The World’s Largest Rock Tumbler is an Unusual Testimony to an Ancient Earth

Evidence that some features of earth’s landscape are the product of processes acting over longer periods of time can be found everywhere we look.  Some of this evidence, such as that from the Grand Canyon, may be familiar to us but other evidence is far less familiar but tells the same story.  For example, high in the Atacama Desert of South America strange boulders strewn over a dry valley tell a powerful story of an ancient landscape.

The Atacama Desert is famously one of the driest places on earth. The location where these particular boulders are found receives an average of only 3mm of rain per year! Some years this boulder field will receive no rain whatsoever.  These boulders have garnered interest by scientists who study the desert.  In particular the peculiar girdled erosion pattern on their sides.  They have smooth sides with rougher top surfaces.   At first glance you might think that someone had tipped them up and been rolling them around but some of these boulders are over 2 tons in weight.  Initial work demonstrated that these boulders were not washed into place nor were they blown to their present locations (yes, some rocks can be blown around – The Sliding Rocks of Racetrack Playa).  So, what could account for these strange smooth-sided boulders?

Being in the right place at the right time to discover the origin of the boulder girdles

About 10 years ago, a single providential event led researchers to the explanation for odd features and locations of these boulders.   While investigating a group of these boulders there was a magnitude 5 earthquake centered not far away.  The researchers report that the boulders suddenly started to rock back and forth and some of them started to slowly turn on the desert floor.  Most significantly there was a very loud sound of boulders bumping into each other.   Suddenly a plausible and observed mechanism to explain the features of these boulders became clear.

A press release summarizing this research (Quade et al. Geology, 2012, see reference below) included the following scenario for the history of these boulders:

“The whole story appears to be that the boulders tumbled down from the hills above — probably dislodged by earthquakes. They accumulated on the sand flat, with no place else to go. Quade compares the situation to a train station where people are crowded together closely, rubbing shoulders as they wait for a train. In this case the boulders have been stuck at the station for hundreds of millennia and the train never comes. So they just get more crowded and rub shoulders more over time.

Analyses of the boulder top surfaces suggest that they have been there one to two million years. That age, combined with the fact that seismic activity in the area generates a quake like that Quade witnessed on the average of once every four months, suggests that the average boulder has experienced 50,000 to 100,000 hours of bumping and grinding while waiting for that nonexistent train.”

We could say that what these scientists discovered is the worlds largest natural rock tumbler, albeit a rock tumbler working at slow speed over a long period of time.   From the picture above you can see that some of these boulders are many meters away from the next closest boulder. It likely takes hundreds or even thousands of years for them to move via vibration over to another rock and come into contact with each other, much less have enough contact to produce appreciable erosion to its surface.

Since this first report of earthquake-mediated erosion of the boulders more research has been done in several additional boulder fields in multiple valleys to test this hypothesis.  That research found additional support for the great age of these boulders and action of earthquakes as an important geological force that has shaped these boulders slowly over eons.  That work (Matmon et al., 2015) showed that the boulders originated from erosion of bedrock sometimes more than 350 meters (1000+ feet) up on the sides of the mountain.   The erosion of that bedrock is occurring at extremely slow rates—as low as 1/10 of a meter per million years or 100 thousandth of a centimeter per year.  Boulders created by erosion—called splaying—of the bedrock at the top of mountain hillsides slide very slowly down the sides of the hill toward the valley below.  In some locations it is estimated that the boulders requires several hundred thousand years or possible several million years to make the 1350 meter trip down the gentle slopes to the valley floor.

Matmon measured boulders at the top of the hills and at intervals all the way down to the valley floor. They discovered that the majority of boulders are much smaller by the time they reach the floor of the valley.  This was consistent with them slowly sliding down the hill and being gradually eroding along the way.  Near the valley floor boulders continued to rub against each other gaining the girdled appearance.   Remarkably, Matmon found that most boulders in the valleys have been exposed for two to six million years some as much as 12 million years (see below for how these measurement were performed) making their surfaces some of the oldest continually exposed rock surfaces on earth.  The boulders at the top of the hills near the their bedrock origin were found to have surfaces that are younger though still very old.

Put simply, these boulders are witnesses of processes that have been taking place for millions of years at this location.   In contrast to this picture of an ancient earth, young earth creationists (YECs) believe these boulders could not have existed in this valley for much more than 4000 years!   How might they account for the origins of these boulders in such a restricted timeline of earth’s history?

Atacama Boulders in the Context of a Young Earth?

These earthquake-jiggled boulders have been discussed in the literature for eight years but I have not found a single young-earth creationist’ attempts to offer a young-earth explanation for these boulders.   However, based on their approach to other geological formations we can assume that their Flood geology model of earth history would require that the boulders have been formed after a global Noahic Flood and therefore must be less than 4350 years.

