Waragai and Hiki Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0254-5(2019) 6:10

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessInfluence of microclimate on the directional
dependence of sandstone pillar weatheringin Angkor Wat temple, Cambodia
Tetsuya Waragai1*

and Yusuke Hiki2

Abstract
Angkor Wat temple, mainly composed of sandstone blocks, displays a type of religious architecture that signifies
the worldviews of the Khmer and Hindu religions. The temple is a part of a UNESCO World Heritage site. However,there are numerous occurrences of hollows, i.e., depression-like notches, that have developed at the base of its
sandstone pillars due to wet–dry and salt weathering. These pillars are variably weathered due to differences in the
directions of the pillar surfaces and galleries in the temple. In this study, we analyze the differences in the hollow
depths based on the sandstone hardness and water content, as well as a 5-year record of the temperature andhumidity in the galleries. The results show that the hollow depths are profound in the outsides of the inner pillars
and shallow on the northern side of the first gallery. The hollow depth increases with increasing values obtained bymultiplying water content and moisture fluctuations, resulting from direct insolation together with rainfall.
Keywords: Sandstone, Microclimate, Angkor Wat temple, Cultural heritage, Moisture condition

Introduction
Cultural stone weathering research

The Angkor complex which is composed of temples
such as Angkor Wat, Angkor Thom, Ta Keo and Ta
Prohm, and West Barai is a religious structure and ahydrological system constructed from the 9th to 13th
centuries of the Khmer period. Therefore, for more than
800 years, it has experienced significant natural andhuman-driven deteriorations.
In terms of natural deterioration, sandstone and laterite building materials rapidly deteriorate under tropicalclimates. Although chemical weathering is a major
process in the tropics, as well as biochemical weathering(e.g., Liu et al. 2018), other forms of mechanical rockweathering such as wet-dry weathering (e.g., Waragai
2002, 2005a; Siedel et al. 2010; André et al. 2011), salt
weathering (e.g., Uchida et al. 1999; Hosono et al. 2006;
André et al. 2008; Siedel et al. 2008, 2010), and root
wedging processes (Archaeological survey of India 2006)
are also significant. For instance, Waragai (2002, 2005a)* Correspondence: waragai.tetsuya@nihon-u.ac.jp
1
Department of Geography, College of Humanities and Sciences, Nihon
University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
Full list of author information is available at the end of the article

suggested that, at the Angkor Wat temple, deterioration
occurs because the sandstone blocks contain clay minerals, which are constantly exposed to severe cycles of
wet-dry weathering. On the other hand, Uchida et al.
(1999) reported salt efflorescence of minerals such as
gypsum, calcite, and phosphate on the sandstone pillarsof the Angkor Wat temple.
These weathering processes are thought to be restricted by direction dependent (Waragai 2005b; Waragai et al. 2006; André et al. 2012; Hiki 2012; Hiki and
Waragai 2014; Waragai 2016), especially for the temples
at Angkor. Regarding cultural stone weathering, previous
studies have shown the importance of direction or aspect related to heating-cooling repetitions and moisturechanges (Meierding 1993; Paradise 1995, 2002; Williams
and Robinson 2000; Waragai 2017). Recently, a relationship between the weathering of sandstone pillars and thepillar water content in conjunction with the microclimatic conditions has been pointed out (Hiki and Waragai 2014). However, in their study, the microclimaticconditions of the air temperature and humidity were
only measured for 1 year or less. Therefore, to obtain arepresentative view of the microclimatic conditions, longitudinal monitoring of the weathering environment© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Waragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

must be performed. It is also necessary to understandthe distribution and volume of the weathering within thetemple. Accordingly, we discuss both the long-termmonitoring of the weathering environment and the directional dependence of the weathering process based on
the depths of the hollows (i.e., notches) in the sandstonepillars, which is regarded as a measure of the weathering
volume.
Study site

