I can’t write a post about the geological timescale without the obligatory quote form James Hutton – in his famous words, time has ‘no vestige of a beginning, – no prospect of an end’. James Hutton (1726-1797) is perhaps most well-known for his work that contributed to the ‘Principle of Uniformitarianism’, the idea that the processes we observe today (such as erosion and weathering etc.) have also shaped the Earth throughout time. He famously described a number of sites known as ‘angular unconformities’, places where layers of one rock type are at an angle to, and truncate, layers of another rock type below.
These structures represent vast periods of time. The older rocks must first be deposited, then tilted to become near vertical, undergo erosion and then have another rock type deposited above them. If these structures are to be created by uplift, erosion and deposition at the slow rates they occur at today, then the Earth must be immensely old!
As time has moved on, we have gained ever more accurate estimates of the age of the Earth – with it having formed over 4 billion years ago – this is a lot of history to deal with. But how do we know the ages of the various rocks we find out in the world today? How do we make sense of these massive timescales?
The Geological Timescale
‘Stratigraphy’ is the field of geology concerned with understanding the relative ages of different rock successions and trying to fit these into an overall timescale. There are various ways to do this so I’ll give a brief introduction to lithostratigraphy, biostratigrapy and geochronometry.
The International Commission on Stratigraphy work to define subdivisions of time in a standard way. Time is divided into various eons, eras, periods, epochs and ages – eon being the largest division and age the smallest. The actual body of rock that is deposited in these various times are referred to as eonothem, erathem, system, series and stage respectively.
These various subdivisions are defined using a ‘Global Standard Section and Point’ (GSSP). Basically, a site is chosen somewhere in the world that has a succession of rocks that span the boundary and a ‘golden spike’ is established there, whereby any rocks above that point are defined as belonging to one division of time and those below to another. These sites are chosen based on various criteria such as how continuous the rocks are over the boundary and whether they include any useful fossils (as I will come onto later).
Other successions of rock, at other locations around the world, are then correlated to these standard defined sections to determine where they fit in the sequence.
It’s worth noting then that this timescale is a relative one, based on the correlations of different rock successions to GSSPs. The absolute timescale that these subdivisions represent (in millions of years) fluctuates as new data is acquired. These absolute dates rely on radiometric dating as I will come onto later.
So we have an international timescale that we can fit our different rock successions into – but how do we actually subdivide rocks we see in the field and how do we correlate these with this timescale? There are various methods to do this, so let’s look at lithostratigraphy first.
Lithostratigraphy is the subdivision of rocks based on their physical characteristics (e.g. grain size, mineral composition etc.) and their relative position. For example, Steno is credited with determining a number of laws that allow interpretation of successions of rock and their relative ages. One of these is the Law of Superposition that states that younger layers of rock overlie older layers. This seems obvious, but it’s a very useful tool for being able to disentangle the relative ages of various rocks.
Importantly, this relies on the strata involved not having been overturned in geological time (i.e. that they are still the right way up). This can be determined by structures known conveniently as ‘way-up’ structures. An example would be cross-bedding that is common in sedimentary rocks (see image below)
Another useful law is the Law of Cross-cutting relationships – this states that if a structure is seen to cut through layers of rock then it must be of a younger age. E.g. dykes are a common example. These are formed when magma infills cracks and breaks in various rock types and therefore cut across the existing layers of the rock.
So various characteristics of different rock types and their position allow us to determine their relative ages.
Looking at the relative position of rocks and also their composition, texture etc allows us to define lithostratigraphic units i.e. bodies of rock with similar characteristics that can be mapped out across an area. The ‘formation’ is the basic mappable unit, but these can be subdivided into various ‘members’ or ‘beds’, as well as grouped together in ‘groups’ or even ‘supergroups’.
A key thing to consider with lithostratigraphic units is that the base of the unit may not (and often doesn’t) represent a set time. Formations, being based on the physical characteristics of the rock type, depend on the location and environment in which that rock type formed. E.g. if you consider rocks that were created in a coastal environment, then coasts clearly shift and change their position. So a deposit from a beach-like environment will, as the coastline shifts, be deposited at different times. Therefore, lithostratigraphic units are often ‘diachronous’ in that they are deposited over a range of time periods.
