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Volcano Trails, Indonesia

PublicDomainPictures via Pixabay

“Remind me that the most fertile lands were built by the fires of volcanoes.”

― Andrea Gibson, The Madness Vase

The term 'fire and brimstone', has been used to denote God's wrath in the New Testament, but it could just as easily describe Indonesia. Indonesia straddles the Pacific Ring of Fire, a horseshoe-shaped zone demarcating the world's most tectonically active regions. The term 'ring of fire' is also archaic, dating to the Greek and Roman era when it was believed that volcanoes were caused by fires burning within the earth.

Around the world, volcanoes were attributed to the wrath of gods. Among the Maori, Rūaumoko is the god of earthquakes, volcanoes and seasons. Pele is the goddess of volcanoes and fire, and the creator of the Hawaiian islands. Lalahon is the goddess of fire, volcanoes and harvests in the Philippines. The Greeks worship Hephaestus and the Romans Vulcan, gods responsible for volcanoes and metallurgy. Some cultures also had deities for specific volcanoes, like Kan-laon in the Philippines who inhabits Mount Kanlaon, or the malignant Guayota, who lives inside the Teide volcano in the Canary islands.

Today, science is better able to explain the occurrence of earthquakes and volcanoes, and how they're connected to climate. We've also been able to look into the past to understand how their destructive influence has been critical to life on Earth.

Indonesia is one of the most exciting landscapes to study or appreciate the earth-shifting, landscape-sculpting action of volcanoes and their myriad features. This newsletter will provide a broad overview of Indonesia's volcanic landscapes, with some tidbits from my travels to 3 volcanoes: Mount Bromo and Mount Ijen (August 2011), and Tangkuban Perahu (Nov 2022).


VOLCANOES - how they erupt

Deep under the Earth's surface, temperatures are hot enough for rocks to melt and flow as magma. As this magma is lighter than the solid rock around it, it rises and pushes through vents and fissures in the Earth's crust. 'Magma', is used for molten rock that is still underground, whereas 'lava' is the molten rock that breaks through the Earth's surface. The composition of the magma determines whether a volcano is explosive or not. Magma can be thick and sticky, or slow-moving and runny. If the magma is thick and sticky magma, gases cannot escape the flow, and they build up pressure to erupt as violent, explosive volcanoes. When the magma erupts, it breaks apart into 'tephra', which can range from tiny, ash particles to giant boulders. Explosive volcanoes are dangerous and deadly, with ashfalls that can suffocate all forms of life. If hot volcanic materials mix with flowing water from streams or melted ice, they form mudflows or lahars, which can bury entire communities. When the magma is runny, gases can escape more easily, and the resultant 'lava flows' move slowly, and are less dangerous.

The pahoehoe lava forms ropy structures at the Kilauea volcano in Hawaii. Image credits: Rufiyaa, CC BY-SA (via Wikimedia Commons). The magma composition also defines how the lava, especially basalt lava, forms. Thick, explosive flows result in angular, chunky aa (pronounced as ah-ah). Slow flows create incredible patterns like the smooth, ropy formations of pahoehoe (pronounced as paw-hoey-hoey). If volcanoes erupt underwater, they may form chubby, pillow lavas (Read about the pillow lava formations near Mardihalli, Karnataka in India). Two ideas were key to understanding how the Earth reshapes itself; the continental drift hypothesis which believed that Earth's continents have been 'drifting' through geologic time, and seafloor spreading, where volcanic ridges in the middle of oceans have been expanding to create new crust. These ideas led to the theory of plate tectonics where the movement of the Earth's outer shell comprising numerous plates, results in earthquakes, volcanoes, mountain-building and the formation3,000-kilometre-long of oceanic trenches.


INDONESIA - a volcanic archipelago

With over 130 active volcanoes along the 3,000 kilometre long Sunda Arc, Indonesia's archipelago has been shaped by subduction — a process where a tectonic plate interacts with another plate, and buckles or slides beneath it. Sumatra, Java, Bali and the lesser Sunda islands, represent a chain of active volcanoes, formed by the subduction of the Indian Plate beneath the Eurasian Plate (the same subduction along its northern edge is responsible for the formation of the Himalayas). This volcanic arc of western Indonesia, is one of the most seismically active regions on Earth, with a history of eruptions and earthquakes. The eastern islands of Indonesia such as the Moluccas and Papuawere formed due to the movements of the Pacific and Australian plates. As a seismic hotspot, we often hear of earthquakes and volcanoes in Indonesia. Just this week, we've heard the news of the 5.6 magnitude earthquake in the town of Cianjur, West Java on 21st November 2022, and Mount Merapi erupting lava since 25th November 2022. Over geologic time, this seismic activity has shaped numerous landscapes across Indonesia. Here are some examples based on my travels or writings:

Stratovolcanoes & cinder cones

Image credits: Andhika bayu nugraha (CC BY-SA via Wikimedia Commons). Most of Indonesia's volcanoes are stratovolcanoes — which form over multiple eruptions, by a combination of lava flows, often occurring at subduction zones associated with continental margins. These are different from shield volcanoes — which have a crater at the summit (like you'd draw a volcano if in school), represent a central vent for magma, and occur on ocean islands above seismic hotspots, or on continents above continental rifts. Mount Bromo in east Java, is among the most active volcanoes, and a good example of a stratovolcano. Mount Bromo may also be classified as a cinder cone — a steep cone-shaped hill of pyroclastic fragments like cinders or solidified magma, ash and scoria (a porous, dark-coloured volcanic rock). Most cinder cones have a prominent crater at their summit.

