And How Do We Know

​Talk to Paleontology Society volunteers about their work and you are certain to hear statements like “Yeah, it was a beautiful camel skull, about two millions years old”.  You might ask how do you know its two million years old?”  That's a good question and an important one. The quick answer is that camel skull was found associated with a particular layer of sedimentary rock – sandstone, mudstone, limestone, etc.  Assuming there was no anomaly, it is almost certain that the camel lived at the same time that the rock layer was being deposited - in other words, the rock and the fossil are the same age.  So the question becomes, how old is the rock layer, and how do we know?  This leads us into the subject of “geochronology”,  and what follows is an oversimplified overview.

Classification of Rocks

Geologists classify rocks into three large families – igneous, sedimentary and metamorphic. 

Because our work involves fossils, we are interested mainly in sedimentary rocks, which are the hardest to “date”.  By “date” we normally mean to determine how much time has passed since the rock in question came into existence. 

​For igneous rocks we have several methods available based on radioactive decay; these are called “radiometric” methods.  Igneous rocks make up volcanoes and the backbones of many mountain ranges.  They form by the freezing of a liquid material from deep in the earth.  We call that liquid “magma”.  (The familiar word “lava” is used for magma that erupts onto the surface of the earth, and later cools and freezes to form a volcanic rock.   Basalt and granite are examples of igneous rock.)  Essentially all igneous rocks have radioactive isotopes (call them R) in some of their minerals.  These change to non-radioactive isotopes (N) at a known rate.  Thus, by measuring the amount of R and N in an igneous mineral grain – and taking a few other factors into consideration – it is possible to determine how many years have passed since the mineral formed.  That – the time passed - is the age of the igneous rock.  The magma itself may have come into existence much earlier in Earth’s history, but not the rock.  With these radiometric techniques we determine the time at which the magma froze.  Fossils do not occur in igneous rocks, so we can skip on to the next rock family – metamorphic rocks.

Metamorphic Rocks

Metamorphic rocks include things like slate, schist, marble and gneiss.  Metamorphic rocks form when igneous or sedimentary rocks are buried deep in the earth, compressed, and held for long periods of time at high temperatures.  When this happens, new minerals may form, and older, pre-existing minerals grow and change shape, or shrink and even disappear completely.  The “age” of a metamorphic rock often is defined by the time at which these changes – called “metamorphism” – took place.  Metamorphism generally takes a very long time – sometimes millions of years.  Under some circumstances the “age” discussed is not the age of metamorphism but rather the age of the “protolith” – the igneous or sedimentary material from which the metamorphic rock was made.  Age-dating of metamorphic rocks also is usually accomplished using radioactive materials.  The processes of determining the age of metamorphic rocks often can be fraught with formidable difficulties.  Fortunately, vanishingly few fossils occur in metamorphic rocks, so we can pass on quickly to – sedimentary rocks. ​

Sedimentary Rock

It is important to understand that the age of a sedimentary layer (bed, formation, etc.) is not the age of the particles that compose the rock.  Sedimentary rocks form at the surface of the earth, in many cases from the deposition of solid particles carried by running water.  These deposits – sand, mud gravel, etc. - in most cases form more-or-less extensive horizontal layers. 

The age of the sedimentary layer is defined as the time elapsed since the material was deposited.  In the Park we have many sedimentary rock units that are only a few million years old, or less, but that nevertheless contain particles (small rock fragments or mineral grains) that are over 100 million years old.  It is the laying-down of the sediment that counts, not the age of the particles.
Determining the age of deposition of a sedimentary layer isn’t easy.  The main problem is that, with virtually no exceptions, radiometric methods can’t be used directly.  You can, of course, remove a few radioactive mineral grains from the sedimentary rock and “date” them.  However, as the last paragraph explains, this will have little to say about the age of the sedimentary deposit, or the age of the fossils it contains.  Determining the correct age can be an exercise in creative thinking.
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The rock-bottom principle in sedimentary geochronology is the “Law of Superposition” - it was a big deal in the 19th century, but fairly obvious to us today.  “Superposition” means piling one thing on top of another, and the “Law” states that – if nothing has been drastically disturbed – the oldest sedimentary layer is on the bottom and the youngest on the top.  This follows, of course, from the fact that each layer is deposited from above, hence on top of the sediment pile.  The further down you go, the older is the rock.  This sort of reasoning was used to develop the Geological Time Scale, familiar to all freshman geology students.  It tells you, for instance, that a Cretaceous sedimentary formation is younger than one that is Jurassic.  However, it tells you nothing whatsoever about the absolute age of the rock. 

Radiometric Methods

Imagine a pile of rock layers in which there is a lava flow.  If that lava flow yields a radiometric date of, say, 100 million years, it follows that any sediment above it is 100 million years old or less, whereas those beneath it are older than 100 million years.  Or, imagine again that a layer of igneous rock (we call them “dikes") has been pushed up through a pile of sedimentary layers.  When the dike is dated using radiometric methods, we know that all the rocks it has penetrated are older – they had to be there already, in order for the dike to penetrate them.  There are many such instances world-wide that enable us to put absolute ages on the time scale.

But there must be many situations where there are no such fortuitous arrangements of sedimentary and igneous rocks.  Absolutely there are, and here is where the fossils themselves become indispensable.  There are many fossil species that existed for only a short time (a few million years, or less).  Some of these were wildly successful, multiplied and populated the earth.  If you find such fossils in sedimentary rock units far removed from one another, you nevertheless can conclude that the age of both rock units is identical, or very nearly so.  Using these so-called “index fossils” we can export ages determined in one place to others, all over the earth. 

​It may sound circular to say that you can use fossils to date fossils, but it’s true.

Magnetostratigraphy

For young sedimentary rocks there is another method available, one that is of particular value in the Park.  The method is called “magnetostratigraphy”.  It is based on the well-know fact that the magnetism of the earth reverses itself every so often.  The time-pattern of these reversals is quite irregular – sometimes the magnetic field will stay the same for tens of millions of years, whereas at others it will reverse its polarity (north pole becomes south and, of course, vice versa) several times in 100,000 years.  Scientists have worked out the timing of these “geomagnetic polarity transitions” in reasonable detail for the past 50 million years or so.  Sedimentary rocks acquire a (very faint) magnetism at the time they are deposited, and with effort, it is possible to detect the pattern of polarity transitions recorded by the rocks.  Under favorable circumstances, this polarity pattern recorded by the rock can be matched to the “geomagnetic reversal time scale” and the age of the rocks determined. 

 So that’s how we know the ages of the rocks.  Not all “age-dates” are equal; some are extremely reliable whereas others are inspired guesses.  Geochronology is a challenging field. 
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Myrl Beck, Anza-Borrego Desert Paleontology Society, 2008