Another method is biostratigraphy and this relies on the subdivision of rocks based on their fossil content. The basic idea is that different species have evolved throughout the history of the Earth and each exists for a certain period of time and then becomes extinct – therefore, if two rock types are found that contain very similar fossils they must have been deposited at a similar time.
The basic unit of biostratigraphy is the ‘biozone’ which is defined in various ways based on the known stratigraphic and geographic distribution of particular species.
Clearly in order for this to method to work, the fossils used need to be carefully selected. Ideally we want organisms that evolve rapidly, as this will allow us to use the various forms of the organism to define small time periods (e.g. if a particular species had survived for an extremely long time it would be of very little use in biostratigraphy as it would be found in rock types of a wide variety of ages). The fossils also need to have distinct morphological features that make different species easy to identify in the fossil record. Also they need to be abundant so they will be found in many rock types of that time. Finally, the species chosen should ideally have been widely spread to allow us to correlate rocks that formed over a wide geographic area e.g. If a particular fossil only lived in one particular small region, it would only be found in the rocks that formed there and therefore useless for correlation of most rock types that formed in that time period.
With these criteria in place (and more) only certain fossils are useful for biostratigraphy. One of these is the graptolite, thought to represent marine colonial organisms made up of many individual ‘zooids’ joined together as a single colony. Graptolites are very abundant, existed in a wide variety of easily identifiable forms, evolved rapidly and were widely dispersed throughout the seas.
Again, you have to be careful with the use of biozones to correlate different rock successions – biostratigraphic boundaries may also be diachronous due for example to migration of organisms. Also, clearly this technique is limited to rock types that contain useful fossils!
So we have both lithostratigraphy and biostratigraphy that can act together to help correlate the relative ages of different rocks and relate these to the international timescale, but where do the absolute ages come from? How do we count the millions of years?
This all comes down to a technique known as radiometric dating. This makes use of the process of radioactive decay.
Certain elements are present as a variety of isotopes – meaning there are a variety of forms with the same number of protons in their nucleus, but differing numbers of neutrons. Some of these isotopes are unstable and will spontaneously undergo the process of radioactive decay to form a more stable isotope.
For each isotope, this sequence of radioactive decay takes a certain amount of time, often quantified by the half-life of the isotope. The half-life is the time taken for half of the atoms to decay.
Using this characteristic half-life we can use measurements of the ratios of different isotopes in rocks to date their time of formation. Basically, we’re assuming that these unstable isotopes are incorporated into minerals when the rock is formed, from which point they then begin to decay. Measuring the amount of decay (by measuring the amount of parent vs daughter isotope in the rock) allows dating of the time period involved.
Again there are certain limitations to this technique – a key one being that we cannot perform radiometric dating on every rock type we see out in the field. Good radiometric dates can most often be obtained for minerals in igneous rocks that are formed by crystallisation from a molten state. Sedimentary rocks are much more difficult to date usefully – dating a mineral in a sedimentary rock may provide the date of formation of that mineral (i.e. when it crystallised from magma) not the date when it was incorporated into and deposited as a sedimentary rock.
To summarise, there are a lot of different techniques that allow us to figure out the relative and absolute ages of different rock units. Each have their own limitations and it is only through a synthesis of these different approaches that we can arrive at a full understanding. Also, there are many more techniques than I have mentioned here – take a look in the references I’ve listed below if you want to know more!
Refs / suggestions for further reading
Introduction to Paleobiology and the Fossil Record by Michael J. Benton and David A. T. Harper – chapter 2 is particularly useful, giving a brief overview of a variety of stratigraphic methods
How to Build a Habitable Planet: The Story of Earth from the Big Bang to Humankind by Charles H. Langmuir and Wally Broecker – chapter 6 offers a very nicely laid out introduction to radiometric dating and its uses
Sedimentology and Stratigraphy by Gary Nichols (Second Edition) – has a number of excellent and quite comprehensive chapters on different areas of stratigraphy
Paleocast – http://www.palaeocast.com/episode-71-graptolites/ – a great podcast on graptolites!
International Commission on Stratigraphy – http://www.stratigraphy.org/ – official website of the International Commission on Stratigraphy: has databases of all the GSSPs, as well as the latest International Chronostratigraphic Chart