Distinctive physical features such as permanent glaciers, an active volcano and coastal lagoons dominate the landscape. However interesting and important geographical features can be found at all scales, ranging down from Big Ben, the 2,745 metre high volcano towering over Heard Island, to coastal wetlands, lava tubes, rocky beaches and even the ever-present glassy volcanic grit.
The pages in this section provide an introduction to the geology, geomorphology, glaciology and wetlands of HIMI.
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 2 million square kilometres in area, and stands around 3,000 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 3 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 9 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 2,745 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).
Geomorphology is the study of the evolution of landforms.
The landforms on Heard Island and McDonald Islands (HIMI) have been sculpted by the interaction of unusually diverse processes.
These include a long, complex geological history, multiple episodes of volcanic activity, erosion and transport by wind, water and glaciers, moulding by vigorous marine processes and the influence of gravity.
The volcanic cones, lava flows, broad glaciers and pro-glacial coastal lagoons, numerous glacial moraines and black sand beaches bear witness to the dynamic landscape of these islands. Heard Island looks like it does because of several geomorphologic factors.
Why Heard Island looks like it does…
Heard Island is dominated by two roughly circular volcanic cones – Big Ben and Mt Dixon – which are about 18–19 kilometres apart. Both are relatively modern, having formed in the last million or so years, possibly considerably less.
Big Ben erupts regularly and the cold lava flows radiating from the summit of Mt Dixon suggest that it may have erupted in the last few hundred years, though not since the discovery of the island in the 1850s.
Big Ben is about 18 kilometres in diameter and rises from a little above sea level (from the top of the Drygalski Formation) to 2,745 metres. Mt Dixon, at the centre of Laurens Peninsula, is 8–9 km in diameter and rises to a little over 700 metres. There are also many small volcanoes around the margin of Big Ben, perhaps a result of the loading of the local crust by the mass of the volcano.
This accretion of sediment carried by the wind, glaciers and water formed the low land, including The Nullarbor, a low gravel isthmus that joins the two volcanoes, and the mobile Elephant Spit at the eastern end of the island. Recent volcanic activity on McDonald Island has provided a source of such material and a spit, mimicking that on Heard Island, is forming at the eastern end of this island.
Cliffs around the island are normally a result of the existence of the Drygalski Formation but may, in places, be a result of undercutting of sub-recent material from the volcanoes.
A major feature of the island is a series of exposed rock ‘buttresses’ which radiate out from the centre of Big Ben. These formed due to erosion of soft, fragmental volcanic rock by glacial activity. The eroded sediment is then carried by glaciers, wind and water to the coast where it is remobilised to provide sediment for The Nullarbor and Elephant Spit.
There has been a large landslip from Big Ben towards the southeast at some relatively recent time. This has led to a debris avalanche amphitheatre forming a semicircular ridge to the northeast around the active Mawson Peak. The small conical volcano of Mawson Peak has grown in the space vacated by the avalanche. Two major buttresses extend from the ends of this amphitheatre, Holmes Ridge-Budd Ridge roughly southeast, and North West Cornice to the northwest. The material from the avalanche flowed to the southeast and is responsible for the shallow marine environment to the southeast of the island.
Heard Island is spectacularly glacierised as a result of both its height (2,745 metres at the summit of Big Ben) and its location south of the Antarctic Convergence – the meeting point of cold Antarctic surface water and less cold subantarctic surface water.
The island has extensive ice cover, with glaciers covering approximately 70% of the land (around 257 square kilometres) and permanent snow covering a further 2%.
There are 12 major glaciers and several minor glaciers, the majority of which radiate from the summit region of Big Ben, with individual glaciers separated by well-developed buttresses.
Many of the glaciers with westerly to south-westerly aspects terminate in the sea, whereas those with northerly and particularly north-easterly aspects terminate on land or in lagoons well inland from the ocean.
There are also extensive snow fields and several smaller glaciers which descend from the the summits of Mt Dixon, Mt Anzac and Mt Olsen on Laurens Peninsula. There are no glaciers on the low-lying McDonald Islands.
The largest glacier is the Gotley, which descends for over 13 kilometres from Heard Island's highest point (2,745 metres at Mawson Peak) to the south coast near Cape Labuan. It covers an area of over 27 square kilometres.
Since the late 1940s, the total area covered by glaciers on Heard Island has reduced by approximately 11%. This recession is linked to regional climate warming over that time, as has also been witnessed at Îles Kerguelen approximately 400 kilometres to the north-west. Any contribution to the retreat at Heard Island from volcanic activity is considered negligible.
Four coastal lagoons have been formed as a result of the retreat of the Brown, Compton, Winston and Stephenson glaciers. Some of the lagoons are closed and allow for easy walking along the coast, but some of the lagoons are open to the sea, and present a challenge to pass.
Although Heard Island, with its glaciers and rocky coastline, is pretty far from most people's idea of a wetland, a wetland it is.
Or more accurately, Heard Island has a number of moist, low-level terrestrial, freshwater and shallow near-shore marine environments (wetlands) scattered around its coastal perimeter.
These wetlands areas are of high conservation significance. They exhibit significant wetland features and processes and provide critical breeding and feeding habitat for a number of animals considered to be wetland species, including southern elephant seals and macaroni, gentoo, king and southern rockhopper penguins.
Six wetland types have been identified at Heard Island, covering a total area of approximately 1860 hectares:
- Coastal ‘pool complex’ (237 ha)
- Inland ‘pool complex’ (105 ha)
- Vegetated seeps mostly on recent glaciated areas (18 ha)
- Glacial lagoons (1,103 ha)
- Non-glacial lagoons (97 ha)
- Elephant Spit (300 ha)
Some wetland areas have been recorded on McDonald Island but, due to substantial volcanic activity since the last landing was made their in 1980, their present extent is unknown.
The Heard Island and McDonald Islands (HIMI) wetland is listed on the Directory of Important Wetlands in Australia and, in a recent analysis of Commonwealth-managed wetlands, was ranked highest for nomination under the Convention on Wetlands of International Importance Especially as Waterfowl Habitat (Ramsar Convention). The analysis found that the HIMI wetland satisfy 6 of the Ramsar criteria for wetlands of international importance. A nomination is currently under consideration.