1. How Old is Stuff?
Dr. Mark McGinley
Honors College and Department of
Texas Tech University
2. 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
• Bishop James Ussher made
the same estimate a
• Concluded Creation
happened about 6000
3. Biblical View of Geology
• Features of the Earth
catastrophes such as
• Some 18th century
4. Birth of Modern Geology
• The field of geology
was developing in the
late 18th century in
• James Hutton
– Scottish Farmer
– Interested in the
environment so plants
5. 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
• Cyclic view
– Old rocks broken down and new rocks formed
6. 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
11. 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
– “the present is the key to the past”
12. Age of the Earth?
• Lyell’s work influence the thinking of Charles
– Geologists, biologists, and paleontologists all
started to recognize that 6000 years was too
• “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
13. Age of the Earth?
• Early Geologists concluded that the Earth was
much older than 6000 years…. But how old?
14. 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
– 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.
15. Radiometric Dating
• A brief Chemistry Review
– Atoms made up of
– Protons and neutrons much heavier than
– Protons (+), electrons (-), neutrons (no charge)
16. Brief Intro to Chemistry
• Elements defined by the number of protons in
– Hydrogen – 1 proton
– Carbon- 6 protons
• The same element can have different isotopes
– Same number of protons, but different numbers
18. 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
19. 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
• 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
– 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
21. Stable Isotopes
• Some isotopes are stable
– They don’t undergo radioactive decay
– Therefore, isotopes often decay from one stable
form to another
25. Practice Estimating Dates With
26. 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.
27. Age of the Earth
• There are more than 70 meteorites, of
different types, whose ages have been
measured using radiometric dating
• The results show that the meteorites, and
therefore the Solar System, formed between
4.53 and 4.58 billion years ago.
28. 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.
29. Estimating the Age of the Universe
• If the universe has a very low density of
matter, then its extrapolated age is
30. 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.
31. 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!