OBJECTIVES

· To learn to read time information from rocks and landscapes

· To understand how absolute ages can be assigned to events in Earth's distant past

CONCEPTS/VOCABULARY

· Uniformitarianism · Unconformity · Radioactive isotope

· Correlation · Index fossil · Half life

BACKGROUND

How do we know anything about the past? You carry information about the past in your memory, but that information includes only your own experiences. Written documents -- from newspapers to cave-drawings -- preserve information about the past and give us glimpses into times and places we did not experience ourselves. Still, these documents record only events that were observed by humans.

Making inferences about events that occurred without human witnesses is the central theme of Geology. Earth's landscapes, rocks, waters and lifeforms all reflect events that occurred at times in the past. They are "documents" containing encoded information about earlier times. Decoding these documents is the key to reconstructing the past. As a starting point, let us assume that the same natural processes and physical laws that shape Earth's surface now also operated in the past. This simple idea is called uniformitarianism and can be summarized by the phrase "the present is the key to the past". While the concept may seem obvious today, it was revolutionary when introduced in the late 1700's. Until then, the prevailing view was that Earth's surface reflected cataclysmic processes that had ceased to occur. A uniformitarian point of view is the first step in reading Earth history from natural features. With an understanding of the modern Earth, it is possible to make inferences about the ancient Earth.

EXERCISES

A. Relative timing of geologic events

1. In the field section of the lab, you learned how the principles of superposition, inclusion and cross-cutting relationships can be used to infer the relative sequence of geologic events in the Keweenaw. Use the same principles to determine the sequence of geologic events recorded in the rocks exposed in the Grand Canyon . At least 7 distinct geologic events can be read from the cross section of the Canyon below. Explain which timing rule(s) you use and note cases in which the relative ages cannot be resolved.

· Cutting of the canyon itself (G)

2. Examine the specimens representing rock types exposed in the Grand Canyon as well as the geologic map of the Canyon. How do the outcrop patterns of the flat-lying rocks in sedimentary sequence A differ from those of the older rocks?

B. Correlation of fragmentary records

You have now applied rules for determining the relative ages of geologic events to rocks in both the Keweenaw and the Grand Canyon. But how are these local sequences of events related to each other? Understanding Earth history on a global scale requires correlation of rocks from place to place.

1. Unconformities . One difficulty in correlation is that no single place on Earth -- not even the Grand Canyon -- has a complete, uninterrupted record of all of geologic time. Instead, there are gaps representing periods when erosion, or at least no deposition, occurred. Such a gap in the sedimentary record is called an unconformity and can be likened to pages missing from an historical document.

a. Identify and label at least one major unconformity depicted in the cross section of the Grand Canyon. Explain why this surface must represent a long period of erosion. (What events must have taken place in the time between the formation of the rocks below the unconformity and those above it?)

b. On another sheet of paper, draw a simple but clearly labelled cross section of the geology of the Keweenaw peninsula (perpendicular to strike) and identify any unconformities. In what types of environments did the rocks or sediments above and below the unconformity form? Can you estimate the amount of erosion or length of time represented by the unconformity(ies)? Explain.

2. Index fossils. Although the record at any one place is incomplete, fossils make it possible to recognize overlaps in the records at different places. By the mid-1800s, the study of fossils (paleontology) had shown that there is a globally consistent order in the appearance and disappearance of various lifeforms in the sedimentary record. With fossils acting as global "page numbers", the geologic records from different places can be ordered and assigned to time intervals recognizable worldwide.

The most useful fossils for correlation are those of plant and animal species that lived for only a geologically brief period of time. Such fossils, called index fossils, are thus diagnostic of a particular time in the geologic past. Species that changed little through time do not make useful index fossils.

a. Consider a collection of cast-off objects from this century and imagine them as fossils. List one item that would make a good "index fossil" for a particular decade in the 1900's. Conversely, list one item that were available during that decade but would not make a good index fossil. Explain.

2. By the late 1800s, careful correlation of fossils in sedimentary sequences from around the world had made it possible to create a global geologic time scale. The absolute ages of geologic periods were not known, however. This exercise is a simple illustration of how the fossil-based time-scale was developed.

