A tabular volcano , also called Tuya or ( Icelandic ) Stapi , is a volcano with a relatively flat and wide surface shape and very steep, often very eroded walls. It occurs when the lava exits through a glacier or an ice sheet . After the ice cover has melted, a steep-walled table mountain remains , which towers over the surrounding, mostly flat landscape.
Tabular volcanoes are caused by subglacial eruptions , which creates a characteristic structure: a main body made of palagonite rests on horizontally aligned basalt at the base , the tip of which is formed from subaerically flowed lava .
The heat of the lava emerging from a chimney leads to the melting of an ever-expanding cavity in the ice during the eruption . The pillow lava that forms at the beginning is displaced by hyaloclastites as it approaches the surface . This creates a steep-walled truncated cone made of basaltic pillow lava, leaning against the ice. On the ice surface is formed above the chimney a caldera , which fills up with meltwater. As the volcanic cone grows and the water level in this caldera falls, the phreatomagmatic eruption creates volcanic rocks that resemble the deposits of marine eruptions. If the eruption lasts for a long time, the overloading water pressure decreases, the cone pierces the ice cover and becomes visible as a nunatak . With the now volcanic eruption on the surface, gases and water vapor explode . The outflowing lava is smashed and deposited around the chimney. More layers of lava build up above the ice surface. The melt water surrounding the volcano is warmed up to 20 ° Celsius and transforms basaltic glasses into palagonite.
After the surrounding glacier has completely melted, the mountain stands free in the form of a tabular volcano.
The tabular volcano shows a typical, flat summit plateau made of subaeric lava flows; these lavas are usually a few meters thick and are typically red in color due to oxidation with oxygen from the air . Subaeric eruption products are characterized by striking red colored oxidized volcanic bombs and lapilli .
The typical external appearance of a tabular volcano clearly distinguishes it from submarine volcanoes, as the supporting ice allows for a much steeper flank formation; they are similar in terms of their internal composition.
Subglacial lavas are characterized by their irregular shape, the presence of volcanic glass, a typical brown color due to insufficient oxidation and the columnar secretion.
The special conditions that are linked to the formation of Tuyas limit their occurrence to certain regions:
- Antarctica ( Brown Bluff )
- Iceland ( Herðubreið , Eiríksjökull , Hlöðufell , Ingólfsfjall )
- British Columbia ( Pyramid Mountain , Tuya Butte , Caribou Tuya , South Tuya , The Table )
- Oregon ( Hayrick Butte , Hogg Rock )
The Canadian geologist William Henry Matthews (1919-2003) published in 1947 a paper entitled Tuyas - Flat-Topped Volcanoes in Northern British Columbia , which described the results of a research trip into the Tuya volcanic field on the Tuya River . The area of this expedition is now protected by the Tuya Mountains Provincial Park .
- MT Gudmundsson, F. Sigmundsson, H. Björnsson (1997): Ice-volcano interaction of the 1996 Gjálp subglacial eruption, Vatnajökull, Iceland , in: Nature, 389: 954-957. doi : 10.1038 / 40122
- WH Mathews (1947): "Tuyas", flat-topped volcanoes in northern British Columbia , in: Am. J. Sci., 245 (9): 560-570. doi : 10.2475 / ajs.245.9.560
- WH Mathews (1951): The Table, a flat-topped volcano in southern British Columbia , in: Am. J. Sci., 249 (11): 830-841. doi : 10.2475 / ajs.249.11.830
- JL Smellie, P. Skilling (1994): Products of subglacial volcanic eruptions under different ice thicknesses: two examples from Antarctica , in: Sedimentary Geology, 91 (1-4): 115-129. doi : 10.1016 / 0037-0738 (94) 90125-2