THE WORK OF DENUDATION
Transference of material from land to sea—The denudation
ratio—Weathering of rocks—The work of chemical
denudation—Summary of the data—Composition of the saline
matter in the oceans and of that annually carried to the
oceans—The work of mechanical denudation—Suspended
and bottom loads of rivers—Mississippi not a good
average case—Dole and Stabler’s work in the United
States—Application to the whole land area—Total
material removed and rate of degradation of land—Marine
erosion—Types and quantities of sediments annually produced.
The purely geological methods which have been devised to investigate
our problem are of two kinds. The first attempts to apply a time-scale
to the sedimentary rocks and was, historically, the earliest to be
proposed; the second, due to Joly, deals with the accumulation of salt
in the oceans. The one is concerned with material carried away from the
land mechanically; the other with the material removed in solution. The
various agents of weathering, of which rain and frost are the chief,
disintegrate the surface rocks and supply the rivers with their load of
detritus. The turbid condition of rivers when in flood, heavily charged
with alluvial matter, is a familiar and convincing proof that the
effect of erosion in conjunction with the transporting power of running
water must always be to wear down the land areas. In the dynamical
study of denudation and sedimentation the first essential is to know
the rate at which the rivers are working. Measurements of their load of
silt and dissolved salts and of their annual discharge to the sea make
it possible to arrive at reliable estimates of their activity.
Incidentally, it is useful to determine the ratio which solvent
denudation bears to the whole. The _denudation ratio_, as it may
conveniently be called, is the ratio of the load of dissolved material
to the total load carried both in solution and in suspension.
The amount of material removed in solution from the surface rocks is
not quite the same as that which is carried to the oceans. A small
proportion is abstracted from the over-ground circulation by the waters
which sink below the surface, and while some of this is undoubtedly
brought up again through the agency of springs, it seems possible, as
Prof. Schwarz has boldly suggested, that certain constituents, such as
iron and magnesium, may be permanently removed from the earth’s crust
by downward migration. In the denudation ratio this possibility is left
out of account as having no bearing on the study of sedimentation.
A rough estimate of the denudation ratio may be made by considering the
weathering and decay of rocks _in situ_. Soluble constituents are
withdrawn by leaching and a residue of the more stable minerals and
alteration products is left behind. Weathering involves not only the
abstraction of material but also the introduction of fresh material
from external sources. Oxidation, hydration, and carbonatisation are
the most typical reactions, and they must be allowed for in determining
the proportion of the original rock lost by solution. This can be done
approximately by assuming that some element—aluminium being usually
chosen—has remained invariable during the course of decomposition.
From the analysis of a large number of fresh rocks and of their altered
equivalents, it is found that on an average 30% is dissolved, leaving a
residue of 70%. The denudation ratio ought therefore to be about 0·3.
The direct determination of the work of chemical denudation requires
three distinct sets of measurements: (_a_) the annual discharge
of rivers into the oceans; (_b_) the analysis of their waters;
(_c_) the measurement of their drainage areas. Mellard Reade was
the first to point out the importance of these factors in the study of
dynamical geology. In 1879 he collected such information as was then
available and deduced from it the quantity of rock material annually
removed from the whole land area, his estimate being 5280 million tons.
Sir John Murray’s corresponding estimate of 1887, which was based on
analyses of the waters of nineteen of the world’s principal rivers, was
4975 million tons.
Until 1909 this figure could not be improved upon, but in that year
there was published by the United States Geological Survey the results
of the detailed and systematic work carried out by R. B. Dole and H.
Stabler. For the first time an attempt had been made to measure the
discharge, drainage areas, salinity, and suspended load of all the
important rivers of a large continental area. The estimates represent
the averages of observations made daily for a year or longer, and in
the case of the discharge, the measurements extended over at least
seven years.
Seasonal variations and the effects of floods are apt to be misleading,
and a long continued series of observations is necessary if their
relative importance is not to be over—or under—estimated. The amount
of material in solution varies but slightly from year to year, and the
average of one year’s results is within 10% of the true mean value. The
annual discharge varies much more than this, and still more inconstant
is the load of suspended material, which, in certain years, may differ
from the average value by 50%. These figures indicate how difficult
it is to introduce exact measurements into geology with any hope of
finality.