Why after the Noahic Flood?   Because these boulders are granodioritic rocks which is a fancy way of saying they igneous in origin.  These igneous rocks lie on top of thousands of feet of fossil-bearing sedimentary rock.   The young earth creationist believes that these sedimentary layers were laid down by a global flood followed by intrusion of ignous material. That magma had to cool and crystallize. Only after all of these actions could the mountains be eroded revealing the granodioritic rocks contained within them the flat bottom valleys between them form.

This series of sequential historical events to form the mountains and valleys should be enough to dispel the idea of a single global flood as the causative agent of these geological formations.  But the boulders on the hillsides and in the valley are an even more powerful evidence of an ancient earth.  The creationist’s timeline requires that all the boulders were formed well after a global flood and even after the valleys had formed.

As we have seen, the conventional geological explanation for these rocks is that these boulders formed via splitting off of bedrock by gradual chemical and freeze/thaw erosion.  Each large boulder then gradually moved down the side of the mountain via intermittent earthquakes and soil erosion.  These processes are very slow given the distances traveled and the lack of other forces of erosion.  The simplest explanation for the existence of boulders in the Atacama desert is that they have become scattered over the valley floors over very long periods of time.

How can a YEC compress all of the changes in these boulders into just 4000 years?  There is no evidence that water has played an important role in these valley.  Water erosion would have left characteristic signs of erosion on the rocks and the sediments they are sitting upon.  Furthermore, water would not produce the type of erosion that the boulders have on their sides. In fact water erosion should have removed the girdling pattern of rubbing erosion if there had been more water in the recent past.

Knowledge of the physics and chemistry processes of erosion informs us that it would take a tremendous amount of jostling, bumping, freezing and thawing to erode three and four-ton boulders at their origin point to one-ton boulders on the valley floor.  The distribution of boulders on the valley floor is itself evidence of great age.  Some valley floors are very flat and yet one-ton boulders are found a hundreds of meters from the edges of the valley floor.  The only way they could have gotten there is by movement caused by earthquake vibration.

I expect the earthquake explanation for the observed boulder distribution and erosion girdles would be compelling to YECs.  I expect they may agree that this is how the boulders were brought to and distributed on the valley floor but they could not accept the time required for this to be achieved.  Therefore I expect they would resort to proposing that earthquakes were 1000X more common after the Flood than they are now.

Is such a vastly higher earthquake rate in the past feasible? Quade et al (2012) calculated that 50,000 to 100,000 hours of shaking at the equivalent of a category 5 earthquake would be necessary to create the erosion they observed.  100,000 hours is the equivalent of 11 years of constant shaking.  But individual earthquakes only last for a few second to a few minutes and would not be expected to be continuous.  In places that experience frequent earthquakes most are very small (< category 3) and would not be able to move these boulders.  Earthquakes over category 5 are understandably rare given the tectonic stresses that must build up prior to the release such enormous energy.  Even if there were two earthquakes a year of this magnitude—which is far greater than the observed rate over the past 100 years—that would only account for 133 hours of shaking over 4000 years.

The formation of these rocks in this pattern with these girdled erosion patterns is exceptionally difficult to explain without the availability of large amounts of time as part of the equation.  YECs are fond of saying that not only is the Earth is young but the evidence actually points to a young earth.   The existence of these boulders demonstrates this just isn’t true.  No one would look at these boulders and come to the conclusion that they formed just a few thousand years ago.  Rather than pointing to a young earth they must be forced into a young-earth view.

Are there other ways to date these rocks aside from estimates of erosion from bumping into each other? 

What if you aren’t convinced that erosion of these boulders surfaces truly requires a long time by earthquake vibration.  Is there another way to date these rocks that is independent of our observations and estimates of earthquakes and wind and water erosion.  Fortunately the answers is Yes!

A lesser-known form of radionuclide dating is Cosmogenic Radionuclide Dating (See HERE for more details). Comsmogeic dating one type of what is called surface exposure dating.  The basic theory is that the boulders were at one time part of a rock wall on the mountain and not exposed to the surface (e.g. more than  meter of rock at least was covering them preventing exposure to air and radiation).   As soon as an individual boulder broke away from the rock wall its surface became exposed to cosmogenic radiation (cosmic rays and eventually neutrons).  Imagine a large boulder breaks off and lands on its side on the hillside below.  As the new top surface of the rock is bombarded by this radiation it physically alters the nuclear composition of those rocks.   The longer the surface of a rock is exposed the greater the differences in isotopic ratios of the minerals in that rock will become compared to their unexposed source rock. As a result, measurements of the differences of the original source rock that hasn’t been exposed with rocks that have been exposed can be used to estimate how long the  latter rocks have been exposed.

Matmon and his colleagues measured isotopic ratios of several elements in the surfaces of boulders in the valleys, on the sides of the hills and the source rocks themselves. What they found was that the boulders exhibited progressively greater isotopic ratio differences the further down the slopes of the mountainsides one went and that the boulders strewn far out in the valleys had the greatest differences indicative of the oldest ages.  These ratios can be used to calculate estimates or real ages.  By the time they reach the valley floor they have accumulated well over a million-years-worth of radiation exposure.

