Measuring Geologic Time

MEASURING GEOLOGIC TIME

•         What is geologic time?
•         What does the principle of uniform processes say?
•           What is relative time?
•           What is the principle of superposition?
•           How do fossils help scientists place events in order.
•          What does a geologic coloumn show?
•           How is radioactivity used to find the ages of rocks?
•          What are the limitations of using carbon-14 for finding the ages of rocks?
•          How are varves used to find the ages of some rocks?

              A. Geologic Time

1.      Geologic Time and The Rock Record

What do you consider a long time? Compared to your lifetime, you might consider one year a long time. It you you were to sit and stare at a clock, one minute might seem like a long time. Your conception of time depends on what you compare it to. Geologists estimate that the Colorado River took about 10 million years to carve the Grand Canyon. Compared to that amount of time, one year is very short and significant.

When thingking about geologic events, such as the development of a canyon, scientists compare the time span of the event to geologic time—all the time that hass passed since the origin of the earth. Geologists estimate geologic time to be about 4.6 billion years. Compare to this number of years, even the carving of the Grand Canyon has occured in a short time.

No written records exist of the events that took place throughtout earth’s long history—the mountains that were lifted up and worn down, the seas that swept over continents and withdrew, the magma that formed volcanoes. These events, however, left records in the rocks of the earth’s crust. Geologists are able to read rock layers, such as those exposed at the Grand Canyon, as if they were pages in a took of the earth’s history. The rocks not only provide evidence of what happened, but also the order in which events took place.

2.      The Principle of Uniform Processes

Geologists base their interpretations of the earth’s past on a very simple and important assumption: that the same processes acting on the earth today have acted on the earth througouth its history. This idea is known as the principle of uniform processes. It states that that the laws of physics and chemistry have not changed through time. Therefore, the processes at work today on the earth have always been acting, although not necessarily at the same rates that they do now.

Today running water carries away particles of rock and soil in the process called erosion. You can observe erosion as rain washes soil from lawns on to sidewalks and as rainwater carves paths in mounds of soil, shown in figure 2. The principle of uniform processes states that erosion has always occured because the physical and chemical laws involved in erosion have always been operation. However, rates of erosion might have been different in the past. For example, if more vegetation was present in a certain area, erosion might have been slower because plant roots tend to hold soil in place.

             B.  Relative Time

1.      Relative Time: Order of Events Without Dates

When  you study history, understanding the order of events is sometimes more important then simple memorizing the exact dates of events. When studying the American Revolution, for example, you may not be required to know all the dates of all the events. But you should keep in mind the order of events that led to the Revolutionary War, such as the Stamp Act and Boston Massacre, and the events that resulted from the war, such as the Treaty of Paris and the Constitutional Convention.

In a similar way, geologists may not always be able to tell the excact dates of events, but they can tell the order of events. Geologists first try to place events in relative time—the order in which events occured without actual dates.

Figure 3 represents one of the challenges of placing geologic events in relative time. Each letter on the drawing represents an events, such as the information of rock beneath a sea, the intrusion of magma through a crack, or the erosion of surface rock. Once these events are recognized, a geologists attempts to place them in order from oldest to youngest. Several basic principles are used for peforming this task. After reading about them, you may be able to place the events represented in the drawing in relative time yourself.

2.      The Principle of Superposition

When you unpack a suitcase, you can be fairly sure that the bottom layer of clothes was put in before the layers of clothes above it. This simple observation is similar into a basic principle that helps scientists determine the order of geologic events.

Notice the many horizontal layers in the rock formation shown in Figure 4a. Each layer represents sediment that settledand formed into rock. The oldest rock layer is at the bottom, just as the layer of clothes that first goes into a suitcase is at the bottom. The principle of superposition states that if a series of sedimentary rock layers has not been overturned, the oldest layer is always on the bottom and the youngest layer is always on the top.