A summary of all the best data now available has recently been given
by Dr. F. W. Clarke, and covers about 28 million square miles of the
drainage areas of the earth. The details are given in the following
table:
-----------+---------------+------------------------
| | SOLVENT DENUDATION.
| DRAINAGE AREA | TONS ANNUALLY REMOVED.
CONTINENT. | IN SQ. MILES. +---------+--------------
| | PER SQ. | FROM WHOLE
| | MILE. | AREA.
-----------+---------------+---------+--------------
N. America | 6,000,000 | 70·5 | 423,000,000
S. America | 4,000,000 | 45·5 | 182,000,000
Europe | 3,000,000 | 90·0 | 270,000,000
Asia | 7,000,000 | 75·0 | 525,000,000
Africa | 8,000,000 | 40·0 | 320,000,000
| ---------- | ---- | -------------
| 28,000,000 | 61·0 | 1,708,000,000
-----------+---------------+---------+--------------
The total land area of the globe is estimated by Murray at 55·7
million square miles, but of this 11·5 million square miles are areas
of internal drainage, such as the Great Basin of the United States
and the Asiatic depressions, which contribute nothing to the ocean.
The circumpolar regions, representing 4·5 million square miles, must
also be left out of account, and the remaining 39·7, or say, 40
million square miles, is that from which the oceans are fed. If the
figures given above be accepted as typical, then the annual addition
of material to the oceans by solution amounts to 2440 million tons.
Some of this, however, is derived from the atmosphere—chiefly as
carbon-dioxide. Applying the necessary correction of nearly 10%, there
remains 2220 million tons as the amount actually derived from the
rocks. Murray’s estimate, it will be noticed, is almost exactly twice
that of Clarke.
The composition of the saline matter carried to the oceans may be found
by suitably weighing each analysis of river water according to the
discharge of the latter. The general mean of all such results is given
in the table opposite, together with the total amount of each substance
in the ocean. The small traces of elements other than those listed are
quite insignificant.
---------------------+----------------------+--------------------
| ANNUAL ADDITION | TOTAL SALINE MATTER
CONSTITUENT. | OF SALINE MATTER | OF THE OCEANS IN
| IN MILLIONS OF TONS .| MILLIONS OF TONS.
---------------------+----------------------+--------------------
SiO₂ | 284 |
Fe₂O₃Al₂O₃ | 67 |
Mg | 83 | 1,535,000,000
Ca | 497 | 490,000,000
Na | 156 | 12,616,000,000
K | 37 | 454,000,000
Cl | 138 | 22,800,000,000
Br | | 78,000,000
CO₃ | 857 | 80,300,000
NO₃ | 22 |
SO | 299 | 3,172,000,000
---------------------+----------------------+--------------------
Total | 2440 | 41,230,000,000
---------------------+----------------------+--------------------
Annual discharge of river water
into ocean = 24·3 × 10¹² tons
Volume of ocean water = 307,496,000 cubic miles
Density of ocean water = 1·026 (mean)
Mass of ocean water = 1,178,270 × 10¹² tons
-----------------------------------------------------------------
It is obvious from a comparison of these two columns that the annual
increment of dissolved matter is not permanently retained by the
oceans. The greater proportion is precipitated by chemical and organic
agencies, and either becomes incorporated with detrital material or
goes to form individual sediments. The chief substances produced are
calcium and magnesium carbonate, gypsum, limonite, and silica. Rather
more than one-third of the carbonates appear to associate themselves
intimately with sands and muds. The greater part of the remaining
two-thirds is deposited as limestone on the continental shelves (after
being used by various organisms in shell-making) in waters which are
comparatively free from terrigenous sediment. The abstraction of gypsum
from the ocean takes place at irregular intervals under suitable
conditions of concentration. Of the limonite and silica, the chief
precipitation takes place on the continental shelves where they
associate themselves with the detrital sediments. The history of
potassium is rather obscure, but on the contrary, that of sodium
appears to be the simplest of all. That it is stored up in the oceans
is an assumption which is granted as justifiable by most geologists.
