Overview of Oceania's Volcanoes: From Indonesia to New Zealand

Eruption of Mount Merapi — *Oceania has many active and dormant volcanoes, concentrated in Indonesia, Papua New Guinea, and New Zealand. Image source: astronoo.com (new window)*

Dynamics and Danger of Oceanic Volcanoes

Volcanoes of the Pacific Ring of Fire

Indonesia and New Zealand are part of the Pacific Ring of Fire, where the subduction of oceanic plates causes explosive gray volcanoes. Merapi, Agung, and Ruapehu are stratovolcanoes with significant explosive activity, generating pyroclastic flows, lahars, and ash plumes.

Basaltic and Shield Volcanoes

Some Oceanic volcanoes produce fluid basaltic lava flows, such as Yasur in Vanuatu or Krakatoa in Indonesia during effusive phases. These red volcanoes shape spectacular landscapes, such as shield volcanoes or low-slope cones.

Secondary Risks and Human Impacts

In addition to explosive eruptions, Oceanic volcanoes cause local tsunamis (e.g., Krakatoa 1883), lahars, and crop destruction. Populations are particularly exposed on densely populated and volcanically active islands.

Table of Main Volcanoes in Oceania

Main Active and Dormant Volcanoes in Oceania
Volcano	Color	Location	Altitude (m)	Type	Last Eruption	Comment	Status
Merapi	Gray	Indonesia, Java	2911	Stratovolcano	2023	Frequent eruptions, regular pyroclastic flows.	Active
Agung	Gray	Indonesia, Bali	3031	Stratovolcano	2019	Explosive volcano, threat to local population and tourism.	Active
Ruapehu	Gray	New Zealand	2797	Stratovolcano	2007	Explosive volcano with acidic crater lake, frequent lahars.	Active
Krakatoa	Red	Indonesia, Sunda	813	Composite volcano / cone	2018–2019	Shield and explosive volcano, major historical eruptions.	Active
Yasur	Red	Vanuatu	361	Shield volcano	Ongoing	Fluid lava, almost continuous eruptions, tourist attraction.	Active
Rinjani	Gray	Indonesia, Lombok	3726	Stratovolcano	2016 (last minor activity)	Volcanic summit with caldera lake, recent moderate eruption.	Dormant
Ol Doinyo Lengaï (Oceania extension)	Red	Papua New Guinea	2960	Stratovolcano	2017–2024	Carbonatite volcano, recurrent but moderate eruptions.	Dormant

Red and Gray Volcanoes: Two Eruptive Dynamics

Red volcanoes have fluid basaltic lava flows, such as Yasur or Krakatoa during effusive phases, with spectacular but moderate activity.

Gray volcanoes are explosive, rich in silica and dissolved gases, such as Merapi, Agung, or Ruapehu, generating ash plumes, pyroclastic flows, and lahars.

Comparison Between Red and Gray Volcanoes
Characteristic	Red Volcanoes	Gray Volcanoes
Magma Type	Basaltic, low in silica (<52 % SiO₂)	Andesitic to rhyolitic, rich in silica (>55 % SiO₂)
Viscosity	Low \(\eta \sim 10^{1-3}\, \text{Pa·s}\)	High \(\eta \sim 10^{5-9}\, \text{Pa·s}\)
Eruptive Style	Effusive: fluid lava flows	Explosive: ash plumes, pyroclastic flows
Visible Manifestations	Red lava, extensive lava fields	Gray ash layers, steep stratovolcanoes
Main Risks	Slow but destructive lava flows	Pyroclastic flows, sudden explosions, lahars
Oceanic Examples	Yasur (Vanuatu), Krakatoa (Indonesia)	Merapi (Indonesia), Agung (Indonesia), Ruapehu (NZ)