8.2 the History of (Relative) Time

CHAPTER 8: GEOLOGIC TIME

SUMMARY

This chapter begins the discussion of geologic time by acknowledging that our personal conceptions of time are firmly grounded in time intervals measured in hours, days, and years. Yet Earth is very old (~4.6 billion), and many slow-acting events have occurred that have shaped it into the world we observe today. Students are reminded that Earth’s environment changed from that of a molten mass, to having solid continents, to an environment that included an atmosphere, and eventually, oceans. Some of those events were unlike changes we observe today and caused catastrophic changes in the face of Earth. Other events were not unlike what we observe today and are uniformitarian (or similar) with events we observe today. Regardless of the type of process involved, geologists use the principles of relative time, correlation, and numerical time to figure out what happened and when. That evidence indicates that primitive life is at least 3.8 billion years old and developed in a hostile environment compared to today. In the past 542 million years, life flourished, but occasionally underwent mass extinctions. Some of those extinctions may have been catastrophic events. Many types of fossils are preserved in the rock record and those changes marked significant modification in the type of life that dominated earlier episodes of Earth history. Humans now have the most dramatic environmental impact of all species on Earth. That may change some day in the future as our species fights to survive as have so many in the past.

8.2 The History of (Relative) Time.

The study of relative time involves placing events in the order in which they occurred. The principles include:

a) Original Horizontality: sedimentary rocks are deposited in nearly horizontal layers (and if tilted, they must have undergone deformation after their formation).

b) Superposition: in a series of sedimentary rocks (that are not deformed so much that the sequence is overturned) the rocks at the bottom of the stack are the oldest and the rocks at the top are the youngest.

c) Cross-cutting relationships: older rocks may be cut by younger rocks or other geologic features (such as faults, joints).

d) Inclusions:younger rock units sometimes incorporate pieces of older rocks. It can be used to identify older pieces of rock surrounded by younger igneous rocks (such as a magma intrusion that incorporates surrounding rocks).

see pages 213-214 and figures on these pages for more on the above principles.

James Hutton's geologic principles helped to invent our modern concept of geologic time (that the Earth is very old and Earth's processes take a very long time to produce any significant change in the shape of the Earth's surface). These principles are known as Uniformitarianism:the same slow-acting geologic processes that operate today must have operated in the past ("The present is the key to the past"); he noted that all land should be worn flat unless some process acts to renew the landscape by forming new mountains (so there are processes of construction, such asmountain building during plate tectonics and processes of destruction such as weathering and erosion).

Another of Hutton's principles is Unconformities: when rock layers are worn down by erosion (while they are exposed at Earth's surface) and then later buried beneath newhorizontal layers of sediments they form a feature called an unconformity (located at the physical boundary between the two sections of rocks)...since erosion removed rocks layers, it is equivalent toremoving geologic time...so an unconformity surface represents a gap in geologic time. Please see pages 218-219 "Geological History of the Grand Canyon" and follow the block diagrams from #1to #8 to get a better understanding of how unconformities form (and to understand other principles of relative time dating techniques).

The above diagram depicts (using block diagrams) how to apply relative age dating techniques to rock units. At (a.) three sedimentary rock units are shown, the oldest bed on the bottom (A) and the youngest bed on top (C) (we know this by the principle of superposition). After deposition of these units they were tilted and eroded to a relatively flat surface (so the environment changed from depositional...such as under water; to erosional...exposed to processes of weathering and erosion at the Earth's surface). Then at (b.) a renewed period of deposition placed (D) on top of the tilted units.At (c.)all of the units are now tilted(remember, the principle of horizontality states that these rock units are originally deposited horizontally). Also note that the physical boundary between (D) and the other tilted units represents an unconformity (gap in geologic time). Keep in mind the processes described above can take millions of years to produce these features.

The above should help you with checkpoint 8.2 and 8.3. Checkpoint 8.3 also shows examples of cross-cutting relationships and inclusions which will help you interpret the sequence of events.

