Difficulties in dating the geologic time scale

Published by Frank Hall Modified over 3 years ago. The trees are recording all of the environmental variables that affect tree growth. Can be used to date objects with annual resolution back 10, years in the best circumstances. One difficulty with the time scale is that we can not differentiate fine time increments. Using radioactivity in dating Reviewing basic atomic structure Nucleus —Protons — positively charged particles with mass —Neutrons — neutral particles.

My presentations Profile Feedback Log out. To get to that point, there is also a historical discussion and description of non-radiometric dating methods. A common form of criticism is to cite geologically complicated situations where the application of radiometric dating is very challenging. These are often characterised as the norm, rather than the exception. I thought it would be useful to present an example where the geology is simple, and unsurprisingly, the method does work well, to show the quality of data that would have to be invalidated before a major revision of the geologic time scale could be accepted by conventional scientists.

Geochronologists do not claim that radiometric dating is foolproof no scientific method is , but it does work reliably for most samples. It is these highly consistent and reliable samples, rather than the tricky ones, that have to be falsified for "young Earth" theories to have any scientific plausibility, not to mention the need to falsify huge amounts of evidence from other techniques. This document is partly based on a prior posting composed in reply to Ted Holden.

My thanks to both him and other critics for motivating me. Much of the Earth's geology consists of successional layers of different rock types, piled one on top of another. The most common rocks observed in this form are sedimentary rocks derived from what were formerly sediments , and extrusive igneous rocks e.

The layers of rock are known as "strata", and the study of their succession is known as "stratigraphy". Fundamental to stratigraphy are a set of simple principles, based on elementary geometry, empirical observation of the way these rocks are deposited today, and gravity. A few principles were recognized and specified later.

An early summary of them is found in Charles Lyell's Principles of Geology , published in , and does not differ greatly from a modern formulation:. Note that these are principles. In no way are they meant to imply there are no exceptions. For example, the principle of superposition is based, fundamentally, on gravity. In order for a layer of material to be deposited, something has to be beneath it to support it.

It can't float in mid-air, particularly if the material involved is sand, mud, or molten rock.

Radiometric Dating and the Geological Time Scale

The principle of superposition therefore has a clear implication for the relative age of a vertical succession of strata. There are situations where it potentially fails -- for example, in cave deposits. In this situation, the cave contents are younger than both the bedrock below the cave and the suspended roof above. However, note that because of the " principle of cross-cutting relationships" , careful examination of the contact between the cave infill and the surrounding rock will reveal the true relative age relationships, as will the "principle of inclusion" if fragments of the surrounding rock are found within the infill.

Cave deposits also often have distinctive structures of their own e. These geological principles are not assumptions either. Each of them is a testable hypothesis about the relationships between rock units and their characteristics. They are applied by geologists in the same sense that a "null hypothesis" is in statistics -- not necessarily correct, just testable. In the last or more years of their application, they are often valid, but geologists do not assume they are. They are the "initial working hypotheses" to be tested further by data.

Using these principles, it is possible to construct an interpretation of the sequence of events for any geological situation, even on other planets e.

Circular Reasoning or Reliable Tools?

The simplest situation for a geologist is a "layer cake" succession of sedimentary or extrusive igneous rock units arranged in nearly horizontal layers. In such a situation, the " principle of superposition" is easily applied, and the strata towards the bottom are older, those towards the top are younger. For example, wave ripples have their pointed crests on the "up" side, and more rounded troughs on the "down" side. Many other indicators are commonly present, including ones that can even tell you the angle of the depositional surface at the time "geopetal structures" , "assuming" that gravity was "down" at the time, which isn't much of an assumption: In more complicated situations, like in a mountain belt, there are often faults, folds, and other structural complications that have deformed and "chopped up" the original stratigraphy.

Despite this, the "principle of cross cutting relationships" can be used to determine the sequence of deposition, folds, and faults based on their intersections -- if folds and faults deform or cut across the sedimentary layers and surfaces, then they obviously came after deposition of the sediments. You can't deform a structure e. Even in complex situations of multiple deposition, deformation, erosion, deposition, and repeated events, it is possible to reconstruct the sequence of events.

Even if the folding is so intense that some of the strata is now upside down, this fact can be recognized with "way up" indicators. No matter what the geologic situation, these basic principles reliably yield a reconstructed history of the sequence of events, both depositional, erosional, deformational, and others, for the geology of a region. This reconstruction is tested and refined as new field information is collected, and can be and often is done completely independently of anything to do with other methods e.

The reconstructed history of events forms a "relative time scale", because it is possible to tell that event A occurred prior to event B, which occurred prior to event C, regardless of the actual duration of time between them. Sometimes this study is referred to as "event stratigraphy", a term that applies regardless of the type of event that occurs biologic, sedimentologic, environmental, volcanic, magnetic, diagenetic, tectonic, etc. These simple techniques have widely and successfully applied since at least the early s, and by the early s, geologists had recognized that many obvious similarities existed in terms of the independently-reconstructed sequence of geologic events observed in different parts of the world.

One of the earliest relative time scales based upon this observation was the subdivision of the Earth's stratigraphy and therefore its history , into the "Primary", "Secondary", "Tertiary", and later "Quaternary" strata based mainly on characteristic rock types in Europe. The latter two subdivisions, in an emended form, are still used today by geologists. The earliest, "Primary" is somewhat similar to the modern Paleozoic and Precambrian, and the "Secondary" is similar to the modern Mesozoic.

