Similan Islands geology

How the Similan Islands geology formed?

Today, we are getting in a time machine and voyaging back a whopping 260 million years to the Mesozoic era of planet Earth. We are not going back to check up on dinosaurs – as much fun as that would be – but to try and figure out how some of the favourite dive sites on the west side of Thailand formed. These include world favourites such as the Similan Islands, Richelieu rock, and almost every other rock outcropping on the Thai west coast. Little did mother nature know that when she pushed out those relatively tiny rocky outcroppings out of the belly of the earth, that millions of years later they would become such a favoured destination for Earth’s future inhabitants.


The Mesozoic period began following the chaos and destruction of the Permian–Triassic extinction event. This event is the most destructive in the 4-billion-year age of our planet. During the course of this extinction event, 97% of all marine life became extinct, as well as 70% of terrestrial vertebrate species. It is the only recorded mass extinction of insects in our planet’s history, in fact, the destruction was so severe that life on earth took up to 10 million years to recover. While the cause of the event is unknown, it ushered in the Mesozoic period, further upheaval to the surface of our planet and the dawn of the age of reptiles.

oceanic platesIt is theorised that during the Mesozoic period, pressure from the Indian plate in the west and from the Shan-Thai Terrane in the east resulted in a subduction event. The Shan-Thai plate boundary runs up from Malaysia to the west coast of Thailand, Burma and then heads west all the way up to Lhasa, the capital of Tibet.
Subduction is a geological event that occurs when two of the earth’s tectonic plates start to move and push towards each other. As the plates touch and push, eventually one of the plates slips underneath the other plate pushing it up as its leading edge heads downwards deeper into the earth. This process is one of the main geological factors responsible for the formation of mountain ranges and island arcs on the surface of the planet. The most well-known island arcs formed by this process is the modern day Philippines.

It is argued that a smaller scale level of subduction events created the many granite islands and undersea pinnacles that litter the west coast of Thailand. This process occurs when the descending plate – in this case the Shan Thai plate – has pinnacles and outcropping that become dislodged and sutured onto the rising plate. As the plate moves forward and is pushed up by the plate underneath it, these pinnacles and outcroppings stick up to form small island chains and underwater seamounts.

Geological composition

similan islands rock formationkoh bon rock formationMost of the islands and sea mounts running up the west coast of Thailand are mainly composed of intrusive granitoids. These rocks are formed as a result of magma bubbling up from deep within the earth and forming protrusions and small mountains over millions of years. These peaks are generally steeper than normal mountains and the molten magma pushes other rock out of the way and creates a more and more columnar shape. This movement happened prior to the above subduction event and these intrusive granitoids are what became sutured to the rising plate.
Over the Eons since the formation of the Similans and many other islands on the west coast of Thailand, the surface has turned slowly but surely into soil and vegetation has taken hold, and life has generally flourished on the islands.
richelieu rock formationelephant head rock

Coral formation

One of the biggest misconceptions about corals is that they are often mistaken for rocks or plants; in fact, corals are animals, and reefs are an amalgamation of coral polyps that live in a calcium carbonate skeleton. Coral polyps filter carbon dioxide from the sea water and transform it to create a limestone skeleton, whilst extracting other nutrients from photosynthesis to grow the coral colony. Since corals require light to grow, it is only the surface of the coral that is alive. For example, when you encounter a large brain coral (Faviidaae), it is only the outer layer of the coral that is alive – the big internal mass of the coral is merely composed of deceased coral polyps’ skeletons. Coral reef explained here.

When coral polyps die, what happens is that the actual animal or polyp expires, and just leaves the dead limestone husk of its former self. This is why a dead reef, with time, looks like a giant limestone rock.

patch reefcoral reef

Not all Islands and reefs are created equally

Once an island or undersea mount is created by tectonic movement, what happens after that can lead to different islands in the same chain having very different geological characteristics. This explains the differences between the granite geology of the Similan islands and other sites like Kho Bon and Richelieu rock which look like limestone to the casual observers.

Depending on the water conditions and temperature after the formation of the islands, corals start to grow, live and die on the rock. With the passing of eons, eventually the base rock becomes covered in a thick layer of limestone coral skeleton, with corals growing on the outside.

With the uplift and subsidence of the rocks over time, these fringing reefs turn into barrier reefs, limestone islands, and atolls. This process can even create the postcard beautiful coral lagoons, since as the centre of the island starts to subside and disappear into the sea floor, it leaves all the coral around its edge intact – creating a circular wall of coral with a beautiful turquoise lagoon in the middle of it.

Research has showed that by boring into most of these limestone blocks, you will ultimately hit a base layer of volcanic rock that is the foundations upon which the island or reef formed. This process also creates undersea coral pinnacles, where the centre has subsided to just below sea level, creating a flat table top of coral. Whether the subsidence continues and creates a lagoon or not is in the lap of the gods.

How long does it take for the process to happen?

Most of the coral reefs on earth today were formed during the last glacial period on earth, this means that the majority of them are less than 10 000 years old, which does not seem like a long time. However, when you take into account that reefs are created by animal activity then it is truly staggering that a small collection of animals have been busy creating these stunning structures.

So what are some of the older reefs on the planet?

Probably the oldest reef on earth at the moment is the Great Barrier Reef, and its immense size belies its age of nearly 20 000 years. The formation of this reef is somewhat different since it started when sea levels were 120 meters lower than today. As the sea level kept rising, the coral kept growing upwards towards the light as more and more of the deeper corals died – leaving a limestone rocky base for the ever expanding reef.

How fast do corals grow?

There are many different growth rates for corals; as a rule soft corals tend to grow much faster than hard corals. This is because hard corals have to extract significant amounts of carbon dioxide form the sea water. This energy consuming process is limited by the amount of limestone that can be laid down. To put it in human terms, it generally takes our bodies much longer to repair an injury to our bones than a soft tissue injury.

Have we been researching corals as long as we have been diving?

No, in fact the study of reefs and coral islands goes back much further than scuba diving; the first great seminal work to be published about coral reefs and islands was in 1842 by none other than the legendry Charles Darwin. In his book The structure and distribution of coral reefs, Darwin put forward several theories to the formation of the various types of reefs and coral islands that over time have proven to be mostly correct.