Relative Dating of Rocks

 Relative Dating of Rocks

Instructions/Descriptions

Worksheet on Relative Dating of Rocks

Laboratory for Physical Geology 111

Stratigraphic Principles for Relative Dating of Rocks

1. Introduction and Goals:

The goal of this lab is to learn about relative dating and how to apply it to rocks, fossils, and geologic events. The history and concepts of stratigraphy will be addressed, as well as the use of fossils for relative dating. You’ll learn about the geologic timescale, how to calculate relative ages, and how geologists date events in Earth history using various approaches. You’ll also gain some hands-on experience with the principles and practices.

1. The Six Principles of Stratigraphy: Knowledge and Understanding:
2. Define the seven fundamental physical stratigraphic laws:

Definition of the Stratigraphic Law

1)

3) Inclusion

4) Fossil Succession is a term that refers to the succession of fossils.

5) Continuity on both sides

6) Horizontality that is unique

7) Unconformity

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1. Non-deposition and/or erosion occurred over a long length of time in between episodes of deposition, resulting in unconformities in the time-rock record. They usually manifest themselves as visible uneven surfaces between two sets or groups of rock pieces, known as formations. Other geologic events such as tilting, folding, faulting, intrusion, and uplift can also be recorded by an unconformity. As a result, unconformities are useful for determining the age of rocks. The three types of stratigraphic unconformities are listed and defined as follows:

Definition of the Unconformity Type

2)

III. Complete the analysis and evaluation of the two geologic cross-sections below to determine relative ages of rocks and geologic events based on stratigraphic order. Do the following for each geologic cross-section:

1. Determine the ages of the rock bodies and other geologic features/events, such as tilting, uplift, faulting, and erosional unconformities, using relative ages.
2. List the sequence of geologic events (each identified with a letter) in chronological order by writing the letters in the column of blanks from oldest (bottom of the list) to youngest (top of the list). You must also specify which stratigraphic legislation was utilized to place each dated event in its relevant time frame for each dated event. The stratigraphic rules are abbreviated as SP = superposition, IN = inclusions, CC = cross-cutting, and UN = unconformity.

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3. Determine and label all lettered unconformities in each cross-section (by kind).

Grand Canyon Cross-Section #1: Geologic Cross-Section #2: Grand Canyon Cross-Section #1: Geologic Cross-Section #2: Grand Canyon Cross

Stratigraphic Law and Age Sequence Stratigraphic Law and Age Sequence Stratigraphic Law and Age Sequence Stratigraphic Law and Age Sequence Strati

Type of Unconformity – X-Section #1 Type of Unconformities – X-section #2

Questions:

1) For dating the sedimentary layers, which stratigraphic principle did you rely on the most?

2) For dating intrusions and faults, which stratigraphic principle did you rely on the most?

3) How did you date rocks directly above and below an unconformity using another stratigraphic principle?

1. The Usefulness of Index Fossils in Geologic Dating
2. What exactly is a fossil? Answer.

The Principle of Fossil Succession and the identification of time-constrained rock blocks called range zones, which contain a unique collection of fossilized plant and animal species suitable for dating, are the foundations of biostratigraphy.

2. Define the Fossil Succession Principle (from the lecture and lab text):

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Answer.

3. Fossils that are used to date rocks are referred to as fossils.
4. What are the essential characteristics of an excellent index fossil?
5. Index of Fossils by Era.

Directions: Find and list the names of some of the most common, helpful groupings of index fossils for each of the three eras using the Geologic Timescale figure below.

1)

1. Rock Age Determination Using Index Fossils

Refer to the diagrams below for instructions. To determine the relative age and absolute age of the sample in each figure, use the chart above and the geologic time scale. Note: If you identified your fossils as dinosaurs (relative age Early Triassic through Cretaceous Periods, absolute age ca. 240–66 Ma) and mammals (Jurassic through Quaternary Periods, absolute age ca. 208–66 Ma), the two groups of organisms’ concurrent or Overlapping Age Range, or Resolved Age, is Jurassic through Cretaceous, which equates to a numeric age range of 208 Ma too, As a result, the resolved age of the rock is the time span during which both fossil species were living at the same time.

