Chapter Chapter 3232

Geologic HistoryGeologic HistoryRelative & Absolute Relative & Absolute

DatingDating

BFRB BFRB Pages Pages

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Who’s got the Who’s got the TIME?TIME?

• RELATIVE TIME -- the order or sequence is known, but not the actual date of occurrence.– Example - the trilobite fossil is

older than the dinosaur fossil

• ABSOLUTE TIME – the actual age of object– Example - the dinosaur fossil is

250 million years old

How How geologists geologists find the find the RELATIVE RELATIVE

AGES of rocks AGES of rocks usingusing

RELATIVE RELATIVE TIMETIME

The “DUH” LawsThe “DUH” LawsI. Law of SUPERPOSITION – sedimentary

sequence will be OLDEST on BOTTOM and YOUNGEST ON TOP (if undisturbed)

II. Law of Original Horizontality – rocks are usually deposited (laid down) flat and level (parallel to the Earth’s surface)

III. Law of Folds and Tilts - the fold or tilt is younger than the rocks that were deposited horizontally. (the rocks were there first so they are older)

IV. Law of Cross Cutting - a fault or igneous rock intrusion is younger than the rock layers it has broken or intruded upon. (cut across)

V. Law of INCLUDED FRAGMENTS - pieces of rock found buried WITHIN another rock must be OLDER (formed first) ex conglomerate.

YOUNGESTYOUNGEST

OVEROVER

OLDESTOLDEST

IN OTHER WORDS, YOOOOO!IN OTHER WORDS, YOOOOO!

I. I. LAW OF LAW OF SUPERPOSITIONSUPERPOSITION

II - Law of Original Horizontality

III – Law of Folds and Tilts

Sediments, such as those at the bottom of an ocean or lake, are usually deposited in flat layers.

When geologists see sedimentary layers that aren’t horizontal, they

know that the layers were deposited first, then tilted later.

STEP 1

STEP 2

IV Law of Cross Cutting

Occurs when magma squeezes into or between layers of pre-

existing rock

OR…

Layers of pre-existing rock are broken by a fault due to

compressional or extensional forces

IV LAW OF CROSS-CUTTING BY IV LAW OF CROSS-CUTTING BY IGNEOUS INTRUSIONS & FAULTSIGNEOUS INTRUSIONS & FAULTS

faults

Igneous intrusions

V. LAW OF INCLUDED V. LAW OF INCLUDED FRAGMENTSFRAGMENTS

UNCOMFORMITY

•A buried surface of erosion separating two rock masses

•Represents a gap in geologic time (because that layer was exposed to the atmosphere for an amount of time in order for the erosion to happen, then another layer came and buried it…)

….a look at the Grand Canyon and 2 types of unconformities...

1 – ANGULAR UNCOFORMITY

2 - DISCONFORMITY

1 - Angular Unconformity

An unconformity in which the beds below the unconformity dip at a

different angle than the beds above it.

2. DISCONFORMITY• An unconformity in which the beds

above the unconformity are parallel to the beds below the unconformity, though layers are “missing”.

““SEQUENCE” of events SEQUENCE” of events of the formation of an of the formation of an

angular unconformity…angular unconformity…1. The lower sediments were

deposited as horizontal layers in a body of water

2. These sediments were then raised above water level and tilted during a tectonic event (what type of boundary?)

3. Streams & other forces of erosion carved a nearly horizontal surface across the tilted beds

STEP 1

STEP 2

““SEQUENCE” of SEQUENCE” of events…events…

4. The land surface subsided (or the water level raised), submerging the erosion surface

5. A new series of sediments deposited in horizontal layers on the erosion surface

6. The complicated sequence of tilted and horizontal rocks was again uplifted, exposing them to erosion and producing the outcrop we see today

STEP 3

STEPS 4-6

Practice – Describe the steps that formed this cross

section.

PLAY THE PLAY THE “WHICH “WHICH CAME CAME

FIRST” FIRST” GAME!GAME!

 1). D or L?

 2.) M or C?

 3.) N or E?

 4.) B or A?

 5.) I or Q?

 6.) Q or J?

 7.) M or A?

ABSOLUTE ABSOLUTE TIMETIME

Measuring Measuring Absolute TimeAbsolute Time

• All methods of measuring absolute time must provide a specific date for that occurrence

• Tree rings can be used to mark the passing of each season and therefore obtain a numerical age (dendrochronology)–Tree rings can be used to date events up to 3000 years ago

DendrochronolDendrochronologyogy

• The dating of past events (climatic changes) through study of tree ring growth

• Discovered by A.E. Douglass from the University of Arizona, who noted that the wide rings of certain species of trees were produced during wet years and, inversely, narrow rings during dry seasons

• Each year a tree adds a layer of wood to its trunk thus creating the annual rings we see when viewing a cross section

RADIOACTIVE RADIOACTIVE DATINGDATING•Radioactive elements decay at

known, steady rate•By comparing the amount of radioactive element that has decayed to the amount of decay product, an absolute age can be calculated–Compare the ratio of PARENT to DAUGTHER materials

http://www.earth.nwu.edu/people/seth/202/DECAY/decay.mov

Understanding Understanding RADIOACTIVITYRADIOACTIVITY

• A radioactive element has an unstable nucleus

• Over time radiation is given off from the nucleus ( Alpha, Beta, and Gamma Rays)

• As the nucleus decays it becomes a different element – For example Carbon–14, a radioactive

element, will change to the stable element Nitrogen-14

HALF-LIFE (decay HALF-LIFE (decay rate)rate)• A measure of the

time required for half of a radioactive material (parent) to decay into a stable end product (daughter)

•The amount of time required for one half life is different for each element

•ESRT page 1 •Example Carbon –14 requires 5700 years for half of its mass to decay into a stable product

•VIF - THE PERIOD OF A HALF LIFE CAN NOT BE CHANGED BY CUTTING THE MATERIAL IN HALF OR BY HEATING OR BY EXTREME PRESSURES UNDER THE GROUND – that’s why radioactive dating is so accurate for dating rocks!!!

How does radiocarbon How does radiocarbon dating work?dating work?

• All plants and animals on Earth are made principally of carbon

• When an organism dies, the Carbon in begins to degrade into Nitrogen

• In the 1940s, scientists succeeded in finding out that it takes 5568 years for radiocarbon to disappear, or decay, from a sample of carbon from a dead plant or animal

• Radiocarbon dating is not accurately able to date anything older than ~50,000 years old because too much of the parent Carbon-14 has turned into the daughter Nitrogen 14 and the relative proportions of the 2 substances is too large to accurately measure.

Okay… so if we have 500g of C-14,Okay… so if we have 500g of C-14,

In 1 half-life (5,700 years) we’ll have __________In 1 half-life (5,700 years) we’ll have __________

In 2 half-lives (__________years) In 2 half-lives (__________years) __________ __________

In 3 half lives (__________years) In 3 half lives (__________years) ____________ ____________

In 4 half-lives (__________years) In 4 half-lives (__________years) ____________ ____________

In 5 half-lives (__________years) In 5 half-lives (__________years) ____________ ____________

In 6 half-lives (__________years) In 6 half-lives (__________years) ____________ ____________