Geologic Time
So far have talked about formation of rocks - crystallization, metamorphism, erosion, sedimentation,- all essential parts of the rock cycle - but how much time to complete these processes. How much time to form a mountain range?, how much time to erode one?, how much time to fill a sedimentary basin?
This suggests other fundamental questions - when did life appear on this planet? how did it evolve? how long did it take for humans to evolve? and the most magnificent of all - what is the age of the Earth
This, the study of geologic time is Historical Geology.
Time scales
[overhead]
o Some geological processes easy to observe and time.
o E/q -seconds; floods - weeks;
o erosion of rivers and coastlines years.
o human history - hundreds to thousands (6 to be accurate)
o opening of an ocean - cms/yr; therefore to have an ocean thousands of miles across it needs millions of years (200 to be more precise)
So for most geological processes we are looking for significant changes over millions of years - This is deep time - a concept that preceeded the astronomical concept of deep space (which might be more familiar) by 200 year ! -- also more contraversial (to this day) ! ! ! !
Geologists therefore changed the way we think about time, and therefore the way we think about our planet (its great antiquity), and even ourselves. Humans, as we will see, have only been inhabitants of the Earth for the briefest moments of its vast history.
So how did geologists work all this out??
Stratigraphic sequences
- this is discipline of stratigraphy
Two principles stratification: Horizontality; Superposition
Sequences of layers (stratigraphic sequences).
Rates of sedimentation, and thickness could form a "stratigraphic clock" - problems of differing rates of sedimentation or even periods of non-sedimentation.
Formations - a particular sequence of distinctive beds, often with particular set of characteristic fossils. Basic unit from which a stratigraphic sequence is constructed. Not a landform !
Unconformities - indicators of missing periods of time. First recognized by James Hutton. [explain]. Implies uplift and erosion.
Unconformities inspired James Hutton ("father of modern geology") to first formulation of immensity of geologic time - "no vestige of a beginning, no prospect of an end". Revolutionary concept at a time when many believed that the world began at 9.00 am Oct 23, 4004 BC ! ! !
Fossils - known to Greeks and Romans, fossils never properly understood until end of 18th century. Hard parts of animals, plants, buried and preserved in sediments. Birth of paleontology.
Fossil assemblages (use historical analogue). William Smith
Noticed that, as go up through layers, same sequence of fossil assemblages occur in different locations
Correlation: assemblages of fossils are a way of determining if one set of beds in one part of the world were formed at the same time as asecond set of beds in another part of the world
.
Reconstructing geologic history
Purpose of this to establish order of events using Horizontality/Superposition/Unconformities
also inclusion and cross-cutting relationships [examples].
Geologic time scale
Above shows way of ordering geologic events. Key are fossils.
After comparing sequences of rocks over large parts of the world was possible to establish a relative time scale of geologic periods (those periods marked my distinctive fossil groups).
A mnemonic:
Cruddy old socks damage most peoples pedicures. They just corrode. People eventually overcome many painful, pedicures, however.
or
Camels often sit down carefully (mighty precisely) & painfully. Their joints creak. Precautionary, early oiling might prevent permanent handicaps
Astronomical ("absolute") Time and the Geologic Time Scale
For long time geologists had no way of expressing time in years (astronomical units).
The key was the discovery of radioactivity at the beginning of this century. Unstable parent isotopes break down to other daughter isotopes in a process called decay. Since this occurs at a definite rate, then can be used to measure geological time. Basis of isotopic dating - also called radiometric dating.
What happens? Statistical process; over a given period of time (different for each parent isotope) 50% of the parent atoms will decay to daughter atoms.
That period of time is called the half-life.
Graph:
Thus, over time parent atoms decrease, daughter atoms decrease, thus if you know proportion of one to the other (ratio) then it is possible to calculate age since daughter atoms began to accumulate.
What resets the clock? Crystallization parent atoms are locked in crystal structure (no daughter atoms because they are different and do not fit in the crystal structure). When parent atoms begin to decay, daughter atoms cannot move out of crystal stucture and so are trapped. Thus -
igneous rocks: gives time of crystallization of melt (ie formation)
metamorphic rocks: time of metamorphism
sedimentary rocks: can't be dated using isotopic methods.
How are parent and daughter atoms measured? Cannot measure individual atoms, but can measure relative weights [check this !!!!] of parent/daughter using mass spectrometer.
Main systems used for isotopic dating
Problems of isotopic dating
Ig/Met rx yes; Sed rx, no
Integrating with geologic time scale. Have to use field relations
Illustration:
Evolution of Life
Side effect of the establishment of the geologic time scale and the use of fossils was the order of evolution of life
Strongly influenced Charles Darwin in developing his theory of evolution
Combined with results from isotopic dating (the link to the astronomical scale), the eventual realization was of the not only the order of evolution of life on the Earth, but its time scale. Strong influence of modern biology.
Tour of evolution of life: extinctions etc.