How Old is Stuff?

Dr. Mark McGinleyHonors College and Department of Biological

SciencesTexas Tech University

Age of the Universe and the Earth?

• The original estimates of the age of the Universe were based on interpretations of the Bible– 17th Century

• John Lightfoot, Anglican clergyman, estimated that creation occurred during 4004 BC.

• Bishop James Ussher made the same estimate a decade later

• Concluded Creation happened about 6000 years ago

Biblical View of Geology

• Features of the Earth created by catastrophes such as Noah’s Flood.

• Some 18th century geologists also thought that catastrophes were important

Birth of Modern Geology

• The field of geology was developing in the late 18th century in Europe

• James Hutton– Scottish Farmer– Interested in the

factors that maintained the environment so plants can grow

Hutton’s Observations

• Rocks exposed to the atmosphere tend to decay and produce gravel and sand

• Many rocks are made up of debris from older rocks that had apparently decayed in the past

• Sedimentary rocks were still forming in the sea• Cyclic view

– Old rocks broken down and new rocks formed• equilibrium

Hutton’s Observations

• These changes occurred very slowly

• Thus, if these processes produce the patterns we see in geology the Earth must be much older than 6000 years

• “What more can we require? Nothing but time.”

Hadrian’s Wall

Geological Stratigraphy

Deposition

Law of Superposition

• The top layers of deposition are the youngest and the lowest layers of deposition are the youngest

• Allows for “relative dating” in sedimentary rock layers

Erosion

Charles Lyell

• British Geologist• Principles of Geology, 1830

– Showed that the geological principles taking place today also took place in the past

– Processes occurring today took place at the same rate as in the past

• Uniformitarianism– “the present is the key to the past”

Age of the Earth?

• Lyell’s work influence the thinking of Charles Darwin– Geologists, biologists, and paleontologists all

started to recognize that 6000 years was too young

• “I could get along very well if it were not for those geologists. I hear the clink of their hammers at the end of every Bible verse”– John Ruskin, 1851

Age of the Earth?

• Early Geologists concluded that the Earth was much older than 6000 years…. But how old?

Estimates of Age of the Earth• John Phillips using stratigraphy - 96 million years old.• Mikhail Lomonosov suggested in the mid-18th century that Earth is several hundred

thousand years old.• In 1779, Comte du Buffon tried to obtain a value for the age of Earth using an

experiment– He created a small globe that resembled Earth in composition and then measured its rate

of cooling. This led him to estimate that Earth was about 75,000 years old.• In 1862, the physicist William Thomson fixed the age of Earth at between 20 million

and 400 million years.– He assumed that Earth had formed as a completely molten object, and determined the

amount of time it would take for the near-surface to cool to its present temperature. • His calculations did not account for heat produced via radioactive decay (a process then unknown to

science) or convection inside the Earth, which allows more heat to escape from the interior to warm rocks near the surface.

• Geologists had trouble accepting such a short age for Earth. Biologists could accept that Earth might have a finite age, but even 100 million years seemed much too short to be plausible.

Radiometric Dating

• A brief Chemistry Review– Atoms made up of

• Protons• Neutrons• Electrons

– Protons and neutrons much heavier than electrons

– Protons (+), electrons (-), neutrons (no charge)

Brief Intro to Chemistry

• Elements defined by the number of protons in the nucleus– Hydrogen – 1 proton– Carbon- 6 protons

Isotope

• The same element can have different isotopes– Same number of protons, but different numbers of

neutrons

Radioactive Decay

• The spontaneous transformation of an unstable atomic nucleus into a lighter one, in which radiation is released in the form of alpha particles, beta particles, gamma rays, and other particles.

• When this occurs, the parent material is converted into the daughter material

Radioactive Decay

• The rate of decay or rate of change of the number N of particles is proportional to the number present at any time– This rate is constant

Half-life

• The half-life is the amount of time it takes for one half of the initial amount of the parent, radioactive isotope, to decay to the daughter isotope. – Thus, if we start out with 1 gram of the parent

isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.

– After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc

Stable Isotopes

• Some isotopes are stable– They don’t undergo radioactive decay– Therefore, isotopes often decay from one stable

form to another

Radioactive DecayParent Daughter Half-life

Uranium-235 Lead-207 0.704 billion years

Uranium-238 Lead-206 4.47

Potassium-40 Argon-40 1.25

Rubidium-87 Strontium-87 48.8

Samarium-147 Neodymium-143 106

Thorium-232 Lead-208 14.0

Rhenium-187 Osmium-187 43.0

Lutetium-176 Hafnium-176 35.9

Radioactive Decay

Thus, the proportion of parent/daughter changes over time in a predictable manner.

Radioactive Dating

• http://www.youtube.com/watch?v=1920gi3swe4

Practice Estimating Dates With Radioactive Dating

• http://www.sciencecourseware.org/VirtualDating/files/1.0_ClocksInRocks.html

Age of the Earth- Oldest Rocks

• Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents.

• The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 billion years old) and the Isua Supracrustal rocks in West Greenland (3.7 to 3.8 billion years old)– these ancient rocks is that they are not from any sort of

"primordial crust" but are lava flows and sediments deposited in shallow water, an indication that Earth history began well before these rocks were deposited.

Age of the Earth

• There are more than 70 meteorites, of different types, whose ages have been measured using radiometric dating techniques.

• The results show that the meteorites, and therefore the Solar System, formed between 4.53 and 4.58 billion years ago.

Estimating the Age of the Universe

• An approach to estimating is the age of the universe is to measure the “Hubble constant”. – The Hubble constant is a measure of the

current expansion rate of the universe. – Cosmologists use this measurement to extrapolate

back to the Big Bang.– This extrapolation depends on the history of the

expansion rate which in turn depends on the current density of the universe and on the composition of the universe.

Estimating the Age of the Universe

• If the universe has a very low density of matter, then its extrapolated age is

1/Ho

Estimating the Age of the Universe

• Many astronomers are working hard to measure the Hubble constant using a variety of different techniques.

• Until recently, the best estimates ranged from 65 km/sec/Megaparsec to 80 km/sec/Megaparsec, with the best value being about 72 km/sec/Megaparsec.

• Thus, estimate the age of the Earth is between 12 and 14 billion years.

Estimating the Age of the Universe

• Measurements by the WMAP satellite (sent up in 2001) can estimate the parameters needed to estimate the age of the universe to about 1%: 13.7 ± 0.13 billion years!