Key to homework-1 for Class G&G 160
1. Measuring the age of the earth
In class, we have been talking in general about the history of measuring the age of the
earth, now we are going to work out some examples.
a)
De Maillet and the Decline of the Sea
: Benoit De Maillet (1656-1738), a
Frenchman, was one of the earliest naturalists to question the validity of the age
of the earth estimated from Biblical chronology. But he was well aware of the
power and influence of the Church, so he wrote his theory as a story in which
several conversations between him and an Indian Philosopher named Telliamed
(his name spelled backwards) over 6 days. Through this Telliamed, he
presented his view about the age of the earth. Now let us repeat his logic. De
Maillet accepted the idea that the earth was once covered entirely with water.
Some marine sediment (containing sea-shell fossils) was discovered on the
mountains far inland as high as 6000 feet. His grandfather had constructed a
hydrographic station on the shore near his village over 75 years earlier that
determined the sea level decreased at a rate of three inches per century. What is
“the age of the earth” based on De Maillet’s logic?
Key:
In order to get an age estimation based on De Maillet’s logic, at least three
assumptions have been implicitly made: 1) when the earth was born, the whole earth was
covered with seawater and the ocean was very shallow; 2) sea level was declining at a
constant rate; 3) the land was not uplifting. Given these assumptions, we can make the
age estimation as follows:
6
6000 feet 12 inch/feet
2.4 10 (years)
3 inch/100 years
t
×
==
×
(1.1)
The problem of this calculation is that none of these assumptions is strictly valid. For
example, we now know that mountains uplift due to plate tectonics and at the same time
sea level changes at a varied rate; in the early stage of the earth evolution, there could be
magma ocean instead of ocean filled with water, and etc. In general, these complications
suggest this calculated age in equation (1.1) underestimates the real age.
b)
Salt in the oceans
: Physics was not the only discipline to provide early
estimates of the age of the earth; chemistry was also set to the task, and in a
rather clever way. Edmund Halley (a British scientist, 1656-1742), of whom the
Halley comet bears the name, had proposed to use the concentration of some
elements in the ocean to determine the age of the earth, but he never did the
real work. John Joly (an Irish scientist, 1857-1933) is famous for implementing
this idea. At the time, the mean ocean depth was estimated to be 3797 m by Sir
John Murray (a Scots-Canadian oceanographer and marine biologist, 1841-
1914), and the ocean area of 1.0372 x 10
8
km
2
by Hermann Wagner (German
scientist, 1840-1929), density of seawater can be approximated to be about 1