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Earth’ s HistoryUnit 6

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Vocabulary List1. Relative Dating

2. Absolute Dating

3. Superposition

4. Cross-cutting relationships

5. Uniformitarianism

6. Original Horizontality

7. Correlation

8. Unconformity

9. Index Fossils

10. Radioactive Decay

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List of Focus QuestionsDate Focus

____ What is relative dating? What principles do we follow so we can put events into order?

____ What is correlation? What two things do we use to correlate rock layers?

____ What is absolute dating? What is C14 used to date?

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A. Order of Geologic EventsWhen studying the history of Earth, we must be able to place items or events in a particular order so we can determine which event happened first, which happened next, and so on.

We order events in two ways:

1. Relative Dating:____________________________

___________________________________________

___________________________________________2. Absolute Dating: __________________________

___________________________________________

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To determine an order of events using relative ages, we use the following four principles:

1. __________________ Layers of rock on the bottom are older than the rock layers above them, provided they have not been overturned.

2. __________________________ Rock layers areolder than any event that cuts across them. Example: __________________________________________________________________________.

Oldest Rock Layer

Youngest Rock Layer

The sedimentary rocks are older than the igneous intrusion. Note the contact metamorphism.

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3. ____________________________________: Processes that occur now (erosion, deposition, plate movement, etc.) occur the same way now as they always have throughout geologic history.

3. ____________________________Sediments that form sedimentary rocks are usually deposited in flatlayers. If we see rock layers that are not flat and

level,we can assume that they were changed (tilted, foldedor faulted) after they formed.

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B. Matching Rock Layers

When studying the history of Earth, we must also beable to compare rock layers found in one place tothose found in another.

Correlation:________________________________________________________________________________________________________________________

Correlation of a single rock layer can often be difficult. Often, it is better to try to correlate a sequence of rock strata. This is often done by a technique called “walking the outcrop”: ___________ ______________________________________________________________________________________

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Walking the outcrop can be limited because the rock strata may be covered by sediment or other rock. Also, part of the rock strata may be missing from a location.

Unconformity: _____________________________

___________________________________________

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Volcanic Eruptions: _________________________

___________________________________________

To be a good index fossil, a fossil must:

a. __________________________________

b. __________________________________

There are two other methods used to correlate rock layers; index fossils and volcanic eruptions.

Index Fossils: _______________________________

__________________________________________

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C. Absolute Age of Rocks

How can the absolute age of rock strata be determined? Some rocks contain elements that are unstable.

Radioactive Decay: __________________________

___________________________________________

Radioactive DecayUranium 238 Lead 206 + Energy

As the uranium decays, it does so in a very predictable manner and at a predictable rate.

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Half-life: ___________________________________

___________________________________________

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As each half-life is reached, __________ of the remaining radioactive element has decayed. Notice that you will never reach a point where all of the radioactive element has decayed. However, there will come a point where: ___________________________

__________________________.

Half-lives may be as short as ___________________ or on the order of _________________________. The front cover of your Earth Science Reference Table has radioactive decay data for several radioactive elements or isotopes.

A radioactive isotope with a appropriate half life must be used to correctly measure age. If an item that is only several thousand years old was dated using an isotope with a half life of 5 billion years, not enough radioactive decay would have occurred to be measurable. If a very old item was dated using an isotope with a short half life, then not enough radioactive element would be remaining to measure.

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Radioactive Dating Example

QUESTION: A sample is found to contain 100 grams total of the radioactive element Carbon-14 and its daughter element Nitrogen-14. If 25 grams of that total is C14 and 75 grams is N14, how old is the sample?

ANSWER: First, determine the number of half lives this sample has undergone.

•After one half life, there is _____grams of C14 & _____ grams of N14.

•After two half lives, there is ____grams of C14 & _____ grams of N14.

So, if the half life of C14 is _______ years (see your ESRT) and there have been ____ half lives, then _________ X _______ = __________ years old!

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D. Absolute Age of Rocks

Geologic time is divided into units based upon:

___________________________________________

___________________________________________

___________________________________________

The units are not based directly on _______!

When dividing Earth history, begin with the largest units _______. They are divided into _____; they are divided into _______; and they are divided into _______.

NOTE: Compared to the overall geologic time scale (about 4.6 billion years), human existence is considered ________________

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Evidence found in rock strata and in their fossils allow scientists to infer what life and the environment were like at that particular time and in that particular location. For example, coral fossils found in limestone would indicate what type of environment? _____________________________

Of all the species on Earth, __________ of them have become extinct since life was first recorded in the rock record.

Most scientists believe the fossil record indicates life forms have ______ (changed) through geologic time, probably to adapt to changes in their environment.

This theory, know as _________________________, is not a change from simple life like one-celled organisms to complex like humans. It is an increase in the variety of life forms.