10th Asian Regional Conference of IAEG (2015)

Rates of rock property changes due to weathering:welded tuff gravel in fluvial deposits in the Miyazaki Plain, Japan
Ken-ichi NISHIYAMA(1)(1) Laboratory of Geology, Institute of Socio-Arts and Sciences, Tokushima University, Japannishiyama@tokushima-u.ac.jp
Abstract
Temporal changes in color and physical properties due to weathering during 240 ka
were examined using welded tuff gravel in fluvial terrace deposits with a known emergencetime that were distributed in Miyazaki Plain, Japan. The time between the age of eachterrace formation (8, 90, 120 and 240 ka) and the present was assumed to be the weathering
period.
The results of the measurements are as follows: (1) rock samples have both the outerweathered layers and inner fresh parts. The outer weathered layers can be subdivided into
two parts, outer weathering rind and inner weathering rind. (2) the a*, b*-values of color
indices increase as the weathering period increases. (3) effective porosity increases during 0
to 90 ka. These results suggest that (1) the increase in pores in gravel that occurs as theweathering period increases is caused by the dissolution of biotite; (2) iron concentrationand the formation of goethite occur in older gravels. Dissolution of biotite and increasing
pore volume play major roles in the weathering of welded tuff.
Keywords: Weathering, Welded Tuff, Osuzu Acidic Rocks, Miyazaki Plain, Rock Properties,
Color Measurement
1.

Introduction
Weathering influences the processes and rates oflandform development. The study of these processes
and rates of rock weathering is, therefore, importantfor engineering Geology. Studies on the changing
rates of rock properties due to weathering have been
limited in number (Kimiya, 1975; Crook andGillespie, 1986; Oguchi, 1999；Oguchi et al. 1999;
Hachinohe, et al., 1999; Oguchi and Matsukura,
1999; Nishiyama and Matsukura, 2006), because ofdifficulties in estimating weathering periods. In
order to solve the rates and mechanism of weldedtuff weathering, this study investigates the rockproperties of welded tuff gravel taken from terrace
deposits in a series of dated fluvial terraces in the
Miyazaki Plain, south Kyushu, Japan. Welded tuff
gravel included Middle Miocene Osuzuyama
Volcano-Plutonic Complex. The rates of long-term
weathering are estimated based on the assumptionthat the weathering period is equal to the period
between the age of emergence of these terraces andthe present. This study can discuss the changes inrock properties due to weathering during 240 ka.
The measured rock properties include thefollowing: (1) rock texture (observation of nackedeye), (2) colors, (3) physical properties (effective
porosity and apparent specific gravity), Using
these data, the relationships between rockproperties and weathering period in an attempt toclarify the rates and mechanism of weathering of
welded tuff gravel.
2. Geological Features of Sampling Points andRock Samples for Analysis
The welded tuff rock was sampled from fluvial
terrace deposits in Miyazaki Plain, Kyushu.
Miyazaki Plain has the best-developed middle tolate Quaternary deposits in Kyushu. The
stratigraphy of this plain has been investigatedusing tephrochlonology in previous studies (e.g.,
Nagaoka et al., 2010). This study assumes that
fresh rocks were deposited on river floodplains andtheir weathering started soon after the terrace
emergence. Thus, the time between the age of each1

10th Asian Regional Conference of IAEG (2015)

0 ka
8 ka
240ka
90ka

120ka

10km

fragments.
samplesetofal,rounded
Fig. 1 Geomorphological map of the Miyazaki Plain of
androck
sampling
pointsAbout(After10Nagaoka
2010)
welded tuff cobbles and pebbles were taken fromeach sampling point. The characteristics of theterrace formation and the present can be assumed
sampling sites and welded tuff gravel sample taken
to be the weathering periods.
from the sampling sites are as follows:
The locations of the eight rock sampling sites(1) Recent river floodplain (0-ka rocks): The
are shown in Figs 1 and 2. All sampling points are
sampling point is located on a gravel bar of river
directly covered with tephra layers, except for thefloodplain in the Nanuki River. The gravel taken
recent river floodplain. All sampling points at eachfrom this site is not visibly weathered. The cuttingsite are above the groundwater level.
surface of this gravel shows that it is dark grayAll examined welded tuff gravel belongs to thecolored and has no weathering rind. This gravel is
Middle Miocene Osuzuyama Volcano-Plutonic
hard, and sounds made by hammer blows to it areComplex. All welded tuff gravel samples areclear.
biotite rhyolite welded tuff with smaller amounts

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10th Asian Regional Conference of IAEG (2015)

Fig.2 Sampling sites.
A: Present river floodplain (0ka),
B: Mikazukibaru terrace deposits (8ka),
C: Karasebaru terrace deposits (90ka),
D: Sanzaibaru terrace deposits (120ka),
E: Chausubaru terrace deposits (240ka)

made by hammer blows to it are dim.
(5) Chausubaru terrace deposits (240-ka rocks):
Gravely deposits of this sampling point are almost
directly covered by the tephra Kr-Iw (40-50 ka). The
covered tephra layer is about 3 m thick. Deposits
contain the tephra Ata-Th (240 ka) in the middle partat the type locality. The gravel taken from this site isstrongly weathered, and its color is reddish brown.
This gravel is brittle, and sounds made by hammer
blows to it are dim.

