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!Indonesia – Mountains of fire

As the world’s largest island country, situated on the Pacific Ring of Fire, Indonesia has about 400 volcanoes, of which appx. 150 are active. So if you want to climb an active volcano and even get into a crater, Indonesia is the place to be. With their magical turquoise, but toxic lakes, fascinating blue fires, pungent smell of sulphur and stunning sunsets, Indonesian volcanoes are a treat for anyone looking for an unforgettable experience.

There are many volcanoes accessible for tourists all over Indonesia. Krakatoa, Merapi, Kelimutu, Mount Batur, Mount Rinjani are only a few. But if you happen to be in Eastern Java, you should try to see Bromo at sunset and get to Ijen Crater at night.

Our journey started in Yogyakarta (Jogja)– city located approximately 425 km east from Jakarta (only 45mins by plane). Yogyakarta is also known as cultural capital of entire Java. If you have some spare time, you should wander around in the city centre, explore the 18th-century sultan palace (still inhabited!) and visit local theatre (link) which regularly shows a spectacular ballet based on old Sanskrit epic poem Ramayana.

Two days around the city will give you also a chance to visit two amazing ancient Buddhist and Hindu templates: Borobudur and Prambanan (see pictures below). Borobudur was built in 9th century and is one of the world’s largest Buddhist templates. You will be fascinated by thousands of fantastic reliefs and over 500 Buddha statues, some of them enclosed in magnificent bell-shaped stupas. Another temple – Prambanan is a Hindu temple built in mid-9th century and is, like Borobudur, one of the biggest in the Hindu world. Three characteristic, tall buildings represent Trimurti i.e. trinity of the three supreme divinities: Brahma (creator), Vishnu (preserver) and Shiva (destroyer). Both temples are listed on UNESCO World Heritage list. The easiest way to get there is to book a trip via local tour operator or through the hotel/hostel you will be staying in during your visit to Yogyakarta. The trip costs around 75000-100 000 Rp (but these normally do not include entry tickets). Unfortunately, entry tickets are quite expensive for non-Indonesians (about US$22/Rp 280,000) however it is possible to buy combined ticket which costs about $32 and some tour operators may offer other discounts.

Once you are satisfied with these fantastic architectonic masterpieces you will like to go see some volcanos, I guess. Again, ask at your hotel/hostel reception and you will probably get plenty of tour operator’s brochures and tour options. We chose a trip that offered a combined trip to Bromo and Ijen volcano (that cost us about - Rp 300 000, but this may vary depending on tour operator). Many brochures do not mention the ‘blue fire’ option for the Ijen trip – yet most operators do offer it, just ask at the tour operator’s office or at your hotel.

We set off early morning and it took us entire day to get to some small village at the edge of Bromo; Cemoro Lawang (Tengger caldera; Bromo National Park, entry fee about Rp 230 000). Early in the morning, before the sunrise, we were taken to a view point located approximately 45mins away (of 4WD driving). Make sure you get up early and do not miss your car, otherwise you may miss a chance to see an astonishing sunrise. It is important to be early because there are literally hundreds of cars going up and traffic jams are unavoidable (it is good to take a photo of your car’s number plate to be sure you will find the right car at the end). Also, remember that although Java is on the equator it may get quite chilly, especially that you are over 2500 m (8200 ft) a.s.l.

So, now, you are standing on a hill with a breathtaking view before you… You can see a few volcanic cones in all their glory. In some distance, you can see high Semeru volcano and in the foreground the “steaming” Bromo volcano and old inactive mount Batok. They all belong to a larger geographical unit – Tengger massif.

Bromo volcano geological map

Bromo volcano seen from the distance raises from a relatively flat sand plane (Sea of Sands; Lautan Pasir) and is flanked by steep walls made of volcanic rocks. It is, in fact, an old crater which collapsed at some point in the geological history (see drawing below). This structure is known as a caldera.

Bromo volcano is still active and its last eruption happened in 2015. During, let’s say, normal conditions, the volcano is continuously emitting “smoke” from the crater. If you want, you can see what is inside the crater. You can climb up to the crater rim and walk along it, contemplating amazing views of crater and the sea of sands. While walking up, look at the small eroded ravines. The rocks you will see are relatively soft and laminated - they are records of the past eruptions and they tell you about the long story of the volcano growth. In some places you may find very porous rock– it is pumice. There are more sedimentary rocks of volcanic origin which represent the old deposits of ash fallouts (tuffs) and violent debris avalanches (so called pyroclastics).

The next stop on our excursion was Ijen volcano (Gunung Merapi stratovolcano; which means Mountain of Fire). Ijen is located approximately 140 km (87 mi) east from Bromo area. It takes a few hours to get there. We travelled along a wonderful coast then turned into a jungle to stay for the night at one of the coffee plantations (or jungle) near Ijen volcano.

Again, we had to wake up very early (around 1am) to see the famous, magnificent blue fire. It is definitely worth the early start! Entry fee is Rp 100 000, take a headlight/torch and face mask if you can, but your guide will most likely have a few to share.

