Marcus Gillespie
Some
of the most beautiful and distinctive art from Southeast Asia depicts a
seemingly magical landscape of magnificent rock pillars that rise above
alluvial valley floors and utterly dwarf any manmade features that may
appear in the work (Figure 1).
Such artwork conveys the sublimity of the natural world and an idealized
state of harmony between humankind and nature.
The landscape is known as tower karst and because of its striking beauty
(Figure 2) it has served as the backdrop in such
movies as the James Bond classic, “Man with a Golden Gun” and more recently,
“The Beach”. The recent episodes
of the TV series “Survivor” are also set among tower karst islands in Thailand.
This article discusses the origin of tower karst in general and focuses
on that which occurs in Southern Thailand near Phang Nga Bay in the Andaman
Sea (Figure 3a and 3b).
Any
landscape whose characteristics derive primarily from the dissolution of
limestone rock is termed a karst landscape.
Limestone is a type of sedimentary rock that forms in the ocean by chemical
processes that result in the precipitation of calcium carbonate.
Over thousands to millions of years, the calcium carbonate can accumulate
to form layers of limestone rock thousands of feet thick.
Once this rock is uplifted above sea level by geologic forces, ground water
percolates through it and slowly dissolves it, creating a variety of solution
features. Thus, if water
is present to dissolve the limestone, karst landscapes can develop in almost
any climatic region, including arctic and arid areas.
However, karst landforms are most likely to develop in temperate and tropical
climates because of the greater water availability.
The
dissolution of the limestone often creates sinkholes and caves, and it
is these solution features that people in the United States and most other
midlatitude countries usually associate with karst.
In the mid-latitudes, these features are common where thick, hard, porous,
and pure (greater than 60% calcium carbonate) limestone layers are present.
For example, Mammoth Cave in Kentucky and Carlsbad Caverns in New Mexico
are cave systems developed in such layers.
Thick, resistant limestone is also necessary for tower karst to develop
and caves are also common in tower karst regions; however, the towers that
characterize tower karst can only develop in the tropics because it is
only there that the right conditions of rock and climate interact to create
these magnificent features.
The most spectacular examples of tower karst occur in the monsoon area
of Southeast Asia, including the Malayan Archipelago and Indonesia.
China contains what is perhaps the most beautiful and extensive tower karst
and it has significantly influenced the culture of this region.
Tower karst also occurs in Papua New Guinea, Australia, Honduras, Cuba,
Jamaica and Puerto Rico.
The
towers that define this type of topography can reach heights of more than
1000 feet and are very steep-sided, with slopes ranging from near vertical
to overhanging (Figure 4).
Individual towers can range in extent from small pinnacles (Figure
5) to blocks that are several square miles in area.
They often occur in groups, and are usually surrounded by a river, alluvial
plain, or, in the case of Phang Nga Bay, by a mangrove swamp (Figure
6). These moist conditions
are largely responsible for the creation of tower karst.
In
Thailand, tower karst occurs predominantly in the southern, peninsular
region of the country, which is 500 miles long and has a maximum width
of 124 miles. This part of
the country is bordered on the east by the Gulf of Thailand and on the
west by either Burma or the Andaman Sea (Figure 3b).
The fossiliferous, light gray limestone (Figure 7)
that forms the tower karst is part of the Ratburi Group, a geologic unit
that was deposited during the Permian Period between 286 and 245 million
years ago. This unit also
contains some sandstone and shale, and ranges in thickness from 2467 feet
to 3000 feet in peninsular Thailand.
This unit is more than 6500 feet thick in other parts of Thailand.
Wherever it is exposed, it forms prominent ridges and towers.The
ridges develop because the limestone outcrops occur in long narrow belts
that follow the lineation of the mountain chains on the peninsula.
This arrangement facilitates ridge development as surrounding rock layers
are removed by weathering and erosion.
Consistent
with the fact that tower karst can only develop in the humid tropics, one
researcher has determined that tower karst development requires a minimum
of 47 inches of precipitation per year and an average temperature of 64°F.
