Indonesia is one of the most volcanically active countries in the world because of its tectonic instability. Being along the equator, it has only a wet and a dry season in a year with a tropical rainforest climate and extremely fertile soil due to the volcanic activity. The islands of Indonesia have hundreds of mountains, most of them either active or inactive volcanoes. Indonesians have a term called “ribu” meaning “thousand” to denote mountains that are above 1,000 meters tall and “spesial,” Indonesian for “special,” to denote mountains shorter than that. One of the ribus is Kelimutu, a volcanic mountain famous for the three lakes on its peak that change color from green to blue to black or red. Here is more about the lakes of Kelimutu.

Kelimutu is a passively degassing volcanic mountain located in the center of Flores Island of Indonesia. It is 1,690 meters (5,544 feet) high at its peak. 

Kelimutu is close to a small town known as Moni and around 50 kilometers east of Ende. It is part of the Kelimutu National Park. Though the park is named after the volcano, the highest peak in the park belongs to Mt. Kelibara at 1,731 meters (5,679 feet). The park has flora such as pines, casuarinas, redwood, and Edelweiss, that are scarcely found anywhere else on Flores Island.

An interesting feature of the mountain is that one side of it is covered in a pine forest while the other side is dry with highly unstable sand and soil. Kelimutu Mountain is held sacred by the local community who believe that it gives fertility to nature.

Kelimutu is famous for its three, summit, crater lakes which attract both tourists and geologists alike because of their varying colors despite being located atop the same volcano. 

The westernmost of the three lakes, Tiwu Ata Mbupu (TAM) or the “Lake of Old People,” is usually blue in color. The other two lakes, Tiwu Ko’o Fai Nuwa Muri (TiN), or the “Lake of Young Men and Maidens,” and Tiwu Ata Polo (TAP), or the “Bewitched/Enchanted Lake,” are separated by a shared crater wall that arcs down to a minimum height of 35 meters (115 feet) above the surface. They are typically green or red in color respectively.

The lake colors vary on a periodic basis and are a popular tourist attraction. According to the Kelimutu National Park officials and Indonesian guidebooks, the changes in lake colors are a result of chemical reactions that occur because of the minerals present in the water and the gas from the volcano. This explanation, however, is considered misleadingly simplistic by the geologists.

Earliest recorded study of the lakes was done by French naturalist LeRoux who provided a description of them in his travel log in 1896. No further significant studies were conducted to understand the phenomenon until late 20th century. 

After LeRoux, Dutch colonial geologist Kemmerling performed a more comprehensive geologic and topographic survey of Kelimutu in the 1920s when he traveled through Flores Island. He reported a visible volcanic activity in the form of steam and gases above the shoreline of TiN. He also reported that 70 years before his visit, the crater wall separating TAP and TiN was as high as the surrounding crater rim and had since eroded.

Scientists from Wesleyan University, Connecticut, conducted geochemical surveys of the lakes. The water in each lake was found to be chemically different resulting in the varying colors. 

During the survey, the temperature, pH, and dissolved oxygen content were taken as well as samples of the waters. TAM is an acid-sulfate, volcanic crater lake which was more active in the 1970s than it is now. TAP is an acid-saline lake and is intermediate in volcanic activity. The frequent changes in its colors are due to changes in the oxidation state of the water.

TiN is a cool, acid-brine, crater lake with exotic sulfur compounds and probably some amount of copper-rich minerals in the sediments. It has the greatest volcanic activity that includes a flux of around 85 tons of sulfur dioxide per day.

To put it simply, according to Dr. Gregory B. Pasternack of the University of California, the changes in the color of the lakes is similar to how the color of blood looks through our skin. When there is a lack of oxygen, the water looks green just like veins in your wrist. Similarly, when the lakes are rich in oxygen, they appear deep red or black.

[sources: wikipedia, UCDavis, IndonesiaTravel]