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THE EARTH IS OLD
THE EARTH IS VERY, VERY OLD
During the 1700s and 1800s, it became evident that the earth was at least millions of years old through a number of lines of observation:
--It would take enormous amounts of time for the salts that freshwater rivers carry into the ocean to produce the level of salinity found in the ocean.
--The extent of erosion that had occurred
in some parts of the world (the
--One type of
rock (sedimentary rock) is formed after the accumulation of various sediment.
The vast quantity of known sedimentary rocks would have taken enormous
amounts of time to form. In fact, there were so many strata in the geologic
column (although from different localities), the geologic column was more
than 100 km thick. By
--The shape of the earth is not perfectly spherical (it bulges a bit at the middle). This implies that the earth was partially molten while its final shape was being determined and this means that an enormous amount of time would have been required for the earth to cool from this semi-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. Some atoms are radioactive which means that they expel particles and/or radiation and 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 billion years old. This consistently measured as the age of meteorites and lunar samples (from Apollo 12, 14, and 15 missions) 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 –4.0 billion years old); the earth was molten during its early history and most 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 radioactive elements: 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 such as thermoluminescence (used to test objects that were once heated, electrons of heated materials return to a more stable state over time), electron spin 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 breaks down over time to form a certain product. Younger rocks have more of the parent element and less product; older rocks have less of the parent element and more of the product.
PARENT ELEMENT CONCENTRATION THROUGH TIME
PRODUCT CONCENTRATION THROUGH TIME
For example, uranium (and thorium) is radioactive elements that can decay to form certain forms of lead (Pb206, Pb207, Pb208). In older rocks, the amounts of uranium decrease while these forms of lead increase in concentration (you need to measure their concentrations relative to Pb204, which is not produced by radioactive decay). Uranium series dating can actually give you 2 or 3 independently derived dates that can be compared.
Are these methods accurate? They can be--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 standard deviation (see chart).
--Not all radiometric techniques are appropriate with all samples. Potassium/argon dating is error prone when dating samples under half a million years old and requires the presence of volcanic rock or ash. Carbon-14 dating is error prone when dating older samples and requires material that 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 acid racemization is extremely limited and is only accurate when dating samples which are impervious to water (such as the egg shells of larger birds).
--Some samples are easier to date than others and some samples cannot be accurately dated at all.
--Some young sedimentary rocks were formed with sediment mixed with pieces of much older rocks (some South African hominid cave fossils will never be dated because of the mixing of source materials).
--Dating techniques which work wonderfully with certain rocks. Potassium/argon dating works well with igneous rocks (made by volcanic action) may be of limited use with sedimentary rocks (and unfortunately, almost all fossils are found in sedimentary rocks). Uranium and rubidium dating do not work with certain minerals such as limestone. Marine rocks cannot be dated well with potassium/argon dating.
--Stony meteorites can be dated well, iron meteorites cannot.
How accurate are the dates which are produced? Below are some examples of sites analyzed by different researchers and different techniques. Please compare the dates in each specific colored box (keeping in mind the standard deviation).
YOU DO NOT HAVE TO UNDERSTAND THE DATING TECHNIQUES LISTED BELOW (this is beyond the scope of this course), SIMPLY EXAMINE WHETHER DIFFERENT SAMPLES, OFTEN ANALYZED BY DIFFERENT RESEARCHERS USING DIFFERENT TECHNIQUES, OBTAIN EQUIVALENT DATES. Each different color represents a different set of samples and should be considered separately.
Dating the boundary between the Cretaceous and Tertiary Periods (K/T Boundary)
Refer to the final yellow portion of the absolute dating table above.
Many samples have been analyzed by absolute dating techniques (K/Ar, Ar/Ar, Rb/Sr, U/Pb) and every sample has fallen between 63.5 to 66.4 million years ago. It is possible to conclude with scientific certainty that the Cretaceous Period ended about 65 million years ago.
Does every sample agree precisely? No. Two samples with different dates could both be accurate; two different samples from two different localities were not necessarily formed at the exact same instant. As you can see above, there is some minor disagreement between the dates and some samples have a larger margin of error (standard deviation) than others do. Some of the error is caused by differences in the technique of dating. A separate lab using a slightly different set of standards to obtain dates of 65.06 and 65.16 mya respectively analyzed the two dates with the asterisk (64.38 and 64.48 mya).
RELATIVE DATING METHODS
Even though not every rock strata can be dated absolutely, there are a number of techniques which can compare different rock strata to show that one is older than another or that one rock strata is equivalent to the strata of a different location. Even if one sample cannot be dated absolutely, it can often be compared to other strata, which can be dated absolutely. The comparing 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 variations when the strata formed (as demonstrated by variations in the different forms [isotopes] of oxygen present, for example), and variations in the types of fossils present (vertebrate fossils, invertebrate fossils, microfossils, pollen grains, etc.). Usually the deeper layers of rock were formed first and are thus older (there are exceptions to this, especially during the formation of mountains in which strata can be overturned).
Some fossil are what are called index fossils:they have been shown through absolute dating to exist only during a certain period of time. After a large number of observations confirm this, this fossil can be used to date strata which cannot be dated with absolute dating. For example, there is a type of bryozoan fossil classified as Archimedes. Although it was once common and widespread, it is extinct today. In fact, it only existed for a brief period in what we will recognize as the Mississippian Period (part of the Carboniferous Period). Every Archimedes fossil that has ever been absolutely dated occurred in the Mississippian Period. Once the observation that Archimedes only occurred in the Mississippian is confirmed again and again and again, Archimedes can now be used to date strata. If a stratum is found that cannot be dated absolutely which contains Archimedes fossils, it is dated through relative dating as being of Mississippian age.
Dating the Cretaceous/Tertiary Boundary by Relative Dating
There are many differences between the rock strata around this boundary. There is a layer which has an enormously high amount of iridium due to an impact from space, there are a specific type of quartz crystals (shocked quartz; this also indicates impact), there is a brief switching of magnetic poles, and there are stratigraphic differences between the rocks in this zone. A major extinction event occurred here so that the assemblage of organisms that lived in the Cretaceous is different from that of the Tertiary. This change can be observed with land vertebrates (such as dinosaurs), other land organisms, marine organisms, microscopic fossils, pollen/spore classes, etc.
SUMMING UP: THE HISTORY OF THE EARTH
Earth’s history can be divided into 3 eons: the Archeozoic, Proterozoic, and Phanerozoic. The Archeozoic and Proterozoic are often lumped together in the term “Precambrian” (the Cambrian is the first period of the Phanerozoic). The Precambrian represents 85% of earth’s history.
The Phanerozoic is divided into 3 eras: the Paleozoic, the Mesozoic, and the Cenozoic. Each of these is divided into periods. These periods can be further divided. We will refer to the divisions of the periods of the Cenozoic Era when we study the evolution of mammals.
Notice that humans have only existed for a tiny fraction of earth’s history. Please refer to the charts below throughout the course.