Photo: J Shaw
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 2745 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.