Geoscience Reference
In-Depth Information
thinkers to attempt to quantify sedimentation rates. He calculated
that it would take 5,000 years for the Red Sea to silt up completely.
This sedimentation theme was taken up nearly 2,400 years later in
England and still later in the United States, and for a while occupied
the thoughts of geochronologers until it was swept away by the pre-
sentation of a new theory.
SEDIMENTATION RATES AND GEOCHRONOLOGY
The basic premise underlying the use of sedimentation rates as a tool
for estimating the age of the Earth is that if one can estimate the
thickness of a modern sedimentary deposit such as a delta, and one
knows the rate at which sediment was added to it over a period of a
year, then a simple mathematical calculation will give the length of
time that the delta has been forming. Similarly, if one knows the
original height of a feature on the surface, such as the Colorado
Plateau, and one measures the depth of the Grand Canyon and
knows the rate of downcutting by the Colorado River into the level
sediments, one can calculate the length of time that has elapsed since
the beginning of canyon formation. Simple, it would seem, but in
actuality this calculation is not so, even for the seemingly straightfor-
ward deltaic model. It is even more complicated when you look at the
thickness of the sedimentary rocks that make up much of the geolo-
gical record. In an ideal situation (but the Earth is never that helpful), it
suffices to say that if we know the thickness of the sedimentary pile
that makes up the complete rock succession and the rate at which it
was deposited we should be able to estimate the age of the Earth.
Denudation, or the breakdown of rocks exposed at the Earth's
surface, leads to the topography seen today. Denudation produces two
different types of product: firstly, particulate matter or 'clasts' of a
variety of sizes that range from cobbles (the largest grains) through
sand and silt, to mud (the finest fraction), and secondly, unseen ions in
solution in water. Both products were used as geochronometers - as
we saw earlier, Edmond Halley discussed how an estimation of the
rate at which a freshwater body became salty might be useful in