Another observation was the similarity of the fossils observed within the succession of strata, which leads to the next topic. As geologists continued to reconstruct the Earth's geologic history in the s and early s, they quickly recognized that the distribution of fossils within this history was not random -- fossils occurred in a consistent order.

This was true at a regional, and even a global scale. Furthermore, fossil organisms were more unique than rock types, and much more varied, offering the potential for a much more precise subdivision of the stratigraphy and events within it. The recognition of the utility of fossils for more precise "relative dating" is often attributed to William Smith, a canal engineer who observed the fossil succession while digging through the rocks of southern England.

But scientists like Albert Oppel hit upon the same principles at about about the same time or earlier. In Smith's case, by using empirical observations of the fossil succession, he was able to propose a fine subdivision of the rocks and map out the formations of southern England in one of the earliest geological maps Other workers in the rest of Europe, and eventually the rest of the world, were able to compare directly to the same fossil succession in their areas, even when the rock types themselves varied at finer scale.

For example, everywhere in the world, trilobites were found lower in the stratigraphy than marine reptiles. Dinosaurs were found after the first occurrence of land plants, insects, and amphibians. Spore-bearing land plants like ferns were always found before the occurrence of flowering plants.

The observation that fossils occur in a consistent succession is known as the "principle of faunal and floral succession". The study of the succession of fossils and its application to relative dating is known as "biostratigraphy". Each increment of time in the stratigraphy could be characterized by a particular assemblage of fossil organisms, formally termed a biostratigraphic "zone" by the German paleontologists Friedrich Quenstedt and Albert Oppel.

These zones could then be traced over large regions, and eventually globally. Groups of zones were used to establish larger intervals of stratigraphy, known as geologic "stages" and geologic "systems". The time corresponding to most of these intervals of rock became known as geologic "ages" and "periods", respectively. By the end of the s, most of the presently-used geologic periods had been established based on their fossil content and their observed relative position in the stratigraphy e.

These terms were preceded by decades by other terms for various geologic subdivisions, and although there was subsequent debate over their exact boundaries e. By the s, fossil succession had been studied to an increasing degree, such that the broad history of life on Earth was well understood, regardless of the debate over the names applied to portions of it, and where exactly to make the divisions. All paleontologists recognized unmistakable trends in morphology through time in the succession of fossil organisms.

This observation led to attempts to explain the fossil succession by various mechanisms. Perhaps the best known example is Darwin's theory of evolution by natural selection. Note that chronologically, fossil succession was well and independently established long before Darwin's evolutionary theory was proposed in Fossil succession and the geologic time scale are constrained by the observed order of the stratigraphy -- basically geometry -- not by evolutionary theory. For almost the next years, geologists operated using relative dating methods, both using the basic principles of geology and fossil succession biostratigraphy.

Various attempts were made as far back as the s to scientifically estimate the age of the Earth, and, later, to use this to calibrate the relative time scale to numeric values refer to "Changing views of the history of the Earth" by Richard Harter and Chris Stassen. Most of the early attempts were based on rates of deposition, erosion, and other geological processes, which yielded uncertain time estimates, but which clearly indicated Earth history was at least million or more years old.

A challenge to this interpretation came in the form of Lord Kelvin's William Thomson's calculations of the heat flow from the Earth, and the implication this had for the age -- rather than hundreds of millions of years, the Earth could be as young as tens of million of years old.


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This evaluation was subsequently invalidated by the discovery of radioactivity in the last years of the 19th century, which was an unaccounted for source of heat in Kelvin's original calculations. With it factored in, the Earth could be vastly older. Estimates of the age of the Earth again returned to the prior methods.

Variant of the same parent atom b. Different number of neutrons c. Different mass number than the parent atom. Spontaneous breaking apart decay of atomic nuclei 2. Parent — an unstable radioactive isotope c. Daughter products — isotopes resulting from the decay of a parent c. Half-life — the time for one-half of the radioactive nuclei in a sample to decay. The percentage of radioactive atoms that decay during one half-life is always the same: However, the actual number of atoms that decay continually decreases c.

Comparing the ratio of parent to daughter yields the age of the sample. Two isotopes of uranium d. A closed system is required b. To avoid problems, one safeguard is to use only fresh, unweathered material.

Geological time scale chart made easy with tricks - memorize geographical time scale in 5 minutes

Half-life of only years 2. Used to date very recent events 3. Carbon is produced in the upper atmosphere.

Radiometric Dating and the Geological Time Scale

Useful tool for anthropologists, archeologists, and geologists who study recent Earth history. Radiometric dating is a complex procedure that requires precise measurement 2. Rocks from several localities have been dated at more than 3 billion years 3. Confirms the idea that geologic time is immense.

Hadean — the oldest eon.

Earth Science, 12e Geologic Time Chapter 11

Collectively, the Hadean, Archean, and Proterozoic eons are often referred to as the Precambrian. Not divided into smaller time units because the events of Precambrian history are not known in great enough detail. Precambrian rocks have been subjected to a great many changes. The grains composing detrital sedimentary rocks are not the same age as the rock in which they occur b.

The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed. Explain the difference between relative and absolute dating of earth materials.

Briefly explain other principles used in relative age dating. List and briefly explain the three types of unconformities. Briefly discuss fossilization, including the origin and types of fossils.


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  7. Discuss the correlation of rock layers using physical criteria and fossils. Briefly explain radioactivity and how it relates to absolute age dating. Discuss the procedure of radiometric dating and explain how it is used to obtain absolute ages. List the isotopes commonly used in the radiometric dating of earth materials. List and briefly discuss the major subdivisions of the geologic time scale.

    Briefly explain the significance of the Precambrian division of the geologic time scale.