Mount Melbourne

Mount Melbourne is a ice-covered stratovolcano in Victoria Land, Antarctica, between Wood Bay and Terra Nova Bay. It is an elongated mountain with a summit caldera filled with ice with numerous parasitic vents; a volcanic field surrounds the edifice. Mount Melbourne has a volume of about and consists of tephra deposits and lava flows; tephra deposits are also found encased within ice and have been used to date the last eruption of Mount Melbourne to . The volcano is fumarolically active.

The volcano is part of the McMurdo Volcanic Group, and together with The Pleiades, Mount Overlord, Mount Rittmann and the Malta Plateau forms a subprovince, the Melbourne volcanic province. The volcanism is related both to the West Antarctic Rift and to local tectonic structures such as faults and grabens. Mount Melbourne has mainly erupted trachyandesite and trachyte, which formed within a magma chamber; basaltic rocks are less common.

Geothermal heat flow on Mount Melbourne has created a unique ecosystem formed by mosses and liverworts that grow between fumaroles, ice towers, and ice hummocks. This type of vegetation is found at other volcanoes of Antarctica and develops when volcanic heat generates meltwater from snow and ice, thus allowing plants to grow in the cold Antarctic environment. These mosses are particularly common in a protected area known as Cryptogam Ridge within and south of the summit caldera.

Description

Mount Melbourne lies in North Victoria Land, facing Wood Bay of the Ross Sea. To the southeast lies Cape Washington and due south lies Terra Nova Bay; Campbell Glacier runs west from the volcano and Tinker Glacier lies north of the volcanic field. The seasonal Italian Mario Zucchelli Station lies from the volcano; the 5th Chinese station in Antarctica (due to be completed in 2022), the Korean Jang Bogo Station, the German Gondwana Station and a neutrino detector are also in the area. Mount Melbourne was discovered and first recognized as a volcano by James Ross in 1841 and named after William Lamb, 2nd Viscount Melbourne, who was then the prime minister of the United Kingdom. The volcano and its surroundings were investigated by New Zealand-based parties in the 1960s, by German ones in the 1970s and 1980s and by Italian-based parties in the 1980s and 1990s. The volcano and its summit can be accessed from the stations by helicopter.

Volcano

Mount Melbourne is an elongated stratovolcano formed by lava flows and tephra fall deposits, with gentle slopes. The volcano is uneroded and forms a cone with a base area of . Viewed from afar, Mount Melbourne has a nearly perfect cone-like profile that has drawn comparisons to Mount Etna in Italy and Mount Ruapehu in New Zealand. Lava domes and short lava flows form the summit while volcanic mounds, cones, domes and scoria cones dot its flanks; from the summit is a large parasitic vent on the north-northeastern slope, which generated several lava flows. Part of the edifice rises from below sea level. Pyroclastic flow depositsa rarity for Antarctic volcanoeshave been reported. The total volume of the edifice is about .

A crater or caldera sits at the top of the volcano. The highest point of the volcano lies east-northeast of the caldera and reaches elevation. The caldera has an incomplete rim and is filled with snow, leaving a depression. The rim of the caldera is covered by volcanic ejecta including lapilli and lava bombs, probably the products of the most recent eruption, which overlie a layer of pumice lapilli. Three small, nested craters formed by phreatomagmatic eruptions occur on the southern rim of the summit caldera. Pyroclastic fall deposits crop out in the northern rim of the caldera and there are more alternating lava-tephra sequences elsewhere in the summit region. There is evidence of past structural instability (collapse structures) on the eastern and southeastern flanks, and an arcuate (with the shape of an arc) scarp on the eastern flank appears to be an incipient sector collapse.

Except for geothermal areas, the ground is bouldery. Some of the coastal areas around the volcano are ice-free and rocky. Frost heave has been observed in the summit region. Small creeks flow down the eastern flank of Mount Melbourne; they are fed by meltwater during summer and quickly disappear when the snow is gone.

Glaciation

The mountain is covered with permanent ice, which extends to the coast and leaves only a few exposures of the underlying rock; rocky outcrops are most exposed on the eastern flank. The caldera hosts a névé that generates a westward-flowing glacier. An icefall lies northwest of the caldera. Glaciers emanating from snowfields on the volcano have deposited moraines; these and tills from both Pleistocene and Holocene glaciations crop out at Edmonson Point.

Tephra layers crop out in ice cliffs and seracs and testify to recent eruptions, including the one that deposited the ejecta and lapilli pumice units on the summit. Tephra bands are also found in other glaciers of the region. They form when snow accumulates on top of tephra that fell onto ice and in the case of Mount Melbourne they indicate eruptions during the last few thousand years. Volcanic sediments from Mount Melbourne are also found in Terra Nova Bay.

Volcanic field

Mount Melbourne is surrounded by a volcanic field consisting of 60 exposed volcanoes, which have the form of scoria cones and tuff rings with hyaloclastite deposits, lava flows and pillow lavas. Some of these volcanoes formed under ice. The volcanic field forms a peninsula which is separated by steep faults from the Transantarctic Mountains to the north. Among these volcanoes is Shield Nunatak southwest from Mount Melbourne, a subglacial volcano, now exposed, that may have formed during the last 21,000 to 17,000 years. The Cape Washington ridge consists mostly of lava, including pillow lava, overlaid by scoria cones, and is the remnant of a shield volcano. Edmonson Point is another volcanic complex in the volcanic field that formed partly while interacting with glaciers and partly through phreatomagmatic activity. Other volcanoes in the field are Baker Rocks, Oscar Point and Random Hills. These volcanoes are aligned mainly in a north–south direction, with palagonitized outcrops that expose dikes. Perfectly preserved scoria cones occur at Pinckard Table north of the volcanic field, while Harrow Peak is a heavily eroded lava plug. The total volume of volcanic rocks is about and their emplacement apparently altered the path of the Campbell Glacier.