(2) Mikazukibaru terrace deposits (8-ka rocks):
Gravely deposits of this sampling point are almost
directly covered by the tephra K-Ah (0.73 cal kaBP) and black humic soils. The covering tephra
layer is about 0.3 m thick. The gravel taken fromthis point is slightly weathered, and its color islight gray. This gravel has no weathering rind. It is
hard, and sounds made by hammer blows to it areclear.
(3) Karasebaru terrace deposits (90-ka rocks):
Gravely deposits of this sampling point are almost
directly covered by the tephra Aso-4 (90 ka). The
covered tephra layer is about 5 m thick. The gravel
taken from this site is slightly weathered, and its
color is light gray to brown. This gravel is brittle,
and sounds made by hammer blows to it are
somewhat dim.
(4) Sanzaibaru terrace deposits (120-ka rocks):
Gravely deposits of this sampling point are almost
directly covered by the tephra A-Iw (70 ka). The
covered tephra layer is about 5 m thick. Deposits
contain the tephra Aso-3 (120 ka) in the Lower partat the type locality. The gravely deposits arecorrelated with the non-marine upper member of the
Sanzaibaru formation. The gravel taken from thissite is strongly weathered, and its color is brown toreddish brown. This gravel is brittle, and sounds

3. Analysis and results
3.1 Rock Texture
Figure 3 lists the characteristics of the rock basedon naked-eye observations of cutting surfaces. Eachsample has a fresh-looking interior with somephenocrysts of quartz, plagioclase and biotite. The
observations of the cutting surface of rock with the
naked-eye are summarized as follows:(1) 0- and 20-ka rock samples have fresh textures
through the rock surface to the interior. They havewhite phenocrysts of quartz and plagioclase andblack phenocrysts of biotite surrounded by a grey
groundmass. No weathering rind can be observed at
the cutting surface of rock samples. The appearanceof quartz, plagioclase and biotite phenocrysts havenot changed.

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10th Asian Regional Conference of IAEG (2015)

Fig. 3 Some of the selected cut-rock samples.
corresponds to white. A positive value of a*expresses red, and a negative value green. A positive
value of b* indicates yellow, and a negative value
blue. Positive a*- and b*-values express an increasein iron minerals such as goethite and hematite(Nakashima et al., 1992; Nakashima, 1994). Colormeasurements were carried out on a surface andcutting interior of each rock sample taken from eachsampling site.
The results of color measurements arranged byweathering time are shown in Table 1. The numberin the parenthesis is standard deviation. The
L*-value ranges from 51.5 to 62.6, the a*-value
ranges from 1.6 to 5.6, and the b*-value ranges from
11.1 to 18.7. The a* and b*-values increases during
increasing weathering periods, and the L*-value has
no trend during increasing weathering period. These
results support that the color of rock samples
becomes more yellowish during the 240-ka period.
The a*-b* diagram (Fig. 4) shows that outerweathering rinds are reddish than interior of rocksamples. The gradient of the trend line from allplotted data is linear.
Nakashima (1994) suggested that an increase in
the b*-value expressed an increase in goethite and anincrease in the a*-value expressed an increase inhematite. Therefore, the present data of color
measurement indicate that goethite increases at theouter weathering rinds during the 240-ka period.

(2) 90- and 120-ka rock samples have both theouter weathered layers and inner fresh parts. The
outer weathered layers can be subdivided into twoparts, outer weathering rind and inner weatheringrind. The thickness of outer weathering rind is
4-8mm, inner weathering rinds is 10-15 mm.
Although the appearance of quartz phenocrysts has
not changed, plagioclase and biotite has been slightly
weathered.
(3) 240-ka rock samples have outer weatheringrind only. The thickness of outer weathering rind is
10-15 mm. Although the appearance of quartz
phenocrysts has not changed, plagioclase and biotite
has been slightly weathered.
3.2 Color
Soil color is closely related to the content ofgoethite and hematite (e.g., Torrent et al., 1982).
Recent researchers in earth sciences have used aspectrophotometer to measure the color of rocks,
minerals and soils (e.g., Nagano and Nakashima,
1989; Nakashima et al., 1992; Nakashima, 1994;
Oguchi et al., 1995; Mitsushita et al., 1998;
Wakizaka et al., 1998; Nishiyama et al, 2011).
The color of rock samples were determined usinga Conica-Minolta SPAD-503 Spectrophotometer as
having components L*, a* and b*, i.e., L* a* b*
color space. The L*-value is the degree of lightness:
L* = 0 corresponds to black, and L* = 100

