III TIME AND CHANGE
Key Idea 2: Geological events can be placed in relative and absolute timescales
‘The geological column provides a means of (i) placing geological events in their correct time
sequence and (ii) defining the absolute age of some events’ (WJEC syllabus)
Geological time is vast and almost incomprehensible to many people. We, as non-geologists, talk
about ancient civilisations that are perhaps thousands of years old. As geologists we have to deal
with time scales that go back billions of years.
Geological time is represented on a geological column that usually shows absolute time
(measured in ‘millions of years ago’ or ‘Ma’ and usually obtained from radiometric dating methods)
and relative time (subdivisions of the Earth’s geology in a specific order. Most of these subdivisions
are recognized globally on the basis of their relative position in the Earth's stratigraphy and their
fossil content.)
Please note that this
geological column
(and most others) is
not to scale!
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The geological column was well developed before geologists had the means to determine absolute
ages of rocks. it existed as a relative timescale. With the development of dating techniques,
absolute ages can be placed on the timescale. It should be noted that several different versions of
the timescale exist, all with slightly different interpretations of absolute ages of geological
subdivisions.
You wil see geological columns in many different formats, but all provide the same information.
The two types of geological time are analogous to the difference between "lunchtime" ("relative
time") and the numerical time on a clock, like 12:00pm to 1:00pm ("absolute time"). "Lunchtime"
occurs after "morning" and before "suppertime", but its position in time and its duration can also be
measured in hours and minutes, just like the Jurassic Period occurs after the Triassic Period, and
before the Cretaceous Period, and spans the time from about 205 million years ago to about 142
million years ago.
Subdivisions of geological time
A useful analogy of the subdivisions of geological time is that of biological classification.
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Kingdom
Phylum
Class
Order
Family
Genus
Species
This classification is used to add detail about an organism at each level of the hierarchy. Similarly,
we can place events in geological history in narrower subdivisions if more detail is known (or
required).
The main geological subdivisions are:
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Eon
Era
Period
Epoch
Age
increasingly smaller
units of time
(The terms you need to be most familiar with are ‘era’ and ‘period’.)
Eons
This is the largest subdivision of geological time lasting for hundreds of milions of years. Currently
we are in the Phanerozoic Eon which has run fron 540 Ma until present. Before this there was the
Proterozoic, Archaean and Hadean Eons. These three eons are generally just lumped together
under the label ‘Precambrian’ (they occur before the Cambrian period). It is rare to refer to rocks or
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organisms as anything other than Precambrian if they are older than 540 My unless a further
degree of detail is required. Cetainly, ‘Precambrian’ will suffice at this level.
Eras
Generally, the Phanerozoic Eon is subdivided into three eras - the Cenozoic or Cainozoic (‘recent
life’), Mesozoic (‘middle life’) and Palaeozoic (‘ancient life’). (The Palaeozoic Era may be
subdivided into ‘Lower’ and ‘Upper’. )
Palaeozoic (Paleozoic) refers to the Cambrian to Permian periods (570 – 245 Ma). Mesozoic refers
to the Triassic, Jurassic and Cretaceous periods (245 – 65 Ma). The Cenozoic refers to the
Paleogene and Neogene periods (lumped together under the name ‘Tertiary Period’) and the
Quaternary Period (65 Ma – present).
Periods
Eras are further divided into Periods, some of which you will probably be familiar with.
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Quaternary
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Neogene
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Paleogene
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Cretaceous (Abbreviated to ‘K’ e.g. K-T boundary)
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Jurassic
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Triassic
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Permian
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Carboniferous (Americans subdivide this into Pennsylvanian and Mississippian)
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Devonian
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Silurian
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Ordovician
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Cambrian
Tertiary
Often put together as ‘Permo-Triassic’
(The Periods may be further subdivided into Lower/Early, Middle and Upper/Late. E.g. Lower
Jurassic. This gets slightly complex – Lower and Upper refer, strictly, to rocks; and Early and
Late refer to time e.g. Lower Jurassic sandstone deposited in the Early Jurassic.)
There is a standard convention of colours used to represent geological periods on maps etc.
For example Cretaceous rocks are usually represented in lime green, Carboniferous rocks are
usually represented in grey and Jurassic rocks in blue
Epochs
Periods of the Cenozoic Era are subdivided into Epochs. We are currently in the Holocene Epoch.
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Activity 1
(a) On the above chart, place the following events in the correct position:
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1. Early land plants
8. Early birds
2. Early shelled organisms
9. Early flowering plants
3. Extinction of dinosaurs
10. First primitive fishes
4. First dinosaurs
11. Earliest hominid fossils
5. First reptiles
12. Coal swamps abunbant
6. First amphibians
13. First multicelled organisms
7. Early bacteria and algae
14. Formation of grasslands
(b) On the above chart place the following major events in the correct position:
A. Oldest known rocks on Earth
G. Himalaya Mountains begin to form
B. Earth’s crust being formed
H. World wide glaciations occurs
C. Primitive atmosphere begins to form
I. Caledonian fold mountain belt formed
D. Break up of Pangaea begins
J. Formation of first iron ore deposits
E. Opening of the Atlantic Ocean
K. Collision of India with Eurasia begins
F. Formation of the Rocky Mountains
L. Eruption of Deccan basalts
Check your answers by using this link: http://en.wikipedia.org/wiki/Geologic_time_scale
Activity 2
Using a geological map of the UK (paper copy or online at BGS make-a-map http://www.bgs.ac.uk/discoveringGeology/geologyOfBritain/makeamap/home.html ) answer the
following questions:
1. Where are Precambrian rocks most common?
2. Where are Cretaceous rocks found?
3. What age are the rocks at outcrop in the London region?
4. What is the general trend in the age of the rocks at outcrop in the UK from North to South?
5. Where are the most metamorphic rocks found?
Find out:
6. The extinction of the dinosaurs occurred at the boundary of which two periods?
7. During which period chalk was deposited in the UK?
8. Which period do the coal deposits in the UK belong to?
9. When the UK was covered by desert?
10. What is the age of the local limestone used in building (Cotswold Stone, Bath Stone etc.)?
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