Introducing Volcano Mount Gede

[BrickFielder]

Introduction
My intention is to produce a series of articles (depending on interest) which introduces ideas about volcanic and earthquake activity. I want to go slightly off the beaten track to explore oddities, volcano hazards, analysis methods, Wonders and Mankind’s impacts. Keep in mind I am not expert (corrections gratefully received), but hopefully these will at least give a flavour of some different places in the world and provide a few minutes escape from people’s troubles. 

Introducing Volcano Mount Gede

Reason for Interest
 Three major cities, Cianjur, Sukabumi and Bogor, are located near to Mount Gede. With the growth of Greater Jakarta and those 3 cities, dense suburban growth has engulfed the fringes of the volcano, home to roughly 4 million people. The Volcano is thought to produce large eruptions in the past and with the huge populations nearby the potential for severe destruction is significant if a large eruption occured. It should be pointed out that most recent eruptions have been at most VEI2 smaller eruptions. What has piqued my interest is the fact that rock crystals analysis of magma suggests that magma must have risen through roughly 35 kilometers of crust in just months during past eruptions.

The Setting
Mount Gede or Gunung Gede (Big Mountain in Sundanese) is a stratovolcano in West Java, Indonesia. The western side of the volcano is bordered by Pangrango, which is an extinct volcano. The volcano is within the conservation area Mount Gede Pangrango National Park. The park contains fast flowing rivers, waterfalls, lakes, montane forest, elfin woodland, subalpine grassland, and unique mountain flora and fauna including an edelweiss meadow. By international standards this is a small conservation area yet it is one of the most visited in the world with an estimated 20-30 million tourists flocked to the dual volcano site.

West Java is a province of Indonesia on the western part of the island of Java, with its provincial capital in Bandung. It is bordered by Jakarta to the west, the Java Sea to the north, the province of Central Java to the east and the Indian Ocean to the south. The province is the native homeland of the Sundanese people, the second-largest ethnic group in Indonesia after the Javanese.IT has rich and fertile volcanic soil with traditional dry rice, coffee, tea, and quinine being cultivated.

The mountains and uplands have a dense growth of tropical rainforests comprising teak, eucalyptus, rhododendron, juniper, banyan, oak, ash, maple, and ironwood. The park is also rich in wildlife and birds such as the Java Gibbon, Javan langur, leopards, pangolin, wild boars, Javan hawk-eagle and the Javan scops owl. The wet season is from November through to April with the driest months being July and August. Humidity is usually very high and temperatures are fairly consistent through out the year at around 28 degrees Centigrade. Temperatures at the volcano summits can dip below 10 degrees Centigrade and get occasional frosts.

The Tectonic environment
 The volcano belongs to the Sunda arc which is associated with the subduction of the Indo-Australian plate beneath the Eurasian plate. Magma originates from partial melting of the mantle wedge driven by the fluids from subducting slab. Large geothermal fields are found in West Java compared to ones in Central and East Java. 

Rock Crystal Analysis

From the rocks released by that 4000-year-old eruption Costa and his colleagues at the Center for Volcanology and Geological Hazard Mitigation of Indonesia in Bandung were able to glean some crucial clues about Gede's behavior. The clues were locked in crystals, most smaller than lentils, embedded in the rocks. Each crystal grew in a soup of magma deep underground, accreting layers that bore witness to the events that preceded the eruption, and most importantly how fast they unfolded. These crystal clocks told Costa's team that Gede's 4000-year-old eruption came roughly 4 weeks after the injection of a fresh batch of magma beneath the volcano. Crystals from four more ancient eruptions gave similar answers. 

The Volcano
Gede volcano is also known as Gedeh, Ageung or Agung. There are 7 summit craters - Gumuruh, Gedeh, Sela, Ratu, Lanang, Wadon and Baru. Currently the most active craters are Lanang and Wadon.The edifice of the volcano consists of 3 major parts having different ages. The main stratocone (called Gumuruh) with 1.8-km-wide summit caldera is the oldest part. The Intra caldera cone (Gede proper) with 900m wide summit crater and the Intra crater infill composed of a lava dome with 3 small explosive craters on its top being the youngest.

On the slopes can be found the Telaga Biru/Blue lake with blue green algae surrounded by sub-montane to montane vegetation.This is a good place to watch the Sunda Cuckoo-shrike and Sunda Thrush to come to drink.A little further up the slope is the Cibeureum Waterfall consisting of three waterfalls, formed from the Cikundul, Cidendeng, and Cibeureum rivers the highest of which is 50 meters high. A red moss, endemic to the mountains of West Java, can be seen growing on the rocky outcrops at the waterfalls and the bats seen flying around come from the nearby bat cave of Gua Lalay.

A little higher are the hot Springs/Air Panas with water temperature as high as 75°C which contain an algae which is remarkably adapted both to hot water and high sulphur levels. The hot springs are a good place to see the Javan Cochoa and Blue-rumped Trogon birds.

At the summit are three semi-active craters grouped together: Lanang (male), Ratu (queen) and Wadon (female). Acid rocks, sulfur rich gas emissions and an inhospitable climate all make for extreme conditions. In response, a fascinating plant community has developed including two rare species of rhododendron.

The famous naturalist Alfred Russel Wallace was so impressed by the areas beauty and uniqueness that he wrote, “by far the most interesting incident in my visit to Java was a trip to the summits of the Pangrango and Gede mountains”. The trail on the south-eastern slopes was found by Sir Thomas Raffles in 1811 which passes through some interesting and beautiful landscapes like Alun-alun Suryakencana the highland meadow covered with edelweiss flowers.

Past Eruptions
During the last few centuries there have been mainly small explosive eruptions of short duration. The largest eruptions were in 1840, and January 1948. The most recent eruption was on 13th March 1957 when following a roar from the volcano an ash plume reached 3km high above the volcano. The SE avalanche deposit from an earlier eruption covers more than 250 km2 on the lower south-eastern flank of the volcano in Cianjur region. It traveled at least 35 km, and had volume of no less than 10 km3 and would be within reach of certain cities today.

Recent Activity

1991 Earthquake Swarm
Earthquake swarms occurred at Gede volcano in 1991. On 29th April there were 100 earthquakes recorded, compared to the background level of 10-15 per day. On 1st May over 40 earthquakes were measured. Seismicity returned to normal after this and there were no changes to surface activity at the volcano.

1997 Earthquakes
Earthquakes were measured at Gede volcano in August 1997. Volcanic earthquakes were concentrated in vertically elongated zone beneath the summit crater. The depth of hypocenter ranged from 2 –8 km below sea level. During the observations, A-type earthquakes with clear P and S-waves were observed at Gede volcano suggesting magma movement.

Small volumes of erupted material suggests that the likelihood of future large volume eruption of the volcano is low with the strongest eruptions of the  volcano thought to be in the VEI3–VEI4 range.

The volcanic edifice of Gede is not exceptionally high or steep and the hydrothermal weakening of the rocks on the upper slopes appears to be of limited extent. Thus, large-scale gravitational collapse of the edifice is of low probability. However, a remnant of the NNE crater rim (Rock Sela) is gravitationally unstable and could collapse during the next eruption or large earthquake.

Discussion

Perhaps It is the large populations nearby that concern me or the propensity to erupt very quickly after a magma injection that concerns me. All evidence suggests eruptions are not likely to be large yet there is a sense of unpredictabilty and regularity of earthquake swarms that niggles. I am also not convinced that the the myth that the spirits of Eyang Suryakencana and Prabu Siliwangi will manage to stop eruptions. Then again perhaps my interest is why some hikers on the volcano think it is a good idea to take nude selfies.