Heard Island and McDonald Islands (HIMI) are surface exposures of the second largest submarine plateau in the world, the Kerguelen Plateau.
The Kerguelen Plateau is approximately two million square kilometres in area, and stands around 3000 metres above the surrounding sea floor.
The central part of the plateau, on which the islands lie, formed above sea level about 90 million years ago and subsequently sank below the ocean surface.
It is thought that Heard Island itself has been formed over three major, and several minor episodes.
The first major interval of deposition occurred around 45–50 million years ago, when oceanic sediment formed white and pink limestone that occurs everywhere beneath the island. This limestone outcrops only along the southern margin of Laurens Peninsula but it continues under the water as a widespread rock unit over much of Kerguelen Plateau.
The second major event was the deposition of a 300–350 metre thick layer of rock known as the Drygalski Formation, which is thought to have occurred some nine million years ago. This enigmatic rock unit seems to be dominated by volcanic sediments, including pillow lavas formed when lava flows into water, and forms the prominent cliffs bordering much of the island.
The Drygalski Formation has been eroded flat on its upper surface and this has provided the basement for the third main event, during which the modern volcanoes have been built over the last million years, probably considerably less.
The modern day Heard Island is dominated by Big Ben, a roughly circular active volcanic cone that rises to a height of 2745 metres above sea level. A second major volcanic cone forms Mt Dixon, which rises to 700 metres and sits in the centre of the Laurens Peninsula.
Numerous eruptions and volcanic ‘events’, such as plumes, have been observed on Big Ben since 1947, most recently by satellite (see photo above and satellite image below).
However, the most recent example of major volcanic activity in the region has been the increase in the size and height of McDonald Island, about 40 kilometres to the west of Heard Island. The island is now twice the size it was in 1980 and almost 100 m higher – the result of lava flows sometime during the 1990s.
Also, the previously separate McDonald Island and Flat Island are now joined by a low-lying isthmus (view a satellite image [PDF] of McDonald Island taken in 2004 overlaid with a shaded area indicating the island's extent in 1980).