The activity, which has produced basaltic rocks, has modified the shape of the volcano and may lead to the formation of an island in the future.

Often, such chains begin in the southeast with volcanoes such as Hawaii that become progressively more eroded northwestward and eventually end as series of atolls.

[19] The crust beneath Macdonald is of Eocene age,[4] and away from the area of shallower ocean it is covered with hills and sediment.

[24] Ongoing volcanic activity may have modified the topography of the summit of Macdonald between surveys in 1975 and 1982, forming another elliptical pinnacle reaching a depth of 29 metres (95 ft) at the northwestern margin of the plateau and raising the summit plateau to depths of 50–34 metres (164–112 ft).

Some hydrothermal alteration products are also found,[17] and a thick ash cover occurs to depths of 2,000 metres (6,600 ft).

[27] Farther down, lava flow fronts form scarps which become particularly noticeable at depths of 620–1,000 metres (2,030–3,280 ft), except on the northern flank.

[17] Save for a debris-covered ridge to the northwest, Macdonald has a circular shape,[29] with a width of 45 kilometres (28 mi) at a depth of 3,900 metres (12,800 ft).

The slopes of Macdonald display radial ridges which may reflect tectonically-controlled rift zones, as well as isolated parasitic cones.

[32] Data obtained in gabbroic rocks expelled by the volcano during its eruptions also suggest that another magma reservoir exists at depths of 5 kilometres (3.1 mi), that is within the crust beneath Macdonald.

[40] The vulcanites are typical ocean island basalts,[41] whose alkaline nature is unlike the tholeiite that is found on other hotspot volcanoes such as Hawaii, Iceland and Reunion.

[45] The first recorded eruptions at Macdonald occurred in 1967 and was followed by additional activity in 1977,[2] although pumice rafts observed in 1928 and 1936 could have been formed by the seamount as well.

[8] The submarine Cyana observed activity directly in one summit crater in the form of intense bubbling,[51] while steam and water fountains were seen on the ocean surface.

[21] Grey-coloured slicks developed on the ocean surface,[51] which were formed by pyrite, sulfur and volcanic glass plus smaller amounts of cinnabar, cubatine and quenstedtite.

Such release occurs in the summit area[57] in the so-called "Champagne Field",[37] but also from a second crater at 2,000 metres (6,600 ft) depth in the southeastern flank.

[59] Hyperthermophilic bacteria have been found on Macdonald, including Archaeoglobus, Pyrococcus, Pyrodictium and Thermococcus as well as previously undescribed species.

These bacterial communities contain both hydrogen- or sulfur- consuming autotrophs and heterotrophs and appear to be capable of long-range propagation, considering that relatives of the species found are known from Vulcano in Italy.