Geologic Time and the Geologic Column
•When we speak of geologic time, we are referring to a different timescale than the one we typically use in our day-to-day lives. Geologic processes can be very rapid, but the most common take place over thousands or millions of years.
•Dating works because of the geologic law of uniformitarianism. This is the concept that those processes that we now see at work on the Earth — both internal and external — have occurred throughout Earth's history, though at different rates.
•Relative dating places events in their proper chronological sequence, but tells us nothing of the starting or ending point of that sequence. Absolute dating applies specific dates for events expressed in years before present.
Principles of relative dating
•Note that these principles refer to sedimentary rock for the most part. This is because while all types of rock (igneous, metamorphic, sedimentary) are forming today, sediment is being formed continuously over the entire Earth. The formation of sedimentary rock is thus not specific to a region or location but occurs in its various forms worldwide. Thus, the sedimentary record is the most complete and easiest to relatively date.
•The Principle of Superposition says that in an undisturbed succession of sedimentary rock layers, the oldest layer is at the bottom and the youngest is at the top. So the relative ages of all the layers in between can be determined to lie between these two layers in sequence.
•Example: Toward the end of a cold winter it is often possible to see a layer of old snow that is compact and perhaps dirty, overlain by fresh, looser clean snow deposited during the latest snowstorm. This is an example of two layers, or starata, that were eposited in sequence, one above the other.
Although usually used for sedimentary rock sequences, the principle of superposition works in many other cases as well, including other planets. In this image of the Moon's crater Eratosthenes, we know that the crater was formed after the smooth volcanic material because the crater's ejecta blanket (the radial spray of debris from the impact) lies on top of the volcanic unit.
•The Principle of Original Horizontality states that sediment settles from water in essentially horizontal layers, parallel to the Earth's surface. Thus, any inclined layers must have been tilted after lithification of the sediments.
•Example: That same sequence of snow might be moved upward by a snowplow. Often the layers can still be seen, but they are no longer parallel. They may be tilted, folded, or even completely overturned, but those layers must have originally been horizontal.
•The Principle of Cross-Cutting Relationships states that anything that cuts through layers is younger than those layers. For example, a volcanic batholith or dike intruding into the surrounding rock must be younger than the rock it intrudes into. One of the most obvious examples of this principle is the Grand Canyon, where the Colorado River is younger than all of the layers it cuts into.
In this image, the magma intrusions are younger than the rocks they have been injected into.
•The Principle of Inclusions refers to situations in which fragments of one rock are contained within another. This principle states that these inclusions are older than the rock layer containing them. This can be seen most easily in sedimentary rocks, made up of fragments of older rock, but this term typically refers to the inclusion of surrounding rock in a batholith or other igneous intrusion.
Breaks in the Geologic Record
•The accuracy of these principles depends on there being no depositional breaks during a rock sequence (that is, the rock sequence is conformable). On a planet as active as the Earth, this is not always the case.
•Unconformities are gaps of significant time in the geologic record as shown in rock layers. An unconformity is also referred to in the record as a hiatus. Unconformities are thus testimony to the fact that interactions between the internal processes of the Earth that form and deform rocks, and the external weathering processes, have been going on throughout the Earth's long history.
•There are three main categories of unconformities: disconformities, angular unconformities, and nonconformities.
Unconformities in a sequence of rock layers disclose a hiatus in sediment deposition.
•A disconformity refers to an erosion surface that is parallel to the layered strata above and below it. This formation would indicate either a pause in the regular sequence of sediment formation, or a period of erosion in which layers between those bordering the unconformity were eroded away. This latter case is usually what has occurred.
•An angular unconformity is an erosion surface lying on tilted/folded rocks underlying younger layers. Such a feature is the result of layers of older strata being deformed or tilted and then truncated by erosion before the younger layers were laid down on top.
•A nonconformity is an erosion surface cut into metamorphic or igneous rocks that has then been covered by sedimentary rocks. A nonconformity is essentially an erosional boundary between sediments and the underlying bedrock.
Correlation and the Fossil Record
•Correlation is the ability to demonstrate that rocks in different areas are the same age.
Rock sequence from the Grand Canyon, Zion and Bryce Canyon National Parks show a large portion of the geologic column. Though they are separated by hundreds of km, the sequences from each park are correlated with the other two.
•Correlation is based both on the principles above, and the principle of fossil succession, that is, that fossils succeed one another through time in a regular, predictable order.
•This method is crucial for use over great distances, where weathering processes have interrupted the sequence of units. Most useful for dating purposes are guide fossils, those fossils representing organisms that were widespread and only lived for a short time, and whose fossils are easily identifiable.