Rocks that have been steeply tilted or overturned no longer show such as a simple sequence. Notice the folded and overturned rock layers in Figure 4b. Because the fold are fairly complete, you can perhaps imagine flattening the rock layers to see clearly which ones formed first, second, and so on. But what if the rocks between the dotted lines were exposed? Would every layer be older than the one above it? To determine the order of events for rock layers that have been distrubed, geologists have to examine other features. For example, they may look for mud cracks. The same kind of cracks you may have seen in dried soil have been preserved in some rock layers. Mud cracks that form in drying sediment are wider at the top than at the bottom, so geologists compare the width of a mud crack through a rock layer to determine the true top of the layer.

3.      Fossils and Constructting Relationships

About 175 years ago, an English engineer named William Smith wa studying rock formations in southern England. He noticed that different rock layers contained different fossils. Figure 5 is based on Smith’s original drawing of some of the fossils and rocks he found. As he traveled throughtout England, he found that the same layers of rock contained similar fossils. By observing the fossils, Smith was able to identify most rock found anywhere in England and determine its relative age.

From the observations of Smith and other scientists came the principle of faunal succesion. Fauna revers to the animal life of a certain region or time. This principle states that, in a series of rock layers containing fossils, the fossils in the bottom layer are oldest, asuming the layers have not been overtuned. Each fossils bearing rock layer contains a unique assemblage, or group, of fossils. Rock layers that contain similar assemblages of fossils formed at about the same time.

From the work  of Smith and others also came the use of index fossils. An index fossil is a fossil that can be used to establish the age of the rocks that contain it. Such fossils are from organisms that were abundant and that lived for a geologically short period of time over a large part of the earth. Therefore, wherever a certain kind of index fossil is found, the rocks in which it is found formed at about the same time. Notice in Figure 6 that index fossils can be used to match rock layers from different areas, sometimes across continents.

4.      Geologic Coloumns

During the early 1800s, scientists used index fossils and the principles of superposition, faunal succession, and crosscutting relationships to learn the relatives ages of rock layers. They made the first accurate geologic maps, which were useful in planning canals, mines, and quarries. The scientists matched uo rock layers within regions, making relative time scales, called geologic coloumns. A geologic coloumn is a diagram of the rock layers in an area arranged in order of age. As shown in Figure 8, informationfrom the exposed rock within a region is brought together in a geologic coloumn.

It is important to remember that a geologic coloumn for an area may not have rocks representing all geologic time. Some rocks may have totally eroded away and others may not have formed during a prticular time.

In the 1800s, geologic coloumns could show only relative ages. As you will learn, scientists today can give actual dates to the coloumns.

            C.     Absolute Time

1.      Radioavtive Elements and Absolute Time

When scientist first began constructing geologic columns, they could only tell if a rock was older or younger than another. Today, geologists have methods for learning the actual ages of rocks or tell when geological event took place in absolute time—time based on years. The metohds used most often for calculating a rock’s absolute age are based on the rate of decay or radioactive elements in the rocks.

Rocks are made of minerals and minerals are made of atoms. Some minerals contain forms of elements, known as isotopes, whose atoms change over time. These isotopes break apart, or decay, and form other elements or other isotopes of the same element. The length of time it takes for half the atoms of a radioactive isotope to decay is its half-time. Form the work of others, scientists today know the half-lives of certain isotopes. To determine the absolute age of a rock, they measure the radio of different isotopes remaining in the rock in a method called radiometric datting. The decaying elements thus function as clocks inside the rocks, ticking off the years since the rocks formed.

A mass spectrometer is one instrument used to measure the decay rate of certain radioactive isotopes. The photograph shows a mass spectrometer at the California Institue of Technology. Even microscopic pieces of rock can be accurately dated with this sensitive instrument

Many radioactive isotopes exist, but only a few are helpful in figuring out the absolute ages of rocks. An isotope of the absolute ages of rocks. An isotope of the element uranium, uranium-238 (²³⁸U), is used to determine the ages of very old rocks.