Geology

Mount Melbourne is part of the McMurdo Volcanic Group, which includes the active volcano Mount Erebus. This volcanic group is one of the largest alkaline volcanic provinces in the world, comparable with that of the East African Rift, and is subdivided into the Melbourne, the Hallett and the Erebus volcanic provinces. The volcanic group consists of large shield volcanoes mainly near the coast, stratovolcanoes and monogenetic volcanoes which formed parallel to the Transantarctic Mountains.

Volcanic activity of the McMurdo Volcanic Group is tied to continental rifting and commenced during the Oligocene. It is unclear whether this is caused by a local hotspot beneath the area or mantle convection in the area of the West Antarctic Rift. The latter is one of the largest continental rifts on Earth but little known and possibly inactive today. The Ross Sea and the Victoria Land Basin developed along this rift and were deeply buried, while the Transantarctic Mountains were rapidly uplifted during the last fifty million years and are on the "shoulder" of the rift. The line separating the two is a major crustal suture, with large differences in elevation and crustal thickness across the suture. Many of the volcanoes appear to have formed under the influence of fault zones in the area, and increased activity in the last thirty million years has been correlated to the reactivation of faults.

Mount Melbourne is part of a volcano alignment that includes The Pleiades, Mount Overlord, Mount Rittmannall large stratovolcanoeswhich with the Malta Plateau form the Melbourne province of the McMurdo Volcanic Group. In addition, this province consists of numerous smaller volcanic centres, volcanic intrusions and sequences of volcanic rocks, and has been active for the past twenty-five million years. Volcanic edifices buried under sediment are also part of the Melbourne province, including a cone southeast of Cape Washington, which has a size comparable to that of Mount Melbourne.

Mount Melbourne and its volcanic field are over a basement of Precambrian to Ordovician age, which consists of volcanic and metamorphic rocks of the Wilson Terrane. The volcano is at the intersection of three geological structures: the Rennick Graben of Cretaceous age, the Victoria Land Basin and the Polar3 magnetic anomaly. The Terror Rift in the Victoria Land Basin runs between Mount Melbourne and Mount Erebus and appears to be related to their existence. Mount Melbourne appears to lie in a graben; the marginal faults on the eastern flank of Mount Melbourne are still active with earthquakes. North–south-trending faulting may also be responsible for the trend in edifice structure, and strike-slip faulting takes place on the eastern flank. Recent offset on faults and Holocene coastal uplift in the area indicates that tectonic activity is ongoing.

Tomographic studies have shown an area of low seismic velocity at depth under the volcano, which may be due to temperatures there being hotter than normal. Anomalies underneath Mount Melbourne are connected to similar anomalies under the Terror Rift. These anomalies above depth are focused under Mount Melbourne and the neighbouring Priestley Fault. A low gravity anomaly over Mount Melbourne may reflect either the presence of low-density volcanic rocks or of a magma chamber under the volcano.

Composition

Trachyandesite and trachyte are the most common rocks on Mount Melbourne, with basalt being less common and mostly occurring around its base. The rocks define a mildly alkaline suite rich in potassium, unlike the rocks elsewhere in the volcanic field. The rest of the volcanic field also features alkali basalts, basanite and mugearite. Phenocrysts include aegirine, amphibole, anorthoclase, augite, clinopyroxene, fayalite, hedenbergite, ilmenite, kaersutite, magnetite, olivine, plagioclase and sanidine. Gneiss, granulite, harzburgite, lherzolite and tholeiite xenoliths are found in the volcanic field and form the core of many lava bombs. Inclusions in xenoliths indicate that the gaseous components of the Mount Melbourne volcanic field magmas consist mainly of carbon dioxide. The rocks in the volcanic field have porphyritic to vitrophyric textures.

The trachytes and mugearites formed through magmatic differentiation in a crustal magma chamber from alkali basalts, defining an alkali basalt-trachyte differentiation series. Basalts were mainly erupted early in the history of the volcano. During the last hundred thousand years the magma chamber became established; this allowed both the differentiation of trachytes and the occurrence of large eruptions. A gap in the rock spectrum ("Daly gap") with a scarcity of benmoreite and mugearite has been noted at Mount Melbourne and other volcanoes in the region. There is no agreement on which processes contributed to petrogenesis in the Mount Melbourne volcanic field but diverse mantle domains and assimilation and fractional crystallization processes appear to have played a role. The magmatic system that feeds Mount Melbourne appears to have a composition distinct from the one associated with the Mount Melbourne volcanic field.

Hydrothermal alteration has affected parts of the summit area, leaving yellow and white deposits that contrast with the black volcanic rocks. Hydrothermal sinter deposits have formed in geothermal areas from past liquid water flow. Clay containing allophane, amorphous silica and feldspar are found in the summit area.

Eruption history

Mount Melbourne was active beginning 3.0–2.7million years ago. Activity has been subdivided into an older Pliocene

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