Dating Events in Earth History
The History of "Time"
The influence of the bible
Bishop Ussher. (Ireland; 1664) Bishop Ussher Calculated the age of the earth based on biblical history by counting up time in patriarchal genealogies. He gave a date and time of the creation: 9:30 AM October 29, 4004 BC. For many in the western world, this and similar chronologies continues to be the standard accepted history of the earth
The Discovery of Deep Time
James Hutton (1726-1797, Scottland) Expounded the principle of uniformitarianism which, at the time, meant that present day processes are the same processes that occurred in the past. Based on the assumption of uniformitarianism, even the largest geologic features, like mountains or canyons, are explained by slow processes that act over vast time spans.
Perhaps the most profound illustration of hutton's thinking is found in Hadrian's Wall--a Roman wall built around 200 A.D. that still stands in Scottland. In Hutton's day, this wall was nearly 1,600 years old, and the earth was thought to be 5,800 years old. Hutton observed that the wall had been but scarcely touched by erosion, while some nearby, much older volcanoes had been nearly levelled by the same processes. In other words, the volcanoes lost thousands of times more material to erosion. In this light, it did not seem reasonable that the volcanoes were a mere 5,800 years old (3.6 times as old as the wall). It was more likely that they were millions of years old, under the assumption of uniformitarianism.
Uniformitarianism is an assumption about the earth that is still one of the most important foundations of modern geology. However, the concept has changed since Hutton's day. Now, instead of saying that "the present is the key to the past" we say that the physical laws that govern geological processes today have not changed. The subtle difference is that the modern view accepts catastrophic events (as long as they are "natural"), while the older view seems to discount the possibility of cataclysmic events--such as huge rocks falling from the sky and wiping out all life. Uniformitarianism still counts an appeal to the supernatural as "cheating", when one attempts to explain the history of the earth.
Using his principle of uniformitarianism when looking at an unconformity, Hutton concluded that the earth must be ancient. An unconformity is a zone in the sedimentary rocks where there is evidence that sedimentation stopped. The figure below shows an unconformity that formed from the following sequence of events:
1. lower layers (yellow, white, brown, light blue, red, yellow, purple and dark blue) were deposited as flat sediments on the ocean floor. These sediments may represent thousands of feet.
2. lower layers were converted from sediment to rock
3. lower layers were folded by horizontal compression during mountain building events
4. lower layers were eroded--again-thousands of feet of erosion.
5. upper layers (pink, green, light brown, gray) were deposited in the ocean.
6. Upper layers were hardened to form solid rock.
Considering the age of Hadrian's Wall the time that it must have taken for the unconformity to form was truly vast--in the millions of years, and when the entire sequence of such unconformities is condidered, then the age of the earth must be measurable in billions of years.
This discovery or deep time, such a blatant contrast to the accepted chronology, may be geology's greatest contribution to human thought. Like the discovery that the earth is not the center of the universe, it was bothersome in that it called into question the central place of the human race in the scheme of things, relegating the entire of human history to the most recent tiny fraction of the history of the earth.
Charles Lyell (1797-1875) Published "principles of geology" textbook (1833), wherein he espoused the principle of uniformitarianism. Geologists accepted this idea, and, having accepted uniformitarianism, they generally agreed that vast amounts of time were needed to explain the earth's features and particularly the sedimentary record.
William Smith (1769-1839), a British surveyor who worked in southern England, discovered by looking at several similar sedimentary rock beds that they could be distinguished by their different fossils. each of the layers contained a different suite of fossils and the fossils always occurred in the same order from bottom to top. He was able to use this observation to create the first geologic maps that reflected the ages of rocks exposed in southern England. This principle of faunal succession was extended.
Charles Darwin (1809-1882) Read Lyell's text and accepted the principle of uniformitarianism. He used the ancient age of the earth to accommodate the evolution of species.
