Geologic Time

Background

Relative approaches to geologic time establish sequences of rock formation and events, such as deformation and erosion. Absolute approaches assign approximate ages (dates) to rocks and events.

Don't use plagiarized sources. Get Your Custom Essay on
Geologic Time
Get an essay WRITTEN FOR YOU, Plagiarism free, and by an EXPERT!
Order Essay

Five principles can be used to work out sequences of geologic events:

Original horizontality: Sedimentary rock layers, or strata, were originally deposited as relatively horizontal layers of sediment.
Superposition: In any undisturbed sequence of strata, the oldest stratum is at the bottom of the sequence, and the youngest stratum is at the top of the sequence.
Inclusions: A piece of rock (inclusion) contained within a larger rock body must be older than the rock body.
Unconformities: These are surfaces that represent a gap in the geologic record, much like pages missing from a book. They are surfaces on which sediment was not deposited for a period of time, or surfaces on which erosion has occurred.
Cross-cutting relationships: A feature (such as a fracture, fault, or intrusive rock mass) that cuts across another rock is younger than that rock.

Absolute dating is accomplished using radioactive isotopes. These isotopes occur naturally in many minerals when they first form. Over time, the parent isotopes change into daughter isotopes. By using a mass spectrometer to measure the ratio of parent to daughter isotopes in a rock sample, a geochemist can estimate the age of the sample. However, there are three major assumptions that must be made:

No daughter product was present when the rock formed.
Radioactive decay occurred at an exponential rate throughout the rock’s history.
After the rock formed, no parent or daughter was gained or lost except through the decay process (a closed system was maintained).

Geologists use various parent-daughter pairs to date rocks, depending on several factors, such as minerals present in the rock and its approximate age. Each pair has its own half-life, the amount of time it takes for half of the original amount of parent isotopes to decay into daughter isotopes. For example, Potassium-40 (40 denotes atomic mass) decays into Argon-40 with a half-life of 1.3 billion years, as expressed in the figure below.

Assignment

1. For the following geologic cross section, determine the sequence of events using the principles described above. Write down the sequence of letters, from oldest to youngest, along the side of the figure. Between each pair of letters, write down the principle you used to determine the relative timing of the rock or event. In this cross section, Z and I are igneous intrusions, T is a fault, C and L are unconformities, and the other letters denote sedimentary rocks.

2. When estimating the age of rock, why should geologists collect a fresh sample inside a rock outcropping rather than use a piece from the weathered exterior of the outcropping?

3. A feldspar crystal in granite was analyzed for Potassium-40 and Argon-40. Measurements showed that approximately 75% of the Potassium-40 remained (had not been converted to daughter). Using the decay curve above, estimate the age of the rock.

4. Certain fossils can also be useful for dating rocks. Index fossils represent organisms that lived a short amount of time (in a geological sense) and covered a wide geographic area. For example, suppose a sedimentary rock contains two different index fossils: A and B. Suppose that, based on paleontological studies, Species A lived from 4-7 million years ago, and Species B lived from 3-5 million years ago. Therefore, both species lived in the overlapping, 4-5 million year old time frame, and we assign that age to the rock.

The following diagram shows age ranges, in millions of years ago (Ma), for different types of clams found in a rock.

Use the method outlined above to determine an age range for the rock. Hint: In what time interval did all five clams live?
What would be the estimated age range if M. subhercynius was not present in the rock?

Source: Physical Geology by Steven Earle, CC BY 4.0 license

Rubric
Assignment 8 Rubric
Assignment 8 Rubric
Criteria Ratings Pts

This criterion is linked to a Learning Outcome
Question 1
State the sequence of events and associated principles: 25% credit for incorrect interpretation; 50% credit for partially correct interpretation; 75% credit for mostly correct interpretation; and 100% credit for correct interpretation

12.0 to >0.0 pts
Full Marks

0.0 pts
No Marks

12.0 pts

This criterion is linked to a Learning Outcome
Question 2
25% for incorrect reasoning; 50% for partial correct reasoning; 75% for mostly correct reasoning; and 100% for correct reasoning

4.0 pts
Full Marks

0.0 pts
No Marks

4.0 pts

This criterion is linked to a Learning Outcome
Question 3
Determine the age of the rock: 25% credit for incorrect determination; and 100% credit for correct determination

4.0 to >0.0 pts
Full Marks

0.0 pts
No Marks

4.0 pts

This criterion is linked to a Learning Outcome
Question 4
For each part (4 points each), evaluate the fossil data to determine the age range of the rock: 25% credit for incorrect determination; and 100% credit for correct determination

8.0 to >0.0 pts
Full Marks

0.0 pts
No Marks

8.0 pts

Total Points: 28.0
Previous

The Homework Market
Order NOW For A 10% Discount!
Pages (550 words)
Approximate price: -

Why Choose Us

Top quality papers

We always make sure that writers follow all your instructions precisely. You can choose your academic level: high school, college/university or professional, and we will assign a writer who has a respective degree.

Professional academic writers

We have hired a team of professional writers experienced in academic and business writing. Most of them are native speakers and PhD holders able to take care of any assignment you need help with.

Free revisions

If you feel that we missed something, send the order for a free revision. You will have 10 days to send the order for revision after you receive the final paper. You can either do it on your own after signing in to your personal account or by contacting our support.

On-time delivery

All papers are always delivered on time. In case we need more time to master your paper, we may contact you regarding the deadline extension. In case you cannot provide us with more time, a 100% refund is guaranteed.

Original & confidential

We use several checkers to make sure all papers are plagiarism-free and do not share them with third parties.

24/7 Customer Support

Our support agents are available 24 hours a day 7 days a week and committed to providing you with the best customer experience. Get in touch whenever you need any assistance.

Try it now!

Calculate the price of your order

We'll send you the first draft for approval by at
Total price:
$0.00

How it works?

Follow these simple steps to get your paper done

Place your order

Fill in the order form and provide all details of your assignment.

Proceed with the payment

Choose the payment system that suits you most.

Receive the final file

Once your paper is ready, we will email it to you.

Our Services

No need to work on your paper at night. Sleep tight, we will cover your back. We offer all kinds of writing services.

Essays

Essay Writing Service

You are welcome to choose your academic level and the type of your paper. Our academic experts will gladly help you with essays, case studies, research papers and other assignments.

Admissions

Admission help & business writing

You can be positive that we will be here 24/7 to help you get accepted to the Master’s program at the TOP-universities or help you get a well-paid position.

Reviews

Editing your paper

Our academic writers and editors will help you submit a well-structured and organized paper just on time. We will ensure that your final paper is of the highest quality and absolutely free of mistakes.

Reviews

Editing your paper

Our academic writers and editors will help you submit a well-structured and organized paper just on time. We will ensure that your final paper is of the highest quality and absolutely free of mistakes.