: Great Basin


The Age of the Earth I, Great Basin and Layer Counting

Pools in Lehman Cave, Great Basin National Park, Nevada.
Pools in Lehman Cave, Great Basin National Park, Nevada.
A bristlecone pine tree on Wheeler Peak, Great Basin National Park, Nevada.
A bristlecone pine tree on Wheeler Peak, Great Basin National Park, Nevada.

Out in eastern Nevada, a long way from almost anywhere, is Great Basin National Park. The jewel of Great Basin is Lehman Cave, one of the most "decorated" caves known, with a wide range of odd cave formations (stalactites and stalagmites, but lots more, too). Lehman Cave is dissolved into marble (metamorphosed limestone) on the side of Wheeler Peak, which rises to more than 13,000 feet (almost 4000 m), and which was glaciated during the ice age—the remnant cirque glacier has wasted away to a debris-covered rock glacier over the last decades. Fewer than 100,000 visitors make the trek to Great Basin each year, and you can find a lot of solitude and wonder in this beautiful place.

map of US with Great Basin, Nevada highlighted.
Credit: R.B. Alley

Far up on Wheeler Peak, Great Basin bristlecone pines are living. These gnarled, straggly trees grow slowly in high, cold places, whereas those bristlecone pines growing in warmer, moister, lower-elevation sites live faster and die younger. In part because of this slow growth, the high-altitude trees can be very old. The oldest known living bristlecone pine is more than 4,600 years old, in the White Mountains of California. The oldest tree known so far was cut on Wheeler Peak in 1964, when the land was still administered by the U.S. Forest Service, as part of a study to learn more about the growth and behavior of the trees. Now known as Prometheus, that tree was 4,950 years old when cut. That one old-looking tree was not notably different from many others in the large grove. Because it is so unlikely that the first such tree cut on Wheeler Peak out of the many there would happen to be the oldest tree on Earth, it is likely that there are older trees out there that have not been sampled yet.

In a seasonal environment, a tree reliably produces a visible growth ring each year. The reasons for this behavior are well-understood, and the annual nature of the rings has been checked many, many times. Rarely, there is a problem (a piece of a ring may be missing if the tree was damaged, by a fire or a burrowing beetle, or a late frost or other odd event may make a ring look strange), but tree-ring daters (dendrochronologists) have learned to recognize these events. In general, tree-ring dating can be practiced with no errors. Many, many tests have been conducted to confirm that this works, that the results match historical records, etc. Most such sampling is done using narrow coring devices, and does not harm the trees.

In studying tree rings, one sees that the width is not the same from year to year. Thick rings grow during “good” years, and thin rings during bad years. This allows tree rings to be used to reconstruct past climates. In a dry area, a good year is a wet one, so tree rings can be used to find out how much rain fell in the past. In a cold area, a good year is a warm one, so the tree rings function as thermometers.

For our purposes here, the pattern of good and bad years (fat and thin rings) is important for dating. On Wheeler Peak, and in the White Mountains and elsewhere, dead trees occur near the living bristlecones. Some of these dead trees sprouted before the living ones and overlapped in age with the still-living trees. Other dead trees can be found in archaeological sites or buried in sediments. A tree-ring specialist can date the good and bad years using living trees. The specialist can then find the pattern of thick and thin rings in the dead tree, and so use the dead tree to extend the record back to when the dead tree first sprouted (see the figure below). By overlapping a few long-lived trees, or many short-lived trees, very long chronologies can be generated.

So How Many Layers?

Such techniques are used to date archaeological sites, including those of the Ancestral Puebloan peoples (also called the Anasazi; at Mesa Verde and several other national parks). Professor Peter Ian Kuniholm and his collaborators at Cornell have for decades been doing amazing work using tree rings to understand classical history in the Aegean region, the Middle East, and elsewhere, confirming, refining, and extending historical accounts. The beautiful agreement between tree-ring and historical accounts as far back as the oldest reliable written records confirms the accuracy of the techniques.

Diagram of matched Tree Rings from a living tree and dead wood.
Tree rings can record history. “Good” and “bad” years produce thick and thin rings, and the pattern of thick and thin rings can be matched between living trees and nearby, older dead wood, allowing tree-ring histories to be longer than the life of a single tree.
Credit: R.B. Alley

But, the tree-ring records extend well beyond reliable written histories. The longest tree-ring record in the U.S. Southwest is now more than 8000 years. The longest known of such records is from tree trunks buried along rivers in Germany, and extends to 12,429 years—before that, closer to the heart of the ice age, conditions were too cold for trees in that region of Germany. Because most trees live for “only” centuries rather than millennia, such records (and a few other really long ones, such as a 7,272-year record, as of 1984, from Irish oaks buried in bogs) represent immense investment of time and effort, but people have devoted whole careers to assembling these outstanding records. Notice that there is much older wood, including the fossil trees at Yellowstone, Petrified Forest, and elsewhere. The more than 12,000 years in Germany are the longest continuous record reaching the present, but surely do not come anywhere close to including the whole history of trees.

