Conquering Thorbjorn!
Thorbjorn is a volcanic mountain located in the Krysuvik Volcanic Area, which is characterized by tuyas, tindars, conic tuyas, splatter cones, lava flows, and the remnants of a series of fissures. The effusive event which formed Thorbjorn Fell occurred sometime during the Ice Age. The main tuya was formed at least 12,000 years ago, though the exact date of its formation is unknown.
Glacial deposits affirm that the eruption occurred underneath a glacier, or more properly, a massive ice sheet, which was at least 500 meters deep – thus making this a subglacial formation. The heat from the eruption melted the ice, and the lava immediately interacted with the resultant meltwater from the glacier – a phreatic eruption, which subsequently formed subglacial pillow basalts. Long after this initial effusive event, in the early 1200s (roughly between 1223 and 1226), a series of fissures resulted in a row of splatter cones in the area surrounding the original tuyas and tindars, which significantly predate these more recent features.
Classification of Structures:
· Tuya – volcanic mountains with a plateau or ‘tabletop’. Magma melts the ice, forming a ‘boat’ of meltwater. As a result, pillow lava forms around the vent. Continued volcanic activity results in the formation of a miniature lake, which begins to fill in with volcanic deposits, forming a delta-like feature. Here, there are at least 4 sets of inclined features with breccia and tuff.
· Tindars – ejected piles of pillow basalt in the area surrounding the vent.
· Conic-shaped tuyas – the lava flow breaks through the ice, forming a more ‘classical’ volcano structure
· Hyaloclastites – the deltaic lake deposits which result from the interactions between the glacier’s meltwater and the lava from the vent – forms at the top and upper sides of the tuya.
Another more recent feature is the formation of a graben near the top of the main tuya, which was extracted and extruded by a series of normal faults.
Another feature formed in the initial effusive event were a series of characteristic rinds – these structures are the interface between the glacial meltwater and the pillow basalts. The pillow basalts cooled from the outside, in, beginning with the rind.
A question for later study: How are the development of fissures in the early 1200s related to the normal faulting which formed the series of alpine grabens?
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