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Cooling Rate Calculations:

Lord Kelvin
    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.

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