The Angkor Wat temple, located in the tropics (N13.412, E 103.867), was built with a masonry style ofconstruction using blocks of sandstone and laterite during the reign of the Khmer King Suryavarman II (1113–
1150 CE). The temple is surrounded by a moat that is
1300 m from north to south and 1500 m from east to
west, with a central sanctuary made from sandstone
blocks 62 m in height (Fig. 1). Three rectangular galleries of varying heights were constructed around the central sanctuary (Fig. 2a). These are called the first gallery,
the second gallery, and the third gallery from the outer
to central parts of the temple, respectively. The first andthird galleries, which were selected as the study area, arecolonnades composed of square sandstone pillars (Fig. 3a,b).
The first gallery is 200 m from north to south and
220 m from east to west and is the longest passagewaywith gopuras (Fig. 2a). The gallery has a height of 6 m
with a vault that is 4 m above the ground (Fig. 2b). A
closed wall supports the roof of the vault, and it is decorated with bas-reliefs as well as inner and outer pillars
joined by beams (Fig. 3c, d). Several outer pillars are reinforced by mortar and/or iron hoops due to the development of vertical cracks and severe deterioration.
Hollows are visible at the bases of the inner pillars (Fig.
3c, e). These hollows are continuing to grow via wet-dry
weathering (e.g., Waragai 2005a; Hara and Waragai
2010; Siedel et al. 2010) and salt weathering (e.g., Uchida
et al. 1999; Hosono et al. 2006; Siedel et al. 2008). The
depths of the hollows are correlated to the directions
faced by the pillars and the galleries.
The third gallery, located approximately 30 m above
the ground, forms a square shape with a length of 70 mfrom north to south, and 70 m from east to west (Fig.
2a). The gallery surrounds the courtyards around thecentral tower of the sanctuary, and an enclosure wallwith window is located along the outermost side of the
sanctuary. The third gallery (Figs. 2c, 3b), which is characterized by a ventilated environment, differs from thefirst gallery, because the inside of third gallery is open
on one side. A combination of 196 inner and outer pillars and the enclosure wall support the roof of the gallery. The pillars, which are reinforced and/or aredeveloping cracks, are minuscule. However, hollows have

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developed at the bases of the inner pillars (Fig. 3f ), producing sandstone fragments.
There is no evidence of hollow formation on the outerpillars at the first and third galleries. It is presumed thatthe differences in the shape of the pillar bases and thevertical loads are responsible for the formation of thehollows (Kambe et al. 2016).
According to Waragai and Hara (2011), the sandstone
block has a specific gravity of 2.64, a dry unit weight of
2.30 g/cm3, a saturated water content of 6%, a porosityof 13%, and a tensile strength (dry) of 9.4 MP.
Meteorological observation at the Angkor Wat temple(the observation point is shown in Fig. 1; AMOS) between 2011 and 2016 shows an annual averagetemperature of 27 °C with a total annual rainfall of1633 mm (Waragai 2018). The monthly averagetemperature is the highest (29.6 °C) in April and thelowest in January (24.9 °C), and the variation in the annual temperature is relatively small (Fig. 4). Conversely,
the monthly rainfall from June to October is 243–
358 mm/month and that from December to February is
0.1–16 mm/month. Therefore, there is a clear distinction
between the rainy and dry seasons at the study area.

Methods/Experimental
Hollow depth measurements

The temple galleries are precisely oriented in the cardinal directions (Fig. 2a), and hollows occur in the basesof all the inner pillars of the galleries. In the first gallery,
there are 64 inner pillars in the northern (1N) andsouthern (1S) galleries, and 38 inner pillars in the eastern (1E) and western (1W) galleries. In the third gallery,
there are a total of 32 inner pillars with hollows: 8 ineach of the eastern (3E), western (3W), northern (3N),
and southern (3S) galleries. Because the courtyards arein the enclosure wall of the third gallery, the pillar arrangement is opposite that of the first gallery (Fig. 2b, c).
We define the inner pillars of the third gallery as pillars
that have hollows at their bases.
To measure the hollow depth, we selected a total of 91
inner pillars from the first gallery and 16 inner pillars
from the third gallery (Fig. 2a). Specifically, in the firstgallery, we selected 28 pillars from the northern gallery(1N), 22 pillars from the western gallery (1W), 19 pillars
from the eastern gallery (1E), and 22 pillars from thesouthern gallery (1S). The selected pillars are equivalent
to approximately 45% of the total pillars in the first gallery. On the other hand, eight inner pillars were selectedfrom each direction in the third gallery (3E, 3W, 3S, and3N; 32 pillars in total).
Although hollows form on all sides of the inner pillars,
we only measured the hollow depth at both the outside
and inside hollows of the pillars using a caliper during
August of 2012. The depth is defined as the maximum

Waragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

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Fig. 1 Location of Angkor Wat temple and the study site. AMOS; Meteorological observation station in Angkor Wat temple

depth between the original surface and the innermost
point of the hollow.

inner pillars in each direction were selected to measure
the hardness.

