Image of Mount Melbourne (?) taken from a RNZAF Hercules C-130 transport aircraft.

One of the benefits of spending time in McMurdo is to make use of the Ross Island Trail System. A series of well-maintained paths allow access to areas around McMurdo station to take in some fresh air and get some exercise while enjoying spectacular scenery.

These trails also allow access to approximately 1.2 million years of volcanic history preserved along the Hut Point Peninsula. These volcanic rocks were erupted as part of a much larger magmatic province, including Mt. Erebus, Mt. Morning, Mt. Discovery and Mt. Melbourne and related to the West Antarctic Rift System, or WARS.

The WARS is a major active rift valley lying between East and West Antarctica and is flanked to the west by the Transantarctic Mountains. The WARS is considered to have been active since at least the late Cretaceous period. At around this time, Australia and Antarctica rifted apart in the last great act of the break-up of the most recent super-continent, Gondwanaland.

Slow rifting (about 2 mm per year) persists, leading to stretching of thinning crust in the Ross Embayment, and continued igneous activity. The geology of the Hut Point Peninsula is dominated by basanite and basalt, with more minor evolved rocks, called phonolite (literally because it rings when you hit it with a hammer). Collectively, these rocks are part of the McMurdo group volcanic rocks.

Close-up of a fragment of olivine-phyric basalt on the Ob Hill Loop.

Ob Hill is a major landmark on the Hut Point Peninsula, and the core of this hill is phonolite with kaersutite (amphibole) crystals. The phonolite here has been dated at around 1.25 Ma (Forbes et al., 1974, Geology, 2, 297-298) and has the form of a plug, surrounded by older olivine-bearing basalts.

The ~1.2 Ma Ob Hill kaersutite-bearing phonolite.

Basanites and basalts in the surrounding McMurdo region can have inclusions of altered phonolite, similar to that found at Ob Hill, within them. These basanites and basalts are between 0.4 and 1.2 Ma (Kyle & Treves, 1974, Antarctic J. 9, 232-234) and are vesiculated, olivine- and clinopyroxene bearing rocks, with no visible evidence for interaction with water or ice.

Inclusion of kaersutite-bearing phonolite within basanite on the Hut Point Loop.

Further to the northeast evidence for sub-glacial volcanism can be found at Castle Rock. This prominent feature on the Hut Point Peninsula is formed from an olivine-augite basanitoid hyaloclastite dated at around 1.1 Ma (Kyle & Treves, 1974). Hyaloclastite forms when cold water and hot magma/lava interact to form a potent and explosive mixture. The evidence for the catastrophic disruption of the lava is evident in the Castle Rock Hyaloclastite.

The ~1.1 Ma Castle Rock olivine-augite basanitoid hyaloclastite.

Given the height about sea level for Castle Rock (>400 m), the evidence favours this landform being formed under the ice about 1.1 Ma, much like Icelandic Tuyas form. Tuyas are under-ice volcanoes and are nowhere better characterized that by Heidubred (sic.), the ‘Queen of the Icelandic Mountains.

Geologist Ioannis Baziotis investigates a rubble pile of xenolith-bearing basanite on the Hut Point Loop.

Remarkable features of the Hut Point Peninsula lavas are the abundance of xenoliths. Xenoliths (or foreign rocks) are fragments of rocks accidentally included within magmas and lavas. The Hut Point peninsula xenoliths are notable for their diversity, including plagioclase-bearing ‘granulites’, clinopyroxenites, clinopyroxene-olivine peridotites, spinel peridotites and dunite.

Cross-polarized light image of a dunite xenolith. Grains in the image are dominantly olivine, and the field-of-view is ~7 mm in the long-dimension.

During cooling of magmas, crystals form and can be separated from the magma, changing the magma composition. The magmas start off rich in compatible elements that are used up during the crystallization of minerals following a predictable sequence (known as ‘Bowen’s Reaction Series). This process, most widely known as fractional crystallization, can explain some, but not all the Hut Point Peninsula xenoliths.

While we focus our efforts on the search for rocks beyond our own planet, much remains to be understood about the geology of the Antarctic. Not least, how the Hut Point Peninsula and surrounding WARS evolved to give us the conditions that we find today, and the relationship between volcanism and glaciation on this vast continent.

Posted by James from the Hut Point Peninsula on 2017-12-08 at 15.00 local