Page 170 - Handbook of Gold Exploration and Evaluation

148 Handbook of gold exploration and evaluation

3.7 Simplified distribution of Sun's radiant energy.

atmosphere. The seasons are determined by the Earth's axis of rotation relative
to the Sun. At present, the Earth rotates with its axis at about 23.5ë in relation to
the Sun, but over time there is evidence that its axis wobbles between 22.1ë and
24.5ë, which causes variations in the amount of sunlight received over the
Earth's surface and modifies its seasons. According to the Milankovitch theory
this phenomenon, which affects the amount of sunlight (insolation) received on
the Earth's surface during summer and winter, cycles every 41,000 years.
In polar regions the Sun's rays have more atmospheres to penetrate than at
lower latitudes and they strike the ground obliquely, thus spreading the rays over
larger surface areas (Fig. 3.8). In permafrost regions, vegetation acts as a cover
over the frozen ground keeping it away from the warming rays of the sun. Thus,
while early miners in Alaska progressed from using fires, then steam and then
water to thaw the frozen gravels underground, they eventually found that by
clearing the surface vegetation, sufficient solar energy was absorbed to thaw the
ground for mining (Woodsend, 1984). Insolation effects are also apparent in the

3.8 Intensity of insolation at surface from (a) direct and (b) oblique radiation.