Hardness measurements

An Equotip 3 Hardness Tester (Proceq SA) was used tomeasure the hardness on the inside and outside of thepillars in August 2012. This device is often used in orderto estimate in situ non-destructive hardness of stones(e.g., Aoki and Matsukura 2007; Wilhelm et al. 2016a,
2016b). Average hardness (the Leeb hardness; Kompatscher 2004) was calculated from five measurementson the innermost surfaces of the inside and outside hollows for all selected inner pillars in the first gallery. In
the third gallery, the inside and outside hollows on two

Water content measurements

An infrared optical moisture meter (JT Engineering Inc.,
JE100) was used to measure the water content of the pillars. This meter measures the water content based on
the amount of absorbance of reflected light converted
from the amount of infrared light that bounces from an
object (Matsukura and Takahashi 1999). For the graysandstone at the Angkor Wat temple, water content iscalculated based on the absorbance measured by themeter (Waragai et al. 2008).

Waragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

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a

b

c

Fig. 2 A plan of the galleries (a) and conceptual cross sections of the first (b) and the third galleries (c). The plan was modified from Glaize (1963). The
inner pillars that measured the hollow depth and hardness are shown in red squares. The air temperature and humidity measurements using dataloggers were carried out on the beam. The water contents were measured at eight inner pillars

Waragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

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a

b

c

d

e

f

Fig. 3 Galleries and sandstone pillars of Angkor Wat temple. a Eastern side of the first gallery (1E). b Eastern side of the third gallery. c The innerand outer pillars in the first gallery. d A data logger on the beam in the first gallery. e The inner pillar accompanied with the hollow of the firstgallery. f The hardness measurement on the inner pillar at the third gallery

To measure the water content, eight pillars (1NW26,
1EN04, 1SE02, 1WS03; 3NW03, 3EN03, 3SE02, 3WS03)
were selected from each cardinal direction in the galleries for each hollow on the inside and outside of the pillars (Fig. 2a). The hollow surfaces of the selected pillars
were measured nine times in the first gallery (around
10:00, 15:50, August 10, 5:50, 7:00, August 11, and 10:20,
11:30, 15:30, 16:20, 17:30, August 12, 2012), and fivetimes on pillars of the third gallery (around 10:00, 11:30,
15:00, 15:50, 16:40, August 14, 2012).

Measurements were obtained in 30- or 60-min intervals
using eight data loggers (Onset Computer Co.,
H08-032-08 and T&D Co., CTD-RTR-52A). Data logger
installation points are shown in Fig. 2a. Four data loggers were installed on the beam connecting the innerand outer pillars of each cardinal direction in the firstgallery (Fig. 3d). Another set of four data loggers was installed on the back of a lintel at each corner of the third
gallery.

Results
Weathering environment monitoring

Hollow depths of the inner pillars

To analyze the effects of the weathering environment
due to the different directions of the galleries, the airtemperature and relative humidity were simultaneously
measured from August 22, 2011 to September 6, 2015.

The results of the hollow depth measurements in thefirst gallery are shown in Table 1 and Fig. 5. Table 1
shows that the depth ranges from 4.0 to 95.4 mm (withan average of 34.1 mm) for the outside hollows, and 0–

Waragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

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Fig. 4 Monthly average temperature and precipitation based on the meteorological observation between 2011 and 2016 at Angkor Wat
temple (AMOS)