Here's a lovely timelapse video of Mount Bromo:

In 2011, when I travelled to Mount Bromo, I couldn't have imagined the sheer size of the volcano — the crater seemed to yawn open before us. The little village I was staying at was perched on the rim of the giant depression and I could see little cones rising in the distance. On my first evening, I ventured on horseback with a motley crew towards the summit. I remember the smoky cone that we trotted toward and the eerie silence of the pony's hooves as it kicked up ash. We couldn't reach the summit before dusk, and had to navigate by the pinpricks of light along the volcano's edge — the soft wind and ash had obliterated our previous tracks.

Image credits: Sharon Ang via Pixabay.

The following dawn saw me with another group, climbing towards a fiery horizon to catch the first smoky glimpse of Mount Bromo. At daybreak, we walked down into the crater, and made our way up the cinder cone — the sight of its incredible maw almost made me lose balance. I remember inching my way along the lip of the crater, on my hands and knees till the vertigo subsided. Then I sat for a long time looking into what seemed like the gateway to hell and wondered at all the mysteries that lay beneath.


Crater lakes

Image credits: David Mark via Pixabay. Lake Toba in north Sumatra, is the largest crater lake in the world, measuring about 100 by 30 kilometres across, with a depth of more than 500 metres. When volcanic eruptions cease, craters often fill up with water, as with Lake Toba. Toba's last recorded eruption dating ~74,000 years ago was termed a 'supervolcano' and it resulted in a global climate-changing event. Several prehistoric civilizations around the world were wiped out during this cataclysm. Yet stone tools indicate that humans who inhabited the Dhaba locality, along the Son river in central India, survived. Read more here.

Image credits: sasaint via Pixabay. Mount Ijen or Kawah Ijen ('kawah' is the Bahasa Indonesia word for crater) is another fantastic crater lake — perfect for the 'fire and brimstone' experience, as the volcano belches sulphur and is rimmed with the pale-yellow mineral. The turquoise lake fringed with sharp, volcanic tephra also seems magical, as you catch glimpses of it through the sulphur smoke. In 2011, I'd read a National Geographic article about the sulphur miners, with stunning moonlit photographs by Olivier Grunewald (the article is behind a paywall now, but the images are archived here). I remember climbing up Kawah Ijen (exhausting, as I was in poor health), and then down into the smoky crater, even trying to lift one of the wicker baskets of sulphur along the way. It was interesting to think of how the volcano affects the lives of people living around it — it may provide them with a livelihood via tourism or mining, yet is slowly poisoning them and is liable to erupt at any point. Though my photos could not capture the sheer beauty of this crater lake, it was an exhilarating experience.


Fumaroles & hotwater springs

Image credits: Devayani Khare (CC BY-SA)

Mount Tangkuban Perahu, located near Bandung, Indonesia, is perfect to see fumaroles and hotwater springs. Fumaroles occur where holes, cracks or fissures near active volcanoes emit steam and volcanic gases, like sulphur dioxide and carbon dioxide. Fumaroles may also occur where magma has risen into the earth's crust but hasn't yet erupted. Hotwater springs around volcanoes like Tangkuban Perahu, are heated by water coming into contact with magma (other hotwater springs rely on geothermal heat, produced by the Earth's fiery interior). Both fumaroles and hotwater springs are signs that the volcano is active. Earlier this month, I visited Tangkuban Perahu — which has 3 craters. Kawah Ratu is the most popular, thronging with tourists, noted for its fumaroles (look for the smoke in the image above) and an ephemeral lake that occurs between October and February, after heavy rains. Kawah Upas, isn't accessible but has more fumarolic fields and was once the site for sulphur mining. Kawah Domas hosts 3 steaming pools at different temperatures, where you can soak your feet, get a massage with volcanic clay or boil eggs in the geothermal springs at exorbitant prices. The locals will also try to sell you volcanic rocks — beautiful, pale-yellow sulphur crystals, glassy, dark obsidian, and even fossilized or petrified wood. From a geological perspective, Tangkuban Perahu is an interesting volcano, as its eruptions often occur suddenly, with few, if any early signs. The 2019 July eruption was the most recent one, though the locals remember the more destructive eruptions of the 1960s. Mount Sunda, once among the highest volcanoes in Indonesia, erupted ~55,000 years ago and left 3 smaller volcanoes behind — Tangkuban Perahu is a remnant of the ancient Sunda volcano.

These are just a few of the volcanic landscapes I've had the privilege to visit in Indonesia — there are so many more worth exploring!



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