The Indo-Australian plate is or was a major tectonic plate. It is in the process of separation into two or three plates, and may be currently separated into more than one plate. It contains the continent of Australia, its surrounding ocean and extends north-west to include the Indian subcontinent and the adjacent waters.
Hunga Tonga–Hunga Haʻapai () is a submarine volcano in the South Pacific located about 30 km (19 mi) south of the submarine volcano of Fonuafoʻou and 65 km (40 mi) north of Tongatapu, Tonga's main island. It is part of the highly active Kermadec-Tonga subduction zone and its associated volcanic arc, which extends from New Zealand north-northeast to Fiji, and is formed by the subduction of the Pacific Plate under the Indo-Australian Plate. It lies about 100 km (62 mi) above an active seismic zone. The volcano rises around 2,000 m from the seafloor and has a caldera which on the eve of the 2022 eruption was roughly 150 m below sea level and 4 km at its widest extent. The only major above-water part of the volcano are the twin uninhabited islands of Hunga Tonga and Hunga Haʻapai, which are respectively part of the northern and western rim of the caldera. As a result of the volcano's eruptive history, the islands existed as a single landmass from 2015 to 2022: they were merged by a volcanic cone in a volcanic eruption in 2014–2015, and were separated again by a more explosive eruption in 2022, which also reduced the islands in size. The Hunga Tonga–Hunga Haʻapai volcano has seven historical recorded eruptions.
The most recent eruption, in January 2022, triggered a tsunami that reached the coasts of Japan and the Americas, along with a volcanic plume that soared 58 km (36 miles) into the mesosphere. It was the largest volcanic eruption since the 1991 eruption of Mount Pinatubo and the biggest explosion recorded in the atmosphere by modern instrumentation, far surpassing any 20th-century volcanic event or nuclear bomb test. NASA determined that the eruption was "hundreds of times more powerful" than the atomic bomb dropped on Hiroshima. It is believed that the 1883 eruption of Krakatoa is the only eruption in recent centuries that rivaled the atmospheric disturbance it produced. The January 2022 eruption is the largest volcanic eruption in the 21st century.
Semeru is the highest mountain on the Indonesian island of Java and an active volcano located in the province of East Java in a subduction zone, where the Indo-Australian Plate subducts under the Eurasian Plate. Semeru is 3,676 m tall at its peak, making it the third tallest volcano in Indonesia.
The name "Semeru" is derived from Meru, the central world mountain in Hinduism, or Sumeru, the abode of gods. This stratovolcano is Mahameru, meaning "The Great Mountain" in Sanskrit.
The Krakatoa Archipelago is a small uninhabited archipelago of volcanic islands formed by the Krakatoa stratovolcano located in the Sunda Strait, nestled between the much larger islands of Java and Sumatra. As of 2018, the archipelago consists of four main islands: Verlaten (a.k.a. Sertung) to the northwest, Lang (a.k.a. Rakata Kecil, or Panjang) to the northeast, Rakata (what remains the main island) to the south, and the currently volcanically active Anak Krakatoa in the north-centre. Together, the islands are a part of the Indonesian island arc system, created by the northeastward subduction of the Indo-Australian Plate. As part of a dynamic volcanic system, the islands have been continuously reshaped throughout recorded history, most notably in the 1883 eruption of Krakatoa.
Administratively, the Krakatoa Archipelago lies entirely within Lampung, Indonesia and forms a part of Ujung Kulon National Park.
The Sunda Arc is a volcanic arc forming the topographic spine of the islands of Sumatra, Nusa Tenggara, Java, the Sunda Strait, and the Lesser Sunda Islands. The Sunda Arc begins at Sumatra and ends at Flores, and is adjacent to the Banda Arc. The Sunda Arc is formed via the subduction of the Indo-Australian Plate beneath the Sunda and Burma plates at a velocity of 63–70 mm/year.
The Louisville Ridge, often now referred to as the Louisville Seamount Chain, is an underwater chain of over 70 seamounts located in the Southwest portion of the Pacific Ocean. One of the longest seamount chains on Earth, it stretches some 4,300 km (2,700 mi) from the Pacific-Antarctic Ridge northwest to the Tonga-Kermadec Trench, where it subducts under the Indo-Australian Plate as part of the Pacific Plate. The chain's formation is best explained by movement of the Pacific Plate over the Louisville hotspot, although others have suggested that it formed by leakage of magma from the shallow mantle up through the Eltanin fracture zone, which it follows closely for some of its course.
Depth-sounding data first revealed existence consistent with a seamount chain in 1972, although some of the seamounts had been described as a ridge in 1964 linked to the Eltanin fracture zone system, hence the name.