Unit 2

Lecture: Geologic TimeConcepts for Relative & Absolute

Dating of Geologic Strata

How do we measure time?

Easiest solution – look at the rate of a convenient natural process. If the rate is constant, then use it as a timer.

Examples: Revolution of the Earth (years)

Rotation of the Earth (days)

But what about geologic time?

Same answer.

Useful “Timing” Processes: Biological – Use tree rings “dendrochronology”

Bristle-Cone Pine Tree: ~ 3 -4000 years

Geological – Counting “varves” (annual sedimentation layers (Baron de Geer, 1878; Bradley, 1929).

Geophysical- Measuring cooling rates for magmas, then extrapolating for the entire Earth. (Lord

Kelvin, 1899)

Geochemical - Measuring thickness of sedimentarylayers and estimating erosion rates

Nuclear – Measure rates of radioactive decay and proportions of parent & daughter isotopes.

Two Kinds of AgesRelative - Know Order of Events But

Not Dates

Civil War Happened Before W.W.II

Bedrock in Wisconsin Formed Before The Glaciers Came

Absolute - Know Dates

Civil War 1861-1865

World War II 1939-1945

Glaciers Left Wisconsin About 11,000 Years Ago

18.2 The beginnings of geology

In 1666, Nicholas Steno, a Danish anatomist, studied a shark’s head and noticed that the shark’s teeth resembled mysterious stones called “tonguestones”.

18.2 The beginnings of geology

Steno theorized that tonguestones looked like shark’s teeth because they actually were shark’s teeth that had been buried and became fossils.

Relative Dating - placing the geologic occurrencein the proper sequence

Which came first and WHY-----

To establish a “relative” time scale, rules were discovered(principles of relative dating) – Nicholas Steno (1636-1686)

o Principle of Original Horizontalityo Law of Superpositiono Principle of Cross-Cutting relationso Principle of Inclusions

RELATIVE DATING & AGE Relative Dating: putting

rocks and geological events in correct chronological order

Relative Age: how old something is in comparison to something else

HOW? Use of sedimentary rocks

Use of fossils

Study of strata

Let’s unravel some geologic history from observations of various formationsand their contacts

Nicholas Steno – 1669 proposed the following relative dating principles

The principle of Original Horizontality:• Sedimentary rock layers are deposited as horizontal strata• Any observed non-horizontal strata has been disturbed

basin

Sediment input

A

B

C

LAW OF HORIZONTALITYSediments are originally deposited in

horizontal layers

Folds or inclines: layers must have been deformed after they were deposited

Folds or inclines: layers must have been deformed after they were deposited

The principle of Superposition In any undisturbed sequence of strata, the oldest

stratum is at the bottom of the sequence and the youngest stratum

is on top.

1

2

3

4

5

Unit 1 = oldUnit 5 = young

LAW OF SUPERPOSITIONFor undisturbed rocks, the oldest layer is on the bottom and the youngest is on top (Supai is oldest)

Superposition:

Mindoro Cut, Wisconsin

The principle of Cross Cutting relationships

• Any geologic feature that cuts across another geologic feature is younger

1

2

3

4

5

6

Unit 1 = olderUnit 6 = youngest

Which came firstunit 5 or Unit 6 ?

The principle of Inclusions

• A piece of rock (clast) that has become “included” in another rock body is older than the rock body it has become part of – why?

Rock body A

Intrusion of pluton B

AA A

Older (rock A was there first)

LAW OF INCLUSIONS

If a rock body (Rock B) contained fragments of another rock body (Rock A),

then Rock B must be younger than the fragments of rock it contained

Which “granites” are older and younger?

OLDERYOUNGER

Youngest

Oldest

Superposition

Original Horizontality

Cross-cutting relationship

A B C Asp Vn

principle of inclusions

Which granite is older?

Older Younger

The principle of Unconformities• Rock surface that represents a period of erosion or non-deposition• Often represent a “gap” in time• Three major types of unconformities

o Disconformityo Angular unconformityo Non-conformity

Unconformity

Disconformity – unconformity in non-disturbed sedimentary layers

Angular unconformity – unconformity lies between angled strata and overlying horizontal strata

Non-conformity – sedimentary strataoverlies crystalline rocks (ign and meta)

Igneous or metamorphic rock

Form

ati

on o

f an

unco

nfo

rmit

y

Layers are formed according to superposition.

Something happens to uplift the areafoldingfaulting, etc.