Development of the Geologic Time Scale
Many researchers contributed to the development of the geologic time scale which was developed using the relative dating principles discussed below. Relative dating utilizes a number of relatively simple concepts to arrange geologic events in a sequence, but does not arrive at an age for rocks in years. Using these relative dating techniques, geologists were able to divide the history of the earth into a number of time periods called eons, periods, and epochs. Each of these time periods is separated from others by differing fossil content, and bounding unconformities, but the actual ages or duration of the periods were completely unknown. This time scale is still the most precise (absolute ages have a range of error that can slop across entire epochs) way to look at geologic time, and representations of the time scale can be seen by clicking on the links below--these time scales do have ages in years marked on them, but keep in mind that the names of the periods were developed before anyone had a method of determining how old any of the rocks were in years.
Below are the major eras and periods of the geologic time scale along with some of the biological developments with which they are most closely associated:
The Precambrian makes up the bulk of geologic time on earth and is divided into 2 major eras:
Paleozoic era: "old life"
Mesozoic era: "middle life"--the age of dinosaurs
The Quest for an Absolute Measure of Age
There was also a long history of attempts to obtain accurate absolute ages for events in earth history. These involved an estimate of how long it took for the oceans to gain there present saltiness, estimates of how long it takes to accumulate a certain amount of sediment, etc. Many of these attempts were easy to find problems with, and some absolute dating methods are discussed in more detail below, but one calculation in particular caused a great deal of problems for geologists.
Lord Kelvin (1824-1907) used the cooling of the Earth as his "hourglass". He assumed originally molten state, and that there are no additional heat sources other than the initial heat of formation of the planet. Thus, taking the temperature of molting for rocks, the size of the earth, and the current rate at which the earth loses heat, he was able to calculate that the earth took 20 m.y, to 100 m.y. to cool from its original molten state. Kelvin claimed to have overthrown uniformitarianism. Lord Kelvin was highly respected as a physicist, and his neat calculations based on accepted physical principles and clearly measurable parameters shut down discussion of the age of the earth. Geologists, whose evidence for a much older earth was based on numerous but less tangible facts, had a difficult arguing their case, and few continued to fight a losing battle. Other geologists and biologists struggled to fit everything into a few million years.
Henri Becquerel (1852-1908) discovered radioactivity
Lord Rutherford (1871-1937) Concluded that the radioactive elements found in the earth were sufficient to have kept the earth from ever cooling down at all, thus invalidating one of the fundamental assumptions of Kelvin's calculations: that there are no additional heat sources within the earth. this left open the possibility of a very ancient earth.
Rutherford also proposed that radioactive decay provides an excellent way to date events in earth history--as we will explain below--and he and other physicists began obtaining radiometric dates which fell much more in line with geologists' expectations
How Rocks and Geologic Events are Actually Age-Dated
A geologic event is anything that has happened in the history of the earth that is recorded in rocks. Examples include the eruptions of volcanoes which leave lava flows and ash deposits, as well as a rise or fall in sea level which lead to sediments being deposited or eroded respectively. Dating these events becomes a problem of dating the rock bodies that they leave behind. There are are actually three approaches used together to determine the age of events in earth history. these are: relative dating, correlation, and absolute dating. Each of these is explained below.
Relative age dating or rocks: foundation principles.
There are three simple common-sense principles used to determine which rocks are older and which rocks are younger. These principles are powerful tools with very high temporal resolution--that is to say, you can conceivably distinguish events that occur seconds apart. But you can't actually use these principles to tell how much time actually elapses between events--seconds or eons.
The three principles are the principle of superposition, the principle of cross-cutting relationships, and the principle of inclusion.
The principle of superposition states that the rock layers on the bottom are older than the rock layers on top. In everyday experience, you know that if you dig through the pile of cloths in the corner of the room, that the cloths on bottom of the pile were put there first. The same reasoning applies to a pile of rocks that originally accumulated as sediments.
The principle of cross cutting relationships states simply that a rock is older than anything that cuts it or that it is older than any fracture found within it. Something has to exist before it is cut or broken.
The principle of inclusions states that sedimentary particles are older than the pile of sediment, or fragments included in an igneous rock are older than the rocks that include them. A more familiar illustration: the chocolate chips found in the cookies had to exist before the cookies were baked.