36.0 mm (with an average of 12.9 mm) for the inside
hollows. The outside hollows were noticeably deeper
than the inside hollows. Figure 5 shows the distributionof the hollow depth, grouped into classes at an intervalof 10 mm depth. Many of the outside hollows exhibited
more than 20 mm depth, except in the northern gallery.
On the other hand, depths of less than 20 mm are dominant (90%) in the inside hollows. Depths are elevated in
the outside hollows, but shallow in the inside hollows.
We then compared the depths of the outside hollows to
the gallery directions finding that 1E (with an average of
46.5 mm), followed by 1S (with an average of 34.8 mm),
1W (with an average of 33.6 mm), and 1N (with an average of 21.4 mm). In particular, the outside hollows withmore than 50 mm depth occupy approximately 42% of
the pillars in the eastern gallery (Fig. 5). Conversely, theoutside hollows with less than 20 mm depth occupy approximately 57% of the pillars in the northern gallery.
The depths of the inside hollows are as ranked as follows: 1W (with an average of 16.6 mm), 1E (with anaverage of 14.2 mm), 1S (with an average of 12.5 mm),and 1N (with an average of 8.4 mm). Therefore, the hollow depths in 1N were shallower than those in the galleries in other directions.
The results of the hollow depths in the third gallery
are shown in Table 1. The depths range from 21.1 to
134.1 mm (with an average of 53.9 mm) for the outside
hollows and 12.7–84.8 mm (with an average ofTable 1 Hollow depths of the inner pillars of the first and third galleries. Italics mean maximum values of the hollow depths
Gallery and direction

First gallery
1N

Outside of the pillar

Inside of the pillar

1E

Third gallery
1S

1W

3N

3E

3S

3W

Number of measurement

28

19

22

22

8

8

8

8

Maximum depth, mm

48.5

95.4

84.0

61.6

74.8

103.9

52.8

134.1

Minimum depth, mm

4.0

9.0

4.2

7.0

27.8

35.5

21.1

35.3

Average depth, mm

21.4

46.5

34.8

33.6

49.5

64.7

37.4

64.0

Standard deviation, mm

13.8

19.7

18.6

15.5

13.6

19.4

12.6

34.2

Average depth of all directions, mm

34.1

53.9

Number of measurement

28

19

22

22

8

6

8

8

Maximum depth, mm

28.3

36.0

29.0

32.0

64.8

68.7

68.7

84.8

Minimum depth, mm

0.0

6.0

0.0

4.9

29.0

40.5

12.7

27.2

Average depth, mm

8.4

14.2

12.5

16.6

43.4

48.4

49.9

45.6

Standard deviation, mm

7.6

7.8

8.4

8.9

14.2

9.0

20.4

20.8

Average depth of all directions, mm

12.9

46.8

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Fig. 5 Hollow depths of sandstone inner pillar at galleries of Angkor Wat temple. Numbers of upright and italic show the average depth of outerhollow and the average depth of inner hollow, respectively

46.8 mm) for the inside hollows. The average depth ofthe outside hollows is approximately 7 mm deeper thanthat of the inside hollows. A depth difference between
the inside and outside hollows is observed for 3E and
3W in the third gallery; however, this difference issmaller than that observed in the first gallery (Fig. 5). As
shown in Fig. 5, when the depths of the outside hollows
are compared with the directions of the third gallery, it
indicates that 3E and 3W are the largest (with an averages of 64.7 and 64.0 mm, respectively), followed by 3N(with an average of 49.5 mm) and 3S (with an average of
37.4 mm). The outside hollows are more than 60 mm
deep for 63% of the hollows in 3E and for more than
50% of the hollows in 3W. On the other hand, the inside
hollows have depths of 49.9 mm in 3S, 48.4 mm in 3E,
45.6 mm in 3W, and 43.4 mm in 3N. The depth rangeof the inside hollows is smaller than that of the outside
hollows, with no influence of the gallery direction.
Sandstone hardness of the inner pillars

The average hardness and standard deviations are shownin Fig. 6. The average hardness in the first gallery was
402 HLD in the outside hollows and 334 HLD in the inside hollows (Fig. 6). The hardness of the outside hollows was noticeably greater than the inside hollows. The
maximum average hardness of the outside hollows, indecreasing order, is 1N (438 HLD), 1W (411), 1W (383),

and 1E (376). The difference between the maximum andminimum hardness is 62 HLD. On the other hand, therange of hardness in the inside hollows is small, ranging
from 349 HLD (1S) to 319 HLD (1W). Therefore, thehardness of the hollows was high in the outside hollows
in 1N, and the difference due to the gallery direction
was negligible for the inside hollows.
The average hardness in the third gallery was 311
HLD for the outside hollows, and 323 HLD for the inside hollows. The hardness difference is small between
both sides as well as gallery directions. The ranges fromminimum to maximum hardness are from 289 HLD(3N) to 341 (3S) for the outside hollows, and from 307(3W) to 333 (3S) for the inside hollows. Therefore, it indicates that the outside and inside hollows, as well as
the gallery directions, have no influence on the hardness.
Water content of the inner pillars

