Sunday, December 31, 2006

Walkabout (part 2)

Over the past few days, I’ve been learning about the geology of the area around McMurdo and I would like to share some of that knowledge with you. Ross Island owes its origin to volcanoes, and is geologically very young. Mt. Erebus is the youngest and largest of the three volcanoes that form Ross Island and is the only one active today with a lava lake in its crater. Rising 3974 meters above sea level it is simply the most dominant feature on the island and begun its eruptive activity less than a million years ago (Ma). The older Mt. Terror formed between 2 and 0.5 Ma. while the still older Mt. Bird formed between 4 and 3 Ma.

McMurdo sits on the southern edge of Hut Point Peninsula, which was formed as a result of minor volcanic centers along its length active between 1.5 to 0.5 Ma. Right on the edge of the peninsula next to McMurdo is Observation Hill which is a remnant of a volcano and has an interesting composition of rocks including dark, fine-grained alkali basalts, pyroclastic flows and brick-red scoria. The stratified pyroclasts occur along parts of the western and northern slopes of Ob Hill, while the red scoria occurs on the northern slope just off the road to Scott Base and on the northeastern slope The scoria is composed of iron rich blobs that have been oxidized giving them the red colour. The blobs almost look like over baked pastries and in some ways they are just that – they were erupted as hot molten blobs, thrown into the air and then rolled down the slopes of the volcano. Fine material that probably flowed down the slopes of the volcano formed the stratified pyroclastic deposits, while the dark, fine rocks formed from the actual lava flow. The dark volcanic rocks dominate the southern slopes of Ob Hill and also occur through to the top.

Ob Hill seen from the road to Scott Base: the brick red colour on the lower slope is due to the oxidized scoria.

Oxidized Scoria at Ob Hill.

Pyroclastic deposits on the western slope of Ob Hill.

Across from Ob Hill along the road to Scott Base is another outcrop of the pyroclastic flow.

Halfway up Ob Hill, the rock formed from lava flow is being carved by strong winds.

The bed rock at McMurdo is laden with xenoliths. A xenolith is any rock caught/trapped within an igneous rock; i.e. the xenolith is a preexisting rock that was caught in the magma forming the igneous rock. Rocks from deep within the crust and even from the base of the crust can be brought to the surface as xenoliths. The xenoliths at McMurdo are bright green in colour and mostly made up of the mineral olivine. They could have been brought up from the mantle just below the crust. Some of the xenoliths are as big as a small apple and they look beautiful, studded in the dark, fine-grained rock.

I hold up a piece of rock containing a bright green xenolith a little bigger than my thumb. (Photo from Robo)

A little to the Northwest of Ob Hill is Crater Hill another volcanic remnant and about 3 miles North of McMurdo is Castle Rock which is a volcanic neck composed of hard rock resistant to the erosive force of the wind. Numerous other small volcanic features have been mapped along the length of Hut Point Peninsula and they definitely add a lively colour to the geological history of McMurdo. I’ve enjoyed observing these rocks; they tell a story of how the land beneath me was formed. It is fascinating to understand that McMurdo is today on land that was an active volcanic field less than a million years ago and would have made a spectacular sight of fire and ice!

Wishing you a Happy New Year!

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