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Rinjani from the evolution of the caldera to geopark
Rinjani, a volcano with a caldera and a crater lake, has now become a national tourist attraction in Rinjani National Geopark on Lombok. This area is being nominated as a member of the UNESCO Global Geopark (UGG) network. The tallest volcano in the Lesser Sunda Islands has a long history of evolution. Mount Barujari—a post-caldera cone of Mount Rinjani—which last erupted in 2015, drew us to tell the history of the evolution of Rinjani.

Rinjani (3,726 m), located on the northern part of the island of Lombok, West Nusa Tenggara, is the second tallest volcano in Indonesia, after Kerinci on Sumatra. This volcano has unique characteristics in the form of a cone that rises on the eastern part of the edge of the caldera, a crescent-shaped crater lake inside the caldera, and new cones that emerge from the lake. The caldera is called Segara Anak, and the new cone is called Mount Barujari. Segara Anak Lake (2,008 m) is the highest crater lake with an active volcano in Indonesia and in the world.

Major eruptions have colored Rinjani throughout the history of its development from the time of its forbear, which was around one million years ago, to the present period of modern human history. The consequences of one tremendous eruption, measuring 7 on the VEI (Volcanic Explosivity Index) scale were felt a year later, in Europe in the 13th century, and only discovered at the beginning of the 21st century.

Caption of the graphic in the original article: East-west cross-section of the caldera Rinjani Tua (Samalas). The part that was demolished and lost due to the eruption of Samalas, forming Rinjani Caldera circa 1257, is shown in yellow. Source: Heryadi Rachmat. Note: Timur means east, and barat means west.

In addition to producing rock diversity, Rinjani eruptions also provide a landscape that is of high aesthetic value. Because of its beauty, Rinjani is now a favorite mountain for climbing, and it and other environments from peaks to beaches compose the national geopark area of ​​Rinjani, Lombok. So, the character of Rinjani needs to be known and the information is packaged attractively and presented in the context of disaster mitigation and its use as a volcanic tourist area.

Mount Rinjani Tua
The history of eruptions that created the morphology of Mount Rinjani as it is today began at the time of the Pleistocene Epoch or about one million years ago. Much earlier, during the Miocene Epoch of the Tertiary Period (about 11 million years ago), Formasi Gunung Api Tua (the Old Volcano Formation) gradually formed as a result of the Australian Plate moving north and colliding with the Eurasian Plate. This rock formation is known as Old Andesite Formation (OAF) and now forms a line in the southern part of Lombok Island, occupying the tourist area of ​​Kute Beach (Mandalika).

The movement of the Australian Plate, which is an oceanic plate, has continued during the Quaternary Period, specifically during Pleistocene Epoch until today, crashing into and penetrating under the Eurasian Plate, which is a continental plate, resulting in a row of Quaternary volcanoes. The Mount Rinjani Tua complex was high before forming the caldera, reaching more than 4,000 m. above sea level. This is the only Quaternary active volcano complex that is rising in the northern part of Lombok Island.

Caption of the graphic in the original article: Volcanic activity information board inside the Rinjani caldera. Photo: Heryadi Rachmat

Mount Rinjani Tua—later known as Samalas—gradually continued to grow to form the volcano in layers in step with the movement of the plates that affected it. Furthermore, because Rinjani Tua's crater hole was blocked by magma activity in the form of lava, the activity gradually moved to the weakest part, namely towards the eastern slope of Rinjani Tua. This process finally formed a new volcano known as Rinjani, whose height almost equaled that of Mount Rinjani Tua.

During the next stage, the magma activity stopped temporarily, and Rinjani's second caldera hole was blocked by the previous lava flow which had solidified. While the second caldera holes was blocked, an increase in magma activity resulted in a large accumulation of gas and a strong thrust of magma. Because the thrust of magma and gas exceeded the strength of the blockage, there was a massive eruption of Rinjani Tua (Samalas) forming the caldera called the Rinjani Caldera. This caldera—which can also be referred to as the Samalas Caldera—has a diameter of about 7.5 x 6 km with an average depth from the rim to the base of the caldera reaching 750 m.

