Dating rocks• Relative dating

•Using a set of principles to put rocks in their proper sequences of formation

• Absolute dating•Using radioactive decay to determine the exact age of rocks

Absolute Dating:any method of measuring the age of an

event or object in years Most common:based on Radioactive

DecayParent daughter

Why does it work?

1. The decay rate is CONSTANT, independent of external conditions in the earth.2. The daughter/Parent ratio can be precisely measured.

Radioactive Decay• The process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element.

• Sounds great but what is an isotope?

• An isotope is an atom that has the same number of protons (atomic #) as other atoms of the same element do but that has a different number of neutrons (and thus a different atomic mass)

Isotopes• Most isotopes are stable, meaning they stay in their original form

• But some are unstable• Unstable isotopes are radioactive

• Radioactive decay is the process in which a radioactive isotope tends to bread down into a stable isotope of the same or another element

How does it work?What does this have to do with the age

of rocks?• An Unstable isotope is called a parent parent isotopeisotope

• The stable isotope produced by radioactive decay is the daughter isotopedaughter isotope.

• Decay is constantDecay is constant• The more daughter isotope- the older the rock!

Radiometric datingRadiometric dating• A method of determining the age of an object by estimating the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope

• Ratio or parent material to daughter material

– Absolute dating• Helps scientists determine actual age of fossils• Rocks near fossils contain radioactive elements – unstable elements that break down into different elements

• Half-life of a radioactive element is the time it takes for half of the atoms in a sample to decay

• Scientists compare the amount of radioactive element in a sample to the amount of the element into which it breaks down

• Scientists use this info to calculate the age of the rock, which then tells the age of the fossil

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

Dating with carbon-14Dating with carbon-14 (radiocarbon dating)

– Carbon is normally found in three forms: stable C-12, stable C-14 & radioactive C-14

– All combine with oxygen to form CO2

– 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

Carbon-14Carbon-14• Carbon-14 is continuously created in the atmosphere by cosmic radiation.

• There is one atom of radioactive C-14 for every trillion atoms of C-12 in the atmosphere

• Plants absorb C-14 directly through their leaves in the form of carbon dioxide

• Animals take in C-14 indirectly when they eat plants

• Although C-14 disintegrates at a constant rate, it is continuously renewed as long as an organism remains alive.

• When an organism dies, it stops absorbing new C-14 and its radiocarbon “clock” is set.

Types of radiometric dating• Potassium-argon method

– K-40 half-life 1.3 billion years– Decays into argon and calcium– Used to date rocks older than 100,00 years old

• Uranium-lead method– U-238 half-life 4.5 billion years– Decays into lead-206– Used for rocks more than 10 million years old

• Rubidium-strontium method– Rb-87 half-life 49 billion years– Decays into strontium-87– Used for rocks more than 10 million years old

• Carbon-14 method

• Lateral continuity– sediment extends laterally in all directions until it thins and pinches out or terminates against the edges of the depositional basin

• Cross-cutting relationships– an igneous intrusion or a fault must be younger than the rocks it intrudes or displaces

Relative-Dating Principles

• A dark-colored dike has intruded into older light colored granite: the dike is younger than the granite

Cross-cutting Relationships

North shore of Lake Superior, Ontario Canada

• A small fault displaces tilted beds: the fault is younger than the beds

Cross-cutting Relationships

Templin Highway, Castaic, California

Back to Steno

www.gly.uga.edu/railsback/1121Steno.jpg

Why are layers tilted?• Deformation of rocks

– Occurs after they are deposited– Important factor in relative dating

• Folding– Anticlines, synclines– Rock bends, but does not break

• Faulting– Normal, reverse, transform– Rock breaks

Folding

www.hill.anorak.org.uk/dhtml/glgchap5.html

Faulting

www.stmarys.ca/academic/science/geology/structural/faults.html

The Map That Changed the World

• The relative geologic time scale has a sequence of – eons – eras – periods – epochs E-or played at Epcot! (Eon, Era, Period, and Epoch!)

– but no numbers indicating how long ago each of these times occurred

Relative Geologic Time Scale

• Large divisions based on characteristics of fossils

• Paleozoic Era – early life dominated by invertebrate animals

• Mesozoic Era – middle life

• Cenozoic Era – recent life

Geologic Time Scale

• Mapping in 1800s using the principles of

– Superposition

– Original Horizontality

– Original Lateral Continuity

– Cross-cutting relationships

– Also Fossil Correlation

How was the timescale created?

• Radiometric dating is the most common method of obtaining absolute ages– calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks

• Other methods?– Tree ring counting– Varves– Ice cores

Absolute Dating

• The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scale

• The geologic time scale is a dual scale– a relative scale– and an absolute scale

Geologic Time Scale

• The concept and measurement of geologic time has changed through human history

• James Ussher (1581-1665) in Ireland – calculated the age of Earth based on recorded history and genealogies in Genesis• he announced that Earth was created on October 22, 4004 B.C.

