PART 2: Calculating the age of the earth using salinity data
Purpose: Students will attempt to calculate the age of the earth using salinity data in
order to gain familiarity with a 19th century method of calculating the age of the
earth and to identify and explain the shortcomings of the method.
Introduction: Edmond Halley (1656 – 1742), of Halley’s comet fame, hypothesized
that seawater became salty with the infusion of salts dissolved from the ground. He
proposed the dissolved salts were carried down rivers to the sea and gradually
accumulated, raising the salinity of seawater over time.
In 1899, using Halley’s hypothesis, an Irish geologist, John Joly (1857 – 1933),
proposed a method for calculating the age of the oceans and perhaps the earth as
well. Joly reasoned that if the salinity of the oceans was known, along with the rate
at which salt was being added by rivers each year, the calculation could be done.
Activity:
See the sodium data and sample calculation below. The calculation uses Joly’s line
of reasoning to calculate the age of the earth using the data. Discuss the calculation
with your group. Later, you will use the same method of calculation using
magnesium rather than sodium.
1)
2)
3)
4)
The volume of the world’s oceans is 1,370,000,000 km3
Sodium makes up approximately 1% of seawater by volume
The world’s rivers empty a volume of 30,000 km3/yr into the oceans
Rivers contain between 3 to 11 ppm (parts per million) Sodium by volume
Sample calculation:
1) Calculate the total volume of sodium in the world’s oceans. This would be
1% of the total volume of water.
So, (.01)(1,370,000,000 km3) = 13,700,000 km3 salt in oceans.
2) Calculate how much sodium enters the oceans from the rivers annually.
Since there is a range of concentrations, calculate both high and low.
3ppm: (.000003)(30,000km3) = .09km3 salt entering oceans from rivers
11ppm: (.000011)(30,000km3) = .33km3 salt entering oceans from rivers
3) Divide total volume of sodium in ocean by annual volume input. Calculate for
high and low river input to get a range.
13,700,000 km3 salt/.09km3 salt/year = ~ 152 million years
13,700,000 km3 salt/.033km3 salt/year = ~ 41 million years
See the data for Magnesium below. Use the sample calculation as a guide and
perform the simplified version of Joly’s calculation using Magnesium.
5)
6)
7)
8)
The volume of the world’s oceans is 1,370,000,000 km3
Magnesium makes up approximately .14% of seawater by volume
The world’s rivers empty a volume of 30,000 km3/yr into the oceans
Rivers contain approximately 4 ppm (parts per million) Magnesium by
volume
Calculation:
Age estimate using magnesium ___________________________
Age estimate using sodium ____________________________
Summarizing Questions.
1. Compare the values you got in your calculations with Na and with Mg. How
close were they? What inferences can you make regarding which ion is
removed from rivers and oceans at a faster rate? Brainstorm reasons for any
differences you noted.
2. How close are the calculated values to your prior understanding of the age of
the earth?
3. Identify any assumptions you can think of that this method of calculating the
age of the earth relies on for validity. Also indicate whether that assumption
is correct or incorrect, and explain your thinking in the last column.
Assumption
Correct/Incorrect
Salt stays put once in the
ocean
Incorrect
Reasoning
Plate tectonics causes
seawater to pass
through the ocean crust
causing salt to be
removed from the water
Scientist ideas:
Joly’s attempt to calculate the age of the earth was innovative at the time, however his
reasoning is based on some faulty assumptions leading to a significant underestimate of
the age of the earth. A better way to describe the values obtained through this calculation
is as “residence time,” or the time that salt stays in oceans before being removed through
one of many processes. Sodium and Magnesium are removed through different processes
and thus at different rates, accounting for the varying values obtained using each ion.
The main assumption that Joly made that led to the underestimate of the earth’s age is his
assumption that salt stays in the oceans once deposited. There are several processes that
remove salt from the oceans including the process of plate tectonics that causes salt
removal from seawater as it passes through the ocean’s crust. Salt is also removed from
deposition of salt in sediments on the sea floor, and through transfer onto land from sea
spray. The method also assumes that the influx of salt in to the oceans has been constant
over time. Weather rates have been known to change drastically throughout geologic
time, so this assumption is likely false.
Magnesium is removed from seawater by various processes that differ from sodium’s
removal. For example, magnesium reacts with calcium carbonate in coral islands,
through this process, the magnesium becomes incorporated into the coral and thus is
removed from the ocean water. The rate at which magnesium is removed is higher, than
the removal of sodium, resulting in a larger underestimate of the age of the earth using
this calculation method with magnesium as the ion under consideration.
Discuss your responses to #1-3 and the scientists’ ideas above with your group.
Would you change any of your answers? Explain your thinking.