Good afternoon!
Today we are taking a trip to Java, Indonesia, away from the gentle effusive eruption at Fagradalsfjall, Iceland, and what looks like the growth of a shield volcano, to the more explosive activity at the subduction zone between the Indo-Australian Plate and the Sunda Plate, focussing on Gunung Merapi.
Merapi has been erupting since the start of the year, with the growth of two lava domes, block and ash falls and pyroclastic flows generated from partial collapses of the lave domes. The current alert level is 3 (Siaga) and the aviation code is orange. (Siaga translates as, using Google Translate, “Stand By” (There was a significant increase in volcanic activity. Eruption is most likely to occur and the area of potential eruption hazard is in the area of Disaster-Prone Areas (KRB) II. The community is prohibited from carrying out activities in the KRB II area). The exclusion zone extends 5km from the summit).
Background
Merapi is a 2,968 m high stratovolcano located 25 km north of the city of Yogyakarta on the island of Java, Indonesia. Volcanic activity at Merapi is believed to have started around 170,000 years ago. Since then, activity alternated between effusive and explosive, the latter with lava domes and pyroclastic flows. While her eruptions are comparatively modest (VEI 1 to 4), her proximity to a large metropolis means she has the potential to do a lot of damage, hence her status as a Decade Volcano. Over 4 million people live within 30 km of the volcano and over 24 million people live within 100 km of the volcano.
Yogyakarta, itself, is a densely populated city in the Special Region of Yogyakarta. 373,589 people live in the city (Census 2020) and over 4 million in the metropolitan area. It is a sophisticated centre for Javanese fine arts, culture and education. Yogyakarta has been home to the seat of power for the Medang Kingdom between the 8th and 10th century and Mataram Sultanate between 1587 and 1755. Explosive eruptions from Merapi, which destroyed many Buddhist and Hindu temples built between 732 AD and 900 AD (and presumably caused other significant damage), may have been a factor in the migration of the Mataram Kingdom to East Java in 928 AD or 929 AD.
Tectonic Setting
Volcanic activity on Java is driven by the northward subduction of the Australian Plate under the Sunda Plate at the Sunda Trench. The western, southern and eastern boundaries of the Sunda Plate are tectonically very active.
We downloaded earthquake data from EMSC for the period October 2004 to July 2021 between 14.04°S 86.16°E to 12.66°N 127.48°E to take a look. This area is larger than the area under discussion to make sure that subduction zone was captured. The following images are from extracts of that data.
Merapi is located at the intersection of two volcanic lineaments: the north – south trending Ungaran-Telomoyo-Merbabu-Merapi; and the east – west trending Lawu – Merapi – Sumbing – Sindoro – Slamet. Ungaran-Telomoyo-Merbabu-Merapi is a double chain volcanic arc, with Ungaran as the back arc and Telomoyo-Merbabu-Merapi as the trench side volcanoes. Merapi is also bounded by two faults: the north – south Merapi – Merbabu fault and the Baribis – Semarang – Kendeng fault.
Growth of Gunung Merapi
Merapi formed in the late Pleistocene and Holocene in three stages: Proto-Merapi, Old Merapi and New Merapi.
Proto-Merapi emerged after 170 ka and includes Gunung Bibi, dated to 190 ± 60 ka, Gunung Turgo, 138 ± 3 ka, and Gunung Plawangan ,135 ± 3 ka.
Gunung Plawangan was home to a volcano observatory until the 1990s but was abandoned due to nuée ardentes from the current cone.
The summit of Gunung Turgo is home to sacred graves including that of Sheikh Jumadil Qubro, a direct descendant of the Islamic prophet, Muhammad.
Old Merapi started to grow around 30 ka, reaching a height slightly more than the current cone; growth ended with flank failures 4.8 ± 1.5 ka. Collapse of the caldera with debris avalanche flows to the south and west left a Somma rim on the eastern slope.
Gunung Batulawang and Gunung Kendil are part of Somma – Merapi. Gunung Batulawang is the highest peak of Old Merapi. Lake Borobudur formed c 3,400 14C years BP.