Scientists have learned that 4.5 billion years must pass before half of a given amount of ²³⁸U decays into an isotope of lead, lead-206 (²⁰⁶Pb). Therefore, the half-life of ²³⁸U is 4.5 billion years. Lead-206 is never present when when rocks form. If a rocks sample contains as much ²⁰⁶Pb as ²³⁸U, then half the ²³⁸U must have changed to this form of lead. Therefore, the rock is 4.5 billion years old.

The diagram shows how the amounts of ²³⁸Uand ²⁰⁶Pb change over time. After 4.5 billion years, or one half-life, half of the ²³⁸U has decayed to ²⁰⁶Pb. The ratio of ²³⁸U to ²⁰⁶Pb is 50 : 50. After 9 billion years, or two half-lives, half of the remaining ²³⁸U has again decayed to ²⁰⁶Pb. The ratio of ²³⁸U to ²⁰⁶Pb is now 25 : 75. This decay process continues regardless of any pressure or temperature changes in the rocks.

Radioactive uranium is used to date rocks that are many millions or billions of years old. To date rocks younger than two million years old, potassium-40 (⁴⁰K) is measured. Potassium-40 has a half-life of 1.3 billion years. It decays into argon-40 (⁴⁰Ar). Some sandstone and a few shales contain a mineral that contains ⁴⁰K. By comparing the amounts of ⁴⁰K and ⁴⁰Ar in a sedimentary rock sample, scientists can determine the age of the rock.

2.      Finding Absolute Age with Carbon-1

Radioactive carbon is used to find the absolute ages of materials that were once alive or part of living things, such as wood, bones, and shells. Carbon-14 (¹⁴C) decays into nitrogen-14 (¹⁴N). Carbon-14 has a half-life of 5730 years, which means that half the ¹⁴C will decay to ¹⁴N in 5730 years.

All living things take in carbon as long as they live.  Only a small percentage of the carbon is ¹⁴C. The ratio of ¹⁴C to other carbon isotopes stays the same while the organism is alive. When death occurs, ¹⁴C starts decaying and its percentage goes down. By comparing the amount of ¹⁴C to the rest of the carbon in dead organic matter, scientists can determine how long ago the organism died.

The drawing illustrates the rate of ¹⁴C decay. Notice that after each half-life, or 5730 years, the amount of ¹⁴C has decreased by one-half. The following example shows how this rate is used to date certain objects and events. Suppose scientists find a pine log buried in volcanic ash. They determine the percentage of ¹⁴C in the log to be only one-fourth as much as living pine trees. As a result,  they conclude that exactly two half-lives have gone by, which means that the pine tree was burried by ash from a volcanic eruption 11.460 years ago.

Samples older than about 40.000 years contain very little ¹⁴C. For that reason, the ¹⁴C method could not be used, until recently, to find absolute ages older than 40.000 years. But scientists now can use a machine, called a particle accelerator, to help date older fossils. Using this instrument, scientists can measure precisely the number of ¹⁴C atoms in organic matter. Fossils as old as 70.000 years can now be dated by the ¹⁴C method.

3.      Using Varves to Calculate Time

Unique sequences of sediment form in takes that freeze in winter. This sediment, called a varve, consists of two layers. A layer of fine-grained, dark-colored clay forms in the winter because the calm water allows the fine grains to settle. In the summer, a layer of sligthly larger, light-colored particles forms because only these particles in the water due to currents that are caused by winds blowing across the surface. Every year a pair of such layers is depodited in the lake.

The varves shown in Figure 1 are in Sweden. Obseve how each varve tends to grade from the light brown summer deposits to the dark grey winter deposits. Each of these varves is about 4-7 centimeters thick and, together, represent about 10 years of deposits.

Varves are useful clocks of absolute time. But they show only the period of time during which sediments were deposited, not how long ago the were deposited.