Erosion wears away the uppermost layers

Area submerges and deposition begins again.

Here’s the unconformity

Xln rocks

Sedimentary rocks

Disconformity

AngularUnconformity

Nonconformity

Sequence 1: Uplift & Erosion

1. Limestone deposited

2. Sandstone deposited

3. Shale Deposited

4. Uplift

5. Erosion

Sequence 2: Faulting1.Limestone

deposited

2.Sandstone deposited

3.Shale deposited

4.Faulting

Sequence 3: Folding1.Limestone

deposited

2.Sandstone deposited

3.Shale deposited

4.Folding

What happened here? Deciphering Earth’s rock record…

Start by listing the events ,such as deposition of .. ,erosion, intrusion of.., faulting of, etc. in

order to piece together the story..1.Deposition of rock layer O2.Deposition of rock layer N3.Deposition of rock layer L4.Intrusion of M (law of inclusions)5.Erosion of surface(unconformity)6.Depositionof H,I,J7.Erosion (unconformity ) above J8.Deposition of K9.Erosion to present day surface

Let’s practice “Reading “ the rocks!!Determine the sequence of events in this geologic cross section:

The sequence of events is as follows:

1. Deposition of sedimentary rocks D 2. Fault B 3. Intrusion of igneous rock C 4. Erosion, forming the unconformity 5. Deposition of sedimentary rocks E

Fossil Formation

Fossils are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks.

Specific conditions are needed for fossilization.

• Only a tiny percentage of living things became fossils.

Rocks can tell where fossils were made and when

Rocks can tell when mass extinctions happened

PALEONTOLOGYthe study of fossils remains of

ancient life

Body fossils vs. trace fossils Body = remain of organism, like bones; Trace = evidence of organism, like

footprints

Commonly Preserved:

Hard Parts of Organisms: Bones Shells Hard Parts of Insects Woody Material

Rarely PreservedSoft or Easily Decayed Parts of

Organisms: Internal Organs Skin Hair Feathers

Fossils can form in several ways.

Permineralization occurs when minerals carried by water are deposited around a hard structure.

Fossil Formation

The remains of an organism are likely to be changed over time.

Molds and casts are another common type of fossil.

Carbonization is particularly effective in preserving leaves and delicate animals. It occurs when an organism is buried under fine sediment.

A natural cast forms when flowing water removes all of the original tissue, leaving an impression.

Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.

Carbonization: occurs when fine sediment encases an organism, as time goes the pressure will squeeze out the gasses and liquids and leave behind a thin film of carbon.

Petrified Wood

Dinosaur Tracks

Impressions

Carbon Film

Amber

Casts

INDEX FOSSIL

Fossil that defines and identifies geologic periods; often in only one layer of rock

Easily recognizable

Short-lived (found only in a few layers of rock worldwide)

Wide distribution (geographic range)

Ex/ INDEX FOSSIL: AMMONITE

Ammonite fossils are found worldwide, but they existed for only a very specific period of time

this means ammonites are found in very specific layers of rock

when an index fossil is found, the age of the rocks it is preserved in can be determined

Fossil Succession The principle of fossil succession

means that fossils can be used to identify the relative age of the layers of a rock formation. The organisms found in the top layers appeared after the organisms found in the layers below them.

PRINCIPLE OF SUCCESSION Fossils are found in a predictable sequence Fossils in rock B are older then fossils in rock A

What kind of rocks are these fossils in?

Which layer is oldest?

Which layer is youngest?

How do you know?

GEOLOGIC TIME SCALE

a series of time intervals that divides Earth’s history

• Each layer of rock represents specific interval of time

• Index fossils help determine specific period

• Time periods divided by specific events like mass extinctions

(RADIOMETRIC DATING)

ABSOLUTE DATING

ABSOLUTE DATING

Absolute Time -

Numerical age determination of strata, events, and geologic structures from radiometric dating techniques.

Absolute Time Think of an Hourglass timer (the term

used by Arthur Holmes).

Some initial quantity reduces, while its product accumulates at a constant rate.

NO “sand” can be added or removed at any point in the process (closed system).

Knowing the rate, and measuring quantities allows us to calculate the TIME duration for the process.

Absolute TimeEarly Attempts

Bishop James Ussher (Prelate of Ireland)

(1600s) Used O.T. biblical chronologies to date the “creation”

October 22, 4004 B.C. (Sunday)

Georges Buffon (1700s) Used a measured cooling

rate from metal & non-metal balls to estimate the age for a molten Earth to cool. Earth’s Age = 75,000 yrs.

