From Pasar Senen to Lempuyangan, via Bumiayu and Karangsambung
Tauhid Nur Azhar
The new time showed 9:14 when I walked cheerfully while inhaling the morning air rich in vehicle exhaust gas near Tugu Tani in front of Arya Duta. Not long after, I crossed the bridge and passed in front of Mako Marinir Kwitang located at Jl Prajurit KKO Usman and Harun no 32. The Argo Parahyangan train I took from Bandung arrived on time, so I had plenty of time to change to Pasar Senen station, where I was scheduled to take the Jaka Tingkir economy train at 11:55 BBWI (West Indonesia Time). A thought of hunger flashed in my stomach, which had been craving for Hainam rice from Yomart store in Bandung Utara station, and that thought convinced me to turn into Pejambon V alley behind Immanuel Church across from Gambir, just to stop by and taste Sri Rejeki’s tongseng. But I remembered, maybe Sri Rejeki’s legendary tongseng was not available yet. So I continued walking towards Senin and started making plans, whether to stop by the row of Kapau vendors along Kramat or maybe turn into a side alley and have a breakfast of Makassar coto. Or maybe move a bit forward towards Salemba and visit my cousin Kuki Mobi and his aunt Tintin’s house to ask them to make me a bowl of Naga porridge? Ah, let’s not be haunted by food alone, isn’t it also fun to stop by the National Awakening Museum, which was originally a Stovia school building? Oh, it would be so long, and I might be late for the train.
While still looking around, I continued my journey towards Senen station. As I approached the intersection with the hustle and bustle of vehicles passing over the flyover and underpass, I saw a restaurant on the left with a man-made waterfall that was so serene. But I felt sorry for it, wanting to be close to nature again, so nature was recreated in the midst of the hustle and bustle of technology we created ourselves. Before I knew it, I was sitting comfortably in the economy class seat of the refurbished Jaka Tingkir train, which was a work of Balai Yasa Manggarai. It was clean, cool, and comfortable, and also quiet. Maybe because it was a weekday and an old date. Because of hunger, I fell asleep. Jatinegara, Cikampek, and even Cirebon passed by without me realizing it. I dozed off when the train entered the central Java track and approached Bumi Ayu. Suddenly, various thoughts and memories about Bumi Ayu flashed back in my mind. The hypothesis that a hungry person can think faster was proven to be true. Puzzle after puzzle about the discovery of ancient human fossils to various geological and archaeological facts taught by Dr. Sofwan Noerwidi, Head of the Arkeometri Research Center BRIN, began to assemble in my mind. It cannot be denied that Bumiayu is an important area in the early geological history of Java Island. Because in Bumiayu, there are many geological evidence of the formation of Java Island since the early stages. Bumiayu played a crucial role in understanding the geological evolution of Java, particularly related to tectonic and sedimentary processes that have been going on for millions of years.
Java Island itself was formed through a series of geological processes that were very long, most of which were triggered by the interaction between the Indo-Australian and Eurasian plates. About 50–60 million years ago, during the Eocene period, most of Java was still underwater. However, during this period, active subduction along the southern plate boundary caused the uplift of the continental crust and the formation of volcanoes in the ocean floor.
During the Oligocene period (34–23 million years ago), tectonic activity became more intense. Part of Java Island began to emerge onto the surface as a small landmass called “volcanic arc islands” formed due to underwater volcanic activity. Bumiayu, in this context, represents one of the areas that began to be uplifted from the ocean floor at that time.
Evidence from the rock layers in Bumiayu shows the presence of volcanic and deep-sea sedimentary rocks from this period. This indicates that Bumiayu is one of the early parts of Java formed through the uplift of the ocean floor due to volcanic and tectonic activity.
During the Miocene period (23–5 million years ago), Java experienced a more significant uplift, and the southern part of Java (including Bumiayu) began to emerge as a larger landmass. During this period, Bumiayu experienced strong sedimentation, including shallow sea deposits and coral reefs, indicating that this area was once near the coast. This uplift process resulted in the formation of the landscape we see today around Bumiayu, such as hills and valleys.
Vulcanic activity continued, and volcanoes began to dominate the landscape in western and central Java. These volcanoes include some ancient volcanoes that are no longer active today, which helped create the highlands in the Bumiayu area and surrounding areas.
During the Pliocene and Pleistocene periods (5 million to 10,000 years ago), the Java Island became more stable and most of it was formed as it is today. However, volcanic activity continued, producing young volcanoes along the southern Java arc. The Bumiayu area, which had already formed as part of Central Java, experienced strong erosion, resulting in deep valleys and the mountainous landscape we know today.
The rock layers in Bumiayu contain fossils that help geologists and paleontologists understand the geological and ecological history of the area. For example, the discovery of ancient animal fossils in this area provides evidence that in the past, the Bumiayu area was inhabited by various large species of fauna, which also indicates changes in the environment from sea to land.
Before I knew it, the train had entered the large Purwokerto station, and my hunger since Jakarta seemed to invite me to get off and stop by Purwokerto. Especially the call of Mie Ayam Daplun in front of Daop 5 office on Jalan Rel was so tempting. Mie Ayam Daplun may be the only mie ayam in Indonesia that gives a peanut sauce recipe. Therefore, it is no wonder that the umami flavor is so strong in my tongue.