Powerful eruption discovered at the beginning of the 21st century
The age of the Samalas Caldera formation was previously known by the Research Team of the Geological Survey of Japan (GSJ) and Indonesia in 2004, to have occurred between 1200 to 1300 (during the 13th century). However, the results were not widely publicized (internationally), so it was not known in many circles. Furthermore, in October 2013, a team of researchers from France, America, Britain, Belgium and Indonesia published the results of their research on the Mount Samalas eruption in the International Journal "Proceedings of the National Academy of Sciences of the United States of America" ​​(PNASUSA). Finally, the world knows that the Rinjani Caldera was created in the 13th century, specifically in 1257.

This massive eruption of Mount Rinjani Tua occurred over a span of between 13 and 22 hours, with a magnitude or VEI rating reaching 7, which resulted in an eruption column as high as 43 km. The volume of sediment produced was between 33 and 44 km3. The Dense-Rock Equivalent (DRE) consisted of 7–9 km3 DRE Plinian of falling debris of pumice, 16 km3 DRE of deposits of pyroclastic density current (PDC), and 8–9 km3 DRE of co-PDC ash. According to Celine M. Vidal (2015), this eruption reached distances of up to 660 km from the source, in fact, up to the slopes of Mount Merapi in Central Java.

The Plinian eruption of Mount Rinjani Tua, which formed the Rinjani Caldera, has been known as "Samalas". The Samalas name was obtained from notes on palm leaves found at the Leiden Museum and those at the West Nusa Tenggara State Museum from the ancient writings in the Lombok Chronicles and the Suwung Chronicles. The number of ancient writings on palm leaves, which tell the story of the history and culture of ancient West Nusa Tenggara, in the West Nusa Tenggara Museum has now reached 1,200.

Post-caldera Rinjani
After the formation of the Rinjani Caldera, the remaining volcanic cone is Mount Rinjani (3,726 m) which previously grew on the eastern edge of the eastern caldera. Based on evidence in the field and the laboratory, after the formation of the Rinjani Caldera, Eruptions of Mount Rinjani which originated from the Segara Muncar Caldera resumed. The simple proof that there was a post-caldera formation eruption of Rinjani is shown by the fact that there have been no deposits resulting from the previously occurring formation of the Rinjani Caldera discovered in the cavity of the Segara Muncar Caldera hole or on the slopes of the Rinjani Volcano.

In the next stage, after the base of the Rinjani Caldera became waterproof, the caldera was filled with rain water forming Lake Segara Anak. Concurrently with lake filling with waters, magma activity started to increase again beginning with the formation of cinder cones and lava flows sourced from Mount Barujari and Mount Rombongan.

As in other volcanoes, Koesoemadinata (1979) states that Rinjani's post-caldera formation volcanic activity is increasing again. The activity is in the form of effusive that produces lava and explosions that create loose deposits (pyroclastic). The lava is generally black, and, when dripping, it looks foamy.

Explosions after the formation of the caldera are relatively weak in accordance with the volcanic eruption cycle. Lava released by the cones of Mount Barujari and Mount Rombongan is relatively andesitic-basaltic compared to the lava produced by the formation of Volcano Rinjani Tua which was basaltic. The possibility of the occurrence of a hot cloud when there is an eruption increases very little. The products of eruptions are generally deposited only inside the caldera. The sequence of post-caldera events is illustrated in the picture and table below.

Danger of an eruption of Rinjani
Disaster Prone Areas (Kawasan Rawan Bencana or KRB) are certain regions that have geological, biological, hydrological, climatological, geographical, social, cultural, political, economic or technological conditions or characteristics that for a period of time cannot prevent, reduce or achieve readiness for the adverse effects of certain hazards. In other words, KRB areas of the region vulnerable to disaster. Complete building planning must include an intervention effort against the vulnerability of the region so as to increase resilience regional's buildings against the possibility of threats or hazards that can become disasters. Volcano Disaster Prone Areas (KRB) are KRB in which the danger they face is volcanic eruptions.

It is important for the map of the Mount Rinjani KRB to be known by various stakeholders, including Rinjani climbers. Based on the map published by PVMBG, the Geology Board, the Mount Rinjani KRB is divided into three levels of vulnerability from high to low, namely: Disaster Prone Area III, Disaster Prone Area II, and Disaster Prone Area I.