The results of the water content measurement in the firstgallery are shown in Fig. 7 accompanied with the maximum
and minimum concentrations. The water contents in thefirst gallery were 2.0–4.5% (with an average of 3.0%) for theoutside hollows, and 4.0–6.6% (with an average of 5.0%) for
the inside hollows. Average water contents of the inside hollows were approximately 2% higher than those of the outside
hollows. Comparing the water content for each direction in
the gallery, the outside hollows of 1N was the highest

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Fig. 6 Relationships between the hardness of the hollow and gallery directions. Bars indicate the standard deviation of hardness

contents (3.8%) on average, followed by those of 1S (3.2%),
1E (2.7%), and 1W (2.4%). The water content of the inside
hollows was the highest for 1E (5.2%), while the lowest for1S (4.7%); however, the differences between the directions
were small. The water contents were high for both the insideand outside hollows of 1N.
The results for the water content in the third gallery are
shown in Fig. 7. The water contents ranged from 2.8 to 5.1%
(with an average of 3.6%) for the outside hollows and from
2.7 to 4.5% (with an average of 3.7%) for the inside hollows,with only a small difference between the two. Comparing
the water contents for each direction of the gallery, the outside hollows of 3N was the highest (4.5%) on average andthose of 3S (3.1%) was the lowest. The water contents of theinside hollows were also the highest for 3N (4.4%) and thelowest for 3W (3.1%). The water contents tended to be highat the northern end of the third gallery despite the direction
having relatively little influence.
Weathering environments of the galleries

To understand the microclimatic environment of thegalleries, daily averages were calculated from themonitoring results of the temperature and relative humidity from 2011 to 2015 (Table 2). However, there aregaps in the data record due to data logger theft and humidity sensor malfunctions. In particular, humidity sensors installed in 3S and 3W (Fig. 2a) experienced anumber of defects for a long period; therefore, they did
not obtain enough data. The results of the relative humidity in 3S and 3W were therefore excluded from
Table 2.
Regarding a comparison between the monitoring results and the directions of the galleries, it is found thatthe annual average temperatures were lowest in 1N(28.7 °C) and ranged from 29.6 to 29.7 °C in the otherdirections of the first gallery. With respect to the humidity, 1N had a maximum of 73% followed by 1E (70.8%),
1S (70.1%), and 1W (69.4%). The range in the humidity
was small in 1N (28%) and was larger in the other galleries (31–36%). The average temperature in the third gallery ranged from 28.6 (3N) to 29.1 °C (3S), a relatively
small range. The average humidity was at a minimum of
70.0% in 3E and a maximum of 70.9% in 3N, also showing a small range. The difference between the maximum

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Fig. 7 Relationships between the water content of the hollow and gallery directions. Bars indicate ranges from maximum to minimum of water contents

Table 2 Statics of temperature and relative humidity of the first and third galleries between 2011 and 2015. Italics mean maximum
values of the air temperature and relative humidity
Gallery and direction

First gallery
1N

Daily air temperature °C

Daily relative humidity, %Third gallery
1E

1S

1W

3N

3E

3S

3W

Number of samples

366

366

366

366

366

366

366

366

Maximum

33.3

34.3

33.3

34.2

32.5

32.2

32.1

31.7

Minimum

22.9

25.6

27.0

25.2

22.7

24.9

25.6

26.3

Average

28.7

29.7

29.6

29.6

28.6

28.7

29.1

28.8

Standard deviation

2.18

1.87

1.37

1.78

2.14

1.76

1.38

1.29

Variance

4.77

3.49

1.87

3.16

4.59

3.09

1.91

1.67

Confidence interval (95.0%)

0.22

0.19

0.14

0.18

0.22

0.18

0.14

0.13

Number of samples

366

366

366

366

365

366

–

–

Maximum

87.2

89.7

87.2

86.6

81.7

84.5

–

–

Minimum

58.9

53.6

56.0

53.6

54.6

56.6

–

–

Average

73.0

70.8

70.1

69.4

70.9

70.1

–

–

Standard deviation

6.71

7.98

8.52

8.64

6.11

7.26

–

–

Variance

45.07

63.76

72.62

74.60

37.39

52.68

–

–

Confidence interval (95.0%)

0.69

0.82

0.88

0.89

0.63

0.75

–

–

Waragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

and minimum relative humidities was small in 3E(27.9%) and 3N (27.1%).
These results indicate that the northern gallery (1N)
has low temperatures and small changes in humidity
throughout the year. In the third gallery, the northerngallery (3N) had low temperatures, but the temperature
difference between 3N and the other galleries was small.
The humidity differences due to the directions of thegalleries were less than 1% in the third gallery.

