Ears Chapter 6 Absolute Dating

Front 1

How did scientists attempt to establish the absolute age of the Earth?

Back 1

- Salts in the ocean
- Accumulation of sediment
- Earth's temperature

Front 2

Why is the accumulation of sediment for the estimation of the earth's age inaccurate?

Back 2

- The stratigraphic record is full of gaps
- Unconformities in the rock record represent even larger breaks in sediment
- The scientists disregarded metamorphic rocks, which would have been very important in dating

Front 3

What was Kelvin's challenge to the uniformitarianism view that the earth had to be really old?

Back 3

- The Earth's interior was really hot when it formed and has been cooling ever since; since it is so hot now, as we would know from going down a mine shaft, the planet must be at max 40 million years old

Front 4

What is wrong with Kelvin's view?

Back 4

- The temperature of the interior of the Earth is not as high as Kelvin thought, because it loses heat not only through conduction, but also through active convection of hot materials from the core
- Radio activity also beat out Kelvin, providing an explanation for the persistence of high temperatures within the earth

Front 5

What are the three important modes of radioactive decay?

Back 5

- Loss of an alpha particle: an alpha particle consists of two neutrons and two protons.
- Loss of a beta particle: A beta particle is an electron and its loss means turning a neutron into a proton
- Capture of a beta particle: This turns a proton into a neutron, changing the parent isotope into the element whose nucleus has one less proton

Front 6

What is isotopic dating?

Back 6

- One finds out the rate that a radioactive element decays.
- Then one can find out how long this decay has been going on by measuring the amounts of both the radioactive parent isotope and the daughter isotope in the rock

Front 7

What is half-life?

Back 7

- After a certain amount of time, an isotope will have decayed to half its amount. After another same certain amount of time, this same isotope will have decayed to half of that amount. And so on. This interval is known as a half-life.

Front 8

How do metamorphic processes affect dating?

Back 8

These processes reposition radioactive isotopes in new minerals that contain none of their decay products. As a result, subsequent decay gives the age of the metamorphic event, not that of the original rock.

Front 9

What kind of isotopes are good for what kind of rocks?

Back 9

It's better to measure younger rocks using isotopes that decay faster and it's better to measure older rocks using isotopes that decay slower.

Front 10

Uranium-lead dating

Back 10

- Uranium 238 and 235 are used in conjunction
- The uranium-bearing silicate mineral "zircon" is widespread in igneous and metamorphic rocks.
- Zircon crystals are ideal for dating, because they don't contain any of the daughter isotopes at formation and also are very resistant to weathering

Front 11

Uranium-thorium/uranium-helium dating

Back 11

- Intermediary between Uranium 238 and Lead 206
- Applied to reef-building corals

Front 12

Rubidium-strontium dating

Back 12

- Generally useful for dating rocks older than about 10 million years

Front 13

Potassium-argon/argon-argon dating

Back 13

- This method measures rocks of all different ages
- Argon-argon dating produces more accurate results by bombarding a sample with argon
- Provided the dates of important events in the evolution of humans in Africa

Front 14

What is the deficiency of argon dating?

Back 14

- Argon can leak from the lattice of a crystal

Front 15

Radiocarbon dating

Back 15

- Can only be used on materials younger than 70,000 years
- For objects of biological origin like body parts
- Useful for studying human culture
- In plants, one can study the nonradioactive carbon 12 in relation to carbon 14, to determine when the plant tissue died
- Carbon 14 forms in the upper atmosphere as a result of the bombardment of nitrogen by cosmic rays
- Changes in bombardment force geologists to make minor corrections to measured radiocarbon dates

Front 16

Cosmogenic Dating

Back 16

- Helps determine how long an object has been exposed at the earth's surface.
- Using cosmic rays to bombard crystalline materials
- Also enables the dating of tectonic events

Front 17

Thermoluminescence and optical dating

Back 17

- Cosmic rays can also measure the time that has elapsed since a geologic object was buried
- In the absence of sunlight, electrons lay trapped within the particles.
- Scientists can count the amount of electrons trapped in a lab to determine how long the material had been buried
- When heat is used to drive out the electrons, it's called thermoluminescence
- When it's light, it's called optical dating

Front 18

What are the problems of isotopic dating?

Back 18

- For one thing, dating stratigraphic boundaries is problematic when appropriate radioactive isotopes are absent from rocks positioned close to them.
- Isotopic dating is usually associated with igneous rocks, so sedimentary rocks are usually only given an approximate date
- Rocks that are dated isotopically have not necessarily remained intact in nature
- Most geologic correlations are still based on fossils

Front 19

Fossil graptolites

Back 19

- Allow for more accurate correlation of sedimentary rocks within particular regions than do isotopic dates
- Graptolites are highly useful index fossils
- They evolved rapidly over a large area

Front 20

What are the periods when isotopic dating is useful for sedimentary correlation?

Back 20

- Only for the really old or really young sedimentary rocks, because the really old ones have not enough fossils, and the really young ones have radiocarbon dating offering little uncertainty

Front 21

Stable isotope stratigraphy (needs more explanation)

Back 21

- Well-dated fossils provide a record of changes in the ratio of strontium 87 to strontium 86 in seawater.
- Once a record of such changes has been established, geologists can date fossil skeletons by measuring their precise isotopic compositions.