Macdonald

Geography & Climate

Macdonald is located in France, within the Austral-Cook Hotspot Volcano Group of the broader Southern Pacific Volcanic Regions. Situated at 28.98° S, 140.25° W in the Southern Hemisphere, the volcano lies within a subtropical climate zone. With an elevation of -39 meters — below sea level — Macdonald is a submarine or submerged volcanic feature. Submarine volcanoes make up the majority of Earth's volcanic activity, though most eruptions go unobserved beneath the ocean surface. The volcanic landform is characterized as a shield, which describes the physical shape and structure of the volcanic edifice as observed from the surface.

Geological Context

Macdonald is an intraplate volcano, meaning it formed away from any plate boundary. These volcanoes are often powered by hotspots — plumes of abnormally hot mantle material rising from deep within the Earth. Hotspot volcanoes like Macdonald tend to produce effusive eruptions with lava flows that can be relatively predictable, giving nearby communities in France more time to prepare. However, they can also produce explosive phases, particularly when magma interacts with groundwater. The dominant rock type is basalt / picro-basalt, a dark, fine-grained volcanic rock that forms from rapidly cooling, low-viscosity lava. Basaltic eruptions tend to be less explosive and produce fluid lava flows that can travel long distances. While less immediately dangerous than explosive eruptions, basaltic lava flows can destroy structures and infrastructure in their path, and volcanic gases released during these eruptions can affect air quality over a wide area.

Eruption History Summary

Macdonald has 12 recorded eruptions in the geological database, spanning from 1928 CE to 1987 CE. This level of eruptive frequency indicates a persistently active volcanic system that warrants ongoing monitoring. The most recent eruption in 1987 CE places this volcano within the modern era of volcanological observation.

Significance

With 12 recorded eruptions, Macdonald has a substantial eruptive history that provides valuable data for understanding volcanic behavior and assessing future hazards in the Southern Pacific Volcanic Regions region. Given its recent activity, Macdonald is closely monitored by geological survey organizations to provide early warning of future unrest.

GVP Reference Summary

Discovered by the detection of teleseismic waves in 1967, Macdonald seamount (also known as Tamarii seamount) rises from a depth of about 1,800 m to within 27 m of the ocean surface at the eastern end of the Austral Islands. The basaltic submarine volcano marks the site of a hotspot that was the source of the Austral-Cook island chain. The summit, named after volcanologist Gordon Macdonald, consists of a flat plateau about 100 x 150 m wide with an average depth of about 40 m. The summit plateau is capped with spatter cones that form steep-sided pinnacles. Most eruptions have been seismically detected, but in 1987 and 1989 pumice emission was observed from research vessels. Pumice rafts observed in the South Pacific in 1928 and 1936 may also have originated here.

— Smithsonian Institution, Global Volcanism Program