Limestone Caves in Phang Nga Bay

The Geology of Phang Nga Bay - Phang Nga Bay is part of what was once a huge coral reef that covered much of what is now Southeast Asia. The islands of Phang Nga Bay were formed by the movements of massive slabs of earth called "plates". These plates, however, were underwater. They were part of the coral reef. They were lifted out of the seas by the movement of the plates.

Our sea canoeing trips in Phang Nga Bay include visits to a couple of sea caves. On 3 and 4 day trips one day is Krabi cave kayaking in the far northeast corner of the bay.

The formation of karst mountains - The Greek word "tektonickos" means to construct. At one point in history, about 200,000,000 years ago, the continents were connected.

One of the explanations used to support the theory is the "continental jigsaw" argument. The continents "fit" together (see drawing below). Continental geology is another reason. The rock layers match where continents were thought to be joined before. And finally, there are fossils of ancient life forms on separate continents that would not live in the same region as they lay today.

Krabi cave kayaking

These plates shift and move on the lower layers. All of the layers are in motion relative to each other. The consequence of this motion is collision. Incidentally, almost all volcanic, seismic (earthquake), and orogenic (mountain building) activities go on along the boundaries created by these collisions.

Plates move in 3 different methods

  1. Away from each other, called Divergent
  2. Toward each other, called Convergent
  3. Slide past each other, called Transform plate


Limestone is sedimentary rock. About 8% of the earth's surface is sedimentary rock. Fossils are a distinguishing feature of sedimentary rock. However, fossils can be found in other types of rocks too.

Non-marine origin limestone occurs due to diagenesis ("dia" is Greek for "through". "Genesis" is Greek for beginning or origin). Sediments are deposited initially as unconsolidated debris. Consolidation comes about gradually due to dewatering (not in the type of limestone in Phang Nga Bay) and/or because of cementing with a binding material (clay, calcium, lime). The rock is altered and gradually changes into limestone through this process called diagenesis. Rocks that have become solid in this way are described by ending with the word "stone". Like, limestone, mudstone, and sandstone.

Phang Nga Bay hong

Almost all sedimentary rock is layered. Reef limestone is generally not layered. They develop as an atoll or fringing reef. Lime is continuously deposited by the coral animal (polyps).

Pure limestone is snow white. Other colors are caused by other rocks:

  • Limonite and siderite cause yellow-brown shades
  • Hematite causes red
  • Glauconite and chlorite cause green
  • Bitumen causes gray to black

Limestone of marine origin

This limestone is a monomineralic (one mineral) rock consisting of a single mineral (calcite) which can make up 95% of the rock. Other rocks found in marine limestone include dolomite, siderite, quartz, feldspar, mica, and various clay materials.

Fragments from the hard parts of marine animals and plants, the parts which contain calcium, form the sediment. The main sources of calcium come from algae, corals, calcareous sponges, foraminiferids (certain plankton), bryozoa (moss animals) , brachiopods (lampshells) , echinoderms (starfish, sea urchins, sea cucumbers, sea lilies) , mollusks (snails, bivalves, chitons, octopus, squid) , crustacea (barnacles, lobsters, crabs, shrimp) , and pteropods (some snails, sea slugs, abalone, cowries, limpets).Limestone caves

When they die, they leave behind either complete units or skeletons. Sometimes the former organism is recognizable (fossils). Other times, it is completely broken down.

Limestone sea caves

Karst is limestone that features internal drainage. Most areas that have karst formations also have heavy rainfall and a thick bed of limestone with a lot of underground flowing water.

Dissolution caves are formed by dissolving limestone. These are generally the largest caves with the most interesting features, including some of the most interesting mineral deposits.

Acid dissolves the mineral calcite which is the primary elemental in limestone. Pure water is not acidic and therefore doesn't dissolve limestone. However, the atmosphere contains carbon dioxide. As rain fall, it picks up some of this CO2 and becomes acidic. Due to the biological activities involved in soil production, there are high levels of CO2 in the soil. When rain combines with soil, it therefore becomes more acidic.

Water sets in pools on top of the islands. It percolates down through cracks in the rocks. It dissolves limestone as it goes, thus enlarging the cracks.

The sides of this "sink hole" erode and dissolve. If a sea cave happens to connect with this hole, the enlarging process is sped up by the action of the sea water.

The Famous sea canoe Hong of southern Thailand

There are certainly many hidden "hongs" in the islands that are not accessible by sea canoe since they don't connect with a cave. The only way into these sink holes is from the top.

Paddle Asia avoids the well known hongs of Phang Nga Bay as they have become too popular. Day trip tourists from Phuket visit these hongs in mass. There are actually traffic jams in the caves! It's far from being a nature tour.

We do go into some hongs, which are just as stunning as the famous ones, and they are not visited by others.

Our sister site: 

Follow us on facebook paddleasia twitter paddle asia Paddle Asia Pinterest Our videos on YouTube.

18/58 Thanon Rasdanusorn, T, Rasada, Ban Kuku, Phuket 83000
Dave's mobile: 081 8936558
Moo's mobile: 081 7974855

Tourism Authority of Thailand License Number 34/00224
Proof of Insurance