Peninsular Thailand, with a minimum rainfall of 51 inches per year and
an average annual temperature of about 82°F clearly meets these climate
requirements. Consequently,
the tower karst in this area is well-developed and spectacular.
Given
that limestone dissolves in water, one wonders how giant towers of limestone
rock could not only form, but grow in height in a hot, wet, monsoon climate?
The answer to this question has to do with vegetation, microorganisms,
the amount of carbon dioxide (CO2)
dissolved in water, and the porosity of the limestone.
In order for water to dissolve limestone, it must first react with carbon
dioxide to form carbonic acid.
Thus, it is actually the carbonic acid that dissolves the limestone, not
water
per se. The carbon
dioxide that reacts to form carbonic acid occurs naturally in the atmosphere,
which is why rainfall is naturally acidic with a pH of about 5.6.
The
acidity of the rainfall is also partly dependent upon its temperature;
specifically, the warmer the water, the less carbon dioxide It can dissolve.
This means that, all other factors being equal, warm, tropical rainfall
is usually less acidic than cooler rainfall in temperate regions.
So, rainfall onto bare, unvegetated limestone in the tropics is not particularly
effective at dissolving it. However, because of the warm, wet climate of
the monsoon tropics, lush vegetation covers virtually all surfaces that
are not too steep to prohibit its growth (Figure 8).
The roots of plants release carbon dioxide to the soil and this elevates
the concentration of this gas to levels that are higher than that found
in the air. In addition, microorganisms
decompose dead plant and animal material in the soil and this also results
in the release of carbon dioxide.
Because of these processes, the carbon dioxide content in tropical soils
is up to 15 times greater than that of the atmosphere.
As a result, the soil water becomes far more acidic than rainfall and aggressively
dissolves the limestone below the surface.
In some swampy, karst areas, water with a pH as low as 3.0 has been recorded.
This level of acidity is equivalent to that of vinegar.
Karst
towers are surrounded by rivers, alluvial plains, or mangrove forests (Figure
9). These environments
provide not only abundant water, but high levels of carbon dioxide as well
because of the abundant vegetation that grows there.
It is for this reason that tower karst occurs in these areas.
These environments result in the dissolution of the limestone along the
base of the towers and, as the soil surface is eroded, the dissolution
of the limestone layer proceeds downward.
Because
the manner in which the towers develop, the sides of the rock towers become
very steep, so very little soil or vegetation can develop on them.
Consequently, the water that runs across their surfaces is not very acidic,
and so the sides of the towers do not dissolve as rapidly as the bases.
In addition, in the hot tropical sun, water evaporates rapidly.
When it does, any calcium carbonate that was dissolved in the water precipitates.
The precipitated material plugs cracks and pores in the limestone and this
reduces the amount of water that flows through it.
This process is called “case hardening," and it further limits the amount
of dissolution. In so doing,
it greatly increases the tower's lifetime.
Reprecipitation also results in the formation of stalactites all along
the sides of the towers (Figures 10a and 10b)
and in the creation of speleothems inside the caves that frequently form
in towers (Figures 11a and 11b).
As a result of this combination of processes, the base of a tower dissolves
more rapidly than its top and sides, and so it grows in height as the surrounding
land surface is lowered by erosion
In
the early stages of tower karst development, solutional surface depressions
known as dolines develop into steep-walled depressions known as cockpits.
If the solution progresses, steep-sided hills will form between the cockpits.
If alluvium that is deposited either by a river or by runoff from hillsides
fills the cockpit depressions, then the contrasting weathering rates described
above will lead to the development of tower karst.
The
contact between the alluvium and the base of the towers is a locus of spring
development and cave formation.
Because of the cracks and pores in limestone, water readily enters it and
flows downward until it either meets an impermeable layer or the rock becomes
saturated. When this occurs,
the water will emerge from the rock as a spring.
An excellent example of this can be seen at Waterfall Cave in Sa Bok Koronee
National Park. Here, a small
stream emerges from a cave at the base of a rock tower, whereupon some
of the calcium carbonate in the water precipitates forming small "waterfalls"
(Figure 12).
Unlike
their counterparts in the temperate mid-latitudes, caves in the tropical
karst regions are shallow and not well developed.
