Cooling Rate Calculations:
William Thomson, more popularly known as Lord
Kelvin, determined in the mid-1800’s that the Earth was between 20
and100 million years old. Kelvin, who was very upset at Darwin’s
ideas about change in organisms over time, undertook studies on the age
of the Earth hoping to demonstrate that the Earth was not old enough to
allow for accumulated change in organisms over time. His
calculations were based on three assumptions (1) the Earth began as a
molten, or melted rock; (2) cooling occurred
at a uniform, continuous rate; and (3) no heat is generated within the
Earth. While the Big Bang theory and subsequent ideas about the
formation of planets suggests that assumption #1 is a good one, we know
that assumptions 2 and 3 are not correct. Melted (igneous) rocks
don’t
cool at either continuous or uniform rates; any igneous rock with
different crystal sizes is proof of that. Rock is also a very
good
insulator of heat, and rates of cooling at the Earth’s surface would be
drastically slower deep within the Earth. Despite these flaws,
Kelvin’s calculations brought observation,
measurement and objectivity to determination of the age of the
Earth.
Darwin and his supporters interpreted Kelvin’s results as proof of the
antiquity of the Earth and the existence of adequate time for evolution
to occur. Kelvin felt otherwise.
Kelvin's
Calculations:
Kelvin assumed a starting temperature of the Earth of 1200°C to
3870°C.
Based on the geothermal gradient (about 1°/foot in deep mines) and
an assumption that at molten rocks existed at depth, he determined from
the geothermal gradient that the cooling rate of the Earth would be
somewhere between 0.00006°/yr and 0.0002°/yr
So, for example, the age of Earth = 1200°/x = 0.00006°/year or
3870°/x= 0.0002°/yr, in other words, around 20 million years
old.