Columns A-D in the figure below depict fossil-bearing sedimentary sequences at 4 sites. The rock records at each site are continuous except at the zig-zag lines, which represent unconformities. All of the fossils can be considered index fossils. By comparing the sequences, determine the relative ages of the index fossils and enter them in the proper 'global' order in the time-scale at right (1 = oldest).

The nineteenth century geologic time scale

The main divisions of the geologic time scale (Table 1) were based on the known succession of fossil life forms and had been established by the end of the last century. Many of the divisions were defined on the basis of mass extinctions in the fossil record. The end of the Mesozoic Era, for example, was defined as the time of simultaneous extinction of the dinosaurs and many other species.

The fossil-based time scale provides a distorted view of geologic time, however. As we will see in the next section, the tme interval designated as `Precambrian' represents 7/8 of the history of the Earth, while all of the other subdivisions represent only the last 1/8.

3. Find out where the bedrock and surface sediments of the Keweenaw fit into the global chronology and indicate their ages on the time scale above.

C. Determining absolute ages of events: Radioactive isotopes as natural clocks

Fossils make it possible to establish worldwide geologic time divisions, but indicate nothing about how long ago these time periods occurred or how long they lasted. The key to determining the absolute ages of geologic events is to find processes that occur at known rates and leave behind measurable products that accumulate through time. By measuring the amount of the product, it is possible to estimate the time elapsed since the process began. Tree rings are a familiar example of this: Because one ring forms each year, the age of a tree can be determined simply by counting its rings. Layers in glacial ice provide similar annual records. Unfortunately, direct time-records like tree rings and glacial layering are useful only for the relatively recent past. How can we determine the ages of older events?

Like the growth of trees or the accumulation of snow on glaciers, radioactive decay is a natural process that occurs at a known rate, creates measurable products, and can be used to date geologic materials and events. To understand natural radioactivity, we need a quick review of chemistry.

All matter consists of elements or combinations of elements. The smallest subdivision of an element that still has properties of that element is an atom. Atoms consist of a central nucleus, containing protons and neutrons, and an outer cloud of orbiting electrons. The number of protons in the nucleus of the atom -- the atomic number -- is the characteristic that gives an element its identity. For example, all oxygen atoms have 8 protons; all gold atoms have 79 protons. In contrast, the number of neutrons in atoms of some elements can vary, and the various forms of such elements are called isotopes. Some isotopes are stable and remain unchanged through time. Others are unstable and tend to break down, or decay over time, by giving off subatomic particles and energy. Some elements that can occur as unstable isotopes are carbon (C), potassium (K) and uranium (U).

So how can unstable, or radioactive, isotopes be used to determine the ages of rocks? In the same way that the number of annual rings gives the age of a tree, the key is to know the rate at which a process occurs, measure its resultant product and calculate how long the process was going on. However, the process of radioactive decay is rather different from the yearly growth of trees. Atoms of unstable isotopes decay in numbers that are a constant proportion of the unstable atoms present, so the number of atoms that decay in a particular time interval is proportional to the number remaining.

though his reserve would grow very small. Conversely, the amount of money his daughter held would always increase, though the amount she received each day would grow smaller and smaller. At any time, an observer could determine how many days the parent had been giving money to his daughter by comparing the amounts held by each.

1. What is the ratio of the amount of money held by the daughter to the amount held by the parent:

Daughter's $/Parent's $

At the beginning:

At the end of the 1st day:

At the end of the 2nd day:

At the end of the 3rd day:

2. If the parent started with twice as much money (16 coins instead of 8), what would be the ratios of the daughter's to the parent's money each day? Find out by completing the following table.

Amount held Amount held Daughter's $/

by parent by daughter Parent's $

At the beginning:

At the end of the 1st day:

At the end of the 2nd day:

At the end of the 3rd day:

3. Would the ratio of the amounts held by the daughter and parent on any given day be different if the parent originally held four times as much money? Ten times as much money? Explain.

