Java Island is known as one of the islands in Indonesia with the highest density of active volcanoes. However, not all volcanoes in Java have the same shape and characteristics. Some volcanoes have an ideal conical shape.
But some are composed of several cones, resulting in a complex (compound) shape and a large depression known as a caldera.
Caldera morphology is a key indicator that a volcano once experienced a very large eruption with global impact. Examples include the Krakatau eruption in 1883 and the Tambora eruption in 1815.
Therefore, understanding the evolution of volcanic morphology is important in the context of volcanic disaster preparedness.
A new study by a research team at the Faculty of Geography, Universitas Gadjah Mada, led by Dr. Indranova Suhendro, reveals that the potassium content in magma potentially determines whether a conical volcano will evolve into a complex-shaped volcano or a caldera.
This finding was published in the Journal of Geomorphology in its July 2025 edition, titled ‘On the possible role of potassium enrichment for controlling the morphological evolution of stratovolcanoes into compound or caldera (Java Island, Indonesia)’.
“In this study, my research team and I analyzed 40 volcanoes on Java Island, including those that are always active, such as Merapi, Raung, and Ijen,” Dr. Suhendro revealed on Tuesday (Jul. 22).
To analyze the morphological evolution of a volcano, Dr. Suhendro combined morphometry with satellite image analysis.
He also used high-resolution digital elevation models (DEM) from NASA and BIG (Geospatial Information Agency).
With the help of QGIS, his software program, he manually digitized the basic volcanic cones.
He then calculated and analyzed them using several parameters such as volume, height-to-length ratio (H/L), average slope, and irregularity index.
“We also collected geochemical data from various scientific publications, especially silica (SiO₂), magnesium (MgO), and potassium (K₂O) content, which represent magma differentiation processes,” he said.
Throughout history, silica has always been considered the most important factor controlling the explosivity of volcanic eruptions.
Interestingly, Dr. Suhendro found that all types of volcanoes on Java Island (strato, compound, and caldera) tend to have the same silica range.
Meanwhile, the chemical element that serves as the main differentiator between each type of volcano is potassium content.
“All types of volcanoes show overlapping ranges of silica and magnesium values, making them difficult to distinguish. But as soon as we traced the potassium content, the difference was immediately visible. Calderas always have magma with high potassium levels,” Dr. Suhendro explained.
The study results show that caldera-type mountains, such as Raung, Ijen, Bromo, and Dieng, have significantly higher potassium content compared to stratovolcano and compound-type mountains.
Several previous experimental studies have shown that a high potassium content enables magma to store more dissolved gases at high pressure.
Thus, when a large volume of magma suddenly decompresses, the release of these gases can create large-scale explosive eruptions.
This can trigger the collapse of the mountain peak and form a caldera.
Besides its geochemical aspects, this research also highlights the tectonic factors that influence the distribution of caldera-type mountains.
From a regional perspective, the subduction plate’s inclination beneath East Java is relatively steeper compared to Central and West Java.
This condition supports the formation of more potassium-rich magma, which is also reflected in the large number of calderas in East Java.
“For example, Ijen, Raung, Jambangan, and Bromo Calderas are all located in the eastern part of Java, where the subduction zone is deeper compared to the central region, like Dieng Caldera, and the west with Rawa Danau and Sunda Calderas,” Dr. Suhendro explained.
Based on all data, the UGM research team proposed three evolutionary paths for volcanoes.
The first path describes the evolution of a stratovolcano into a caldera without going through a compound phase due to a drastic increase in potassium levels.
The second path shows a volcano growing into a compound mountain before finally forming a caldera, also characterized by magma evolution towards high potassium levels.
The third path is a condition where a stratovolcano only develops into a compound and is very unlikely to form a caldera due to low potassium levels.
Dr. Suhendro stated that this finding not only enriches geoscience studies but also has significant implications for volcanic disaster mitigation.
So far, the potential for large eruptions has often been estimated based on silica content or mountain size.
However, this research shows that potassium can be a new parameter in early warning systems.
“Potassium can be used as a new indicator in monitoring volcanic activity, especially for assessing the potential for large eruptions that trigger caldera formation,” Dr. Suhendro concluded.
This research is part of UGM’s contribution to strengthening multidisciplinary disaster research.
Through collaboration between the Faculty of Geography and the UGM Center for Disaster Studies (PSBA), UGM continues to strive to make earth sciences or geoscience the basis for scientific data-driven disaster mitigation policies.
With a solid analytical approach and a strong geospatial database, UGM continues to affirm its role as a reference institution in disaster research in Indonesia.
Author: Triya Andriyani
Post-editor: Afifudin Baliya
Photographs: UGM Faculty of Geography Research Team