The Earth Is Old
THE EARTH IS VERY, VERY OLD
During the 1700s and 1800s, it became evidentthat the earth was at least millions of years old through a number oflines of observation:
--It would take enormous amounts of timefor the salts that freshwater rivers carry into the ocean to produce thelevel of salinity found in the ocean.
--The extent of erosion that had occurredin some parts of the world (the Grand Canyon and the AppalachianMountains compared to the Rocky Mountains for example)would take enormous amounts of time.
--One type of rock (sedimentary rock) is formed after the accumulation of various sediment. The vast quantity of known sedimentary rocks would have taken enormousamounts of time to form. In fact, there were so many strata in the geologiccolumn (although from different localities), the geologic column was morethan 100 km thick. By Darwin’s day, it wasestimated that with a rate of sediment deposition equivalent to that ofthe Mississippi, more than 12million years would be required to produce the sediments known from England.
--The shape of the earth is not perfectlyspherical (it bulges a bit at the middle). This implies that the earth was partially moltenwhile its final shape was being determined and this means that an enormousamount of time would have been required for the earth to cool from thissemi-molten state to its present temperature.
With the development of radiometric dating,it was possible to absolutely date rocks (from the earth, the moon, meteorites,etc.) with techniques which were reliable, repeatable, and which could be confirmed by other independent
radiometric techniques. Someatoms are radioactive which means that they expel particles and/or radiationand in the process become other kinds of atoms. This occurs at a regular rate and each radioactive element has what is known as a half-life--the amount of time needed for the transformation(or decay) of half of the original quantity.
The earth is now estimated to be 4.6 billionyears old. This consistently measuredas the age of meteorites and lunar samples (from Apollo 12, 14, and 15missions) through several dating techniques (such as Potassium/argon,Uranium series and Rubidium/Strontium dating). The oldest rocks on earth are slightly younger than that (3.9 –4.0billion years old); the earth was molten during its early history andmost of the oldest rocks have since eroded.
ABSOLUTE DATING METHODS
Radiometric dating can be used on a sample to produce an actual age in years. There are many independent techniques that test for different radioactiveelements: carbon-14 dating, potassium/argon dating, argon/argon dating,uranium series dating (which can make use of isotopes of lead, thorium,neodymium), lutetium/hafnium, rubidium/strontium dating, and others. Other types of absolute dating techniques suchas thermoluminescence (used to test objects that were once heated, electronsof heated materials return to a more stable state over time), electronspin resonance (ESR; based on similar principles as thermoluminescence),and amino acid racemization are also beginning to contribute to data analysis.
General model of how some of these methods work: a radioactive element breaksdown over time to form a certain product. Younger rocks have more of the parent elementand less product; older rocks have less of the parent element and moreof the product.
For example, uranium(and thorium) is radioactive elements that can decay to form certain formsof lead (Pb206, Pb207, Pb208). In older rocks, the amounts of uranium decreasewhile these forms of lead increase in concentration (you need to measuretheir concentrations relative to Pb204, which is not producedby radioactive decay). Uraniumseries dating can actually give you 2 or 3 independently derived datesthat can be compared.
Arethese methods accurate? They canbe--they can produce dates which are reliable, repeatable, and confirmed by other independent radiometric techniques. There are points to be considered, however.
--No measure(no bathroom scale, no yardstick, no thermometer, no scientific technique)is 100% accurate. All measurements,including dates obtained through radiometric dating, include a standarddeviation (see chart).
--Not all radiometrictechniques are appropriate with all samples. Potassium/argon dating is error prone whendating samples under half a million years old and requires the presenceof volcanic rock or ash. Carbon-14dating is error prone when dating older samples and requires materialthat was once alive. Carbon 14 dating has been calibrated using a 26,000 year record of tree rings, corals, lake sediments, ice cores, and other sources (Balter, 2006). Amino acidracemization is extremely limited and is only accurate when dating sampleswhich are impervious to water (such as the egg shells of larger birds).
--Some samplesare easier to date than others and some samples cannot be accurately datedat all.
--Some youngsedimentary rocks were formed with sediment mixed with pieces of mucholder rocks (some South African hominid cave fossils will never be datedbecause of the mixing of source materials).
--Dating techniqueswhich work wonderfully with certain rocks. Potassium/argon dating works well with igneousrocks (made by volcanic action) may be of limited use with sedimentaryrocks (and unfortunately, almost all fossils are found in sedimentaryrocks). Uranium and rubidium datingdo not work with certain minerals such as limestone. Marine rocks cannot be dated well with potassium/argondating.
--Stony meteorites can be dated well, iron meteorites cannot.
Does every sample agree precisely? No. Two samples with differentdates could both be accurate; two different samples from two differentlocalities were not necessarily formed at the exact same instant. As you can see above, there is some minor disagreementbetween the dates and some samples have a larger margin of error (standarddeviation) than others do. Someof the error is caused by differences in the technique of dating. A separate lab using a slightly different set of standards to obtaindates of 65.06 and 65.16 mya respectively analyzed the two dates withthe asterisk (64.38 and 64.48 mya).
RELATIVE DATING METHODS
Even though not every rock strata can bedated absolutely, there are a number of techniques which can compare differentrock strata to show that one is older than another or that one rock stratais equivalent to the strata of a different location. Even if one samplecannot be dated absolutely, it can often be compared to other strata,which can be dated absolutely. Thecomparing rock strata (stratigraphy) can be based on: strata composition, variations in the earth's magnetic field at the time of the rocks' formation (paleomagnetostratigraphy), temperature variationswhen the strata formed (as demonstrated by variations in the differentforms [isotopes] of oxygen present, for example), and variations in thetypes of fossils present (vertebrate fossils, invertebrate fossils, microfossils,pollen grains, etc.). Usually thedeeper layers of rock were formed first and are thus older (there areexceptions to this, especially during the formation of mountains in whichstrata can be overturned).
Some fossil are what are called index fossils:they have been shown through absolute dating to exist only during a certainperiod of time. After a large numberof observations confirm this, this fossil can be used to date strata which cannot be dated with absolute dating.
GEOLOGIC TIME SCALE