The analogy of the parent and daughter illustrates the essential principles behind isotopic dating. The parent gave his daughter half his money each day; similarly, every radioactive isotope has a characteristic half-life -- the length of time for half of the original or parent atoms to decay to daughter atoms. The half-lives of some geologically useful isotopes are listed in Table 2. The number after each element name is the isotope's atomic mass number (sum of the number of protons and neutrons in the nucleus).

As you showed above, the ratio of the amount of accumulated daughter isotopes to the remaining parent isotopes does not depend on the original amount of parent material. So comparison of the relative amounts of daughter and parent isotopes allows determination of time elapsed since the parent isotope began to decay. In the analogy of the parent and daughter, the "decay" process began when the parent began to give his daughter money. For most geologically useful isotopes, like the potassium-argon (K-Ar) and uranium-lead (U-Pb) systems, the starting point is the time at which igneous or metamorphic rocks cooled to a particular temperature. For carbon-14 (¹⁴ C), the isotopic "clock" is begun when an organism dies and ceases to take atmospheric carbon into its tissues.

4. What was the "half-life" in the parent-daughter analogy? Explain.

5. Uranium-lead analysis of an igneous rock shows that the ratio of daughter ²⁰⁷Pb to parent ²³⁵U isotopes is 7. How many years have elapsed since the rock crystallized? Explain.

In another igneous rock, the ²⁰⁷Pb /²³⁵U ratio is 15. How many half-lives have elapsed since it crystallized? How many years does this represent?

In a third igneous rock, the ²⁰⁷Pb /²³⁵U ratio is 11. About how many half-lives have elapsed since it crystallized? How many years does this represent? Hint: Make a plot of daughter/parent ratio vs. time.

6. How would the apparent isotopic age of an igneous rock be affected if some of the daughter material had escaped from the rock? (This can happen if a rock is reheated during metamorphism). Explain and/or provide a simple mathematical illustration to justify your answer.

7. Speculate about why it may be difficult to obtain a meaningful isotopic age from a sedimentary rock.

8. Organic materials (bone, wood etc.) older than about 70,000 years cannot be dated accurately using the ¹⁴ C method. Why do you think this is so?

9. Ocean water can be dated by the ¹⁴ C method, and deep ocean waters commonly yield ¹⁴C ages of >3000 years. Why do you think it is possible to get a carbon date from seawater and what is the signficance of the age determined?

D. The modern geologic time scale

Gradually, using isotopic age determinations together with correlation and inferences about relative ages, absolute ages have been assigned to the fossil-based geologic time scale:

EON ERA PERIOD EPOCH ABSOLUTE AGE

Phanerozoic Cenozoic Quaternary Holocene Began 10,000 yrs

before present

Pleistocene

1.64 million yrs

Tertiary

65 million yrs

Mesozoic Cretaceous

145 million yrs

Jurassic

208 million yrs

Triassic

245 million yrs

Paleozoic Permian

290 million yrs

Carboniferous

362 million yrs

Devonian

409 million yrs

Silurian

439 million yrs

Ordovician

510 million yrs

Cambrian

544 million yrs

Pre- Proterozoic

cambrian 2.50 billion yrs

Archean

Origin of Earth 4.55 billion yrs

1. Integrating relative and absolute ages

Isotopic ages for rocks of the Grand Canyon are shown in the figure below (my=millions of years). Use these to answer the following questions about the geologic history of the region.

c. What is the range of possible absolute ages of the layer containing trilobites? How did you infer this? During which geologic period was the layer probably deposited? Explain your logic.

2. Age of the Earth. From the beginning, Earth's surface has been dynamic and changing. The face of the early Earth was so mobile, in fact, that no rocks of the original crust survive. The oldest rocks discovered on Earth so far are gneisses from northwestern Canada, dated isotopically at 3.964 x 10⁹ (3.964 billion) yrs. The age of the Earth, however, is generally cited as 4.55 x 10⁹ yrs.

If there are no Earth rocks this old, what could have been dated to obtain this estimate? Hint: What are the only accessible, unaltered remnants from the earliest days of the Solar System? Also note that the oldest Moon rocks yield isotopic ages of `only' 4.47 x 10⁹ yrs.