Principles and Tools of Correlation
Geologic correlation is the process of matching rocks of the same or similar ages that are not necessarily connected to each other physically. The simplest illustrations of correlation are ones that involve correlating sedimentary rock layers.
The principle of original horizontality states that sedimentary layers (beds) are laid down nearly flat; if they are not flat, they were later disrupted by folding or faulting. This principle is used to reconstruct beds to their original flat lying positions and will be discussed further as we discuss faulting and the origin of earthquakes.
The principle of lateral continuity states that sedimentary layers are laterally continuous, so you can connect beds across gaps, such as between canyon walls, or across areas where the rocks are unknown, such as where rock layers crop out in one area, go underground, then crop out in another area. If there is some characteristic of the layers that indicates that they are the same bed, then it can be assumed that the beds were once connected.
Getting around the Facies problem
The problem with sedimentary rocks is that, though beds themselves may be laterally traceable or continuous, their composition may change over long distances.
Picture a beach (not hard eh?) now picture what happens to the sand as you go out into deeper water...it turns to mud. Here we have sand and mud that are deposited at the same time. Now if we picture these layers turned to stone--we might not think to correlate a sandstone in one area with a mudstone in another area.
The fact that the same bed of rock can have a different characteristic in different places is recognized by the concept of facies. In our example, we move from the sandstone facies to the mudstone facies. Facies change is a significant problem with using the principle of lateral continuity to trace from one area to another. How can you tell that two sedimentary beds are the same age?
There are two ways around the problem: the principle of faunal succession, and the use of discrete event beds which can be correlated through several facies. While faunal succession is more generally useful, event beds give the highest precision of correlations
The principle of faunal succession states that each species has a time of origination and a time of extinction, and because of this, different times in earth history are characterized by different fauna and flora. Therefore, if sedimentary rocks from different places contain the same fossils, then they were deposited at the same time.
Event Beds are layers of rock formed from very distinctive time-limited geologic events such as volcanic ash falls, fallout from meteorite or comet impact, beds disrupted by major hurricanes, beds that form from disruption and turbidity (muddy water) flows that are triggered by earthquakes.
Of these, ashfall beds and impact fallout beds, have proven the most useful. This is because these events are relatively rare, the deposits cover large areas, and the different deposits can be distinguished from each other based on chemistry. Also, each occurs in a matter of hours or days, so the correlations are incredibly precise. The most famous example is the iridium layer found at the boundary between the Cretaceous and Tertiary periods around the world which apparently resulted from the impact of an asteroid which is implicated in the extinction of the dinosaurs. Ashfall deposits, called bentonites, have recently been used to make extremely fine scale correlations in the Ordovician period between the eastern US and Northern Europe.
The (relative) geological time scale (better, see book, page 10).
Absolute dating differs from relative dating in that it yields an actual number of years, not just the knowledge that one thing is older or younger than another.
The method currently used to derive absolute (numeric) ages of rocks, particularly volcanic and other igneous rocks, is Radiometric Dating. Radiometric dating is based on the constant rate of decay of radioactive materials. Radioactive materials are isotopes that are unstable; the radioactivity refers to the particles and energy released by these materials as they revert to stable isotopes.
So what is an Isotope?
To explain the concept of isotope we must explain the concept of an atom; atoms are made up of protons, neutrons, and electrons. Protons and neutrons are bound tightly in the center of the atom as the nucleus, and electrons swarm in a cloud around the nucleus. The number of Protons is called the atomic number and determines the identity of the element; the number of neutrons determines the isotope.
Protons and neutrons each have the same mass which is 1 atomic mass unit. The atomic mass of an atom is the sum of the number of protons and the number of neutrons. This number is expressed as a superscript to the atomic symbol. For example, C12, pronounced "carbon twelve"contains 6 protons like all carbon atoms, and it contains 6 neutrons. C14 (carbon fourteen) also contains 6 protons (which is what makes it carbon), as well as 8 neutrons.
