Geologic Time

How much time ?

Determining geological ages

Relative age dates – placing rocks and events in their proper sequence of formation

Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating)

• BUT “number of years” is just an arbitrary time scale we use

How old is the Earth?

Early attempts to calculate the Earth’s Age:

Rate sediments are deposited

•Determine rate of sedimentation

• determine total sediment thickness

•---> get the age of the Earth

BUT values ranged from 3 million years to 1.5 billion!

Correlation of rock layers

Matching of rocks of similar ages in different regions is known as correlation

Correlation often relies upon fossils• William Smith (late 1700s) noted that

sedimentary strata in widely separated area could be identified and correlated by their distinctive fossil content

younger

Determining the ages of rocks using fossils

So, how old is the Earth?

Up to now we only discussed the relative age of rock units

Using:

•the principle of superposition (oldest is always on the bottom)

•Fossils that existed during a certain time and then disappeared

BUT: how do we know the absolute age??

How much time?

Time in the most general sense is a measure of how long something takes (measure of duration).

Time itself does not make things happen. It’s not a force.

It’s the processes that operate over some duration that result in changes

We decided to use time as a measure of duration

We decided that one year is how long it takes for the Earth to rotate around the sun.

We decided that one day is the time it takes the Earth to rotate around its axis once.

So, to figure out the duration the Earth has been existing,

We need to find a natural process that occurs throughoutthe Earth’s history and that we can observe.

We need an absolute clock that started ticking when theEarth was formed and is still ticking now.

Radioactive decay of elements is the “smoking gun”

This process occurs over a long duration and we can measurethe products that result from this process.

Parent atom Daughter atomdecay

If we can measure the rate of decay (or how long it takes for a parent atom to change into the daughter atom) we have a clock.

We know that radioactive decay occurs because of

the release of harmful radioactive emissions that can

destroy organic cells and destroy life.

The radioactive emissions are a product of the decay process

Marie Sklodowska Curie was one of the first woman scientists to win worldwide fame, and indeed, one of the great scientists of the 20th century. Winner of two Nobel Prizes (for Physics in 1903 and for Chemistry in 1911), she performed pioneering studies with radium and contributed profoundly to the understanding of radioactivity

...carried out an extensive test of all chemical elements and their compounds for radioactivity, and found that thorium emits radiation similar to that of uranium.

Comparing radioactivity of uranium ores with that of metallic uranium, she noticed that ores are about five times more radioactive than would be expected from their uranium content. This indicated that the ores must contain small amounts of some other radioactive substances much more active than uranium itself.

she separated poloniumand radium it was two million times more radioactive than uranium.

Using radioactivity in dating

Reviewing basic atomic structure• Nucleus

– Protons – positively charged particles with mass

– Neutrons – neutral particles with mass

– Electrons – negatively charged particles that orbit the nucleus

Using radioactivity in dating

Reviewing basic atomic structure• Atomic number

– An element’s identifying number

– Equal to the number of protons in the atom’s nucleus

• Mass number– Sum of the number of protons and neutrons

in an atom’s nucleus

Using radioactivity in dating

Reviewing basic atomic structure• Isotope

– Variant of the same parent atom

– Differs in the number of neutrons

– Has the same number of protons

– Results in a different mass number than the parent atom

An Example of Isotopes: let’s look at Hydrogen (H)

Has mass number 1-> it has one proton in the nucleus and

one electron in the shell-> has zero neutrons in the nucleus

An isotope of Hydrogen is Deuterium (D)

Has mass number 2-> it has one proton and one electron-> it has one neutron

It is twice as heavy as Hydrogen but has the same charge

1+0 = 1

1+1 = 2

An Example of Isotopes: let’s look at Carbon (C)

Has mass number 12-> it has six protons in the nucleus and

six electron in the shell-> has six neutrons in the nucleus

An isotope of Carbon is Carbon-14 (C-14)

Has mass number 14-> it has six protons and six electrons-> it has eight neutrons

It is heavier than C-12 but has the same chargeIt is also unstable and decays to Nitrogen-14

6+6 = 12

6+8 = 14

Atomic number: number of protons

C6

12.0107Carbon

Atomic number: number ofprotons

Mass of Carbon (g/mol)

This mass includes contributionsfrom all carbon isotopes(C-12, C-13, C14…)that occur in nature

You see: mass is very closeto 12. So, C-12 is by far themost abundant isotope in natureName

Why are isotopes important to understand?

Radioactive decay can produce a different isotope

from a parent atom

238

92U

Mass number

Atomic number

Uranium

U238.0289Uranium

Atomic number: number ofprotons

Mass of Uranium (g/mol)Occurring in nature

92

Using radioactivity in dating

Radioactivity• Spontaneous changes (decay) in the

structure of atomic nucleiTypes of radioactive decay

• Alpha emission– Emission of 2 protons and 2 neutrons (an

alpha particle)– Mass number is reduced by 4 and the atomic

number is lowered by 2

Alpha Decay

238

92U

234

90Th + alpha particle

The alpha particle consists of 2 protons and 2 neutrons (it is a Helium atom)

Mass number

Atomic number

Using radioactivity in dating

Types of radioactive decay• Beta emission

– An electron (beta particle) is ejected from the nucleus

– Mass number remains unchanged and the atomic number increases by 1

– This produces radioactive gamma rays

40

19K

40

20Ca

Mass number

Atomic number

+ beta particle

The beta particle is an electron (e-)

Fermi showed in 1934 that beta decay is the transformation of a Neutron into a proton and an electron.

