Heard Island and McDonald Islands

Heard Island and McDonald Islands form an Australian External Territory in the southern Indian Ocean, approximately 4,100 kilometres southwest of Perth and 1,600 kilometres north of Antarctica. Heard Island hosts the only two active volcanoes on Australian sovereign territory: Big Ben (2,745 metres), an active stratovolcano whose summit remains frequently active, and McDonald Island, a separate small active volcanic island approximately 44 kilometres to the west that produced a major submarine eruption in the 1990s that doubled its land area. The islands sit on the Kerguelen Plateau — an enormous oceanic igneous plateau formed approximately 110 million years ago by the Kerguelen Hotspot — and their volcanic activity today reflects the continued presence of a hotspot beneath the region. The tectonic setting places them on the Indo-Australian Plate, away from active plate boundaries, meaning their primary seismicity is volcanic-tectonic in origin from the hotspot and magmatic systems rather than from plate boundary interactions.

Volcanic-seismic activity at Heard Island and McDonald Island has been documented through satellite imagery and occasional scientific expeditions. Big Ben on Heard Island has erupted multiple times in recorded observation, with the most active periods in the 1980s, 1990s, and 2000s documented through thermal anomalies detectable by satellite sensors. McDonald Island, previously considered extinct, began showing renewed volcanic activity in the 1990s, erupting significantly in 1992 and in the period 1996–2001 and doubling in area due to lava accumulation; this reactivation was entirely unexpected given the island's long apparent dormancy. Earthquake swarms associated with these volcanic eruption episodes are presumed but difficult to document given the complete absence of permanent seismological instrumentation on the islands. Global seismological networks have detected occasional [[magnitude]] 4–5 events from the Heard Island region, attributed to volcanic-tectonic processes. No [[tsunami]] hazard to populated coastlines has been documented from Heard-McDonald volcanic or tectonic events.

The Kerguelen Plateau is one of the world's largest oceanic plateaus, a massive igneous province created when the Kerguelen Hotspot erupted enormous volumes of basaltic lava onto the ocean floor during the Early Cretaceous. Heard and McDonald islands represent the currently active expression of this hotspot, the southeastern tip of the Ninety East Ridge hotspot track that stretches northward across the Indian Ocean. The plateau stands approximately 2 kilometres above the surrounding Indian Ocean floor and is over 2,000 kilometres long. The oceanic crust beneath the plateau is anomalously thick due to the vast volumes of volcanic material extruded during the plateau-forming phase. Modern volcanic activity at Heard Island generates lava flows and explosions primarily from the summit crater of Big Ben, with episodic activity documented over the past several decades. [[seismic-wave]]s generated by large regional earthquakes from the nearby Indian Ocean ridge systems and from distant global events are detected by seismometers placed on the islands during periodic scientific expeditions but there is no permanent monitoring capability.