Tag Archives: Law of Superposition

Geological Time Part 1- Concepts of Geological Time

In some of my posts I have made mention of certain geological time periods, such as Precambrian, so I thought I’d write a few posts about geological time.  I’m only going to do a brief overview as there are many other sources in which you can get more information.  Most geological text books will cover the subject and even the wikipedia article is worth a look for a little more info.  Oh and in case you were wondering the featured imaged is of the Isle of Hoy, part of the Orkney Islands.  I took the photo several years ago and if you look carefully you can see the geological strata in there.  The cliff itself is about 300m high.

4004 BC, Catastrophism & Uniformitarianism

For most of the past 2000 years the western world has been dominated by Christian theology and this extended to concepts in science.  Genesis was taken literally by many people for the early history of the world, including those who wanted to understand it.  One of the most famous calculations about the age of the earth came from James Ussher, Archbishop of Armagh (lived 1581-1656).  He used the genealogies in the Old Testament to calculate that the world began the night before Sunday 23rd October 4,004 BC.  This calculation became popular (and in some circles this figure is still considered to be the ages of the earth).  Not long after Bishop Ussher scientists – most notably Isaac Newton and Comte de Buffon – started to make their own calculations based on experimentations.  One such experiment was to heat up iron balls and measure how long it takes for them to cool down.  By using such experiments De Buffon eventually calculated that the world was about 74,832 years old.  The venerable physicist Lord Kelvin even weighed in on the issue and using similar ideas thermal diffusion estimated the age of the earth at 20 millions years.  All of these estimates are considerable older than the biblical chronology would have it, but considerably younger than the 4.6 billion years of the current estimate for the earth’s age.

As part of the bible-influenced ideas, one that prevailed was that the geology & geomorphology of the world was shaped by short-lived, dramatic events, in particular Noah’s Flood.  These events were used to explain such things as why their were fossil shells in mountainous areas and why the deep, wide valleys of northern Europe contained rivers that seemed too small to carve them out.  This idea is known as catastrophism.

It didn’t take to long for early geologists working in the field to realise that ideas of catastrophism weren’t seen in the rocks they studied.  What was seen was evidence of processes going on today; rivers depositing sediment, glaciers carving out valleys, sand dues creating angular layers etc.  One of the first geologists to recognise this was James Hutton and he developed the concept of uniformitarianism.  This idea is basically that the laws of nature and the processes that go on now are the same as those that went on in the past.  Catastrophes were not needed to explain the earth’s geology, and in most cases the slow but continuous processes like uplift, erosion and deposition explain what is seen much better.

In modern geology uniformitarianism is seen as being basically true in the ‘day-to-day’ processes, but there is the recognition that dramatic events do happen.  Meteorites impact the earth, large volcanic eruptions and flood basalts do take place and such events can have a dramatic impact on the local (and very occasionally global) geology.

Geological Time Scale

As the study of geology developed several distinct time periods started to be noticed.  Along with uniformitarianism a very simple principle was recognised; the law of superposition.  This is a simple concept; sedimentary layers are deposited in a time sequence with the older layers being at the bottom and the younger layers at the top.  Uplift, folding & faulting can disrupt this sequence, but such processes are usually evident.  Fossils were used to help define such layers and a sequence began to emerge with simpler organisms at the bottom and more complex ones higher up the sequence (yes this is a bit of a generalisation I know).  The presence of the same fossils in different rocks around the world is usually an indication that the rocks are the same age.

Below is a simple diagram of the geological time scale as we currently understand it.

Geological Time 1

The earth is currently estimated to be around 4.6 billion years old.  The above diagram shows the Eons that earth’s history is divided into.  It is roughly to scale.  The Proterozoic, Archaean and Hadean Eons are collectively known as the Precambrian and represents the early earth.  Rocks from the Precambrian form the base of the continents and are often metamorphic in nature.  To give you an idea of scale the most famous extinct organisms, the dinosaurs, lived in the highlighted section of the Phanerozoic (green box).  Being more recent the rocks of the Phanerozoic Eon are more prolific, easier to define and better preserved.  Starting from about 542ma the Phanerozoic is divided as below.  The dates to the side are approximations as there is a margin of error of a couple of ma either side, with some sources quoting slightly different dates.

