Halong Bay (Vietnam): Karst Towers
1. Zoom from the coast of Belize to Halong Bay (Vietnam)
2. In the previous lectures we have been talking about caves and sinkholes.
Geologically speaking, these represent early stages of karst geography.
Suppose now that you took that region, with sinkholes developing, and you
rain on it a lot, for a very long period of time. You rain so much that almost all
of the rock has dissolved away. What you are left with is Halong Bay,
Vietnam.
a. Demonstrate how the limestone dissolves by taking some and
dropping some acid on it.
3. Halong Bay is a spectacular region of the coast of Vietnam that consists of
thousands of islands that primarily consist of conical limestone karst towers,
some more than 200m high.
a. You need to look at this scenery with the eyes of a geologist - - - look
at what is MISSING, not just at what is there.
b. Remember, the sea level has gone up 400 feet over the past 20,000
years, so this area was all way above water not too long ago. But now
the whole area is flooded with a shallow layer of water.
c. In fact, the erosion of the limestone towers shows the evidence of
many different sea levels over time.
4. But I’m getting ahead of myself --- the Halong Bay has had a fascinating
geological history.
a. Many of the limestone rocks here go back about 500 million years,
when the whole area was flooded by the ocean for long periods of
time. [much like the Grand Canyon]
b. However, unlike the Grand Canyon, this area underwent a tectonic
collision that caused the layers of rock to be tilted and even folded up
in places. (about 350 million years)
c. By 70 million years ago, mountain building processes (associated with
the arrival of India crashing into China, forcing the lands of SE Asia
toward the east) pushed the rocks of Halong Bay up to form tall
mountains, which immediately began eroding down.
d. Even more importantly, it created a broad set of parallel cracks in the
rock called “joints.” These joints have provided places for the rain
water over tens to hundreds of millions of years to percolate down
into the rock, causing the development of fissures and caves
underground. Over time these cracks widened, until there are large
spaces between the towers.
e. [Could demonstrate this with a block of clay. I don’t think I did this in
HTEW]
f. And it sure rains a lot! Currently about 20 m per year of rain, and
more at times in the past.
g. The greatest erosion of the region occurred within the past few
million years.
h. The percolating groundwater has created extensive caves throughout
the towers. When parts of the towers collapse and fall down, they
often reveal new and unknown caves within.
i. The rising and falling sea level affects the towers as well, creating
notches in the bases of the towers, keeping them steep-sided.
j. The islands are covered with lakes that are collapsed sinkholes.
k. One well-known pair of rocks is the Kissing Cocks.
5. Halong Bay also has a very long human history.
a. Soi Nhu peoples, starting around 16,000 years ago. Diet of freshwater
shellfish shows that it was a mountainous area at that time, not near
the ocean (remember, the sea level was hundreds of feet lower).
b. By 5000 yrs ago the sea level had risen to close to its current level,
and the Cai Beo peoples here had developed boats and were exploring
islands.
c. Interesting cultural myth: From Wikipedia: “According to local legend,
when the Vietnamese were fighting Chinese invaders, the gods sent a
family of dragons to help defend the land. This family of dragons
began spitting out jewels and jade. These jewels turned into the
islands and islets dotting the bay, linking together to form a great wall
against the invaders. The people kept their land safe and formed what
later became the country of Vietnam. After that, dragons were
interested in peaceful sightseeing of the earth, and then decided to
live here. The place where the mother dragon descended was named
Hạ Long, the place where the dragon's children attended upon their
mother was called Bái Tử Long island (Bái: attend upon, Tử: children,
Long: dragon), and the place where the dragon's children wriggled
their tails violently was called Bạch Long Vỹ island (Bạch: whitecolour of the foam made when Dragon's children wriggled, Long:
dragon, Vỹ: tail).”
d. In 1288 a famous sea battle was fought here. The Mongols had taken
over most of Asia, and Kublai Khan [Grandson of Ghenghis; famous for
converting China to Buddhism] had a navy that was attacking along
the coasts of SE Asia. The Vietnamese navy tricked Kublai Khan’s navy
to be trapped in a labyrinth of channels of the Bach Dang river there.
When the Mongols tried to escape, the found that the Vietnamese had
put metal-tipped spears in the water, and all their boats were impaled
on them. The Vietnamese had rafts of fire that they floated down into
the Mongol fleet, and had an army ready on the surrounding banks, so
the mongol boats burned and their soldiers were killed as they swam
to shore.
e. Unfortunately, that was not the last warfare this region has known.
Many of the channels around the islands are impassable today
because of mines the US installed during the Vietnam War.
f. Today over 1500 people live in 4 floating fishing villages among the
karst towers.
6. Top 5:
a. Guilin (China), along the Li River: This was a tough call – could easily
have edged out Halong Bay. Spectacular towers that have inspired
Chinese art and poetry for centuries.
b. Phang Nga Bay (Thailand) – 42 islands similar to Halong Bay –
includes famous “James Bond” rock
c. Chocolate Hills (Philippines) – strange conical grass-covered
limestone hills
d. Tsingy (Madagascar) – sharp, razor-edged karst formations. Very
different from other locations (very little rain there, so most
weathering occurs slowly from groundwater). Show my own pictures
and describe.
Questions:
1) What might control the distance between karst towers in terms of how they
initially form?
2) When you look at all of these places that have these tall karst towers, what
kinds of geologic history to they have to share to be so similar?