Explain: Clocks in Rocks
Entry Task
 How do rock layers provide evidence of
change across time?
 Order the layers from oldest to youngest
(sketch and label the picture in your
notebook)
Explain: Clocks in Rocks
Start a new header/thread “Clocks in
Rocks” and date 11/8/2013.
Learning Target: I can describe the
process of radioactive decay and how
radiometric dating techniques can be
used to date rocks.
Update your TOC – don’t forget the
date!
Explain: Clocks in Rocks
Read the Introduction p. 288
We are going to do the process and
procedure a bit out of order.
P&P 3,4,5 first – class activity
P&P 1,2,6-8 - individually
Explain: Clocks in Rocks
P&P 3,4,5: Half-life simulation
Type
of
atom
Trial
#
Parent 1
2
3
total
Daugh 1
ter
2
3
total
0s
10s
20s
30s
40s
50s
60s
Explain: Clocks in Rocks
Complete P&P 1,2,6-8
Make sure you take notes on the
reading.
Write a summary of the reading (2
sentences)
Complete R&C #1-4 p. 293-296
Read/Notes on “FYI: Changing Rates of
Change” p. 294-295.
Summary at the end!
Entry Task
Read Predator and Prey Process
and Procedure p. 296-300.
Radiometric Dating notes
 In the late 1800’s, scientists discovered that
some minerals in rocks contain radioactive
elements.
 Isotopes are atoms of the same element that
contain different numbers of neutrons.
This means that isotopes have the same
atomic number (number of protons), but
different atomic masses (number of protons
and neutrons).
The nuclei of some isotopes are unstable –
they break down into isotopes of different
elements which are more stable.
Radiometric Dating notes
 Radioactive decay is the process by which the
nucleus of an unstable atom emits particles of
radiation to become a more stable isotope of a
different element.
 Particles that are emitted can be alpha particles
(He nucleus), beta particles (electrons) or
gamma rays (energy)
 alpha decay – results in a new element
 beta capture - results in a new element (a proton
becomes a neutron)
 beta decay - results in a new element (a neutron
becomes a proton)
 Gamma decay – element doesn’t change, energy is
released.
3 primary ways of decay
alpha decay (Z ≥ 58)
particle has 2 neutrons and 2 protons
U238
Th234
92 protons
90 protons
beta decay (n0 = p+ + e-)
breakdown of neutron into an
electron and a proton and loss
of the electron to leave a proton
(result is gain of one proton)
K40
Ca40
19 protons
20 protons
electron capture (e- + p+ = n0)
capture of an electron by a proton
and change of proton to neutron
(result is loss of proton)
K40
Ar40
19 protons
18 protons
Radiometric Dating Notes
 The original element is called the parent
element.
 When the parent element decays, the new
element is called the daughter element.
 The half-life is the amount of time it takes for
exactly half the parent atoms to decay to
daughter atoms.
 Each radioactive element has a unique halflife. (the rate of decay is constant)
 Some half-lives are small (days – years), some
are very large (millions to billions of years).
predictable ratios at each half-life
exponential decay (half always remains)
exponential decay: never goes to zero
exponential
linear
example: Uranium 238 decay to Lead 206 (stable)
several steps
(each has its own half-life)
most common dating systems
• uranium-thorium-lead dating (previous example)
U-238, U-235, Th-232
each of these decays through a series of steps to Pb
U-238 to Pb-206
U-235 to Pb-207
Th-232 to Pb-208
half-life = 4.5 by
half-life = 713 my
half-life = 14.1 my
• potassium-argon dating
…argon is a gas--may escape
(ages too young--daughter missing)
K-40 to Ar-40
half-life = 1.3 by
• rubidium-strontium dating
Rb-87 to Sr-87
half-life = 47 by
Radiometric Dating Notes
 This is useful in dating rocks because
scientists can measure the amount of
parent and daughter atoms (elements)
present and calculate the amount of time
that has passed (the number of half lives
that have passed).
 By measureing the amount of various
isotopes, geologists can determine the
absolute age of rocks.
Radiometric Dating Notes
 Radioactive isotopes can be found in rocks from
volcanoes – ash layers and lava flows
 The clock “starts” when the rock is formed
 The decay products begin to accumulate in the
minerals when the rock solidifies
 Most volcanic rocks contain more than one
radioactive element, so scientists have multiple
data sources they can compare.
 Volcanic ash layers are deposited world-wide, so
they provide valuable information.
R&C #1
 Explain how radiometric dating works.
 Radioactive isotopes are unstable and break down into
more stable isotopes through a process called
radioactive decay. The rate of decay for a particular
isotope is a constant and can be expressed as half-life.
The half-life is the amount of time it takes for exactly
half of the parent isotope to decay into daughter
isotope.If scientists measure the amount of parent
isotope and daughter isotope in a mineral of a rock, they
can calculate how many half-lives have passed.
Geologists use isotope systems that let them measure
ages of millions or billions of years.
R&C #3
 How do fossils, rock layers, and radiometric dating
technologies work together to provide evidence for
change across time?
 Rock layers (strata) provide evidence for the order that
sediments were laid down. You can determine the
relative age of fossils based on what rock layer they are
found in. Radiometric dating of rock layers such as ash
can give an accurate age for that layer. Then the age
range of the surrounding rock layers can be determined.
With all three pieces of information, we know
environments have changed and when those changes
occurred.