9.2 Chemical Denudation
solution is responsible for only a sma11 percentage
(up to a maximum of ca. 10%) of the total carbonate
dissolution (Bögli 1978). Despite the fact that lowering of the land surface in karst regions often proceeds very irregularly, one can assume and calculate
from river output a certain mean denudation rate for
such regions.
Silicate Rocks
Crystalline rocks, free of carbonates and other easily
soluble constituents, display reduced denudation
rates (Fig. 9.5) and yield lower quantities of dissolved matter than the rock types discussed so far.
The chemie al characteristics of groundwater from
silicate rocks (including their soils) are shown in Fig.
9.6 for some common rock types compared with
limestone. Water-rock interaction has norma11y not
reached thermodynamic equilibrium conditions in
these systems, and the concentrations of dissolved
matter varies considerably between the different rock
types.
The mostly dark minerals of ultrabasic, basic
(such as basalt and gabbro), and intermediate rocks
(such as andesite) are rich in iron, magnesium, and
calcium. They break down more readily than most of
the light minerals, such as feldspars, muscovite, and
quartz, which are abundant in granitic rocks. Among
the plagioclase group, anorthite rich in calcium is
less stable than albite (rich in sodium) or K-feldspar;
biotite is altered more quickly than muscovite. From
these and other observations we can draw the fo11owing conclusions:
- In the case of basic and ultrabasic rocks, a11 principal minerals are readily altered and therefore contribute substantia11y to the composition of groundwater
and river water. The concentration of dissolved speeies tends to be relatively high (<: 150 mg/I, Fig. 9.7),
and the waters are fairly rich in Ca and Mg.
- Waters from granitic rocks, on the other hand,
draw their ions essentia11y from the weathering of
plagioclase (rich in Na), biotite, and K-feldspar.
Since most of the potassium is fixed in clay minerals,
such waters tend to be relatively rich in sodium, but
poor in calcium and magnesium; their total dissolved
species concentrations are low (50 to 100 mg/I, Fig.
9.7).
- SiOz released by the breakdown of unstable primary minerals is partly retained in secondary minerals. Therefore, the silica acid content of natural waters (R 4 Si0 4 , here written as SiOz) derived from igneous rocks is limited and far below the values
known for amorphous silica dissolved in water. In
the first case, the aqueous silica content mainly refleets equilibrium conditions with newly formed
aluminosilicates.
427
- Except for environmental conditions with very low
pH-values, a11 the Al z 0 3 released by the weathering
of primary minerals is retained to form secondary
minerals.
- Quartz is dissolved in substantial quantities only
under tropical to subtropical conditions (alkaline soil
waters) and is therefore of minor importance for the
composition of groundwaters outside these regions.
It is enriched in the alteration products within the soil
zone and thus in the sediments derived from these
sourees.
- Iron, although present in many rocks in considerable quantities, is rarely a substantial component in
near-surface groundwater. It is generally fixed in the
soil zone as oxide or hydroxide and may later be mechanica11y eroded and carried away as solid river
load.
- The presence of sulfate ions in waters from crystalline rocks commonly results from sulfide weathering.
- Groundwater as we11 as river waters from crystalline rocks are relatively rich in hydrogen carbonate.
In fact, RC0 3 is commonly the most important constituent of these waters.
In the case of silicate rocks, RC03 is entirely derived from
the atmosphere, whereas the weathering and subsequent
precipitation of carbonates only intermittently draws CO 2
from the atmosphere (cf. Sect. 5.6.2). Weathering of silicate rocks has therefore been quoted by many authors as an
important factor controlling atmospheric CO 2 and affecting
global climate. The ongoing debate on this topic is not further discussed here (e.g. Blum et a1. 1998).
Meteoric: waters passing metamorphic rocks, which
do not contain substantial amounts of carbonate,
have chemical characteristics similar to those of
rnany claystones and shales, impure sandstones, and
acidic and basic magmatic rocks. Groundwaters from
mica schists (Fig. 9.7) do not deviate much from
those of intermediate or basic igneous rocks.
Very low mineralization of near-surface groundwaters is usually observed in quartzitic sandstones,
shales and slates devoid of carbonate. These rocks
consist of the most stable minerals which are not dissolved much under normal weathering conditions.
Shales containing pyrite may deliver waters relatively
rich in sulfate.
When sedimentary or metamorphic rocks contain
carbonate (e.g., marly or dolomitic claystones,
carbonatc-cemented sandstones, and greenschists
with lenses of marble), their groundwater chemistry
approaches that of limestones or dolomites with high
concentrations of earth alkali ions and hydrogen carbonate, RC0 3 ·• The latter comes partly from the atmosphere or soil zone, and its concentration increases in temperate humid climates with acid soils.
solution is responsible for only a sma11 percentage
(up to a maximum of ca. 10%) of the total carbonate
dissolution (Bögli 1978). Despite the fact that lowering of the land surface in karst regions often proceeds very irregularly, one can assume and calculate
from river output a certain mean denudation rate for
such regions.