The only new factor required in order to estimate the mechanical work
of denudation is the load of material carried by the rivers. Besides
the silt transported in suspension, larger fragments are carried by
rolling along the stream bottom. Measurements of the bottom load are
lacking except in a solitary case—that of the Mississippi—in which
it amounted to about 10% of the whole. It is difficult to define any
precise difference between bottom load and suspended load, the former
being only a limiting case of the latter. When the water is fully
charged with rock debris the highest proportions are found near the
bottom and sides, and at a point in mid-stream at about one-third the
depth from the surface—the position of the stream lines of maximum
velocity. In clear water the rolling power reaches its maximum value,
for apart from fluid friction, energy is expended in no other way. Most
rivers fall between these extremes. In making actual measurements,
samples are taken from representative points in the river and used to
give the average over the whole section. In the final estimate it seems
probable that a large proportion of the bottom load is accounted for.
Even if a correction ought to be applied it would be pedantic to do
so except for rivers which have been under observation for several
consecutive years, since the variation from the mean annual load is
very great from year to year. The Nile varies by 40% and the Potomac
by as much as 100%. In the United States the mean variation is about
50%. With uncertain data of this kind a correction of less that 10% may
safely be disregarded.
Of all the rivers of the world, the Mississippi has been most favoured
by measurements of the kinds required, and many estimates of the rate
of continental degradation, of the rate of deposition of sediments and
of the age of the earth have been based upon them. There is no doubt,
however, that the Mississippi is working more rapidly than any other
river of importance in North America, except, perhaps, the Colorado
River. The high declivity in the west, the Tertiary elevation of the
plains to which the streams are not yet adjusted, and the abundance
of easily eroded glacial drift are all factors which promote this
activity. In the case of rivers other than those of North America for
which data are available, the same high rate of denudation obtains; the
Rhone and the Po, for example, being amongst the most energetic workers
in the world. Generalising for the whole earth from these rivers alone,
would obviously give misleading results. The work of Dole and Stabler
again comes to our aid, and in the following table their aggregate
measurements for the whole area of the United States are tabulated.
Similar evidence for four widely separated rivers is also given:
----------------+-----------+----------------------+----------
| | MILLIONS OF TONS OF |
| | MATERIAL REMOVED |
DRAINAGE BASINS.| AREA IN | PER YEAR. | DENUDATION
| SQ. MILES.+-----------+----------+ RATIO.
| |IN SOLUTION| IN |
| | |SUSPENSION|
----------------+-----------+-----------+----------+----------
United States | 3,088,500 | 241·5 | 468 | 0·34
Mississippi | 1,265,000 | 122 | 304 | 0·29
Nile | 1,100,000 | 21 | 52 | 0·29
Uruguay | 150,000 | 7·5 | 15 | 0·33
Rhone | 34,800 | 8·5 | 36 | 0·19
----------------+-----------+-----------+----------+----------
Leaving out the Mississippi because of its inclusion in the United
States and weighing each result according to the area over which it
holds, the mean denudation ratio is 0·31, a figure which agrees very
well with our previous estimate. If now we use the denudation ratio to
calculate the material removed by mechanical denudation over the whole
land surface, we should not be far from the truth. It is clear that
if from the 40 million square miles which drain into the oceans the
quantity of material carried in solution represents 0·3 of the total
material removed, then, as the former amounts to 2440 millions of tons
annually, the quantity carried away mechanically must be 5700 million
tons. That this figure is of the right order is favoured by another
consideration. The mean elevation of North America is very nearly that
of all the land areas of the earth. Moreover, according to Clarke’s
figures the rate of denudation over North America is slightly higher
than the average for all the lands, but more closely approaches it than
does that of any other continental area. We may therefore take the rate
of denudation of North America as a fair average and apply it with some
confidence to all the drainage areas of the globe. Doing this, the
total amount of suspended material annually discharged into the oceans
is computed to be 6000 million tons.
