Google maps of Lombok island with Rinjani and Tambora to the east
This is a short follow-up to the Rinjani post Granyia wrote last week.
Rinjani sits in a rough geologic neighborhood. It is some 170 km west of Tambora which is on West Nusa Tenggara (Sumbawa). It is some 110 km east of The Batur Caldera on Bali. Bali also has The Buyan – Bratan Volcanic Complex and Gunung Agung. These are active, relatively recent calderas which indicate robust magma feeds from the subducting Indo-Australian Plate. The string of islands east of Java is referred to as the Lesser Sunda Islands.
Lesser Sunda Islands – http://geoseismic-seasia.blogspot.com/2013/06/the-lesser-sunda-islands.html
The basic tectonic forces of eastern Indonesia are driven by the subduction of the Indo-Australian Plate under the Eurasian Plate. The subduction location is marked by the Java Trench which transitions to the Timor Trench as you travel along it toward New Guinea. At the location of Lombok Island and Rinjani, the motion of the subduction is generally south to north.
Generalized plate tectonics of Indonesia – http://www.visionsofthecosmos.com/Plate%20Tectonics.htm
As you travel along the Java Trench eastbound, the plate interaction becomes quite complex as the collision between the Indo–Australian and Eurasian Plates shatters weak regions at the end of the Eurasian Plate. We have the Timor Microplate on the southern edge of the line of impact. North of it is the Banda Sea Microplate. North of that is the Molucca Microplate (Sulawesi). North of that is the Bird’s Head Microplate. Travel a little further north from that junction, and you end up with the Philippine Plate. Travel a little farther east and you end up with the Pacific Plate.
The absolute tectonic mess at the eastern end of the Eurasian Plate – http://www.wikiwand.com/en/Bird’s_Head_Plate
Interestingly (at least to me), it appears that the vigorous volcanic activity in the Lesser Sunda islands is driven primarily by subduction and manifests itself as front arc volcanism. I wonder why the jumbled tectonic mess that is eastern Indonesia and the various basins and platelet boundaries would not have introduced more weaknesses through the Eurasian plate allowing more opportunity for basaltic magmas to reach the surface as they have around intra-plate blocks in New Mexico and the Bering Sea. Perhaps you need a significant and wet magma source, which is what the subducting plate provides. Appears that magma source does not exist at the eastern end of the Eurasian Plate, though if you go east of New Guinea, you run into the Ontong – Java Plateau, a relatively recent Large Igneous Province.
Another view of the plate collisions in eastern Indonesia – http://earthjay.com/?p=2035
The Lesser Sunda Islands are divided into four tectonic structural units. From north to south, these include the back arc region which is mainly the Flores Sea. There are two major zones of back arc thrusting in the Flores Sea and surrounding islands.
The inner arc region includes most of the islands of the Lesser Sunda. The width of the islands decreases as you travel east along their line. The islands are marked by extensive volcanism with that centered around Lombok Island being the most recent. Not only do these islands contain significant volcanic structures, they are also being uplifted via folding of the crust underneath the plate by the collision offshore. Uplifting has not ended and appears to still be underway.
Lombok is located some 300 km north of the Java Trench. The Benioff Zone lies some 170 km beneath the island. This is the location of melting plate that supplies the volcanic activity. Crustal thickness under Lombok is some 20 km. https://en.wikibooks.org/wiki/The_Geology_of_Indonesia/The_lesser_Sunda_Islands
As usual, I am not an expert. Merely a hobbyist.
Volcanic belts in Indonesia – http://www.vangorselslist.com/Regional.html
Additional information
agimarc
Spica
Granyia
volcanohotspot.wordpress.com 
Thank you agimarc for working your way through this tectonically tricky corner of the world! I read that there are still new papers coming out suggesting different interpretions of the “jumbled tectonic mess”. But that’s mostly about the niceties, the overall picture seems not too hard to understand once you go into it.
Looking at the last graphic – unfortunately I can’t read the legend, it seems the coloured bands are rock formations of the different geologic times – strange how they converge in eastern Kalimantan, as if that island is a completely dead area (tectonically speaking), or as if it had been a pole everything else rotated around.
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Howdy Granyia –
Apologies for the final graphic. The bands are what are called magmatic arcs dating since the Cretaceous (70 My ago). Can’t see the legend either, but age of the bands are generally older the farther north and west you get. I included it because it kind of showed me how the volcanism of the region has grown / moved over the last 70 My or so.
I thought the sharp corner in the most recent magmatic arc (black dots are volcanoes on the second / red arc) may be related to whatever is going on in Sulawesi, which you touched upon with your Colo post some months ago. What makes that sort of corner in what was previously a smooth line of subduction? Cheers –
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Update for Rinjani today: 11 – 16 Nov. 2015 – The eruption has been going on but with less intensity, and flight conditions are back to normal. Lava flows down the NE and NW of the cone. – It is estimated that a buildup of 4.5 million m^3 (0.0045 km^3) of pyroclastic material has accumulated in this eruption. Temperature of the lake water has risen fom 21°C (June 2015) to 38°C. – With the increased volume of erupted material in Lake Segara Anak water level has risen 1 m (!), causing an over/outflow of more than double the usual rate. There is an imminent danger of flooding along the river Kokok Putih, especially in heavy rain conditions.
Info Siaga: Mount Marapi in western Java has had an ash eruption Saturday night (14 Nov.), but the ~500 climbers up there for the weekend were not affected. Volcano Marapi (not to be confused with infamous Mt. Merapi in E Java) has been on alert level 2 since 08/2011 but was quiet lately.
Volcan Colima in Mexico did a brilliant show at dawn this morning:
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