Discussion
As mentioned above, the hollows develop at the bases of
the inner pillars and the depths of the outside hollows
are deeper than those of the inside hollows in the firstgallery. The hollows in the third gallery are deep for theoutside hollows, except in 3S. In addition, the hollow
depths are shallow in the northern gallery, and show adifference due to the gallery direction. In previous studies, Takahashi and Matsukura (2006) point out global
solar irradiance, as a major factor of directional dependence for depressions that developed on sandstone bridgepiers. Paradise (2002) argues the importance of wet-dry
changes in rocks from weathering rates of sandstone
quarry surfaces in Petra, in addition to temperaturechanges due to insolation.
Several weathering processes are present for sandstone
weathering at Angkor (e.g., Uchida et al. 1999; Waragai
2002, 2005a; Archaeological survey of India 2006; Hosono et al. 2006; André et al. 2008, 2011; Siedel et al.
2008, 2010). However, there are very few discussions regarding the directional dependence of these processes(e.g., Waragai 2005b, 2016; Waragai et al. 2006; Hiki and
Waragai 2014). This is due to the availability of climate
data at the study area. Therefore, from the viewpoint of
the microclimatic environment, we consider several reasons for the differences in the hollow depths due to thedirections of the hollows and the galleries.
Difference in the hollow depth due to pillar directions

The average hollow depth in the first gallery was
34.1 mm deep for the outside hollows, and 12.9 mm
deep for the inside hollows (Table 1). In the third gallery,
the outside hollows were on average 53.9 mm deep andthe inside hollows were on average 46.8 mm deep. Such
depth differences between the inside and outside hollows
results from changes in the water content related to theoccurrence of wet-dry and salt weathering processes.
In the first gallery, since the inside of the inner pillars
face a closed wall, the pillars are rarely exposed to direct
insolation and ventilation (Fig. 2b). While the outsidesof the inner pillars experience weather changes due to
direct insolation and rainfall, since they face open air.
Therefore, the microclimatic environment is considerably different between the inside and outside of the

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pillars. In particular, the environment brought a difference in the moisture conditions of the hollow surfaces.
The water content during the rainy season was approximately 5% on average for the inside hollows and approximately 3% on average for the outside hollows(Fig. 7). Although these measurements were only performed during the rainy season, this indicates that thewater content in the inside hollows is higher than that in
the outside hollows throughout the year.
Pillar moisture changes due to rainfall are supposedfrom experimental results at the Angkor Wat temple (Hikiand Waragai 2014). They monitored water content at thebase of the pillar by absorption after watering to the floorof the inner pillar (EN8) in the first gallery in August
2012. Based on the results, the water content of the insideand outside of the pillar increased by 2–3%, approximately
30 min after the watering arrived. Although the watercontent of the outside returned to its initial value after 60
to 90 min, the water content for the inside did not return
to its initial value even after 400 min. This indicates thatchanges in the water content become large on the outsidewhenever it rains, but small on the inside of the pillar.
The relationship between the water contents and theaverage hollow depth of the pillar in the gallery is shownin Fig. 8a. The depths in the first gallery tended to be
deep for the outside hollows, which indicated low watercontent. Since the third gallery is a ventilated area, thewater content is low in both the inside and outside hollows. The reason that the hollow depths in the third gallery are deeper than those in the first gallery is due tothe large changes in the water content. The outsides of
the inner pillars in the first and third galleries are easily
deteriorated due to the larger changes in the water content based on changes in the weather.
According to the results, the hardness of the pillars in
the first gallery showed an average of 402 HLD and 334
HLD in the outside and inside hollows, respectively, and ahard in the outside hollow (Fig. 6). The average hardness
in the third gallery is smaller than that in the first gallery,
and the difference between the outside hollows (311
HLD) and inside hollows (323 HLD) is small. As shown in
Fig. 8b, although the hardness tended to decrease with increasing water content, a hardening of the surfaces in thehollows due to the efflorescence of gypsum (Uchida et al.
1999) might be predominant in the first gallery.
As for the reason why the hollow depths are deeper on theoutsides, it is likely that the large and frequent moisturechanges on the outsides of the pillars cause these surfaces tosuffer wetting and desiccation via direct insolation.
Differences in the hollow depth due to the gallery
directions