They usually consist of a network of small tunnels (Figure
13) rather than an extensive system of caverns such as that which comprise
Carlsbad Caverns in New Mexico.
The difference results from the differing rates at which soil water becomes
saturated in these climate regions.I
n the tropics, the water dissolves the limestone more rapidly near the
surface because of its greater acidity.
When water becomes saturated with calcium carbonate, it cannot continue
to dissolve limestone at greater depths.
So, deep caves cannot form in tropical karst.
As discussed previously, the saturated water redeposits the calcium carbonate,
thereby reducing the porosity of the limestone and further limiting cave
development. Finally, because
the tower karst landscape is fragmented into towers, this also hinders
the development of substantive cave systems.
Nonetheless, some larger, shallow caves do occur and, when they do, they
are associated with through-flowing rivers such as that which forms Waterfall
Cave.
One
important cave in southern Thailand occurs on the tower karst island of
Ko Phi Phi Lay in the Andaman Sea (Figure 14
and Figure 15).
This island is the home of a type of bird that produces nests using its
own saliva. For generations,
indigenous people have risked their lives to build bamboo scaffolding to
reach the nests at the top of the cave.
Without any protection or specialized climbing gear, these people climb
over a hundred feet up the scaffolding, the tops of which are secured by
ropes to stalactites and ledges in the dark heights of the cave.
The motivation for doing this is, in modern times, purely financial.
The nests are used to make a soup that is believed to invigorate those
who eat it. In Japan, the
soup will sell for as much as $100 a bowl.
The people who harvest the nests claim that the nests can be harvested
up to three times a year without overly stressing the birds.
If
the alluvium at the base of a tower remains in contact with the base at
a given elevation for an extended period, overhangs may develop as soil
water erodes the base laterally.
Wave action and salt weathering also erode the base of towers that rise
from the sea (See Figures 7, 10b
and 16).
These overhangs can be as much as 20 feet high and 66 feet wide in some
areas and, were it not for the natural strength of the limestone, they
would quickly collapse. Eventually,
however, the strength of limestone will be exceeded and huge slabs along
the sides of towers will sheer off (Figure 17).
By this process, as well as gradual dissolution, the tower will shrink
in size until nothing of it remains.
The sheering of the side of a tower is undoubtedly a spectacular event,
though quite dangerous to anyone who happens to be nearby.
At
Tiger Cave Monastery in the province of Phang Nga, Buddhist monks have
established a temple, part of which extends under a karst overhang.
Access to this part of the monastery is by a stairway that ascends a tall,
nearly vertical ridge of limestone and then descends on the other side
into a closed karst valley that is lush with the dark green vegetation
of a tropical rain forest.
In this natural sanctuary, in which trees more than one thousand years
old can be found, monks have placed a larger-than-life Buddha image under
an impressive overhang. A
series of small caves extend into the limestone walls under the overhang
and the entrances to some of them are occupied by Buddhist monks who have
built small shelters there to protect them from the daily rains.
That monks consider this valley and its caves to be sacred is further evidenced
by their placement of numerous Buddha images and offerings on formations
within the caves. In this
same area, a large reclining Buddha has been placed under an overhang (Figure
18). Veneration of caves
by the Thai people is a common practice, as it has been for most peoples
throughout the history of humankind.
The
overhangs and caves that develop along the contact between the tower base
and alluvium can be used to trace the developmental sequence of alluvial
valleys. This is because their
development represents a period during which lowering of the alluvial plain
slowed and enough time was available for lateral solution to occur.
Conversely, the lack of overhangs indicates a more rapid rate of lowering
of the alluvial plain-one that did not permit sufficient time for overhangs
and caves to develop. If human
artifacts or other dateable materials can be found within the caves, they
can be used to date the stages of development of the towers.
Using
paleomagnetic data obtained from caves at different levels in a tower in
China, one researcher found that the tower had grown by the lowering of
its base at a rate of less than 0.91 inch per 1000 years during the last
one million years. Based on
this rate, it would take approximately 1,318,700 years for a 100-foot tower
to develop. The tower in
China, from which this rate of development was determined, has been developing
for at least 2.5 million years.