Disaster Prone Area III is an area that has the potential to be hit by hot clouds, lava flows, falling rocks (incandescent), possible base surge and/or toxic gases. 'This area is divided into two, namely: 1) Areas prone to mass flow in the form of hot clouds, lava flows, possible base surge and toxic gases; and 2) Areas prone to falling rock (incandescent), heavy ash rain, and possible mud rain (hot) at a radius of 3 km from the center of eruptions.

Disaster Prone Area II is an area that has the potential to be hit by hot clouds, lava flows, falling rock (incandescent), heavy ash rain, hot mud rain, and toxic gases. This area is divided into two, namely: 1) Disaster-prone areas for seasonal flow in the form of hot clouds, lava flows, and toxic gases; and 2) Disaster-prone areas for falling rock material (incandescent), heavy ash rain, and hot mud rain with a radius of 5 km from the center of eruptions.

Disaster Prone Area I is an area that has the potential to be affected by lava and falling material in the form of ash rain. If the eruption enlarges, this area has the potential to be hit by ash rain and possibly falling rock (incandescent). This area is divided into two. First, disaster-prone areas of mass lava flow. This area is located along valleys and riverbanks, especially in the peak area. Second, the area prone to falling material in the form of ash rain without regard to the direction of wind gusts, at a distance of 8 km from the center of  eruptions.

''On the map, the area in dark pink is prone to hot clouds, lava flows and toxic gases. The area in light pink is prone to hot clouds, lava flows and lava rain. The area in yellow is prone to lava rain.''

Utilization as a geopark
Indonesia, within the past roughly 750 years, has produced three calderas, namely: the Rinjani Caldera was formed in 1257 with a diameter of 7.5 x 6 km; Tambora Caldera produced in 1815 with a diameter of 7.2 x 6.5 km; and the Krakatoa Caldera which was born in 1883 with a diameter of 7.5 x 7 km. The third of the aforementioned calderas is not only well-known domestically but also widely known in the volcanology arena at the global level and is always a topic of discussion among experts. Accordingly, Indonesia has a very high potency in its outstanding situation (excellence) of geological diversity (geodiversity) related to volcanoes, one of which is the Rinjani Caldera.

In addition to creating rock diversity, Rinjani's eruption activities also produce volcanic morphology formations that have high aesthetic value. Furthermore, in this volcanic area, land is also thriving and covered with dense forests, and there has developed a diversity of local flora and fauna (biodiversity). This biological diversity area has become an integral part of the Mount Rinjani National Park (Taman Nasional Gunung Rinjani or TNGR) area which has an area of ​​around 41,330 hectares.

In the end, as a result of human interaction with the natural and biological diversity, in the area of ​​Rinjani and its surroundings the local community's cultural diversity grew and developed. The Rinjani area has also become an attractive destination for climbing and cultural tourism which is visited by many domestic and foreign tourists. The management of this volcanic geological resource involves many parties such as TNGR, the West Nusa Tenggara Provincial Government, tour guide associations, non-governmental organizations, nature lovers groups, and local communities. Volcanic geotourism has become one of the important activities in the Rinjani area.

In accordance with its potential and supported by strong willingness and encouragement from various stakeholders, the Rinjani area was nominated to become a national geopark area and obtained this status on 7 October 2013, with the name "Geopark Rinjani, Lombok, NTB". With that status, Rinjani must be ready as both a conservation and education area, and local economic development will rely on volcanic geotourism as well as general tourism and other tourism for support. For this reason, there are as many as 22 geological sites (geosites), 8 biological sites and 17 cultural sites in the Rinjani Geopark area. Now, since 2014, this area is being considered to become a world geopark or UNESCO Global Geopark (UGG).

Rinjani is now developing from its beginning as a volcano whose activity has always been monitored to become a place for climbing and and other geotourism in the geopark area. Presence of mind faces the fact that Rinjani is actually an active volcano, and its environment is susceptible to damage due to waste dumped carelessly, climbing conditions that require caution, and other things that are important to consider, so there must always be control over the tourists using the Rinjani National Geopark.

Heryadi Rachmat

The author is the Chief Engineer at the Museum of Geology, Geology Board.

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