We may sum up the work of denudation in round figures as follows:
Material annually removed in solution 2500 million tons
Material annually removed in suspension 6000 ” ”
-----------------
Total 8500 million tons
These figures may also be expressed in terms of the rate at which the
land areas are being worn down. By solvent denudation a degradation
of one foot in 30,000 years is implied, and by mechanical denudation,
one foot in 12,000 years. Taking both together the average rate of
denudation is found to be _one foot in 8600 years_. It should
be clearly understood that individual areas may be lowered at rates
very different from this. The maximum rate is attained in the Irawadi
basin, one foot of which is removed in 400 years. The Po is also an
exceptional river, and lowers its basin by one foot in 850 years.
On the other hand, in the Hudson Bay district of North America the
drainage only carries away one foot in 47,000 years.
No minimum figure can be given, for wherever deposition of sediment
takes place on the land areas the temporary rate of denudation locally
becomes negative. In making these calculations, the density of rock
material is taken as 2·6; the weight of a cubic foot as 165 lbs;
and the weight of a cubic mile as 10,800 million tons. Although the
surface covering of loam or earth weighs only about 100 lbs per cubic
foot, the denser and more closely packed underlying rock need alone be
considered, for it is by its decay and expansion that the superficial
blanket above is produced.
Our final problem is to determine the nature and quantity of the
sediments which are ultimately formed on the continental shelves. This
can only be done roughly, but the results will suffice to serve our
purpose. First of all, two serious difficulties must be met before the
way is open to take this step. So far, marine denudation has been left
out of account. It is not yet possible to make a wholly satisfactory
estimate of the relative magnitude of the supply of detritus captured
directly by the sea. The unknown factor is the average encroachment
of the sea upon the coasts. For the British Isles, Croll suggested
an average of three feet per century, and the figure assumed by Sir
A. Geikie about the same time was ten feet per century. A much later
estimate by Prof. Watts places the average retreat of the English
coast at a hundred feet per century. Along parts of our East coast
marine erosion is still more rapid than this, the conditions being
exceptionally favourable. On the other hand, Geikie considers that all
the force of the Atlantic beating upon the N.W. coast of Scotland may
not wear it away at more than one foot per century. What the average
between these extremes may be can only be guessed at. If for
convenience we accept Geikie’s figure as affording a likely average for
all the coast lines of the earth (125,000 miles), and if the average
height of the cliffs be taken as 150 feet, then the mass of material
annually removed will be about 700 million tons.
The other difficulty is concerned with the annual amount of material
which remains in the oceans in solution, and also of that which is
deposited on the ocean floor outside the limits of the continental
shelves. For the former, a knowledge of the age of the oceans is
necessary. Considering all the evidence, the amount retained at the
present day seems to be about 200 million tons, but this is certainly
too high as a figure representing the average increase throughout the
history of the oceans.
For the deep-sea deposits little more than a guess is possible,
although we can now approximate to the right order of magnitude by
considering the circulation of radium. The radium in the material
removed from the lands is redistributed between the sediments on the
continental shelves, the deep-sea deposits and the water of the oceans.
Applying our present knowledge of the distribution of radium (see p.
131) the annual mass of the deep-sea deposits is found to be about ¹/₃₀
of the whole, i.e. about 300 million tons.
The difficulties can scarcely be avoided by balancing them against
each other. There still remain 200 million tons (700 - 300 - 200)
to be added to the 8500 million tons already found as the total for
sub-aerial denudation. This gives us 8700, or as an extra safeguard,
say 9000 million, tons as the mass of sediment annually deposited on
the continental shelves. It is unfortunate that to arrive at this
figure an element of doubt should be introduced by associating the
results of careful experimental work with the vague conclusions just
arrived at. The bugbear of the whole investigation is marine erosion;
but if it is remembered that the figures given in that connection are
meant to be suggestive rather than final, no erroneous impression need
be carried away.
If the sediments ultimately formed are shales (20% quartz), sandstones
(75% quartz) and limestones (75% calcium carbonate), their proportions
will be as follows:
Shales 70% or 6300 million tons.
Sandstones 16% “ 1440 ” ”
Limestones 14% ” 1260 ” ”
--------------------------
Total 100% = 9000 million tons.
In the two following chapters the application of denudational
statistics to the measurement of geological time will be considered.