Geoscience Reference
In-Depth Information
erosion of young glacial deposits by the Niagara River. h is disparity led the
geologists at the time to reject Croll's entire theory, and, sadly, he died an
unappreciated scientist.
milankovitch cycles
Fortunately, Croll's ice age theory was revived two decades later by a young
Serbian mathematician, Mulatin Milankovitch, who had read Croll's topic
and was so inspired that he made it his life's goal to understand the earth's
past climate. He began with calculations of the amount of solar radiation
received by the earth's surface at dif erent latitudes and seasons over the past
130,000 years, based on the orbital cycles. Milankovitch calculated that vari-
ations in the tilt of the earth's rotational axis seemed to produce the largest
changes in solar radiation (and temperature) on the earth's surface. h e tilt
of the spin axis with respect to the earth-sun plane, currently 23.5°, l uctuates
between 22° and 24.5°. h e importance of this becomes more evident when
we consider that the earth's tilt controls our seasons: when the Northern
Hemisphere tilts away from the sun's rays, it experiences winter, and when
the Northern Hemisphere tilts toward the sun, it experiences summer. Each
day, the earth completes a rotation around this axis that is slightly dif erent
than the day before. h e dif erence again is tiny, but it forms a tick in a long
cycle that takes 41,000 years to complete, during which our seasons become
more or less pronounced.
Contrary to Croll's i ndings, Milankovitch calculated that the most likely
time for the growth of large ice sheets is not during the Northern Hemisphere
winter but during its summer. When summer temperatures in the mid- to
high latitudes of the Northern Hemisphere are cooler, the snow that has
accumulated over the winter persists during the summer, rather than melt-
ing. Year by year, snow continues to build up, i nally turning to ice under the
accumulated weight and pressure, eventually growing into a large ice sheet.
evidence from the deep sea
h e i nal link in the great ice age puzzle was discovered in the 1950s, when
geologists began drilling cores in the sediments of the deep seabed. h ese sedi-
ments include tiny carbonate microfossils (the foraminifera) that continuously