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10th Asian Regional Conference of IAEG (2015)

Table 1 apparent specific gravity, effective porosity and color values of rock samples.
weathering timenumber of samples

0 ka（surface）

apparent specific gravity, Gn

11

0 ka（interior)

2.56 (0.04)

effective porosity ne （％）
3.7 (1.3)

the difference between the color values of surface and interior
8 ka（surface）

2.48 (0.08)

6.2 (2.3)

the difference between the color values of surface and interior
90 ka（surface）

2.23 (0.15)

13.2 (4.5)

the difference between the color values of surface and interior
120 ka（surface）

2.35 (0.09)

10.3 (2.9)

the difference between the color values of surface and interior
240 ka（surface）

2.41 (0.11)

8 (2.8)the difference between the color values of surface and interior

15

b*

10
interior
surface-5-20Fig. 4

2

a*

4

6

3.8
11.6 (2.7)

52.2 (3.9) 1.1 (1.0)

7.0 (2.3)

1.6

4.6

59.1 (2.9) 5.6 (1.2)

18.7 (2.5)

54.5 (5.3) 1.8 (1.3)

9.1 (4.8)

3.8

9.6

62.6 (3.9) 4.3 (1.4)

16.8 (2.2)

56.9 (9.8) 0.8 (1.1)

5.5 (2.8)

3.5

11.3

52.4 (2.8) 5.0 (1.3)

14.4 (1.8)

54.8 (4.6) 1.6 (1.0)

6.8 (3.5)

3.4

7.6

Naked-eye observation on cutting surfaceof rock sample show that 90-, 120-, and240-ka rocks have weathering rinds. The
weathering rinds are subdivided into twozones based on naked-eye observation, outerweathering rinds and inner weathering rinds.
The color of welded tuff gravel changes toyellowish or reddish as the weathering period
increases. The color of rocks is affected by
the iron minerals contained therein(Nakashima et al, 1992). This suggests thatwhen welded tuff becomes reddish due toweathering, it is caused by an increase in ironminerals such as goethite and hematite. Fe2+ isformed by leaching of biotite due to weathering. Fe2+
is easily oxidized to Fe3+ in an oxidativeenvironment such as in terrace deposits. Fe3+ isprecipitated within the rock because Fe3+ has a low
solubility in natural water. Hydroxides such as
ferrihydrite are formed by precipitation of Fe3+. The
results of color measurement suggest that ironminerals in rock samples change from ferrihydrite togoethite (α-FeOOH) during the 240-ka period. Basedon the results of naked-eye observation, ironoriginates from biotite in welded tuff gravel.
Changing iron by oxidation, hydration andmineralization during weathering ultimately
produces goethite. The rock structure and color of
welded tuff gravel seem to be affected by changes ofiron minerals.

20

0

0.5

51.5 (4.4) 2.7 (0.8)

-2.4

25

5

7.3 (3.2)

5.7

10

240 ka（interior）

53.9 (4.1) 1.1 (1.2)

4.6

8

120 ka（interior）b*
11.1 (1.4)

-0.7

10

90 ka（interior）

a*

5.1

12

8 ka（interior）

L*

59.0 (2.4) 1.6 (0.5)

8a*-b* diagram

3.3 Physical Properties
The effective porosity, ne (％), and the apparent
specific gravity, Gn, are widespread indices forphysical properties of rocks. The methods of analysis
were used by the Japanese Geotechnical Engineering
Society (1989). The measurements were carried out
using 8 to 12 samples taken from each sampling site.
Temporal changes in the effective porosity and theapparent specific gravity are shown in Table 1. The
effective porosity ranges from 3.7 to 13.2 ％, andthe apparent specific gravity ranges from 2.23 to
2.56. The effective porosity increases during 0 to 90
ka, while the specific gravity decreases during 0 to
90 ka.
4. Mechanism of Weathering in welded tuff
Gravel5

10th Asian Regional Conference of IAEG (2015)

5. Conclusions
Temporal changes in color and effective porosity
due to weathering during 240 ka were examinedusing welded tuff gravel in fluvial terrace deposits
with a known emergence time that were distributedin Miyazaki Plain, Japan. The results of themeasurements are as follows: (1) rock samples haveboth the outer weathered layers and inner fresh parts.
The outer weathered layers can be subdivided into
two parts, outer weathering rind and inner
weathering rind. (2) the a*, b*-values of color
indices increase as the weathering period increases.
(3) effective porosity increases during 0 to 90 ka.
These results suggest that (1) the increase in pores ingravel that occurs as the weathering period increases
is caused by the dissolution of biotite; (2) ironconcentration and the formation of goethite occur inolder gravels. Dissolution of biotite and increasing
pore volume play major roles in the weathering of
welded tuff.

synthetic

minerals:

Rep.

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