Brown Bluff, named for the color of its cliffs, is a basalt tuya that rises over Trepassey Bay on the Tabarin Peninsula of Antarctica. It is one of the more accessible landings on the continent itself, and known for its Gentoo and Adelie penguin colonies.
Less noticed by visitors, perhaps, is Brown Bluff’s remarkable volcanic geology. These bluffs were formed approximately 1.2 million years ago, when a volcano erupted into a lake under a thick overlying glacier.
A tuya is a distinctive type of volcano that forms when lava erupts beneath or through a thick ice sheet. Lava cools very quickly under such conditions, and does not travel far. Where lava meets the water, outwardly dipping beds of volcanic rock form. Eventually, the volcano breaks through the ice cover, either by melting it or erupting through it, and deposits lava in a flat cap over the hill. The result is a plateau-like formation with steep sides, such as you see above you.
The following illustration is of a somewhat generalized tuya:
Reproduced with permission of Prof JL Smellie, School of Geography, Geology and the Environment, University of Leicester, UK
Volcanic Rock Types of Brown Bluff
To aid in understanding the formation of Brown Bluff, some pertinent volcanic rock types are described here.
Pillow lava: named for its shape, pillow lava forms when lava is extruded beneath water or glacial ice. Under these circumstances, surface magma is cooled very quickly and forms a skin around a more slowly cooling interior. Pillow lavas are typically up to a meter in diameter.
Hyaloclastite: a volcanic accumulation (breccia), consisting of glass that has been fragmented when quenched during an underwater or subglacial extrusion of lava. It typically appears as angular flat fragments millimeters to a few centimeters in size.
Hyalotuff: a layered rock formed by explosive fragmentation of glass during eruptions at shallow water depths, or within aquifers or enclosed glacial lakes. The layering is thought to result from oscillations in discharge rate over periods of several minutes. Due to the explosive nature of the eruption, the grains are relatively fine and can appear gravelly.
Volcanic Evolution of Brown Bluff
Here at Brown Bluff, several stages of tuya evolution have been identified. It should be noted that different stages and visible layers do not imply separate volcanic events over time. Rather, Brown Bluff is thought to have formed over a relatively short time, months to a few years, from a volcano with an original diameter of 12 – 15 km. The overlying glacier has been estimated to be around 400 m thick.
The evolutionary stages of this volcano, chronologically, are:
A. Pillow volcano: A geothermally-created lake likely existed between the bedrock and the glacier at the initial stages of eruption. Non-explosive lava flows into that lake created pillow lava.
B. Hyalotuff cone: The volcanic eruption became more explosive, erupting into the lake and forming a thick layer of hyalotuff.
C. Slope failure: The overlying lake drained, with a dry debris avalanche on the northeast flank.
D. Lake recharging and continuing hyalotuff cone formation: The lake refilled and explosive eruptions similar to (B) above continued, covering the layers deposited by (B) and (C) with more hyalotuff.
E. Hyaloclastite delta and lava capping: The volcano vent emerged fully from the overlying water, with subaerial lava first forming a hyaloclastic delta around the vent where the lake still covered parts of the cone. Finally, the lake drained a second time, and further subaerial lava flow capped the plateau, completing tuya formation.
Not all these stages and rock layers are easily identifiable from the usual landing area. However, you should be able to appreciate some important features while standing on the beach.
To claim this EarthCache, please message or e-mail me with your answers to the following questions:
1. Look up at the cliffs in front of you. Describe the major layers you see, in terms of colors and composition (a set of binoculars or a telephoto lens might help you see interesting detail in the higher cliffs).
2. Do you think the cliffs are eroding quickly or slowly? What do you see that supports your answer?
3. Near the listed coordinates are several large rocks.
A. What is their approximate size and coloration? Describe any structural features you see, such as layering, inclusions, erosional features.
B. Do you think these objects were formed where you find them today, or have they moved since their formation? What features of these objects led you to your conclusion? Can you see where they might have come from?
C. Given the descriptions of the three volcanic rock types above, which of the three types do you think these rocks are?
D. What force(s) other than the volcano have contributed to the shape of these rocks?
Skilling, I. P. (1994) Evolution of an englacial volcano: Brown Bluff, Antarctica. Bulletin of Volcanology, Volume 56, Issue 6-7, pages 573-591.
Smellie JL, Skilling IP (1994) Products of subglacial volcanic eruptions under different ice thicknesses: two examples from Antarctica. Sed Geol 91:115–129.
Smellie, J.L., Johnson, J.S., McIntosh, W.C., Esser, R., Gudmundsson, M.T., Hambrey, M.J. and van Wyk de Vries, B. 2008. Six million years of glacial history recorded in the James Ross Island Volcanic Group, Antarctic Peninsula. Palaeogeography, Palaeoclimatology, Palaeoecology, 260, 122-148.