Silicate Rocks
Crystalline rocks, free of carbonates and other easily
soluble constituents, display reduced denudation
rates (Fig. 9.5) and yield lower quantities of dissolved matter than the rock types discussed so far.
The chemie al characteristics of groundwater from
silicate rocks (including their soils) are shown in Fig.
9.6 for some common rock types compared with
limestone. Water-rock interaction has norma11y not
reached thermodynamic equilibrium conditions in
these systems, and the concentrations of dissolved
matter varies considerably between the different rock
types.
The mostly dark minerals of ultrabasic, basic
(such as basalt and gabbro), and intermediate rocks
(such as andesite) are rich in iron, magnesium, and
calcium. They break down more readily than most of
the light minerals, such as feldspars, muscovite, and
quartz, which are abundant in granitic rocks. Among
the plagioclase group, anorthite rich in calcium is
less stable than albite (rich in sodium) or K-feldspar;
biotite is altered more quickly than muscovite. From
these and other observations we can draw the fo11owing conclusions:
- In the case of basic and ultrabasic rocks, a11 principal minerals are readily altered and therefore contribute substantia11y to the composition of groundwater
and river water. The concentration of dissolved speeies tends to be relatively high (<: 150 mg/I, Fig. 9.7),
and the waters are fairly rich in Ca and Mg.
- Waters from granitic rocks, on the other hand,
draw their ions essentia11y from the weathering of
plagioclase (rich in Na), biotite, and K-feldspar.
Since most of the potassium is fixed in clay minerals,
such waters tend to be relatively rich in sodium, but
poor in calcium and magnesium; their total dissolved
species concentrations are low (50 to 100 mg/I, Fig.
9.7).
- SiOz released by the breakdown of unstable primary minerals is partly retained in secondary minerals. Therefore, the silica acid content of natural waters (R 4 Si0 4 , here written as SiOz) derived from igneous rocks is limited and far below the values
known for amorphous silica dissolved in water. In
the first case, the aqueous silica content mainly refleets equilibrium conditions with newly formed
aluminosilicates.
427
- Except for environmental conditions with very low
pH-values, a11 the Al z 0 3 released by the weathering
of primary minerals is retained to form secondary
minerals.
- Quartz is dissolved in substantial quantities only
under tropical to subtropical conditions (alkaline soil
waters) and is therefore of minor importance for the
composition of groundwaters outside these regions.
It is enriched in the alteration products within the soil
zone and thus in the sediments derived from these
sourees.
- Iron, although present in many rocks in considerable quantities, is rarely a substantial component in
near-surface groundwater. It is generally fixed in the
soil zone as oxide or hydroxide and may later be mechanica11y eroded and carried away as solid river
load.
- The presence of sulfate ions in waters from crystalline rocks commonly results from sulfide weathering.
- Groundwater as we11 as river waters from crystalline rocks are relatively rich in hydrogen carbonate.
In fact, RC0 3 is commonly the most important constituent of these waters.
In the case of silicate rocks, RC03 is entirely derived from
the atmosphere, whereas the weathering and subsequent
precipitation of carbonates only intermittently draws CO 2
from the atmosphere (cf. Sect. 5.6.2). Weathering of silicate rocks has therefore been quoted by many authors as an
important factor controlling atmospheric CO 2 and affecting
global climate. The ongoing debate on this topic is not further discussed here (e.g. Blum et a1. 1998).
Meteoric: waters passing metamorphic rocks, which
do not contain substantial amounts of carbonate,
have chemical characteristics similar to those of
rnany claystones and shales, impure sandstones, and
acidic and basic magmatic rocks. Groundwaters from
mica schists (Fig. 9.7) do not deviate much from
those of intermediate or basic igneous rocks.
Very low mineralization of near-surface groundwaters is usually observed in quartzitic sandstones,
shales and slates devoid of carbonate. These rocks
consist of the most stable minerals which are not dissolved much under normal weathering conditions.
Shales containing pyrite may deliver waters relatively
rich in sulfate.
When sedimentary or metamorphic rocks contain
carbonate (e.g., marly or dolomitic claystones,
carbonatc-cemented sandstones, and greenschists
with lenses of marble), their groundwater chemistry
approaches that of limestones or dolomites with high
concentrations of earth alkali ions and hydrogen carbonate, RC0 3 ·• The latter comes partly from the atmosphere or soil zone, and its concentration increases in temperate humid climates with acid soils.