The geologic column
•The geologic column was constructed in the 19th century by scientists in Great Britain and Western Europe, using relative dating methods. Divided into manageable sections (eon, era, period, epoch), the geologic column lays out the known history of the Earth based upon the rock and fossil record. It translates the sequence of rock layers into a timetable.
•In the 20th century, with the discovery of radioactivity and radioactive decay, absolute ages began to be assigned to the sections of the column.
•Before the discovery of radioactive decay, all attempts to assign absolute ages to the geologic column failed, because they were based upon incorrect assumptions.
•Example: Attempts to date the Earth based upon sediment deposition assumed that the rate of deposition was constant. However, the rate of deposition of sediment fluctuates wildly based on many variables, including climate and weather patterns.
•Radioactive decay is a useful method for dating because it is constant regardless of conditions.
•Radioactive decay is based upon the fact that some atoms are unstable enough in their nucleii to spontaneously decay into more stable isotopes.
•Because the rate of decay is different for each isotope, we usually speak of an element or material's half-life. This is the length of time it takes half of the atoms in a material to decay from the parent element to the more stable daughter element.
•The nature of the decay rate is such that it works exponentially. That is, after one half-life, half the atoms are daughter atoms. After two half-lives, three-quarters of the atoms are daughter atoms, and so on. Each half-life sees the remaining amount of parent atoms halved.
•So in any closed system (that is, the radioactive element is not added or subtracted), measuring the number of parent and daughter atoms will reveal the time that material has existed. This time is based on the length of the half-life of each isotope.
•Some examples of atoms unstable enough to spontaneously decay into more stable atoms: U, Th, K, Rb, C14.
•The length of an isotope's half-life determines what sort of dating it is useful for.
•C14 has a half-life of 5730 years, + or - 30 years. Thus, this best-known dating method is actually only useful for the past 70,000 years or so (otherwise all the C14 atoms would be gone). It is crucial for archaeology, but less useful for geology.
•C14 is a component of the atmosphere. It's ratio is well-known, and is absorbed into living things at that constant ratio with C13 and C12. When those living organisms die, the C14 begins to decay into C12, starting the radioactivity clock.
•K40 is a naturally-occurring isotope that decays into the noble gas Ar40. The half-life of 1.3 billion years makes it useful for dating the minerals in many formations. Because argon does not bond well, it is commonly released at high temperatures. This means that this method of dating measures the amount of argon accumulated during the time since a mineral was cool and able to trap argon.
•Cross-checking the absolute date of a rock is important because it is not always possible to be assured of proper conditions for dating.
Example: The material to be dated may not have decayed in a closed system. It is important to know when the radioactivity clock started.
•Example: Sediments have radioactive elements, but dating these can only tell us the age of the source rock the sediments derived from, rather than the age of the sedimentary rock.
Earth's History: Dating the Earth
•The oldest rocks we have found on the Earth are 4.2 billion year old sediment fragments. This suggests that the source rocks for these fragments must have been even older.
•We can understand times previous to this one only by looking at the surface of a planet that does not have the effective weathering that Earth does. Luckily, we have a nearby planet for which the geologic record is nearly unaltered from the earliest times in the Solar System: the Moon.
The surface of the Moon is much older than that of the Earth. That is, the surface has not changed much since it was first formed. The Earth's surface, by contrast, is under constant change.
•The ancient surface of the Moon contains the geologic record for most of the history of the Solar System, unchanged. This makes the study of the Moon invaluable to geologists.
The Earth's history in a year
•Commonly, the geologic timescale is not shown according to scale, so we have a tendency to overestimate the length of time organisms with which we are familiar actually existed. If we encapsulate the entire history of the Earth into one year, we can begin to understand our place in the timeline of our planet.
•If the Earth is born on 1 January, we find that the first elemental life arises around 16 February. The first organisms able to leave behind fossils come into being around noon on 14 March.
•It was not until 14 November that the first shelled animals appear, and not until 21 November that we begin to see fish in the oceans.
•Land plants appear on 28 November, but the first amphibians don't make an appearance until the afternoon of 2 December and reptiles show up on 5 December.
•Around the 12th and 13th of December, the Appalachians form, the supercontinent of Pangaea has come together — and dinosaurs have begun to roam the Earth. They will live for about 180 million years — about two weeks of our year. Also around this time, the first mammals appear.
•Birds make their first appearance on the afternoon of the 19th of December. And about 66 million years ago, while the Rocky Mountains are forming, on Christmas, the dinosaurs and the majority of other species go extinct.
•The Alps and the Himalayas, geologically very young mountains, form around 27 December.
•Finally, at about 9pm on New Year's Eve, the first human ancestors appear. Modern humans make their first appearance on Earth in the last 72 seconds of the year.