Geological Time 2

Again this table is roughly to scale.  Click on the image and you should get a better resolution.  The Palaeozoic (formally know as Primary) has early life and sees the evolution of arthropods & fish in the earlier periods, amphibians, reptiles & proto-mammals in the later periods.  The Mesozoic (formally Secondary) is the age of dinosaurs and tends to be the era most people think of when you mention prehistoric life.  You also had further development of mammals and later birds, along with flowering plants.  The Cainozoic is often called the age of mammals with the Neogene and Palaeogene often being called the Tertiary.  The current time period is the Quaternary and is usually defined as being the last 2 million years up to the present, and contains the so called ice ages.

Telling Time Geologically

Until the discovery of radioactivity there was no absolute way of determining how old a rock was, hence the variations in trying to determine the age of the earth.  Early geologists could only date rocks relative to each other; so called relative dating.

Relative Dating: this uses the law of superposition and uniformitarianism to date the rocks relative to each other.  Unless the layers have been disturbed through processes such as faulting, folding or intrusions, the older layers of rock will be at the bottom and the younger ones at the top.  You can then compare the fossils in each layer to see for differences.  Extinction and evolution will lead to some creatures disappearing and being replaced by other organisms in a sequence up through the rock layers.  This can then be compared to rocks from elsewhere and if you find the same fossils then the rocks will be of a similar age.  This then allows you to compare the fossils & layers in the new site for differences there and so.  This allows you to compare & contrast layers to each other so that you know how old a rock layer is in relation to others around it.  You can then compare current rates of erosion and deposition to then make estimates of how long it took to for the geographic process (say a river depositing a layer of silt) to form a layer of rock that think.  Thus giving you an idea of how old the rock may be.  Even in areas where the rock has been deformed you can study how the layers have changed and get a good idea of how old they are in relation to each other.

Absolute Dating: after the discovery of radioactivity and radioactive decay it was realised that you could get a more accurate date for rocks.  The most common method used is radiometric dating.  I’m only going to do a brief intro to this as to give it a full description would take pages.  The most well known type of radiometric dating is radiocarbon dating.  This uses the radioactive carbon-14 to determine a date.  It can only be used for objects containing carbon (e.g. living material) and only for objects up to about 70,000 years old.  Other radioactive isotopes can be used to for older materials (such as Uranium-Lead, Potassium-Argon and Rubidium-Strontium).  Regardless of the isotopes used the method is the same.  As the atoms decay they lose electrons, protons & neutrons and the original atom (parent atom) becomes a different atom (daughter atom).  This radioactive decay is a constant (there are a few special circumstances such a nuclear fusion that resets this clock).  By determining how long it takes for half of the parent atoms to decay to the daughter atoms you can then measure the ratio between the parent & daughter atoms in a sample of rock.  This will then give you a good estimation for the age of the rock.  This can only be done to igneous rocks as when the molten rock solidifies the crystals that contain the radioactive material are fixed in the rock until you take the sample.  With sedimentary rocks the original material has been eroded away, the crystals are no longer complete and so you cannot get a good ratio reading.  Even if you did get an accurate reading you’d be getting a reading for the crystal not the sedimentary rock it sits in.  For metamorphic rock the crystals have been deformed or broken up by heat & pressure again preventing you from getting an accurate reading.s7001758.jpg

(Metamorphic Lewisian Gneiss, with granite mixed in from north-west Scotland)

There are other absolute dating methods that can be used; dendrochronology, fission-tracking, sediment layers (e.g. in mud), annual snow fall/melt layers in ice to name but a few.  By combining absolute and relative dating methods geologist can determine the age of the earth and the various rock layers that make up its surface.

If you wish to know more about geological dating methods I’d suggest you find a decent geological text book, that’ll describe these methods in greater detail.  For the purposes of my blog writing this brief introduction should suffice.  For part 2 I have been working on something a little special and it has taken a bit of time to gather the necessary materials.

References: Fundamentals of the Physical Environment (3rd Edition) by P. Smithson, K. Addison & K. Atkinson (2002).  Geology (2nd Edition)by S. Chernicoff (1999).  Geology of Shropshire (2nd Edition) by P. Toghill (2006).  The Geology of Britain – An Introduction by P. Toghill (2006).  Fossil Revolution – the finds that changed our view of the past by D. Palmer (2003).