Using radioactivity in dating

Types of radioactive decay• Electron capture

– An electron is captured by the nucleus

– The electron combines with a proton to form a neutron

– Mass number remains unchanged and the atomic number decreases by 1

14C

14N + beta particle

87Rb

87Sr + beta particle

Mass number

238U

207Pb + alpha particles

Series of decays

232Th

208Pb + alpha particles

Important decays for dating geologic materials

40K + e

40Ar 11% of K

40K

40Ca + beta particle

remaining 89%

Why does radioactive decay offers a dependable means of keeping time?

•Average rate of decay is fixed•Does not vary with any changes in chemical or physical conditions

Once a quantity of a radioactive element is created somewhere in the universe, it starts to act like a balance wheel of a clock, steadily firing off one atom after another at a steady rate

To tell time we need:

Some kind of reference (like the numbers of a digital watch)

Numbers we use to read the radioactive clock aresupplied in the form of new atoms (the daughter elements)that form from the parent elements.

Need to count the daughter elements and if we knowthe rate of decay, we can work back to the time when there were no daughter elements but only parents.

This assumes that the daughter elements are exclusivelyproduced by radioactive decay. (40Ar only produced by 40K)

Rate of decay varies from one element to another

Half Life: the time it takes for one half of the original numberof radioactive atoms to decay

14C

87Rb

5570 years

Half life

47 billion years

Atom

40K 1.3 billion years

After (half lives) Fraction of atoms left

1 1/2 55702 1/4 11,1403 1/8 16,7104 1/16 22,2805 1/32 27,850

Years passed

C-14

C-14 is usually used to date events that happened less that 30,000 years ago. After that, too fewatoms are left to measure accurately.

After (half lives) Fraction of atoms left

1 1/2 1.3 billion2 1/4 2.6 billion3 1/8 3.9 billion4 1/16 5.2 billion

Years passed

K-40 decays to Ar-40

SO: K-Ar method of dating rocks is very good. Lots of minerals have K (I.e. Feldspars).Half life is long enough so that we can still accurately measurethe parent and daughter for geologically relevant processes

After (half lives) Fraction of atoms left

1 1/2 47 billion

Years passed

Rb-87

Rb-87 is used to date very old rocks.

After 4 billion years only about 1/10 of a single half lifehas passed.

SO: in the oldest rocks we can measure the amount of Rb-87 and the amount of Sr-87. We know the half life and can calculate the age of the rock.

Using radioactivity in dating

Parent – an unstable radioactive isotope

Daughter product – the isotopes resulting from the decay of a parent

Half-life – the time required for one-half of the radioactive nuclei in a sample to decay

A radioactive decay curve

When does the clock start?

It starts once the minerals in a rock are formed.

Once minerals are formed the daughter elements cannot escape that mineral and are trapped in the mineral.

U

Pb

Molten magma: daughter escapes

U

Pb

Mineral:daughter trapped

Using radioactivity in dating

Radiometric dating• Sources of error

– A closed system is required (no Ar escape)

– To avoid potential problems, only fresh, unweathered rock samples should be used

– No re-melting or metamorphism must have occurred. That would re-set the clock by allowing earlier formed daughters to escape.

Using radioactivity in dating

Dating with carbon-14 (radiocarbon dating)

• Half-life of only 5730 years• Used to date very recent events• Carbon-14 is produced in the upper

atmosphere• Useful tool for anthropologists,

archeologists, and geologists who study very recent Earth history

14CTree takes up

C-14 to makeorganic material

Tree dies:Does not take up C-14anymore.

C-14 decay replenished withNew C-14 from atmosphere

C-14 decay not replenished withC-14 from atmosphere -> clock starts

Using radioactivity in dating

Importance of radiometric dating• Radiometric dating is a complex

procedure that requires precise measurement

• Rocks from several localities have been dated at more than 3 billion years

• Confirms the idea that geologic time is immense

Recent developments: Dating of Meteorites

Big surprise: All Meteorites are of the same age (4.6 billion)

No matter of their composition, or when they fell downon the Earth.

The fact that there are no meteorites of any other age suggests strongly that they originated in other bodies in the solar systemand formed at the same time the Earth formed.

How old is the Earth?

Oldest Earth rocks (Isua Stones) 3.8 billion

Oldest Moon rocks 4.2 billion

All Meteorites 4.6 billion

Isua stonesoldest rocks on Earth, 3.8 billion years old

GreenlandGodthaabfiord

Andy Warhol(1928 - 1987)

Geologic time scale

The geologic time scale – a “calendar” of Earth history

• Subdivides geologic history into units• Originally created using relative dates

Structure of the geologic time scale• Eon – the greatest expanse of time

Geologic time scale

Structure of the geologic time scale• Names of the eons

– Phanerozoic (“visible life”) – the most recent eon, began about 540 million years ago

– Precambrian - everything before 540 million

– -> 4600 million to 540 million

Geologic time scale

Structure of the geologic time scale• Eras of the Phanerozoic (540 Ma to now)

– Cenozoic (“recent life”)

– Mesozoic (“middle life”)

– Paleozoic (“ancient life”)

• Eras are subdivided into periods

540

245

66

4600

Ma

0

Era Period

Geologic time scale

Precambrian time• Nearly 4 billion years prior to the

Cambrian period• Not divided into smaller time units

because the events of Precambrian history are not know in great enough detail

– First abundant fossil evidence does not appear until the beginning of the Cambrian

Time

It is a measure of duration and tells us in a scale so that weunderstand how long something takes.

Our time scale is “designed” so that it scales to our life-time and daily lives. Years, months, weeks, hours, minutes.

Time scales outside of this customary scale are difficult for us to intuitively understand.

Long time scales: 1 Billion years (1,000,000,000 years) Short time scales: 1 nano second (0.000000001 second)