Clearly, these towers develop slowly and last for great periods of time.
Researchers believe that, in other parts of China, some tower karst began
forming at the end of the Cretaceous Period 65 million years ago.
If this estimate is accurate, this is remarkable because few landforms
on earth, other than mountain ranges, are anywhere near this old.
So, as these examples show, tower karst features can represent truly ancient
landforms and can be of great use to geologists, geomorphologists, and
archaeologists who seek to understand the past.
Because
of on-going crustal plate collisions in Southeast Asia, the Malay Peninsula
is experiencing a slow deformation that is causing the east coast of the
peninsula to emerge from the Gulf of Thailand, while the west coast is
slowly dipping and subsiding beneath the Andaman Sea.
The result of this gradual deformation of the region has been to create
an emergent coastline along the east coast that is characterized by wide,
sandy beaches. The west coast,
however, has few beaches and is characterized by drowned river valleys,
prominent headlands, mangrove forests, and isolated islands of partially
submerged tower karst (Figure 19).
The island of Ko Phi Phi Lay, mentioned above, is one of these islands.
Submergence was also facilitated by the rise in sea level that occurred
at the end of the Pleistocene epoch ten thousand years ago.
At that time, the great ice sheets that had covered much of North America
and Europe melted and raised sea level about 350 feet.
Because of this submergence, the west coast of Thailand, especially in
the region of Phuket, is especially fascinating and beautiful, and attracts
tourists from all over the world (Figure 20).
Rising
abruptly and dramatically from the sea, the rock towers provide scenery
of unparalleled beauty. These
tower islands can be accessed by boats, and around many of the towers,
small, sandy beaches have developed that make it possible to stop, have
a picnic, and do some snorkeling on the magnificent coral reefs that grow
on the submerged portions of the towers.
Amazingly, several of these islands are inhabited, with the local population
being confined to a narrow strip of sand along the base of the vertical
towers. Imagine what life
must be like on these islands.
The scenery is one of vast horizontal expanses of open ocean, punctuated
by vertical walls rising several hundred feet above the water (Figure
21). Movement is severely
limited by water on the one hand and rock cliffs on the other.
Hurricanes, or typhoons as they are known in this part of the world, would
easily submerge the only level land around, so life is delicately balanced
between extremes.
The
inhabitants of one village, known as Panyee, dealt with their space limitation
by building the entire village on stilts (Figure
22). Even the school "soccer
field" is nothing more than a "pier" between buildings (Figure
23). The villagers use
the rock tower next to their perched village exclusively for burial of
the dead and for the siting of a mosque (Figure 24).
Tourists can reach Panyee by boat, but tourism is changing this community.
Selling cheap souvenirs has replaced fishing as the primary source of income
for the people of Panyee (Figures 25a and 25b).
The
tower karst of Thailand, like that found elsewhere in the tropics, is unlike
any landscape to be found in North America or Europe.
For those who have the opportunity to see it, its grandeur is unforgettable
(Figure 26).
Learning of the origins of this beautiful and fascinating landscape only
enhances one's appreciation of it.
Photographs
1.
Chinese painting depicting a tower karst landscape.
2.
Sunrise at Phang Nga Bay on the Andaman Sea.The
high humidity and salty sea air create perfect conditions for beautiful
sunrises.
3a.
Maps of Thailand and Southeast Asia.Phuket,
Krabi and the Andaman Sea are shown on the southwest portion of the peninsula.Sources:
http://www.amazing-thailand.com/Images/MainMap.jpg and http://www.cia.gov/cia/publications/factbook/
3b.
Larger-scale map of peninsular Thailand. Source: http://www.amazing-thailand.com/Images/South.jpg
4.
Tower karst at Phang Nga Bay exhibits the tall and steep sided character
of karst towers.
5.
This remnant rock pinnacle is called Nail Head Island.With
continued salt weathering along its base and wave action, it will eventually
topple.Dozens of tourists visit
this site by boat each day from resorts located along the coast.
6.
The acidic waters of a mangrove swamp create ideal conditions for the development
of tower karst.The tidal fluctuation
at this location in Phang Nga Bay is about 10-12 feet.
