WEATHERING AND EROSION
All rocks at and near the surface of the earth crumble or decay.
The term "weathering" includes all the processes whereby rocks are
broken up, decomposed, or dissolved. A mass of very hard and seemingly
indestructible granite, taken from a quarry, will, in a very short
time, geologically considered, crumble (Plate 1). During the short
span of the ordinary human life weathering effects are generally of
very little consequence, but during the long ages of geologic time the
various processes of weathering have been slowly and ceaselessly at
work upon the outer crust of the earth, and such tremendous quantities
of rock material have been broken up that the lands of the earth have
everywhere been profoundly affected.
Most of us have noticed buildings and monuments in which the stones
show marked effects of weathering. A good case in point is Westminster
Abbey, London, in which many of the stones are badly weathering, some
of the more ornamental parts having crumbled beyond recognition since
the building was erected in the thirteenth century. In many countries,
tombstones and monuments only one or two centuries old are so badly
weathered that the inscriptions are scarcely if at all legible.
What are some of the processes of nature whereby rocks are weathered?
In cold countries, and often in mountains of generally mild climate
regions, the alternate freezing and thawing of water is a potent agency
in breaking up rocks where the soils are thin or absent. On freezing,
water expands about one-tenth of its volume and exerts the enormous
pressure of over 2,000 pounds per square inch. Nearly all relatively
hard rock formations are separated into more or less distinct blocks by
natural cracks called "joints" (Plate 8). Very commonly the rocks also
contain minute crevices, fissures, and pores. Repeated freezing and
thawing of water which finds its way into such openings finally causes
even the most resistant rocks to break up into smaller and smaller
fragments. A very striking example of difference in climatic effect
upon a given rock mass is the obelisk in Central Park, New York. For
many centuries this famous monument stood practically without change in
the dry, frostless climate of Egypt, but very soon after its removal to
the moist, frosty climate of New York, it began to crumble so rapidly
that it was necessary to cover it with a coating of glaze to protect it
from the atmosphere.
Temperature change, especially in dry regions, is also an important
agency for mechanical breaking up of rocks. On high mountains and on
deserts, a daily range of temperature of from 70 degrees to 80 degrees
is frequent. Due to heat absorption, rocks in desert regions, during
the day, not uncommonly reach temperatures of fully 120 degrees, while
during the night, due to heat radiation, their temperature falls
greatly. During the heating of the outer portion of the rock, the
various minerals each expand differently, thus setting up a series
of stresses and strains tending to cause the minerals to pull apart.
The outer portions of the rocks which are subjected to unstable and
relatively rapid temperature changes, often crack or peel off in slabs
or flakes, this process being called exfoliation. Stone Mountain in
Georgia, and some of the mountains of the southern Sierra Nevada range
in California, are excellent examples of mountains which are being
rounded off by exfoliation. The principle is the same as that which
causes the "spalling" of stones in buildings during fires.
Masses of débris consisting of more or less angular rock fragments of
all sizes commonly occur at the bases of cliffs and mountains. They
represent materials which have weathered off the ledges mainly by frost
action and temperature changes.
Where electrical storms are frequent, lightning often shatters
portions of rock ledges. Many such cases have come under the writer's
observation in the Adirondack Mountains of New York. The total effect
of lightning as a weathering agency is, however, relatively small.
Another minor weathering effect is the mechanical action of plants. The
principle is well illustrated by the breaking or tilting of sidewalks
by the wedging action of the growing roots of trees. In many places the
roots of plants growing in cracks in rocks, exert powerful pressure
causing the rocks or blocks of rocks to wedge apart.
Let us now briefly consider some of the chemical processes of
weathering. The solvent effect of perfectly pure water upon rocks is
very slight and slow. But such water is not found in nature because
certain atmospheric gases, especially oxygen and carbonic acid gas, are
always present in it, and they notably increase the solvent power of
the water. Such water has the power to slowly but completely dissolve
the common rock called limestone which consists of carbonate of lime.
This material is then carried away by the streams. Rocks, like certain
sandstones which contain carbonate of lime cementing material, are
caused to crumble due to removal of the cement in solution. Carbonic
acid gas in water also has the power to chemically alter various
minerals in many common rocks and thus the rocks fall apart and the
carbonates which result from the action usually are carried away in
solution. One of the most important changes of this kind takes place
when the very common mineral feldspar is attacked by water containing
carbonic acid gas and the mineral alters to a soluble carbonate, kaolin
(or clay) and silica.
The oxygen, both of the air and that which is contained in water, is
a very important chemical agent of decomposition of many rocks. Water
at the surface and the upper part of the crust of the earth as well
as moisture in the air are also important chemical agents which bring
about rock decay. We are all familiar with the rusting of iron which
is due to the chemical union of the iron with oxygen, thus forming an
iron oxide which in turn commonly unites with water from air or earth.
