The Beware of Movies! series is meant to point out some of the scientific inaccuracies of popular movies, specifically in points related to the geological sciences.
This post will present some basic information about geologic time, how we know how old things are, and how movies often get these things wrong.
How do we know the order in which geologic events happened? And how do we know exactly when they occurred?
Uniformitarianism. This is an important concept used throughout the geological sciences. The short definition is “the present is the key to the past,” meaning that the processes that we observe on the modern Earth are identical to processes that occurred in the Earth’s past. Mountains exist today because of the motions of tectonic plates, thus ancient mountains also formed due to the interactions of plates.
This concept is useful for much of the Earth’s history, but might not be applicable to all of it, so it should be used with some caution. At least for the most recent 600 or so million years, it’s a safe assumption. Older than that, some important conditions on the Earth were different. One thing that is true, however, no matter how old of rocks we observe: Chemistry still works the same. Chemical reactions behaved the same 10 billion years ago as they do today. This is very important later on…
There are two basic ways of assigning ages (or dating) in the geological sciences: Relative and Absolute (or Numerical). Relative dating is used to place geological events in order of which came first, second, third, etc. Relative dating does not assign any ages (like ten thousand years ago) to events.
We’ll begin with relative dating, as this is the basis upon which our geologic time scale was originally developed.
There are six important principles used to assign an order to geologic events. Many of these apply especially to sedimentary rocks. Many of these will seem very, very obvious:
- Principle of Superposition – When looking at a pile of rocks, the oldest rocks are on the bottom. Because rocks don’t just float in space with big gaps below them.
- Principle of Original Horizontality – When sediments are deposited, they are deposited in horizontal layers. They’re flat. Thus, if we see rocks that are tilted in any way, we can assume that they were tilted after they were deposited.
- Principle of Original Continuity – Rock layers are deposited over wide areas, not just in the one place where we see them exposed. We assume that a rock layer in one area is continuous with similar rock layers in other areas, even if we don’t see the direct connection. This is one of the most important principles needed to understand the development of the geological time scale.
- Principle of Cross-Cutting Relationships – If there is a fault in a rock, or an obvious erosional surface, then we assume that these features occurred after the rock was deposited. That makes sense, because you can’t fault or erode something that does not yet exist!
- Principle of Inclusions – If there are two rock types (rocks A and B) next to each other, and one (rock A) contains pieces of the other (rock B), then the rock containing inclusions of the other rock must be younger. Rock A is younger than rock B in this example.
- Principle of Baked or Chilled Contacts – When magma comes into contact with pre-existing rock, reactions happen. The pre-existing rock is much cooler than the magma, causing the magma to cool rapidly and crystallize (making a chilled contact). At the same time the heat of the magma heats up and bakes the pre-existing rock, resulting in a baked contact. A baked rock is older than the igneous rock in contact with it. A chilled rock is younger than the rock it sits against.
Using these principles we can place geological events in relative order. We can trace rocks from one area to another and compile all the rocks in an area, and even on a continent into relative order. It is based upon this that the geologic time scale was developed. The divisions of the geologic time scale (like the Jurassic Period) get their names from the area in which rocks of that age were first described (like the Jura Mountains). Some divisions are also named based upon the types of rocks that characterize that division. The Cretaceous Period gets its name because many of the rocks are composed of chalk. The Latin word for chalk is “creta.” Using relative dating methods much of the Earth’s rocks deposited over the last 600 million years have been put in order.
We can then add to this fossils with which we can determine a fossil succession using the principles above. It is from this that much about the evolution of life on Earth is understood.
Biostratigraphy is the use of fossils found in a rock to assign a relative or absolute age to that rock. Biostratigraphic units do not depend upon rock type and are thus defined according to the presence of a particular organism (an index fossil) or a complete fossil assemblage. Biostratigraphy is often used to correlate rocks of similar age but different rock types.
It is through principles of relative dating and biostratigraphy that we know that dinosaurs and humans have never co-existed.
Absolute (Numerical) dating is a means by which we can assign an number age to a rock or a fossil (or a geologic event). The method that most people have heard of is radiometric dating. To understand this, we have to talk a little about chemistry.
The chemical elements come in many forms. Some are stable and some are unstable. The unstable ones are also called radioactive. Some elements can come in multiple forms, some stable and some radioactive. The difference is in how many neutrons are in the nucleus, or what isotope the element is in. Carbon, for example, has three isotopes: Carbon-12, carbon-13, and carbon-14. Carbon-12 and carbon-13 are stable. Most of the carbon in the universe is carbon-12. There’s a little carbon-13, and even less carbon-14. Carbon-14 is radioactive, however. It doesn’t stay around forever. At some point it decays (or self-destructs), which is why radioactive elements are so dangerous.
Carbon-14 breaks down into Nitrogen-14, an electron, and an electron antineutrino, which sounds pretty awful. (And it is, if it happens inside your body! Those little extra bits can cause damage, which can lead to cancer.) Other radioactive elements break down (decay) in similar ways. The original element (in this case, carbon-14) is called the ‘parent.’ What’s left behind (Nitrogen-14) is called the ‘daughter.’ The decay of the parent into the daughter products occurs over a specific period of time, called the half-life, which varies from parent material to parent material. For carbon-14, the half-life is 5,730 years.