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Index Fossils Current Age Range: (in million years ago = mya)

The age of the sample has been determined to be between and years ago.

Index Fossils Current Age Range: (in million years ago = mya)2.

The age of the sample has been determined to be between and years ago.

Which another stratigraphic theory (principle of Fossil Succession) is essential to the logic of utilizing fossils for dating?

1. Radiometric Absolute Dating Principles
2. How do we figure out how old a rock is?
3. Stratigraphy Principles – “A is older than B” Relative Dating – “A is older than B”

Use the Principles and Techniques of Radiometric Dating to quantify the date in years.

1. Radiometric Dating Principles

Natural radioactive materials degrade into other materials at predictable rates. Radioactive decay is the term for this. Stable daughter elements are formed when radioactive parent elements decay. In 1896, Henri Becquerel discovered radioactivity. Rutherford and Boltwood used the principle of radioactive decay to date rocks and minerals in 1905 (using Uranium decaying to produce Helium), and Boltwood dated a sample of granite based on uranium/lead ratios in 1907. Amazingly, all of this was done before isotopes were known, and before decay rates were known accurately. As geologic clocks, a variety of radioactive materials can be used. Each radioactive element decays at a rate that is almost constant. By comparing the number of radioactive parent elements and the number of stable daughter elements, geologists can determine the period of time that decay has been occurring.

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Radioactive parent isotopes and their stable daughter products are two examples.

Suitable Minerals Radioactive Parent Stable Daughter Half-life Constant

K-spar, hornblende, biotite, muscovite 40 Argon 40 1.3 billion years K-spar, hornblende, biotite, muscovite

Feldspars, hornblende, biotite, muscovite Rubidium 87 Strontium 87 47 billion

14 billion years Thorium 232 Lead 208 Zircon, monazite, titanite, apatite

Lead 207 713 million years Uranium 235 Zircon, monazite apatite, sphene

Uranium 238 Lead 206 4.5 billion years Zircon, monazite, Uranium 238 Uranium 238 Uranium 238 Uranium 238 Uranium 238 Uranium 238 Ur apatite, sphene

Organics 5730 years Carbon 14 Nitrogen 14

Note that the number in the table above is the mass number (the total number of protons plus neutrons). Because of the different amounts of neutrons in each element, the mass number may vary. Isotopes of a given element are elements with different numbers of neutrons. The half-life of each radioactive isotope is different. The half-life of a parent radioactive element is the time it takes for half of it to decay into a daughter product. The rate of radioactive decay is continuous exponential or geometric. The amount of parent atoms present determines the rate of decay.

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The ratio of father to daughter informs us how many half-lives there are, which we may use to calculate the age in years. One half-life has elapsed if there are equal amounts of parent and daughter, for example. Two half-lives have elapsed if there are three times as many daughters as parents. Radioactive decay releases particles and energy (as shown in the two figures below). Uranium produces subatomic particles, energy, and lead as it decays.

rates You Can Date Isotopically to Find Out Your Rock Age

Many typical rock-forming minerals contain radioactive isotope parent-daughter pairs, which are radioactive isotope parent-daughter pairs.

It’s possible to utilize it for absolute dating. Because the vast majority of minerals in an igneous rock formed as the magma cooled, the isotopic age closely matches the rock-forming period, igneous rocks are by far the best rock for isotopic dating. Some of the most suitable minerals for the three most frequent types of isotopic-pair radiometric dating techniques are:

Potassium 40 (parent) and Argon 40 (daughter) can be found in the following substances:

feldspar feldspar feldspar feldspar feldspar feldspar (orthoclase)

 Muscovite

 Amphibole

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Rubidium 87 (parent) and Strontium 87 (daughter) can be found in the following substances:

Feldspar is a mineral that is found in nature (orthoclase)

 Muscovite

 Hornblende

 Biotite

Lead – Uranium 235 and 238 (parents)

Relative Dating of Rocks

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