YECs must propose that the boulders are relatively the same age having been formed within a short period of time. They may also wish to explain the boulders change in size as resulting from rocks tumbling down the mountainside and shattering.  But the cosmogenic dating definitively rules these hypotheses out.  Boulders in the valley have upper surface that have been exposed for millions of year longer than their side or bottom surfaces or the boulders at the tops of the mountains.  All boulders appear to have slide to the present location with very minimal overturning. This is indicative of slow motion movements via earthquake vibration vs large floods or rolling and tumbling.

How confident can we be about the dates for the boulders?  

A young earth skeptic probably will be tempted to dismiss cosmogenic dating methods are just another form of radionuclide dating which they believe are all unreliable. They will claim that either we can’t trust the data–and implicitly the data collector–or that we can’t assume that cosmic rays and other forms of solar radiation have not been constant enough over time to use them as a clock.

These would be nothing but red herring dismissals.  Even if cosmogenic dating  has an error rate of thousands of years, the difference of 4000 vs 4 million is a massive difference in measurable isotopes.  What if this method were completely without merit as YECs are apt to claim? Maybe the Atacama desert is really young.  If this dating method were bogus then we should find that sediments in other deserts in the world should also exhibit very old ages because they should have experienced the same presumably changing rates of cosmogenic radiation.  But the ages of many other desert surfaces in other parts of the world  such as Death Valley in California are shown by the same method to be far younger.  This suggests that material has been moved around indicating the climate of that area has been different in the past.   But in the YEC timeline of earth history the Atacama and Death Valley deserts, and all other deserts for that matter, should be about the same age.

Cosmogenic dating frequently corroborates the dating of rocks by other methods and other studies that have been used to estimate how long landscapes have remained stable.   In this case we observe boulders that appear to have required a long time to have eroded to the point that we find them today. The cosmogenic dating confirms our intuition that these rocks have been lying on this desert floor exposed to the surface for a very long time.   Had all these rocks simply gotten jostled off the mountain recently by a very large earthquake, they would lack the signature erosion patterns and isotope differences compared to the rock formations in the hills above from which they originated.

The bottom line is that causal observation of the boulders and the Atacama desert region led many to intuit that this is one of the oldest environments in the world.  Closer inspection has confirmed what our eyes and intuition tell us. These boulders are far older than most boulders found on mountain sides in other environments.  The young-earth model provides no explanation for why such great differences in age are found and provides no intuitive sense for why these boulders are so —different from those in places such as Death Valley.

What about wind erosion? Could that cause some of the smooth sides of these boulders?

Many might wonder, what about wind? Can’t that cause interesting erosion patterns on rocks?  It certainly can but it would not explain the erosion patterns we observe here.  These boulders have erosion patterns that are clearly from contact with other rocks and not from sand blasting.

What would we expect from wind-born sand erosion? Look at the picture below of another rock formation in the Atacama desert.   This is one of many “tree” rocks which appear in some areas of the desert.  These are rocks that are clearly shaped by wind-born sand erosion.   In this case the sand is near the surface and so any rock that stands more than 6 feet tall will experience greater erosion near its base creating these odd formations.

Even if wind could explain the girdling pattern of erosion it would not help to explain how the boulders have moved out over the valley floor or why the upper surfaces are measured to be so ancient by cosmogenic surface dating.

Final thoughts

The present-day geological features of the Atacama desert are the product of a series of historical events that have taken place over long periods of time.  Any attempt to compress all these events into a mere 4000 years must explain all of our observations not just one of them.  The lack of explanatory power of YEC flood geology theory is on full display when one considers these desert boulders.   On the other hand, earthquakes over very long periods of time in a climatically stable environment provide an intuitive and well-support hypotheses which accounts for all of the observations that have been made of these strange boulders strew about the hills and valleys of the inner Atacama desert.

References:

Placzek, CJ, A. Matmon, DE Granger, J. Quade, and S. Niedermann. 2010.  “Evidence for active landscape evolution in the hyperarid Atacama from multiple terrestrial cosmogenic nuclides.”  Earth and Planetary Science Letters.  295: 12-20.   This article provides the evidence for the age of the stones that have the funny erosion patterns.  Cosmogenic nucleotide dating is reported.

Quade, J. P. Reiners, C. Placzek, A. Matmon, M. Pepper, L. Ojha, and K. Murray.  2012.  “Seismicity and the strange rubbing boulders of the Atacama Desert, Northern Chile”  Geology 40: 851-854.

Matmon, A., J. Quade, C. Placzek, D. Fink, M. Arnold, G. Aumaitre, D. Bourlès, K. Keddadouche, A. Copeland, and J. W. Neilson. “Seismic origin of the Atacama Desert boulder fields.” Geomorphology 231 (2015): 28-39.