In the late eighteenth century James Hutton, an Scottish geologist and naturalist, shocked his contemporaries with his observation of rocks that led him to see "no sign of a beginning- no prospect for an end". He deduced that the layered rock were sediments laid down in marine and continental environments by procxesses and at rates currently observed. This became to be known as the principle of uniformitarianism and led to the establishment of relative ages of the various rock layers Measurement of rates of sedimentation then led directly to the discovery of the great antiquity of the Earth.
Relative Age is determining the sequence of geologic events that have occurred in a given area and can be established on the basis of several intuitive principles:
1) Original horizontality: Sediments deposited in water were originally flat lying.
2) Superposition: the old rocks are at the bottom of the pile.
3) Cross-cutting: the disturbed rocks are older than the disturbing rocks.
Correlations of events from one region to another are based on:
1) Physical continuity
2) Similarity of rock types
3) Correlation of fossils
Hutton, Werner, and Lyell developed these concepts to establish the relative ages of the sedimentary rocks of Britain and Europe. The geologic evidence led these workers to advocate an age of teh earth of hundreds to thousands of millions of years which put them in direct conflict with the catastrophism of the contemporary church. Charles Darwin was a contemporary of Lyell, and agreed that vast expanses of time were reuired for biological evolution to achieve the observed degrees of complexity.
The geologists and biologists then encountered arguments based on thermodynamics for a much younger age of the Earth. Lord Kelvin, a British physicist who had done much to advance our understanding of heat generation and conservation of energy, assumed that the sun's energy was due to gravitational collapse and calculated the amount of energy that could be thus derived. From the measured rate of solar radiation, he calculated that the energy source could last no more than 40 million years. The debate raged over the final decades of the nineteenth century until the discovery of radioactivity which provided an alternate, and much larger, source for the energy of the sun (thermonuclear fusion) and a series of radiometric clocks by which we can calculate the absolute age of rocks.
The types of radiation emanating from nuclear reactions are listed in the following table:
Particle composed of: Mass# Atomic # Example __________________________________________________________ alpha 2 neutrons+ 4 2 U, Th, 2 protons beta- electron 0 -1 40K beta+ positron 0 +1 40K gamma photon 0 0 all nuclear reactions neutron neutron 1 0 235U ___________________________________________________________Absolute Age is based on radiometric dating. The Earth contains a number of different radioactive isotopes that can be used for radiometric dating.
Isotope Daughter Half-life
40 K 40 Ar 1.3 x 109 y 40 K 40Ca 1.3 x 109 y 87 Rb 87Sr 4.9 x 1010 y 238 U 206 Pb 4.5 x 109 y 235 U 207 Pb 7.1 x 108 y 232 Th 208 Pb 1.4 x 1010 y 14 C 14 N 5.7 x 103 y
With the exception of 14C, all of these are long-lived isotopes that are remnants of the precursor star from which our solar system formed 4.5 billion years ago.
14C is formed in the atmosphere by cosmic ray bombardment of13C and is used to date organic material derived from atmospheric CO2.
When a mineral such as orthoclase (KAlSi3O8) crystallizes from a melt, it has no Ar. (Ar is an inert gas.) Therefore all the 40Ar in the sample is the decay product of 40K. To date the mineral, we just heat the sample in vacuum to release the 40Ar and measure the released gas in a mass spectrometer. If we know the concentration of40 Ar and the current concentration of K and the 40K/39K ratio, we can calculate how many half-lives must have elapsed from the time of crystallization. Metamorphic events can reset this clock by releasing all or part of the Ar by recrystallizing the orthoclase.
The U decay series goes through a series of radioactive intermediaries before reaching the stable lead (Pb). There are three distinct series: 232Th,235 U, and 238U. All of the intermediary products are alpha-emitters (an alpha particle is a 4He nucleus). Alpha particles do not travel far in air or human tissue, but they do a lot of damage. One of the radioactive intermediaries is radon (Rn) a chemically inert gas which leaks into well water or basements. Radon is a problem in areas with U-rich rocks like granite and homes with private water wells.
The geologic time scale:
Eon Era Period Epoch Began (millions of years ago)
Phanerozoic Cenozoic Quaternary Holocene (recent) 0.01 Pleistocene 1.6 Tertiary Pliocene 5.3 Miocene 23.7 Oligocene 36.6 Eocene 57.8 Paleocene 66.4 Mesozoic Cretaceous 144 Jurassic 208 Triassic 245 Paleozoic Permian 286 Pennsylvanian 320 Mississippian 360 Devonian 408 Silurian 438 Ordovician 505 Cambrian 570 Proterozoic 2500 Archean 4500
Note the grossly non-linear time scale. Here is a link to an image of the geologic time scale.
The Archean is nearly half of Earth history, and so is the Proterozoic. Life is thought to have begun on Earth early in the Archean. The atmosphere became oxidizing (had free oxygen gas) in the early Proterozoic as the result of photosynthetic bacterial action.
The Phanerozoic represents only about 15% of the total. It is the Eon of animals that had internal or external skeletons that have left abundant fossil remains. Most of our unmetamorphosed sedimantary rocks date from this age.
The basal Cambrian is marked by high sea levels and the proliferation of shallow-water aquatic life with hard skeletal parts that are preserved as fossils.
The Mississippian and Pennsylvanian are together known as the Carboniferous and are the period of the proliferation of land plants.
The Mesozoic was the age of dinosaurs. They were around (180My) for nearly three times longer than they have been extinct (66My). The extinction of the dinosaurs is thought to have resulted frm a catstrophic meteorite impact into sulfate and carbonate rocks at a site in the Gulf of Mexico just north of Yucatan.
The Tertiary is the age of mammals and flowering plants. The earliest Hominid fossils are mid Pliocene (3My).
Note that the Cretaceous was longer than the entire Tertiary.
The Pleistocene is the age of continental ice sheets. The last major ice disappeared from North America and Europe only about 10,000 years ago. Antactica and Greenland still have continental ice sheets.
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