There may be several isotopes of the same element. Some of these isotopes are stable while others are unstable or radioactive. Radioactive isotopes emit nuclear radiation in the form of rapidly moving particles or high energy electromagnetic waves. The particles are emitted from the nucleus itself and their removal results in changing the atom from one isotope to another. This change may occur once or emission of particles may continue until the atom becomes a stable isotope.
The radioactive starting isotope is termed a parent isotope, while the resulting stable isotope is termed a daughter isotope. The change from radioactive parent to stable daughter occurs randomly, with any one atom of the unstable isotope subject to a certain probability of decaying in a given time period. This means that in that same period of time, a certain percentage of a group of atoms of unstable isotope will decay. This is similar to the way money in a bank account earns interest: since it is a percentage change (like interest), the actual number of atoms that decay depends on the number of unstable atoms that are present, just as the number of dollars your bank account earns depends on the number of dollars that are in the bank account.
However, while the rate at which your bank account earns interest is expressed as a percentage per year (Annual Percentage Rate), the decay rate for radioactive materials is expressed as amount of time that it takes for 50% of the material to decay--a half life (you can express interest rates as "doubling time" as well--instructive in itself). Half lives range from fractions of a second to billions of years. After 1 half life, half the original material remains, after 2 half lives 1/4 remains, after 3 half lives, 1/8 remains, etc.
Using half life to determine age
The rate of decay of a given radioactive isotope is constant. This provides a clock by which rocks can be dated. Materials tend to crystallize in a pure form, so when the crystals of a rock form, they incorporate certain elements and do not incorporate others. For this reason, minerals that incorporate a radioactive isotope tend not to incorporate the stable daughter isotope. Any stable daughter isotope found in a crystal is likely to be the result of decay after the crystal was formed.
In order to determine the age of a rock, you need to know the percentage of the original parent material that is still radioactive. Since all the atoms were once parent isotope, one need simply determine the amount of atoms of the parent and of the daughter isotope. The total is the number of original parent material. The ratio of parent to this total is the percentage of the original material that remains radioactive. From this you can determine the number of half lives that have passed since crystallization. From the number of half lives that have passed, you can determine the age of the rock in years.
Incorporating Radiometric Dates.
So how old is the earth?
A granite in South Africa is 3.5 b.y. old
A granite in Greenland is 3.7 b.y. old
Some metamorphic rocks in Minnesota are 3.7 b.y. old
In western Australia there is a conglomerate that contains crystals of zircon (inclusions) which are 4.1 to 4.2 b.y. old
Most meteorites are about 4.5 b.y. old
The oldest moon rocks are about 4.53 b.y. old
the helium/hydrogen ratio of the sun suggests its age to be about 4.6 b.y. (based on the physics of star formation)
Since the earth is thought to have accreted about the same time that the sun was formed, the earth must be about 4.6 billion years old. It is presumed that most of the original surface of the earth has been remelted and recycled, leaving only younger rocks to be dated.
The Magnitude of Geologic time.
As an illustration of how long a time the age of the earth is, consider this: If a year is represented by 1 mm, then 100 years is represented by 10 centimeters, and 1,000 years is represented by a meter, which is slightly bigger than a yard (the width of a sidewalk). Historic records go back no later than about 6,000 years, taking our measure out to 6 meters--beyond this there is good evidence that settled civilizations were in place about 10,000 years ago. This is represented by 10 meters, about the width of a street. The Ice age began about 2,000,000 years ago. This is represented by a distance of 2 km, which is about 1 1/4 mile. Now, the entire age of the earth is 4.6 billion years, which is 4,600,000,000 years, represented by 4,600,000 meters, or 4,600 km (=2,858 miles). This is approximately the distance from Los Angeles to New York City. Thus the entire time span of human civilization compared to the age of the earth, is like the width of a street compared to the width of a continent. If the age of the earth were represented by the length of a football field, then the entire span of human civilization would be represented by the thickness of a blade of grass (that's the short way through the blade--not the long way across the blade).
This should give you a feeling for how profound and unsettling the concept of geologic time was when Hutton introduced it to Europe, not so long ago (about 2 centuries).