It takes approximately 2hrs to get to the Ijen crater rim and then another 40mins to walk down into the crater centre. When you descend into the crater you will find yourself in a strange, hostile world, like you’ve never seen before. You will be walking through a toxic and penetrative sulfur gas and you will eventually see massive yellow blocks of sulfur –and magnificent blueish glow of the melted sulfur among those blocks (the pungent smell of sulfur inside the crater is inescapable and believe me, you will not get rid of it even after 10 laundries). Sulfur is pouring out from so-called fumaroles (solfotares, gas vents). The melted sulfur is solidified and then collected by local miners who carry it on their shoulders out of the volcano and sell in the village. It is indeed a horrible job; people for years work in this unwelcoming, toxic environment. They carry up to 80 kg of sulfur in one run. Sometimes they repeat this trip two or three times a day to collect more sulfur and sell it for an only a few dollars. Their shoulders scar under the weight of the sulfur-filled baskets. Their noses get used to the smell, so they don’t need to wear face masks even where going deeper into the crater, where tourists are not allowed, and sulfur odour becomes unbearable. Only some of them, after years of work as sulfur miners, will become tourist guides.

When the sun rises and you’re lucky, and there’s no toxic fog, you will see an amazing, turquoise lake. It is the largest highly acidic crater lake in the world, with pH as low as 0.13!, which occupies a volcanic crater.

Native sulfur crystals are common feature in volcanic areas. These are products of sublimation or incomplete oxidation of the H2S. Sulfur in general is a minor component of magma. This element is common in volcanic gases and, if released in a massive volcano eruption, can considerably influence Earth’s climate. Obviously, the amount of sulfur in magma varies and depends on many factors such as magma composition, oxidation and sulfur oxidation state. It is well known that in volcanic arcs (e.g. Java), the oxidised and water-rich basaltic magmas are rich in this element. It tends to concentrate in water-rich vapour phase (very often being pre-eruptive gas phase) and most of that element is released during volcanic explosions.

So why does Java have so many volcanoes?

Do you remember subduction zone which we described in one of our last posts? Just imagine the entire ocean bottom being consumed under another tectonic plate. The rocks are getting sucked into the mantle and they start to melt. The hot bubbles of melted rock (magma diapirs) start to rise and eventually reach the surface. This is how volcanoes form. When multiple volcanoes start to coalesce, a volcanic arc is created.

As you can see on the geological map (based on various maps, see further reading for details), eastern Java can be subdivided into smaller pieces mostly based on tectonic position and age of formation. In the south we can see Southern Mountain Arc zone, Sunda volcanic Arc, Kendang Basin and Rembang zone.

Southern Mountain Arc (also known as southern slope regional uplift) is represented mostly by mid-Miocene intrusive and volcanic rocks (andesites) and some carbonates representing mid-Miocene reefs (carbonate platform). These rocks are currently heavily karstified due to relatively recent tectonic emersion. Volcanic rocks (Oligo-Miocene age) are represented mostly by alkaline andesite rocks (“Old andesite”).

Sunda Arc is a modern volcanic arc with 45 active volcanos in Java island. Based on radiogenic ages of the volcanic rocks three phases of volcanic activity have been recognized with the oldest records from Eocene to early-mid Miocene (approx.45-20Ma). This arc, as you probably already figured out, is still active.

Kendang and Rembang zones are represented predominantly by sedimentary rocks (deep water and shelf deposits) which developed in a so-called back-arc basin. This basin is connected to a much bigger geological unit, which is located further north known as Sunda zone (Sunda continental plate). The thick sedimentary succession developed in back-arc basins have big hydrocarbon potential.

Based on some recent geodynamic reconstruction, the subduction of the Indian plate (known also as Australia-Indian plate) beneath the Sunda margin started in Eocene. The volcanic arc is representing a classical calc-alkaline magmatism.

Bromo and Ijen volcanoes are amazing places. Destructive forces are still accumulated beneath Java land. These are manifested by volcano eruptions, which may have dramatic consequences for people living nearby. But this destruction is just temporal. In fact, without volcanoes, Java island would not exist. So volcanoes are, in fact, powerful construction sites.

Bromo means Brahma in Javanese who is a god of creation.

More reading:

Métrich, N. & Mandeville, C.W. 2010. Sulfur in Magmas. Elements 6: 81-86.

Mandeville, C.W. 2010. Sulfur: A ubiquitous and useful tracker in Earth and Planetary Sciences. Elements 6: 75-80.

Darman, H. & Sidi, H. (eds.), 2000. An Outline of the Geology of Indonesia. Indonesian Association of Geologist (IAGI)

Smyth, H.R., Hall, R., Nichols, J.G., 2007. Cenozoic volcanic arc history of East Java, Indonesia: The stratigraphic record of eruptions on an active continental margin. GSA Special Papers 2008, v. 436, p. 199-222

East Java Geological Map based on: Blemmen (1949) and Geological Map of Java (1963), adapted from maps from various articles e.g. Smyth, H.R., Hall, R., Nichols, J.G., 2007. Geological map of Bromo volcano based on Google Earth.


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