The latest Somma collapse was around 1,900 14C years BP.
New Merapi, the current cone, grew soon after the growth of Old Merapi ended. Partial edifice collapse of New Merapi may have occurred 1,130 ± 50 14C years BP.
Gunung Selokopo Ngisor, Gunung Pusunglondon, Gunung Patukalapalap, Gunung Dengkeng, Gungung Selokopo Duwur and Gunung Gadjah Mungkur are hills which are thought to belong to New Merapi.
Volcanic Activity
Merapi’s lavas are typical for a subduction zone, being andesite / basaltic andesite, trachyandesite, basaltic trachyandesite with some basalt / picro basalt and trachybasalt / tephrite basanite. Her lavas have evolved over time. Her early lavas were effusive basaltic. The K2O content of the lava has increased over time; Old Merapi had lower K2O lavas than New Merapi and nearby Gunung Telomoyo and Gunung Merbabu.
According to GVP, there have been 111 Holocene eruptions, ranging from VEI 1 to 4. The most recent eruption, currently a VEI 1, started on 31 December 2020, with new lava domes extruded in January and February 2021, and, at the time of writing is ongoing. The most recent VEI 4 was the devastating October 2010 to July 2012 eruption in which partial dome collapse caused pyroclastic flows that destroyed villages, led to the evacuation of more than 300,000 people and caused 386 fatalities; the ash plume reached 18km between 4-6 November; and, the largest pyroclastic flows occurred on 26 October 2010 and 5 November 2010 – the latter produced the widest pyroclastic flow seen in Indonesia for 100 years. By mid-November, eruptive activity subsided, to be followed by lahars as the main hazard.
The 26 October eruption started 19 hours after tsunamis caused by earthquakes on the Sunda Trench swept away villages on the Mentawai Islands, killing 428 people and displacing thousands. The earthquakes were a 7.7 M, preceded by a 5.8 M and followed by aftershocks that included a 6.1 M and 6.2 M. Whether or not the earthquakes triggered the eruption, itself, is open to debate; the volcano was ready to erupt. Since 2007 swarms of volcanic earthquakes had been occurring; deformation and gas emissions increased in September 2010; and, seismicity increased between 15 – 26 October, ramping up during 20 – 26 October. The alert levels were raised to level 2 on 20 September 2010, level 3 on 21 October 2020 and level 4 on 25 October 2020.
A phreatic eruption on 11 May 2018, heralded a new phase of lava dome growth. In November 2020, evacuations were ordered due to decreasing stability of the lava dome. Eruptions commenced on 4 January 2021, leading to further evacuations in the Yogyakarta region.
Here’s hoping the current eruption remains a VEI 1.
The Armchair Volcanologist
©Copyright remains with the author; all rights reserved, 2021.
Sources and Further Reading:
Raw earthquake data, EMSC: https://www.emsc-csem.org/Earthquake/info.php
The Smithsonian Institution’s Global Volcanism Program (GVP): Merapi
Ralf Gertisser, Silvain J. Charbonnier, Jörg Keller, Xavier Quidelleur, “The geological evolution of Merapi volcano, Central Java, Indonesia”, Bulletin of Volcanology (2012) 74: 1213=1233. DOI: 10.1007/s00445-012-0591-3
Newhall, C. G., Bronto, S., Alloway, B., Banks, N. G., Bahar, I., del Marmol, M. A., Hadisantono, R. D., Holcomb, R. T., McGeehin, J., Miksic, J. N., Rubin, M., Sayudi, S. D., Sukhyar, R., Andreastuti, S., Tilling, R. I., Torley, R., Trimble, D., Wirakusumah, A. D., “10,000 Years of explosive eruptions of Merapi Volcano, Central Java: archaeological and modern implications”, Journal of Volcanology and Geothermal Research 100 (2000) 9 – 50. DOI:10.1016/S0377-0273(00)00132-3
Wikipedia:
Mount Merapi: Mount Merapi – Wikipedia
Yogyakarta: Yogyakarta – Wikipedia
An Armchair Volcanologist 