Several other types of annually layered deposits exist. For example, some lakes in cold regions freeze every winter. When the lake is thawed in the summer, sand and gravel are washed in by streams. When the lake and its surroundings freeze, the streams slow or stop, and the only sediment settling to the lake bottom is the very fine silt and clay particles that were washed in during the summer but require months to fall. A coarse layer capped by a fine layer marks one year. Many such layered, or varved, lakes have been studied, and found to contain thousands of years to more than 14,000 years. Many of these lakes occur in glacier-carved basins, and so their records extend only back to the melting of the ice. One lake in Japan, Lake Suigetsu, has a spring bloom of diatoms—algae with silica "shells"—that make a light-colored layer, alternating with darker mud during the rest of the year. 45,000 annual layers have been counted in that lake.

Most lakes lack annual layers. If there is a lot of oxygen in the deep waters, worms will thrive in the mud beneath, and their burrows will disturb the layers. If the lake is shallow, waves will disturb the deep muds. But enough lakes exist with annual layers to be useful. And, simply seeing layers doesn’t prove they are annual; lots of tests have to be done, as described below for the ice cores.

A difficulty in lakes—and other archives such as annually layered stalagmites—is that an annual layer must be thick enough to be recognized, but a lake or a cave will fill up quickly if layers are thick, so the records cannot be extremely long.

Longer records are possible from the two-mile-thick ice sheets. Dr. Alley has been very active in this work, and Dr. Anandakrishnan has contributed in important ways. In central Greenland and some other places on ice sheets, the temperature almost never rises high enough to melt any snow and ice. However, summer snow and winter snow look different because the sun shines on the snow in the summer, “cooking” the snow and changing its structure, but the sun does not shine on the winter snow, which is buried by new storms before the summer comes. You can count many, many layers by looking at an ice core, and Dr. Alley did so, working especially on one core called GISP2.

To verify that the layers are annual, several things were done. First, one person (Dr. Alley) looked at the core, waited a while, and then looked at it again to see that the counting is reproducible. Then, other people counted the layers visible in the core (without cheating by finding out what Dr. Alley had gotten), just to make sure they agreed.

There are many annual indicators in ice cores, probably more than a dozen. For example, the isotopic composition of the ice is a thermometer that records summer and winter. And sunshine makes hydrogen peroxide in the air in the summer when the sun shines, and the peroxide falls on the ice quickly, but there is almost no peroxide made and deposited in the dark winter. So annual layers have been counted using several different indicators, and they agree closely.

This is still not good enough. When a large volcano erupts, it throws ash and sulfuric acid into the stratosphere. These spread around the Earth. The bigger pieces of ash fall out quickly, often in days or less, while the sulfuric acid may take one to a few years to fall (and, until it falls, affects the climate by blocking a little of the sunlight). You can use electrical or chemical techniques to find the layers of volcanic fallout in ice cores. The key sections can then be cut out, melted, and filtered, and any volcanic ash that is found can be analyzed chemically and compared to that from known volcanic eruptions. So, if you count back to the year 1783 in a Greenland ice core, you are in the year of the great Icelandic fissure eruption of Laki, which spread dry fogs across Europe and is well recorded in histories—Ben Franklin commented on the fogs in Paris while he was ambassador there for the fledgling United States. In fact, ash of the composition of Laki occurs in Greenland ice cores at the level dated 1783 by layer counting—the layer counting is right (or very close—some counts missed by a year or two initially). Similarly, ash from many other historical volcanoes has been found, back as far as historically dated volcanoes are known.

Comparison of counts of strata by one person at different times, by different people, and by different methods, and comparison to volcanic fallout, yield almost the same answers, within about one year in one hundred (so one person may count 100 years, and another will count 99, or 100, or 101, but not 107 or 93 or some similarly large error).

There are a few more tests yet. There were very large and very rapid climatic changes at certain times in the past. Ice cores record the climatic conditions locally (how much snow accumulated and how cold it was), regionally (how much dust and sea salt and other things were blowing through the air to the ice from sources beyond the ice sheet), and globally (by trapping bubbles of air, which contain trace gases such as methane that are produced across much of the Earth’s surface). Changes in all of these indicators occur at the same level in the ice cores, showing that the climate changes affected much of the Earth.

These changes left their “footprint” in the ice of Greenland, and the lakes of Switzerland and Poland, and the trees of Germany, etc. So one can date such changes in the annually layered deposits of all of these different places. And, the dates agree closely. These events also have been dated radiometrically (we’ll cover this soon), and the dates also agree closely. One event, for example, was a return to cold conditions during the end of the ice age, and is called the Younger Dryas. Close agreement as to its age is obtained from all of these different layered deposits and from radiometric ages—the Younger Dryas ended about 11,500 years ago.

