Geoscience Reference
In-Depth Information
came from the geology of the Alps, from the uplift and subsidence of continents,
and from radioactivity.
Rock formations in the Alps have sometimes been folded like toothpaste and
then the folds pushed over on their sides. The forces that had produced these struc-
tures had not been vertical, as Suess's model held, but horizontal. Moreover, when
geologists mentally restored the Alpine folds to their original length, they found
that the rocks had been compressed and shortened as much as five times: a fol-
ded and twisted belt that is now, say, ten kilometers wide, as an original, unfolded,
horizontal rock layer would have been fifty kilometers wide. To extrapolate that
degree of shortening to mountain ranges all around the Earth would require an im-
possible amount of shrinking.
TheideabehindthesecondblowtoSuess'smodelarosewhensurveyorsinIndia
realized thattheirleveling plumbbobsswungawayfromthemountains ratherthan
toward them. This led to the idea that a deficiency of mass beneath high moun-
tain ranges offsets their obviously substantial bulk. If mountains are made of rocks
less dense than the surrounding plains, and if mountains have deep roots, then
they would exert less gravitational pull, and a plumb bob would swing away from
them. Mountains may be like icebergs, extending downward in proportion to their
height. But that would require that, like icebergs, mountains float in a fluid sub-
stratum. The American geologist Clarence Dutton (1841-1912) gave the process
by which large blocks achieve gravitational balance the name
isostasy
, Greek for
“equal standing.” One can think of it as Archimedes' principle applied to large
blocks of the Earth's crust.
Dutton thought that as the weight of sediments accumulating on a continental
margin depresses the crust, the material at depth has to shift inward, where it up-
lifts the adjacent continent, which then erodes faster, shedding more sediment, and
so on in a kind of feedback loop. He acknowledged that this requires that, at depth,
apparentlysolidrockcanbehaveasafluid.But,Duttonnoted,Naturehasprovided
an example in “the motion of the great ice sheet which covers Greenland.”
8
Radioactiveheatdeliveredthethirdanddecisiveblow.Becauseeachradioactive
decay event releases heat, the Earth may not be cooling, contracting, and running
down. Instead, radioactive heating could just as well have brought the Earth to a
stable temperature or even have caused it to warm and expand.
A Science Without a Theory