• a century later it was considered heresy to say Earth was more than about 6000 years old

Changes in the Concept of Geologic Time

• During the 1700s and 1800s Earth’s age was estimated scientifically

– Georges Louis de Buffon (1707-1788) calculated how long Earth took to cool gradually from a molten beginning •used melted iron balls of various diameters

•he estimated Earth was 75,000 years old

Changes in the Concept of Geologic Time

– Others used rates of deposition of various sediments and thickness of sedimentary rock in the crust •gave estimates of <1 million •to more than 2 billion years

– Or the amount of salt carried by rivers to the ocean and the salinity of seawater •John Joly in 1899 obtained a minimum age of 90 million years

Changes in the Concept of Geologic Time

• Lord Kelvin (1824-1907)– knew about high temperatures inside of deep mines and reasoned that Earth is losing heat from its interior

• Assuming Earth was once molten, he used • the melting temperature of rocks • the size of Earth• and the rate of heat loss

– to calculate the age of Earth as between 400 and 20 million years

Crisis in Geology

• This age was too young for the geologic processes envisioned by other geologists at that time – leading to a crisis in geology

• Kelvin did not know about radioactivity as a heat source within the Earth

Crisis in Geology

• The discovery of radioactivity destroyed Kelvin’s argument for the age of Earth

• Radioactivity is the spontaneous decay of an atom’s nucleus to a more stable form

• The heat from radioactivity helps explain why the Earth is still warm inside

• Radioactivity provides geologists with a powerful tool to measure absolute ages of rocks and past geologic events

Absolute-Dating Methods

• Understanding absolute dating requires knowledge of atoms and isotopes: we have it!

• Atomic mass number = number of protons + number of neutrons

• Isotopes: different numbers of neutrons

• Different isotopes have different atomic mass numbers but behave the same chemically

• Most isotopes are stable – but some are unstable

• Geologists use decay rates of unstable isotopes to determine absolute ages of rocks

Absolute-Dating Methods

• Radioactive decay is the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different element

• Three types of radioactive decay:– alpha decay, two protons and two neutrons (alpha particle) are emitted from the nucleus

Radioactive Decay

• Half-life of a radioactive isotope is the time it takes for one half of the atoms of the original unstable parent isotope to decay to atoms of a new more stable daughter isotope

• The half-life of a specific radioactive isotope is constant and can be precisely measured

Half-Lives

• The length of half-lives for different isotopes of different elements can vary from – less than 1/billionth of a second – to 49 billion years

• Radioactive decay – is geometric not linear – a curved graph

Half-Lives

• In radioactive decay, during each equal time unit, one half-life, the proportion of parent atoms decreases by 1/2

Geometric Radioactive Decay

• By measuring the parent/daughter ratio and knowing the half-life of the parent which has been determined in the laboratory geologists can calculate the age of a sample containing the radioactive element

• The parent/daughter ratio is usually determined by a mass spectrometer– an instrument that measures the proportions of atoms with different masses

Determining Age

• For example: – If a rock has a parent/daughter ratio of 1:3 a parent proportion of 25%

– and the half-live is 57 million years, how old is the rock?

Determining Age

– 25% means it is 2 half-lives old.

– the rock is 57 x 2 =114 million years old.

• Most radiometric dates are obtained from igneous rocks

• As magma cools and crystallizes, radioactive parent atoms separate from previously formed daughter atoms– they fit differently into the crystal structure of certain minerals

• Geologists can use the crystals containing the parents atoms to date the time of crystallization

What Materials Can Be Dated?

• The isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent material left is measurable– Such isotopes include:

Parents Daughters Half-Life (years)

Long-Lived Radioactive Isotope Pairs Used in

Dating

Uranium 238 Lead 206 4.5 billionUranium 234 Lead 207 704 millionThorium 232 Lead 208 14 billionRubidium 87 Strontium 87 48.8 billionPotassium 40 Argon 40 1.3 billion

How do we know the Earth can’t be older than about 6-7 b.y.?

• Moderate half-life isotopes (1 b.y.)

• If Earth was > 6-7 b.y. old, there wouldn’t be many parents left

• Carbon is found in all life

• It has 3 isotopes – carbon 12 and 13 are stable but carbon 14 is not

– carbon 14 has a half-life of 5730 years– carbon 14 dating uses the carbon 14/carbon 12 ratio of material that was once living

• The short half-life of carbon 14 makes it suitable for dating material < 70,000 years old

• It is not useful for most rocks, but is useful for archaeology and young geologic materials

Radiocarbon Dating Method

• Carbon 14 is constantly forming in the upper atmosphere – when a high-energy neutron, a type of cosmic ray , strikes a nitrogen 14 atom it may be absorbed by the nucleus and eject a proton changing it to carbon 14

• The 14C formation rate – is fairly constant – and has been calibrated against tree rings

Carbon 14

• The carbon 14 becomes part of the natural carbon cycle and becomes incorporated into organisms

• While the organism lives it continues to take in carbon 14– when it dies the carbon 14 begins to decay without being replenished

• Thus, carbon 14 dating measures the time of death

Carbon 14

• The age of a tree can be determined by counting the annual growth rings in lower part of the stem (trunk)

• The width of the rings are related to climate and can be correlated from tree to tree– a procedure called cross-dating

• The tree-ring time scale now extends back 14,000 years!

Tree-Ring Dating Method

• In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans

Tree-Ring Dating Method

Summary• Uniformitarianism holds that

– the laws of nature have been constant through time

– and that the same processes operating today have operated in the past

– although not necessarily at the same rates

Summary• The principles of superposition

– original horizontality,– lateral continuity– and cross-cutting relationships– are basic for determining – relative geologic ages and for interpreting Earth history

• Radioactivity was discovered during the late 19th century – and lead to radiometric dating – which allowed geologists to determine absolute ages for geologic events

Summary

• Half-life is the length of time it takes for one-half of the radioactive parent isotope to decay to a stable daughter isotope of a different element

• The most accurate radiometric dates are obtained from long-lived radioactive isotope/daughter pairs – in igneous rocks

Summary

• The most reliable radiometric ages are obtained using two different pairs in the same rock

• Carbon 14 dating can be used only for organic matter such as – wood, bones, and shells – and is effective back to about 70,000 years