Fossils and Chronology:The most straightforward method of correlating sedimentary rocks is to compare their fossils. Fossils can be the key to predicting the sequence of layers in a given location and to match outcrops of similar rocks between different locations. The best fossils to use for correlation are called index fossils: species that existed for relatively short periods of geologic time and are found over large geographic areas. Paleontologists have used index fossils to correlate rocks around the world!

Checkpoint 8.6: Outcrops of rock are examined in four different locations in a state. The rock types and the fossils they contain are illustrated in the following diagram. Which fossil would be the best choice to use as an index fossil for these rocks? Which fossil is least characteristic of a specific set of geological conditions?

a) Fossil 1

b) Fossil 2

c) Fossil 3

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8.3: Geologic Time

Read on this! Geologic Time and Mass Extinctions. The geologic time scale is like a calendar of Earth's history. Earth's past can be divided into three big chunks of time known as Eons. Today we are in the Phanerozoic Eon, which is further subdivided into three Eras: the Cenozoic (new life), Mesozoic (middle life)and Paleozoic (ancient life). Note the boundaries between the Cenozoic/Mesozoic Eras is marked by a mass extinction (the K-T event that ended the reign of the dinosaurs about 66 million years ago) and the Mesozoic/Paleozoic Eras is marked by an even greater mass extinction (the P-T event that killed off 96% of marine species and 70% of land species about 251 million years ago). These Eras are further sudivided into Periods (see Table 8.1). Finally, note that the beginning of the Paleozoic Era (the Cambrian Period) is marked by an explosion of organisms with hard skeletons about 542 million years ago. Prior to this, fossils are rare in the geologic record.

8.4: Numerical Time

Atoms of the same element with a different number of neutrons are called isotopes (the mass number is the combined number of protons and neutrons in the nucleus). Unstable isotopes can breakdown by radioactive decay and change to a new element accompanied by the release of energy. The unstable original isotope is termed the "parent" isotope and the product of decay is termed the "daughter" atom. Radioactive decay of specific isotopes occurs at a constant rate regardless of the physical or chemical conditions. By keeping track of the relative amounts of isotopes in individual minerals we can calculate an age for the rock containing the mineral.

To calculate the age we use half-lives. The half-life of an isotope is the time taken for half of the parent isotopes to convert to daughter atoms.As time passes the amount of parent isotope decreases and the quanitity of daughter atom increases. After one half-life, 50% of the parent remains and the other 50% of the originalatoms are converted todaughter atoms. After twohalf-lives, the number of parentisotopes ishalved again (25%) and the number of daughter atoms increases by an equivalent amount (to 75%). The relative proportion (ratio) of parent isotopes and daughter atoms can be used to determine how many half-lives have passed, and the time for these half-lives to occuris know for certain isotopes (see Table 8.3).

Checkpoint 8.15: The half-life of a radioactive isotope is 500 million years. Scientists testing a rock sample discover that the sample contains three times as many daughter atoms as parent isotopes. What is the age of the rock?

a) 500 million years

b) 1,500 million years (1.5 billion)

c) 1,000 million years (1 billion)

d) 2,500 million years (2.5 billion)

When answering the above question is it helpfull to refer to Figure 8.21 on page 230. Once you figure out how many half-lives have passed, multiply the time for the half-life to occur by the number of half-lives that have passed.

LEARNING OBJECTIVES

1. Students will explain concepts related to geologic time.

2. Students will explain rules that are used to determine the sequence of geologic events: the principles of superposition, cross-cutting relationships, and original horizontality.

3. Students will apply the principles to determine the order of geologic events.

4. Students will recognize the importance of index fossils and identify the best index fossil in a sequence of rocks and fossils.

5. Students will describe characteristics of the eons of the geologic time scale.

6. Students will determine how the age of a rock is related to the relative proportions of parent and daughter isotopes.

7. Students will distinguish between high- and low-magnitude events and list examples of each.