Geologic time, Lectures 2 & 3 Flashcards
Pinatubo, Phillippines
when, what, causes
- June 1991,
- one of the largest eruptions of the 20th century
- Rich in SO2
- Aerosol dispersed globally in both hemispheres
- cooling of 0.2 degrees C for 1-2 years
Charleston earthquake 1886
where, deaths, spatial scale, timescale
South Carolina, USA
60 deaths
spatial scale: 100-1000km2
timescale: seconds/minutes
How has the relative geologic timescale been constructed?
4 principles
- principle of original horizontality
- principle of superposition
- principle of cross-current relationships
- principle of faunal success
Principle of original horizontality
sediments are deposited as horizontal beds
Principle of superposition
Each layer of sedimentary rock in a tectonically undisturbed sequence is younger than the one beneath it.
Principle of cross-current relationships
An igneous intrusion or fault must be younger than the rocks it intrudes or cuts.
Principle of faunal succession
The sedimentary strata in an outcrop contain fossil in a definite sequence.
Fossils
Remains of ancient organisms
Palaeontology
the study of ancient life and Earth’s history from the fossil record
Body fossils
4
shells, teeth, bomes, impressions of plants
Trace fossils
4
records of biological activity: burrows footprints feeding trails coprolites
Another name for trace fossils
ichnofossils
Coprolites
Pieces of fossilised dung
Who came up with the principle of faunal succesion
William Smith (1769 - 1839)
‘Father of English Geology’
surveyor at coalfield and coal canal in Somerset
published first geologic map in 1815
Absolute ages (of rocks and sediments)
dates in years
When was radioactivity discovered?
1985
Isotopic dating
the use of naturally occurring radioactive elements to determine rock ages
Who introduced isotopic dating and when?
1905, Ernest Rutherford
All matter is made up of…
elements which are composed of atoms
Every atom has a…
nucleus containing most of the mass of the atom, surrunded by a negative cloud of electrons
Isotopes
Atoms of the same element with different numbers of neutrons but the same number of protons.
Difference between isotopes of the same element?
atomic mass
Similarity between isotopes of the same element?
chemical properties
Are isotopes unstable or stable?
some are stable but some are unstable and radioactively decay
What is used for radioactive/isotopic dating?
radioactive decay
What happens to atoms of radioactive elements?
they spontaneouly disintergrate (radioactively decay) to form atoms of a different element
Radioactive parent elements…
decompose to form radioactive daughter elements
Half-life
the time for one half of the original number of radioactive atoms to decay
Why is radioactive decay used as a clock?
- decay rates/half lives of radioactive atoms known and do not vary with changes in T or P
- can measure the ratio of parent to daughter atoms in a rock samples
Use of a mass spectrometer when carrying out isotopic dating in a lab
- count the daughter element atoms
- know the rate of decay
- calculate the time when there were no daughters (only parents)
= time of rock formation
The isotopic age of a rock/sediment/fossil?
time since the isotopic clock was ‘reset’ when the isotopes were locked into the minerals, eg: crystallisation from melt or during metamorphism
Radioactive isotopes are used to date
3
- time of death of organisms (fossils in sedimentary rocks)
- time of cooling and crystallisation of magma (igneous rocks)
- time elapsed since metamorphism (metamorphic rocks)
Age of the Earth
and how was this calculated?
4.6 billion years
relative and absolute dating techniques
Relative vs absolute dating
relative = no specific dates, to determine if one object is older than another absolute = origin dates, time ranges, age range in years