The moisture conditions of the pillars also differed withthe gallery direction. In the first gallery, the averageWaragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

aPage 11 of 14bFig. 8 Relationships between the hollow depth and water content (a), and the sandstone hardness and water content (b). Dashed lines accompaniedwith the equation and correlation coefficient (R) indicate regression lines of the first gallery

hollow depth varied with the gallery direction, and was
deeper in the eastern gallery (the outside hollow is
46.5 mm and the inside hollow is 14.2 mm), followed by
the southern (34.8 and 12.5 mm), western (33.6 and16.6 mm), and northern (21.4 and 8.4 mm) galleries (Fig.
5). Such differences in the hollow depths may be a consequence of the annual and daily wet-dry cycles.
Figure 9 shows the frequencies of the temperature andthe relative humidity for each period: from June to October (the peak rainy season), and from December to February (the peak dry season). Seasonal temperaturechanges are small (Fig. 9a, b), whereas the humidity
changes are large (Fig. 9c, d). For instance, the humidity
is frequently between 60 and 70% in the dry season(Fig. 9c), whereas it switches to between 80 and 85% in
the rainy season, varying by approximately 20% fromseason to season.
The humidity also has daily cycles and is a major factor in sandstone weathering (Paradise 2002). Therefore,
an integrated daily range of relative humidity (IDRH),
i.e., the difference between the daily maximum andminimum during the rainy and dry seasons, is used tocompare with the average depths of the outside hollows(Fig. 10). It means that the IDRH is an index of the intensity of the wet-dry changes in a pillar. A high IDRH
index indicates a large difference between the daily maximum and minimum humidities and therefore a highfrequency of wet-dry changes in a pillar.

Figure 10 shows that the IDRH is approximately
5400% in the eastern gallery (1E; i.e., the gallery with the
maximum hollow depth), while the northern gallery (1N;
i.e., the minimum hollow depth) has an IDHR of approximately 4400%. This indicates that the eastern gallery has a higher frequency of diurnal changes andtherefore has more occurrences of wet-dry changes compared to the northern gallery. According to Waragai(2007), the surface temperature of the outside pillars
reaches approximately 52 °C due to morning sunshineon the eastern side of the first gallery. The maximum
surface temperature of the pillars in the western gallery,exposed to the afternoon sun, reaches 47 °C and ishigher than that of the northern gallery (approximately
41 °C). Insolation therefore dries the pillar and induces alarge change in the pillar moisture.
There is also a relationship between the IDRH and thehollow depth in the third gallery. The hollow depths are
the same in both the eastern (outside hollows 64.7 mm,
inside hollows 48.4 mm) and western (64.0 and
45.6 mm) galleries, whereas the northern (49.5/
43.4 mm) and southern (37.4/49.9 mm) galleries areshallow (Fig. 5). As shown in Fig. 10, the IDRH of 3N
tended to be low compared to that of 3E in the third gallery. In addition, because the outside of the pillars in thethird gallery face an open space (Fig. 2), the wet-dry
change occurs more easily on the outside of the eastern,western, and northern galleries, and the insides of theWaragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

Page 12 of 14

a

b

c

d

Fig. 9 Temperature and relative humidity frequencies of the galleries in the rainy and dry seasons. a Frequency of the temperature at the firstgallery. b Frequency of the temperature at the third gallery. c Frequency of the relative humidity at the first gallery. d Frequency of the relative
humidity at the third gallery. In the legend; r rainy season (from June to October), d dry season (from December to February)

