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CLEMSON / Environmental Resource Science / Env 1010 / When did certain things occur?

When did certain things occur?

When did certain things occur?

Description

School: Clemson University
Department: Environmental Resource Science
Course: Physical Geology
Professor: Alan coulson
Term: Fall 2015
Tags: Geology and Physical Geology
Cost: 25
Name: Week 6 Notes
Description: These notes cover lectures 10 and 11, and they cover material that will be on the next exam.
Uploaded: 02/19/2016
10 Pages 52 Views 1 Unlocks
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Lecture 10—2/16/16


When did certain things occur?



Geology in the News

Tin cans found embedded in the walls after Taiwan earthquake

Shows that buildings weren’t made from reinforced concrete like they  were supposed to  

Part 1- Geologic Time 

Why do we care?

When did certain things occur?

Even non-scientists wonder

Dating Methods

2 approaches

Relative: put things into a sequence of events

Which came first and which came last

No hard numbers

Before, during, or after?

Absolute: a precise number of when something occurred

No longer comparing the date

Why do relative dating?

Seems outdated and we are accustomed to knowing numbers for anything Absolute dating is very expensive and requires difficult to use lab  equipment


Does gap represent 100 yrs or 100 million yrs?



Sometimes don’t even need the absolute date

Relative dating is relatively free

Relative Dating

Fossils: any evidence of past life on earth

Really found just in sedimentary rock

People have been finding fossils

Stratigraphy: study of strata (layers of sand or sedimentary rock); people started  studying strata so they could better understand why the fossils were found there Ideal: find layers and layers representing  

Unconformities: breaks or gaps where time went by, but you don’t have any rock  to represent it

Why?

1. run out of sediment: if you don’t have sediment, you won’t have  evidence

2. run out of accommodation space: if the basin is where sediment is  deposited is full, you can’t put more sediment there


Why do trees vary in growth rings?



We also discuss several other topics like How do you identify secondary amines?

3. start eroding sediment: sediments that have been deposited are getting  eroded faster than they’re being deposited

Types of Unconformities

3 types, classified by comparing data above and below the gap

1. Disconformity: when there is one type of sedimentary rock on one side,  and a different type of sedimentary rock on the other side

2. Non: sedimentary rock on one side and a different type of rock on the  other side (not sedimentary Don't forget about the age old question of What are different types of training?

3. Angular Disconformity: rocks below unconformity are tilted up at an  angle, while rocks above the disconformity are horizontal

Several steps involved

Long time

Layers are formed, then folded, then eroded, then more layers form on top  horizontally

Problems

1. Identification: it’s very difficult to spot an unconformity

2. Duration: don’t know how much time was lost

Does gap represent 100 yrs or 100 million yrs?

Part 2- Stratigraphic Principles 

1. P. of Original Horizontality: when you first make sedimentary layers, they’re  going to be horizontal

Makes sense: gravity won’t let them form at an angle without anything to  support it

When there are layers at different angles, it’s due to something that  happened later on

2. P. of Superposition: when looking at a stack of layers, oldest is on the bottom  and youngest is on the top; in between is a progression of time

3. P. of Cross-Cutting: whatever did the cutting is the youngest

When a fault line intersects or cuts through rock layers, the fault line is  younger than the rock it cuts through

4. P. of Faunal Succession: when going through a bunch of strata, fossils will be  found in a very specific order If you want to learn more check out Who created the swan neck flask?

Younger fossils will be found in younger rock, and older fossils will be  found in older rock

Useful for comparing different layers in different areas

Correlation with fossils

Not all fossils are great for correlation

Want to identify short spans of time

Therefore, big fossils like dinosaurs and saber tooth cats aren’t very useful Want to find fossils that give evidence of something that is more specific Index fossils: any species that is really good for correlation

1. numerous: want a big population

2. widespread: global distribution would be awesome

3. went extinct quickly: want small units of time

4. are easy to identify: don’t want to get hung up on whether or not fossil  was identified correctly

Other correlation tools

Fossils aren’t the only way to correlate Don't forget about the age old question of How many major plates are on the surface of the earth?

Lithostratigraphy: correlate areas based on having the same rock types Can have many glitches depending on what type of rock is found Can have problems with unconformities

Sequence Stratigraphy: correlation based on patterns of unconformity Works best with a lot of unconformities present, and near the shore (due  to changes in sea level)

Chemo stratigraphy: can use chemical signals from the rock

Have a wide range of options

Look for certain chemical signals such as a certain isotope, trace metals,  elements

Ex) iridium anomaly at the Cretaceous-Tertiary boundary

Really only find iridium in high impact and in space, implies a  

meteorite

Magnetostratigraphy: look at magnetic record within the rocks

Correlate by looking for magnetic patterns that are similar between two  areas

Difficult- can be like comparing two bar codes until finding a match Part 3- Geologic Time Scale 

Originally built via stratigraphy Don't forget about the age old question of Why conjugated dienes are more stable than isolated dienes?

Fossils were key for defining boundaries

Eons are subdivided into eras, which are subdivided into periods, etc Geologic Time Units

Eons

Only 3 or 4 recognized

1-Hadean

4.5-4.0 Ga (disputed) We also discuss several other topics like What is the difference between a variable and a constant?