7.
The side of this rock tower reveals the typical gray color of the limestone
that comprises the Ratburi Group.However,
iron oxides from tropical soils often stain it orange.
8.
The manner in which vegetation cover can vary based on slope is clearly
shown in this image of a small tower island.The
unvegetated sides of karst towers experience case hardening, which makes
them resistant to weathering.
9.
Towers rising from a mangrove forest at Phang Nga.Before
the submersion of this region, these towers would have been surrounded
by an alluvial plain.The multiple
roots at the base of the mangroves help trap sediment and organic matter.The
decay of the organic matter produces acids that dissolve the base of the
towers.
10a.
Stalactites form on the outside of the towers when calcium carbonate precipitates
from the water in which it was dissolved.A
cave is located at the base of this tower and a Buddhist shrine has been
placed inside it.For scale, note
the people standing at the foot of the cliff.
10b.
Stalactites are visible along the sides of this sea arch that has developed
in a small tower in the Andaman Sea.
11a.
Although caves are not as developed in the tropics as they are in the mid-latitudes,
they can develop some beautiful speleothems, such as this orange-colored
flow stone.The orange is due to
the iron oxides that form in the tropical soils above the cave.This
cave is located near the city of Phang Nga.
11b.
Stalagmites and columns in the cave near Phang Nga.
12.
The small waterfall at Sa Bok Koronee National Park.As
the small stream flows out of a cave at the base of the tower in the background,
calcium carbonate precipitates to form the flowstone that creates the pool
and "water fall".
13.
This is the entrance to the cave near Phang Nga.Note
the relatively small size of the cave.
14.
This map of the Krabi region of Thailand shows the Island of Ko Phi Phi
Lay.The small islands in this area
are surely some of the most beautiful in the world.Source:
http://www.thailand.com/travel/pic_map/map_krabi_2_2.gif
15.
Bird Cave, on the island of Ko Phi Phi Lay, is visited by tourists who
come to see the bamboo "towers" used by local people to harvest the bird
nests high inside the cave.The nests
are used to make soup that sells for up to $100 a bowl in Japan.
16.
These children are standing under an overhang at the base of the tower
located adjacent to Nail Head Island.The
fact that the overhang can be so large gives some sense of the strength
of the limestone in which tower karst develops.
17.
When an overhang becomes large enough, the overlying rock eventually breaks
free and sometimes forms a perfectly flat shear wall such as this one.
18.
This larger-than-life Buddha image near Tiger Cave Monastery was placed
under an overhang created by the dissolution of the limestone at the base
of tower karst.The surrounding area
is a rain forest that has acidic soils.
19.
The west coast of peninsular Thailand is slowing subsiding below sea level
because of tectonic forces acting in this region.As
the area subsides, towers that were once surrounded by mangrove forests
become surrounded by the sea.
20.
This is one of several beach resorts in Phuket.
21.
Notice
the small homes located at the base of this isolated tower in the Andaman
Sea.It is difficult to imagine what
it must be like to live in a place surrounded by vast expanses of sea punctuated
by the shear vertical walls of rock towers rising from the water.
22.
The village of Panyee in the Andaman Sea was built almost entirely upon
stilts.
23.
The schoolyard at Panyee.When the
author visited a few years ago, it was made of wood.On
his most recent visit in 2000, it had been paved with concrete.However,
it is a concrete surface on a pier above the water.If
a child kicks a soccer ball too far, he or she has to have a boat to retrieve
it!
24.
Southern Thailand has a large Moslem population, and this is true of Panyee
as well.The mosque shown near the
center of the photograph was built on solid ground, which is also used
for burials.
25a.
Tourism has substantially changed the way of life for the people of Panyee.Once
a fishing village, it has become a tourist trap that peddles all sorts
of trinkets, such as those shown here.
25b.
Even the tower adjacent to Nail Head Island has become a tourist trap.Tourists
can buy film, postcards and other items on this small patch of sand in
the middle of the sea.Kids can even
have their pictures taken holding a hawk--for a fee.
26.Sunrise
at Phang Nga Bay.