Now, many rocks contain iron, not as such, but held in combination with
other substances in the form of various minerals, and this iron of the
rocks, where subjected to the oxygen and moisture of air or water,
slowly unites with the oxygen and water to form a hydrated iron oxide
which is essentially iron-rust. The minerals containing considerable
iron are, therefore, decomposed and the rocks crumble. There are
various iron oxides, usually more or less hydrated, ranging in color
from red through brown to yellow, and these constitute probably
the most common and striking colors of the rocks of the earth. The
gorgeously colored Grand Canyon of the Yellowstone River is a very fine
example of large scale coloring due to development of much hydrated
oxide of iron during the weathering of lava rock, the process having
been aided by the action of heated underground waters.
Most of the soils of the earth are the direct result of weathering.
Important exceptions are soils which have been transported by the
action of water, ice, or wind. Although the process of weathering is
very slow and relatively superficial, it is, nevertheless, true that
in many places, the products of weathering form faster than they can
be carried away. Such weathered materials accumulate in their place
of origin to form soils. The upper few hundred feet of the earth's
crust is everywhere more or less fractured and porous and the rocks
are there affected in varying degrees by most of the ordinary agents
of weathering. In such cases, outside the areas which were recently
covered by ice during the great Ice Age, it is common to find the loose
soil grading downward into rotten rock, and this in turn into the fresh
practically unaltered bedrock. Soils of this kind are generally not
more than ten or twenty feet deep, though under exceptional conditions,
as in parts of Brazil, they attain depths of several hundred feet.
In order to make still clearer some of the above principles of
weathering and also to give the reader some understanding of the most
common types of residual soils, we shall consider what happens to a
few rather definite types of ordinary rocks when they are subjected to
weathering. A very simple case is that of a sandstone, the mineral
grains (mostly quartz) of which are held together by carbonate of
lime. The lime simply dissolves and is carried away, while many of
the mineral grains may remain to form a soil of nearly pure sand.
Where oxide of iron forms the cementing material, the rock yields less
readily to weathering, and the sandy soil will be yellowish brown or
red according to the climate. Another simple case is that of limestone
which when perfectly pure yields no soil because it is all soluble.
Pure limestone is, however, rare, and the various mineral impurities
in it, being to a considerable degree insoluble, tend to remain to
form a residual soil which may vary from sandy to clayey, and usually
brown or red due to the setting free of oxides of iron. According to
one estimate a thickness of about 100 feet of a certain fairly impure
limestone formation in Virginia must weather to yield a layer of soil
one foot thick. Soils of this kind, which are usually rich, are common
in many limestone valleys of the Appalachian Mountains. In the case of
shale rock, which is hardened mud, the cementing materials are removed,
some chemical changes in the minerals may take place, and the rock
crumbles to a claylike soil. What happens to a very hard, resistant
igneous rock like granite when attacked by the weather? Such a rock
always consists mainly of the two very common minerals feldspar and
quartz, usually with smaller amounts of other minerals such as mica,
hornblende, augite, or magnetite. The feldspar, which when fresh is
harder than steel, slowly yields when attacked by water containing
carbonic acid gas and crumbles or decays to a mixture of kaolin (clay),
carbonate of potash, and silica (quartz). Clay is an important
constituent of most good soils, while the carbonate of potash is
essential as a food for most plants. Due to yielding of the grains
or crystals of feldspar, the granite falls apart (see Plate 1). The
grains of quartz remain chemically unchanged, though they may be more
or less broken by changes of temperature, and the other minerals, which
are mostly iron-bearing, yield more or less to weathering, resulting
in a variety of products, among which are oxides of iron. A typical
granite, therefore, gives rise to a good heavy soil which is yellow,
brown or red according to climate. Such granite soils are common in
many parts of the Piedmont Plateau from Maryland to Georgia. Most of
the dark-colored igneous rocks, like ordinary basaltic lava, contain
much feldspar, various iron-bearing minerals, and little or no quartz.
Such rocks yield to the weather like granite but, because of lack of
quartz, the soils are more clayey. Rich soils of this kind occur in the
great lava fields of the northwestern United States and in the Hawaiian
Islands.
The importance of the breaking down of feldspar under the influence of
the weather, as above described, not only from the standpoint of soil
development, but also as regards the wearing down of the lands of the
earth, is difficult to overemphasize because that mineral is by far the
most abundant constituent of the earth's crust.
The term "erosion" is one of the most important in geologic science.
It comprises all the processes whereby the lands of the earth are
worn down. It involves the breaking up of earth material, and its
transportation through the agency of water, ice, or wind. Weathering,
including the various subprocesses as above described, is a very
important process of erosion. By this process much rock material is
got into condition for transportation. Another process of erosion,
called "corrasion," consists in the rubbing or bumping of rocks
fragments of all sizes carried by water, ice, or wind against the
general country rock, thus causing the latter to be gradually worn
away. A fine illustration of exceedingly rapid corrasion of very hard
rock was that of the Sill tunnel in Austria, which was paved with
granite blocks several feet thick. Water carrying large quantities of
rock fragments over the pavement at high velocity caused the granite
blocks to be worn through in only one year. Ordinarily in nature,
however, the rate of wear is much slower than this. Pressure exerted
upon the country rock by any agency of transportation may cause
relatively loose joint blocks, into which most rock formations are
separated, to be pushed away. This process, called "plucking," is
especially effective in the case of flowing ice.