Everything was in its molten state, when denser material  Almost no material on the surface to study this

2-Archean

4.0-2.5 Ga

First evidence of continental material

Still didn’t have oxygen

3- Proterozoic

2.5 Ga- 550 Ma

Oxygen is finally present

4- Phanerozoic

When fossil record shows up

3 eras

1- Paleozoic (550Ma-200Ma): Cambrian Explosion, when you  see a huge diversity of life

2- Mesozoic (200 - 65 Ma): Dinosaurs

3- Cebozoic (65 Ma - now): mammals are dominant

Lecture 11 – 2/18/16

Geology in the News

New info why quakes occur deep in subduction zones

Scientists confused on how subduction zones can move under so much pressure

Water released from a mineral called lawsonite enables the fault to move despite  the high pressure environment

Part 1- Absolute Ages 

Two approaches:

- Non-radiometric

- Radiometric

Non-radiometric Methods

1) Varves: really thin alternating bands of light and dark sediment  

Need winter temperatures to get cold enough to get the top of the lake to  freeze

Light layers are what have been deposited during the summer

Dark layers are what have been deposited during the winter

One light band + one dark band = one year  

Uses:

Tell you about the history of that area

A stretch of no varve represents winters that weren’t cold enough  to freeze lake

Restrictions:

Only tells you about that immediate area

Can’t take that data and use it for the surrounding areas

Easy for layers to get disturbed and mix together

Organisms living in the sediment would disturb the varves

2) Dendrochronology: counting the growth rings inside of trees

Dark area on ring is when that ring stops growing

How many rings represent how many years

Useful on a local scale, but hard to expand it to another area

Uses:

Find trees that overlap in age so that can go father back in time

By overlapping trees, can go back almost

Restrictions:

Identify species because some species don’t have annual growth  rings (some form one every few years or some form several times  per year)

Why do trees vary in growth rings?

Different climates, some trees are susceptible to  

environmental stresses (not enough water, forest fires)

Radiometric Dating

Use of radioactive materials for dating specimens

Isotopes: atoms of the same element, but with different numbers of neutrons Most isotopes are stable, but some are unstable

Radioactive decay: atoms emitting particles and energy to stabilize that atom Radiation: combination of energy and particles that are being emitted Parent atom: unstable atom that you start with

Daughter atom: atom that forms after radioactive decay of parent atom

Sometimes the daughter is still unstable and will have to undergo  radioactive decay again

Decay Series (aka chain): having multiple radioactive daughter atoms until a  stable one forms

Misconception: watching atoms ‘pop’

The rate of radioactive decay is actually constant

There’s no way to tell when a specific atom will go through decay, so you  can’t focus on one particular atom and wait for it to decay

Rate of decay of the isotope can help us figure out time it takes to decay

Half-life: amount of time it takes for half of parent atoms to decay into daughter  atoms

Follows exponential curve- won’t reach zero

Each radioactive isotope has its own half-life

-238U = 4.4 billion yrs (almost age of the earth!)

-210Pb = 22.3 yrs

If you’re measuring something that happened quickly, you’ll need an  isotope with a small half-life, and vice versa

Half-life does not vary with any environmental factor!

Crucial because it’s what’s allowing us to measure time in our  

universe

Requirements:

Must always think about your specimen

1- Radioactive isotopes must be present in your specimen

If it doesn’t contain radioactive isotopes, that we can’t use  

radioactive dating

2- Need measureable amounts of the parent & daughter in your  

specimen

Without parent, radioactive decay has stopped

Without daughter, radioactive decay hasn’t started yet

3- Can only go so far back in time

Problem: the parent eventually runs out

After one half-life, 50% is already gone

After two, 25% is gone, etc.

4- Closed system

Your material is not exchanging anything back and forth  

with its environment

Problems with open system:

Add parent or lose daughter, will look like little radioactive  

decay has happened, and vice versa

Part 2- Case Study: Carbon 14 

Only radioactive carbon isotope

14C ???? 14N + particle + energy

Half-life = 5730 years

Track through 10 half-lives, which is an exception to the general rule that you can only  track through 5

Can’t apply carbon dating to everythingQ: Earth is older than 57,000 years, so why  haven’t we run out of 14C?

Carbon is always forming on Earth

Forming Carbon 14

Global distribution- carbon is found all over Earth

Reactions are in a steady state: amount we are creating is the same as the amount  we are taking down 

How do we know it works?

Radiometric techniques are developed by first dating things that we already  know the age of to  

Ex) Egyptian mummies- we have detailed historical records that state when  certain people died, so we know how old the mummy is. So we test to see if  carbon dating works by testing it for things we already know the age of

Can’t carbon date everything

Carbon dating is horrible for dating rocks, but great for dating organic material  such as fossils

Commented [A1]:

Why can we date fossils?

In atmosphere: 14C + O2 = 14CO2

14CO2 incorporated into food chain

Calculating the age

Age = [ln(Nf/N0)/-0.693]*half-life

Nf/N0 = % of 14C in the sample relative to amount of food in living tissue - How much parent is left

Assumptions for Carbon-14

1- System remains closed after death

Is this a good assumption?- No, but there are ways to double-check this  assumption

2- Amount of 14C in living tissue doesn’t vary

Problem 1: production of carbon varies over time

Variations are small and on short timescales

Over a long time scale, the amount of 14C will be the same

Problem 2: fossil fuel burning has changed the relative amount of the  carbon isotopes in the atmosphere

The method has been corrected to compensate for changes

Finding a problem with a method doesn’t invalidate a method

Limit of Carbon-14 Dating

Some organisms don’t get carbon from the atmosphere or the food chain, which  can change the dating

Can’t just date anything! Think about your specimen!

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