pillars in the southern gallery, all of which are exposed
to direct insolation. The rear parts of these galleries,
which receive very little insolation, are not exposed towet-dry repetitions.
The hollow depths in the third gallery were deeper
than those in the first gallery, although the IDRH islower (Fig. 10). This is possibly due to a previous periodof vegetation cover. The first gallery may have been covered with dense tropical vegetation from the end of the
Khmer Empire until the temple’s rediscovery (1431–
1860 CE). There has been a sustained, ventilated environment in the third gallery since the construction of thetemple. Therefore, in the first gallery, which is located ata lower level, the weathering of the sandstone pillars
may have been delayed during periods of high humidity.
According to André et al. (2008, 2012), the weathering
rate of the sandstone blocks at the Ta Keo temple(3.5 km from the Angkor Wat temple) increased due tothe enhancement of sandstone desiccation via tree cutting in the 1920s. Cutting trees causing the root

wedging, as a part of the conservation effort, induces
ironically sandstone desiccation, increasing the impact of
wet-dry weathering. Therefore, it is possible that thesandstone block deteriorates faster at present than it didin the past.
The hollow depths vary with gallery direction because
the northern side of the first gallery maintains a high humidity condition with a small humidity range, which iscontrary to the conditions in the other galleries. The intensity of the wet-dry changes in the northern gallery(1N) is low compared to those of the other galleries; accordingly, there is very little sandstone pillar weatheringin the northern gallery.

Conclusions
In this study, we examined the determinants of the hollow depth differences based on 5-year temperature andhumidity observations, which sufficiently represent theenvironmental conditions in the galleries at the Angkor
Wat temple. The results indicate that the outside

Waragai and Hiki Progress in Earth and Planetary Science(2019) 6:10

Page 13 of 14

Fig. 10 Relationships between the average depth of the outside hollow and the integrated daily range of relative humidity (IDRH) of the galleries. A blue
dashed line accompanied with an equation and correlation coefficient (R) indicates a regression line of the first gallery. A red dashed line indicates a trendline of the third gallery. Bars indicate the standard deviation of the hollow depths

hollows of the inner pillars are deeper than the inside
hollows in the first gallery. In the third gallery, the outside hollows are also deeper, except the 3S hollow. Thisis due to the large and frequent moisture changes on theoutsides of the pillars caused by frequent precipitationand desiccation via direct insolation.
The hollow depths are deep in the eastern gallery andshallow in the northern gallery and vary with the gallery
direction. A possible reason for this directional dependence in the hollow depth can be attributed to the highfrequency of humidity changes on the eastern side of thefirst gallery (1E). The intensity of the changes in the
wet-dry cycle in the northern gallery (1N) is low compared those in the other galleries, i.e., in the northerngallery, the sandstone pillar weathering is negligible. Further investigations, including monitoring of the watercontent and surface temperature of the sandstone pillars,should be performed to understand the directional dependence of the weathering processes in detail.
Abbreviations
1E: Eastern gallery in the first gallery; 1N: Northern gallery in the first gallery;
1S: Southern gallery in the first gallery; 1W: Western gallery in the first gallery;
3E: Eastern gallery in the third gallery; 3N: Northern gallery in the third
gallery; 3S: Southern gallery in the third gallery; 3W: Western gallery in thethird gallery; IDRH: Integrated daily range of relative humidity
Acknowledgements
The authors wish to thank APSARA (the Authority for the Protection andManagement of Angkor and the Region of Siem Reap) for the permission to
conduct this research. The authors also thank Professor Y. Ishizawa (Sophia
Univ.) and Mr. S. Miwa (Sophia Univ.) for their cooperation. We thank Dr. A.
Hada (Ryukyu Univ.), Dr. T. Kajiyama (Nihon Univ.), and Mr. T. Maeda (Nihon
Univ.) for their assistance in our field experiments.

Funding
This work was supported by JSPS KAKENHI Grant Number 22401005 and26300008, in addition to the College of Humanities and Sciences of Nihon
University.
Availability of data and materials
Please contact author for data requests.
Authors’ contributions
TW proposed the topic, conceived, and designed the study. TW and YH
carried out the field survey, analyzed the data, and shared theirinterpretation. YH collaborated with the corresponding author in theconstruction of manuscript. All authors read and approved the final
manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Department of Geography, College of Humanities and Sciences, Nihon
University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan. 2SuijyoHigh School, 2-1-45 Shiraume, Mito-shi, Ibaraki 310-0804, Japan.
Received: 25 April 2018 Accepted: 3 January 2019

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