Geology of Rikutsar

Rikutsar is famous for its enormous volcanoes, 4 of which are still visible on the surface. The archipelago formed approximately 11 mya when a mantle plume caused a small flood basalt eruption, resulting in a tectonic rift in Earth's crust to the northwest of Australia. In the center of the hotspot, the now-submerged Subwazsweornár volcano began to breach the surface of the young Indian Ocean. The hotspot also formed the Wazsweornár volcanoes 5 mya, and the Sweorlac volcano 1.2 mya.

Uzgárac
The Uzgárac caldera lies to the northwest of Sarainár and is an older, dormant stratovolcano. It is thought that an eruption approximately 20kya blew off a significant mass of the volcano. The only active volcanism seen at Uzgárac are various hot springs and a single geyser, known as Chuthore or ”The Gusher.” The current caldera floor lies at an elevation of around 800 meters, and the caldera walls rise a further 300 meters.

Cazegár
The Cazegár shield volcano, known for being potentially the largest volcano on Earth, is the largest visible caldera on the island. It developed to the west of the crumbling remnants of older volcanoes 280 kya. Cazegár is responsible for the rocky beaches, crags, and outcrops of Uchen, being remnants of old lava flows. Approximately 67 kya, Cazegár underwent a catastrophic eruption, one of the largest in the past million years, comparable in size to the largest Yellowstone eruption, and formed a sizable 91 km-long caldera. The eruption caused the extinction of all life on the island, and all native life on the island today is from later periods. Most of Cazegár's eastern flank fell into the ocean during the eruption. The debris from the landslide accumulated enough to form a small island, known today as Pretaoc. After the eruption, a resurgence dome formed in the center of the caldera, called Oighicupre. When the caldera filled with water to create Cazegár, Oighicupre became an island, upon which the capital city of Cazegár Luch was later built.

Sarainár
Of the 4 visible volcanoes, the youngest is Sarainár. It is a pyroclastic shield volcano around 2,000 meters high and is very young, forming around 16,000 years ago. Measuring 240 km by 370 km, it is the largest volcano in the world by surface area. The last period of dormancy for Sarainár lasted until 1967, and its last major eruption episode was the High Ítserachicu Eruption of 2011, which destroyed the ancient village of Enégtoroc.

Sarainár formed over the Rikutsar hotspot and, despite its flat and spread-out nature, it is by far the most prominent elevation feature of the island. It is the only active volcano on Rikutsar, although multiple older volcanoes dot the archipelago.

At the discovery of Rikutsar in 430 AD, Sarainár was dormant, and is estimated to have been for centuries beforehand. Activity began again in 506 AD, with small tremors and lava fountains from mountainside fissures. This culminated in a small effusive eruption from the summit caldera and various flank vents on 16 July 510, which was a relatively gentle VEI 0 event. This would be the first of the 5 recorded eruptions in its history. Other minor eruptions occurred on 1 October 879, 19 February 1320, and 6 May 2011. Sarainár was completely dormant from its 1556 eruption until 1987.

The 1556 eruption, Sarainár's only major eruption, is worth examining in more detail. It happened during an exceptionally cold winter that caused excessive snowfall and ice buildup at the peak of Sarainár. The volcano was already showing signs of another eruption, and on the morning of 9 July an eruption began that was initially only effusive, as is usual for shield volcanoes. However, not only was the snowcap melted very quickly causing contact with lava, but the water from the winter snow and ice had seeped into the rocks and pressurized. This resulted in the near simultaneous occurrence of a phreatic explosion and a boiling liquid expanding vapor explosion at roughly 1:00 PM. The resulting detonation, caldera collapse and pyroclastic flows killed almost 10,000 people (~7% of the population at the time) and remains the most deadly single event in Rikutsar's history. The old-growth forest along the slopes and at the foot of the mountain, Sárronsao, was completely destroyed. The eruption registered as a colossal VEI 6, comparable to the Krakatoa eruption, and was the 10th deadliest eruption in human history.

Later geological research showed that the volcano has erupted violently many times in the past, and that Rikutsar has been blessed with a period of calm, effusive eruptions. It was initially thought to be a standard shield volcano.

Minerals (maybe retcon this)
The selenium used for television tubes is relatively common in Rikutso, as the volcano which formed the island was originally a rich hydrothermal vent field that deposited Aguilarite (Ag4SeS) and Naumannite (Ag2Se), along with Acanthite (Ag2S), and Pyrargyrite (Ag3SbS3).

Rikutsar makes high use of Silver due to its abundance, as well as due to the very low numbers of deposits containing cheaper metals such as Aluminium or Iron.