But I tried to strengthen my intention and strengthen my faith to be loyal to Mangut Lele Girli Kutoarjo, which I had ordered through the Access by KAI application, which would be picked up at the largest station at the end of Daop 5 PWT. Not to mention when I remembered, when I got off at Lempuyangan, I would be greeted by the chubby duo, Dika and Enno, who had promised to be loyal in front of Pak Parno. The promise to always wait patiently for the maturation of Ndas Wedhus tongseng and nasgorkam, whose heat was burning like my wife in a blanket.
And as usual, Jaka Tingkir rested briefly at Kebumen station. After speeding from Kroya and passing through the Ijo tunnel, it arrived at Gombong and Kebumen, which is one region with Karangbolong and Karangsambung.
The face of Tecto Grandiva, a geology expert from Unpad, a mentor in the WA group in terms of geology, began to appear. Many basic geology knowledge, such as stratigraphy, he taught in one opportunity when we traveled together in a vehicle to Rumah Kayu Kanyaah village in the foot of Patuha mountain.
Now, that knowledge seems to echo like a mysterious echo without a speaker. One of the important lessons that Tecto often repeated is the matter of Melange formation. And yes, that formation is indeed unique, endemic, and can be seen in Karangsambung. Not far from this station, only separated by a distance of several dozen kilometers that can be reached by online ojek in less than an hour.
What is Melange formation? Melange formation is one of the types of geological formations that are formed due to complex tectonic processes, usually related to subduction zones, where two tectonic plates collide and one of them moves under the other plate. The word “melange” itself comes from the French language, which means “mixture,” and this describes the main characteristic of this formation, which is a heterogeneous mixture of various types of rocks that are accumulated in one mass.
Melange formation consists of various types of rocks such as sedimentary rocks, igneous rocks, and metamorphic rocks. These rocks can come from different environments, such as oceanic crust, continental crust, or volcanic material.
Melange formation also often shows a condition of fragmented or crushed rocks, due to strong tectonic forces involved in its formation. Large blocks of rocks can accumulate together in a mass that is usually surrounded by finer materials such as shale or clay.
Melange is often formed in subduction zones, where the sinking ocean plate brings material from the oceanic crust, deep-sea sediments, and fragments of the continental crust. When the subducting plate moves downward, deformation, uplift, and accumulation of various materials occur, which eventually become mixed into a melange mass.
This formation often contains large tectonic blocks called olistoliths, which come from their original place, then fragmented and brought to a different environment through tectonic processes.
Therefore, according to the theory above, the Melange formation in Karangsambung was also formed as a result of tectonic processes that occurred in the subduction zone between the Indo-Australian Plate and the Eurasian Plate, which also played a role in the formation of the volcanic chain in Java Island. The formation of Melange and volcanoes is related because both are triggered by the subduction of the oceanic plate towards the continental plate.
The Melange formation in Karangsambung began to form during the Late Cretaceous to early Tertiary period (more specifically during the Paleocene epoch), around 80 million years ago to 60 million years ago. At this time, the subduction between the Indo-Australian Plate and the Eurasian Plate occurred actively.
Karangsambung is part of the accretionary wedge that formed in the subduction zone where the Indo-Australian Plate moves under the Eurasian Plate. During subduction, the sinking oceanic crust brings deep-sea sediments, fragments of oceanic crust, and volcanic material from the ocean floor to the accretionary zone, which is then compressed, deformed, and uplifted to the surface.
During the subduction process, material from the sinking plate does not always go directly into the Earth’s mantle, but some material accumulates in the accretionary zone, forming the Melange formation. This occurs because chunks of oceanic crust, including basaltic rocks from the ocean floor, are carried to the subduction zone. Deposits formed on the ocean floor also get dragged and accumulated in the Melange formation.
While strong compressive forces in the subduction zone will cause material from the oceanic crust and sediments to fragment, mix, and accrete into a heterogeneous geological formation known as Melange.
The Melange formation in Karangsambung is one of the classic examples of this phenomenon, with a mixture of rocks from the oceanic crust and deep-sea sediments arranged in one geological unit. This process has been ongoing for tens of millions of years and has produced a unique mixture of sedimentary, igneous, and metamorphic rocks.
The subduction process that created the Melange formation in Karangsambung is also responsible for the formation of the volcanic chain in Java Island. When the Indo-Australian Plate sinks under the Eurasian Plate, part of the sinking plate undergoes partial melting at the Earth’s mantle. This melting produces magma that then rises to the surface, creating volcanoes.
The volcanic chain in Java, including Mount Merapi, Semeru, and others, is the result of volcanic activity triggered by the subduction of the plate. In other words, the subduction that formed the Melange formation also caused partial melting of material in the subduction zone, producing magma that eventually formed the volcanic chain.
Thinking about the general lecture on the geological history of Java Island, especially about Karangsambung from Kang Tecto, is making me even more miserable due to the unbearable hunger. Why? Because the aroma of Mangut Girli is becoming more and more torturous, as the Kutoarjo station is getting closer. So, it’s better to end this article here, so I have time to prepare for the arrival of Mangut Girli with its chili sauce that needs to be appreciated with a sincere effort when enjoying it. Alhamdulillah.