And others…

John Joly (1889) acting upon a suggestion from Edmund Halley, estimated the ocean’s salinity & its rate of increase.

Age: 90 million years

Lord Kelvin (1899) estimated the Earth’s thermal gradient. Comparing this to cooling rates for known materials he said:

Age : 20 – 100 million years (max)

A few more…

Various geologists (1800s) estimated sedimentation & erosion rates and compared these to sediment thicknesses.Age : ~ 3 million to 1.5 billion years

Arthur Holmes (1900s) first to use Uranium decay techniques.Age of Earth: ~ 4 billion yearspЄ boundary: ~ 600 million

Basic Atomic Structures

Orbiting the nucleus are electrons, which are negative electrical charges.

Atomic number is the number of protons in the atom’s nucleus.

Mass number is the number of protons plus the number of neutrons in an atom’s nucleus

ISOTOPES: Isotopes are atoms of an element that differ in their number of neutrons.

neutrons protrons

RADIOMETRIC DATING

Radioactivity is the spontaneous decay of certain unstable atomic nuclei.

Radiometric dating provides an accurate way to estimate the age of fossils.

Radiometric dating uses the decay of unstable isotopes.

Radiometric Dating

Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.

Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes

Radiometric Dating Techniques

Radioactive elements decay at constant rates.

There are various decay processes. see chart →

If we can measure:

number of Parent & Daughter isotopes, and the decay rate, then we can calculate an age

Radiometric Dating

As a radioactive isotope decays, atoms of the daughter product are formed and accumulate.

Each radioactive isotope has its own unique half-life. A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

The key is the radioactive “Half Life” The idea is: Parents decay into Daughters

P radioactive → D stable

The rate of this decay is constant. A period of time exists during which ½ of

the P isotopes will decay into D’s. This is called the half life, t ½ . Since the rate is constant, so is the t ½ .

Isotopes used for geologic dating are called:

“Geochronometer Isotopes”

Okay, so does this work?

Let’s not get too technical. What we do is use a radioactive isotope’s “half life”.

If we know how long a half-life is, then all we need to do is measure the number of half-lives that have elapsed for a particular sample.

An example would be nice…

Okay. We measure P & D in a rock sample. The ratio of P:D is 1:3.

Or…25% of P remains Look at chart. 25% P corresponds to

2 half lives. If a half life is 200,000

yrs, then this sample is:

2 hl x 200,000 yr/hl 400,000 years old

“hl” – half life

– A half-life is the amount of time it takes for half of the isotope to decay.

Radiometric dating uses decay of unstable isotopes.– Isotopes are atoms of an element that differ in their number of neutrons.

Radiometric Dating: Half-Life

What if there’s been 2.4 or 1/3 of t ½ ?

Okay. In the “real world” of geochronology things can get a bit more tricky. We have equations that we use to calculate ages that don’t really use the t ½ approach…directly.

Like: Age = 1/λ ln (D/P +1)

This is good because we can then use statistics to evaluate the reliability of the age we’ve found. If an age passes the test, its called an isochron age. If it fails, then it’s called a errorchron, and isn’t used.

Dating with Carbon-14

Radiocarbon dating is the method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample.

When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.

Present Radiometric Dating Methods

Cosmogenic C-14 5700 Yr.

Primordial K-Ar (K-40) 1.25 B.Y. Rb-Sr (Rb-87) 48.8

by U-235 704 M.Y.

One last thing

“Radiocarbon” dating is rarely used in geology.

The t ½ of 14C is only 5730 yrs. After 4 - 5 t ½’s it’s reliability becomes questionable.

Also,14C is created in the atmosphere at uneven rates.

14C decays into 12C leaving 14N behind…so the P/D ratio only tells you the “age since death” for living things. It is useless for rocks…but absolutely great for archaeologists, who use it as far back as ~ 50,000 yrs

Don’t confuse radiocarbon with geologic dating!

In practice, both relative and absolute dating are combined, following a procedure like this: Igneous rocks, such as lava flows, volcanic ash beds, and intrusions) are dated radiometrically. The dates of fossil-bearing sedimentary rocks are in a certain area are bracketed using the dates of associated igneous rocks which have been dated radiometrically. The fossil-bearing sedimentary rocks are correlated with sedimentary rocks in other areas which contain the same fossils. The age of the rocks in other areas is determined indirectly, from the ages of the fossils they contain..