Thus far, the layers in the ice cores provide the longest reliable records. Over 100,000 layers have been counted. High accuracy was achieved younger than about 50,000 years, with somewhat lower reproducibility (maybe 20% or so, and with well-understood reasons for the lower accuracy) older than about 50,000 years. Still older ice exists, but the layers in Greenland have been mixed up by ice flow and no longer give a reliable chronology. Thus, we have high confidence of more than about 100,000 years from the ice cores. (Really old ice in Antarctica, to 800,000 years or so, got less snowfall in a year than the height of a snowdrift, so annual layers are not preserved and other dating techniques must be used.)

Why Are We Emphasizing This?

One of the great results of geology has been the concept of “deep time.” The world was once believed in some cultures to be only as old as the oldest historical records. The Archbishop Ussher of Ireland, in the year 1664, declared that based on Biblical chronologies, the creation of the Earth dates from October 26, 4004 BC, Adam and Eve were driven out of the Garden of Eden on Monday, November 10 of that year, and Noah’s Ark landed on Mt. Ararat on Wednesday, May 5, 1491 BC. Other experts obtained slightly different dates, but with broad agreement that the world was not very old.

Ussher’s date rested on a literal reading of the particular translation of the Bible he used, and on quite a number of questionable interpretations of the text—the Bible itself never gives an age for the Earth. Early geologists nonetheless struggled with the constraints provided by such chronological readings—how could all of geologic history fit into 6000 years? The early geologists ultimately reached the conclusion that the world looks MUCH older than 6000 years; either the world is older than this, or we have been deliberately fooled by some powerful being who crafted a young world to look old. As scientists, we work with the observable part of the world, and we have no way to detect a perfect fake, so we treat this as an old world. The geologic record speaks of “deep time,” billions of years, Shakespeare’s “Dark backward and abysm of time.”

Most biblical scholars have reached the same conclusion: the chronologies of Genesis do not allow one to fix the age of the Earth precisely, and are perfectly compatible with an old Earth. Most of the large Christian denominations, for example, have accepted an old Earth based on Biblical and on scientific interpretations. In 1996, the pope added the Catholic Church to the wide range of protestant denominations that accept an old Earth.

It remains that some denominations and people insist on what they call a “literal” reading of the Bible. In addition, a few very vocal people continue to argue that the Earth looks young. Many more people hear all of this commotion and figure that maybe there is something wrong with the science, because “where there’s smoke, there’s fire.” Other people take it as an element of faith to disbelieve the scientific evidence, and even to accuse scientists of being bad people for opposing the fundamentalist interpretations.

In this course, we go to some length to show you a small bit of the evidence that the Earth does not look young—it bears the marks of a deep and fascinating history. The annual-layer counts by themselves require an old Earth, because the tree rings, the lake sediments, and the ice cores all extend to older than the historical chronologies. The Irish oaks preserve rings from more than twice as many years as Archbishop Ussher of Ireland would have said were possible since Noah's flood, so his prediction was tested, and failed. Geologic and other scientific evidence from tree rings, lake sediments, ice cores, archaeological sites, and more match historical records well as far back as those historical records go. But as we shall see in the next sections, those annual layers and other “young” things are only the tip of a very old, very deep iceberg.

Please note that it is not the author’s intent to insult or belittle anyone’s beliefs here. Science, you may recall, has no way of verifying whether it has learned the truth; it is a practical undertaking designed to discard ideas that fail, save the ones that don’t fail as provisional approximations of the truth, and push ahead. The hypothesis of an Earth that is no older, and looks no older, than historical records, leads to all sorts of predictions. Geologists began seriously testing those predictions in the 1700s, and found that those predictions were not supported, whereas predictions of an old-Earth hypothesis worked well—with very high confidence, the rocks look very old.

Consider two people, Sam and Pam. Sam has decided that belief in his literal interpretation of his favorite translation of the Bible is the most important thing in his life, as it controls the fate of his eternal soul and his relation with the most powerful being in the universe. Is it possible for Sam to look at the rocks, trees, ice and lakes, and find some way to explain what he sees in the context of that literal belief? The answer, obviously, is yes; many people do so, and some of them will be mad at us for what we write here. Next consider Pam, who is working in an oil-company laboratory trying to refine dating of petroleum generation and migration. Which works best for her in making sense of the sedimentary record, Sam’s interpretation or that of the geological profession? The answer is equally clear; Sam’s view is unhelpful, and geology works. Finally, ask whether it is possible for Sam to be a geologist and use the old-Earth tools even as he believes in his book, or whether it is possible for Pam to be a religious believer while doing geology, and the answers are yes; some people can hold a variety of ideas in mind at the same time. But recognize that the scientific evidence for an old Earth (and later, for evolution) is about as clear as science gets, and that the level of scientific disagreement on these issues is about as low as disagreement ever gets in science. Within the scientific community, the argument about whether the Earth really is older than historical records is akin to the scientific argument about whether the Earth is roughly spherical. (Lively discussions clearly continue in the blogosphere and in other many non-scientific circles, but those discussions are at best rather weakly linked to the science.)