The half-life is how long it takes for half of the parent material to decay into the daughter product. Here’s an important thing about half-lives, however. This does not mean that after two half-lives, all the parent product is gone. With each half-life, half of the parent product decays. You never really get rid of all the parent material, though there does come a point where it is so small that it becomes impossible to measure.
|Half-life number||Percent parent material present||Percent daughter product present|
Radiometric dating uses this relationship to assign ages to rocks. One need only to measure the relative amounts of parent material and daughter products in a rock and know the half-life of the parent material in order to calculate the age of a rock. Different parent materials have different half-lives ranging from days to billions of years. A scientist will use the parent-daughter system that works the best for the age of the rocks their interested in. Here are a few examples:
|Carbon – Nitrogen (radiocarbon dating)||5730 years|
|Potassium – Argon||1.25 billion years|
|Uranium-238 – Lead-206||4.47 billion years|
|Uranium-235 – Lead-207||704 million years|
For all of these, there are caveats. Firstly, it is important that all the materials being dated actually originally contained the parent material and has not lost any of the daughter product. This can be a problem for potassium-argon dating, for example, because argon, as a gas, can escape. Radiocarbon dating is only good to about 40,000 years before present, before there is so little of the parent material left that it no longer can work.
It is also important to realize that for all of these methods, time zero (or ‘now’) is actually not right now in 2013. It’s actually 1950, which is when the methods were first established. For most radiometric dating methods, this doesn’t matter a whole lot, but for radiocarbon, it can be problematic. Nothing younger than 1950 can be dated using radiometric carbon.
Beware of movies: In the movie Time Cop, with Jean-Claude Van Damme, there’s this shipment of gold that gets transported from the past into the future. This gold is radiocarbon dated (so they say) which informs the time cop agency that it was stolen from the past. Two problems: 1) There’s no carbon in gold. What exactly did they date? 2) If the gold came forward in time, via time machine, it should seem brand new. It should not date to the past. Unless somehow, radioactive decay speeds up in the beaming forward process.
Other radiometric dating methods:
Detrital Zircons: Most of the methods described above are best used to assign ages to igneous rocks. Only radiocarbon dating really works well for sedimentary rocks (but even then, is only useful back to about 40,000 years before present). Radiometric methods can be used to assign ages to sediments when applied to ‘detrital zircons.’ Zircon is a mineral that forms in igneous rocks as they cool and can be dated using the uranium-lead methods noted above. These zircons are very resistant to weathering and become part of sediments that form new sedimentary rocks.
Zircons can be isolated from sedimentary rocks and dated, which gives the age of the igneous rock that they came out of. From this, we can determine where the sediments came from. We also know that the sedimentary rock cannot be older than the youngest zircon that’s in it. Thus, we can derive a maximum age for the sedimentary rock, which can be useful to know.
Fission-track dating: When radioactive elements decay, they leave trails of damage (or tracks) in the matrix of a crystal. These little trails are obvious under the microscope and most often form from the decay of uranium-238. Counting these tracks can be used to assign an age to the mineral and thus the rock that they came from in ways similar to detrital zircon analysis.
Some other absolute dating methods:
Thermoluminescence (TL) dating is used to determine how long a mineral (and the rock that it is in) has been exposed to sunlight. As the mineral is heated, to emits a weak light signal, which is proportional to how much sunlight it was exposed to and therefore how long it sat on the surface. This can tell us how old a material is (like an archaeological artifact) or how long a surface (like a river terrace) has existed.
The use of cosmogenic nuclides for dating surfaces has also come to prominence of late. As it happens, cosmic radiation bombarding an exposed rock surface can cause the appearance of new elements that wouldn’t be there otherwise. A scientist can measure the amount of these so-called cosmogenic nuclides and assign an age to an exposed rock. This can be used to, for example, assign ages to the advances and retreats of glaciers.
There are other methods used by scientists in assigning ages to rocks and fossils, or the parts thereof. For example, one could simply count rings!
Dendrochronology is also known as tree-ring dating. Most trees have annual growth rings which can be used to count years from the initial growth of the tree to its death. If the tree is still alive, one can correlate events down to the exact calendar year. Dendrochronology can help us study paleoclimate and paleoecology, and has been used to calibrate radiocarbon ages.
Sclerochronology refers to the study of growth lines in the hard tissues of animals and plants. Clams show growth lines, as do corals. Some teeth do as well. These growth lines aren’t necessarily annual and may be annual, monthly, fortnightly, tidal, daily, and smaller increments of time. Study of these can help us understand the biology of ancient and extinct organisms.
Ice cores also have annual layers, due to yearly cycles of dust. It is possible to count the rings in ice cores that go hundreds of meters down and study ancient climate patterns, calibrated to precise years, using other geochemical methods. This is how we know much about global warming, glaciations, and climate changes.
Beware of movies: Actually, this is something they got right in “The Day After Tomorrow.” Ice cores are commonly used to measure the concentrations of greenhouse gasses in the Earth’s past atmosphere. They use layer-counting to get the ages right.
Some methods used for dating depend upon comparing patterns of change with similar patterns derived from rock sections of known age.
Magnetostratigraphy is a technique used to date sedimentary and volcanic rocks. The Earth’s magnetic field has not always been such that the north end of the compass needle points toward the north pole. The field has reversed itself many times, and these reversals have not been regular. Scientists can go out and collect rock samples through a series of rocks and measure which way the magnetic poles were pointing at the time the rocks were deposited. This pattern is then compared with the ‘geomagnetic polarity time scale’ for the Earth (which has ages assigned to it). Where the patterns match gives an age for the rocks.
Chemostratigraphy or correctly termed Chemical Stratigraphy is the study of the variation of chemistry within sedimentary sequences. Much like magnetostratigraphy, variations of particular chemical markers also provide useful time markers. For example, the Paleocene-Eocene boundary (~55 million years ago) is defined by a huge spike in the amount of carbon-12 in the Earth’s atmosphere, which is recorded in the rock. Chemostratigraphy can also be used to track environmental changes, since chemical markers change when climates and environments change.
This post has covered most, but not all, of the potential methods by which geological units and events might be dated by geoscientists. If there are other methods that you’ve heard of, comment about them and I can explain those too.