Geology Picture of the Day
Top: Stitched photo from the northern summit of Raung. Middle: Annotated with the unconformity between Gadung and Raung as well as recent features such as the main vent and rock fall. Bottom: Illustration of the above photos and annotations.
There are a few geo-nerds who read my blog, so I want to share this photo from the top of Gunung Raung with you. Raung is the large stratovolcano due north of my site. I also work at its observatory on the weekends. Gunung Raung is 3332m tall, quite a bit short of Java’s tallest volcano Gunung Semeru (3676m). The difference between Raung and Semeru, in shape, however, is that the summit of Raung is a >2km wide crater while the summit of Semeru is the tip of a relatively perfect geometric cone. In other words, Raung is a HUGE volcano. If the flanks of Raung extended past its 2km-wide summit up to the tip of a cone, it would certainly be taller than Semeru (Semeru is approximately 1.8wide at the the elevation of Raung’s summit).
Though it is not immediately obvious, Raung is actually a huge volcano on the flanks of another older, bigger volcano named Gunung Gadung. Gadung is the ‘ancestral’ volcano that formed before Raung, but Gadung has since collapsed. The timing of this collapse is unknown (tens of thousands to hundreds of thousands of years), but the numerous hills scattered along the curving 70km path between the old mountain and the Indian Ocean indicate that it was one of the largest volcano avalanches in the known geologic record (have you seen videos of the Mount St. Helen’s collapse in 1980? Think ‘Gadung’s collapse was 10x that large’).
Illustration of the Raung Caldera built on the east flanks of the ancestral Gunung Gadung. This schematic diagrams several innacuracies in that Gunung Gadung was probably taller than Raung—tall enough that its eastern flanks would be visible on the inside of Raung’s caldera. [Figure Credit: Seibert 2002, Landslides resulting from structural failure of volcanoes, Fig. 25].
At some point before, during, or after the collapse of Gadung, magmatic material began to come out of the east flank of Gadung. This eventually built the edifice that is Raung today (Is it possible that the building of Raung on the east flank caused Gadung to collapse towards the west flank?). Because Gadung has since collapsed, an untrained eye only sees one large volcano north of my site: Raung. There are several things you can look for, however, to clearly identify the remaining presence of Gadung. One, there is actually a jagged cirque (horse-shoe shaped opening of a mountain; this word is usually reserved for glacier origins) on the west flank of Raung. This is the cavity left behind from Gadung’s collapse. A small, resurgent cone sticks out of that cirque in the same spot that Gadung used to sit.
This photo looks ENE at Gunung Raung. The dashed line marks the the collapse rim of the ancestral Gunung Gadung. The solid line marks the resurgent cone of Gunung Pajungan. [Figure credit: Seibert 2002, Landslides resulting from structural failure of volcanoes, Fig. 24, SI’s Global Volcanism Program].
Two, the flanks of Raung are slightly lopsided to the east. From certain angles, particularly from my site to the south, the flanks of Raung look perfectly symetrical, but this appearance actually comes from the fact that you are seeing the flanks of both Raung and Gadung. From a SE vantage point—where the Raung observatory sits—you can see that Raung’s east flank actually extends out much farther than the west flank. This is because Raung formed on the east flank of Gadung. As lava flowed out of Raung, it was blocked by Gadung to the west and most easily flowed to the east.
The third piece of evidence is the specific focus of this post’s picture. The old flanks of Gadung can be seen on the inside crater walls of Raung’s summit. Raung was formed up through and on top of Gadung’s east flank. Now that a collapse or explosion has created the deep pit at the top of Raung, the geology of this building process is visible. From the summit of Raung, you can see both the ancestral flanks of Gadung as well as the ash and lava layers that formed Raung. Rocks that belong to Gadung or Raung can be differentiated based on color. The rocks of Gadung are much older and have experienced much longer processes of weathering and oxidation, thus they bear a darker brown and yellow color. The rocks of Raung are fresh and look pink and grey, much closer to their original material. This color difference is immediately obvious to the untrained eye. The west side of the crater is primarily brown and yellow, and the difference slopes down to the east wall where the color is almost entirely pink and grey. The reason for this color differentiation was, honestly, not obvious to me the first time I went up to Raung’s summit, but the whole geologic story dawned on me during the second visit. I was not looking at just any odd weathering phenomenon, I was looking at the bodies of two separate and very differently aged volcanoes.
The ability to investigate the innards of two different volcanoes makes the summit of Raung very special. Not only do you get to see the geometry of one mass built on top of another, you get to see the daunting complexity of volcano morphology. Geology 101 tells us that rocks are deposited in flat, relatively continuous layers that are deformed by identifiable faults and folds. Introductory diagrams depict beautifully continuous layers at volcanoes as well. These diagrams couldn’t be further from the truth. In actuality, lava gets squirted out as lobes here and there, ash falls wherever the wind takes it, and chunks of broken rock stop wherever gravity and friction decide. Raung’s crater shows (though it’s not very apparent in the pictures) that a majority of lava flows went to the east, away from Gadung, leaving only ash on the west side of the volcano.
Above are three ‘nice’ diagrams of a volcano. By ‘nice,’ I mean they look nice and neat but are way too stylized for the structure of an actual volcano. I pulled the top right image from a website where credibility might not be expected, but the other two images are courtesy of the U.S.G.S (left and bottom right via NOAA). I am poking fun at these cartoons but not criticizing their authors. Volcanoes—of which there are hundreds in many different forms—are inherently tricky things to illustrate. During my year at Michigan Tech, a scientist came over from France and asked us to sketch a volcano with the purpose of pointing out how even Masters students, PhD candidates, and professors turn only to iconic representations of their subject when they conceptualize volcano structure. The simplest representation of something is often the best first representation of that thing.
No strata on a volcano, not even those of a stratovolcano, are continuous. This fact is a huge perturbance to volcano seismologists. Seismic waves generated at an earthquake reflect and refract off of all of these geterogeneous layers ad nauseum until they arrive at the seismometer, essentially distorted. Two of the seismometers I observe at Kawah Ijen are placed within 50m of each other, but the they produce signals that are often different in both shape and size. The difference is due to the material underlying each of them—one sits above layers of solid rock (probably lava flows) while the other sits above layers of looser material (likely ash fall).
Whether you are inclined to search for the patterns of geologic structures or not, the top of Raung is a site to behold. The view into the crater, the urban lights of Bondowoso to the north, and the line of peaks extending east-west across Java are all impressive. The climb is arduous and rarely traveled in comparison to surrounding mountains. I hope you at least enjoyed the photo at the top.