Geological monitoring of Surtsey, Iceland, 1967-1998

Aspects of the geological inoilitorillg of the volcailic island of Surtsey 1967-1998, are described.A hyclrothermal system was developed within he tephra craters in late 1966 to early 1967.Temperat~~res in a drill hole, situated at the eastern border of the hydrothermal area, illdicate that the l~ydrothermal system at that site has been cooling at an average rate of < 1°C per year since 1980.The tephra was altered rapidly within the hydl-othermal area, producillg the first visible palagoilite tuff ill 1969.A substantial part of the tephra pile above sea level was probably collvertecl to tuff by 1972.The visible area of tuff has gradually increased since then, primarily due to erosioil of tephra at the surface.By 1998 52 % of the exposed tephra area had been converted to palagonite luff.By volume, however, some 80-85% of the tephra pile above sea level had been coilverted to tuff in 1998.The area of Surtsey has shrunk from its original 2.65 kin' (1967) to 1.47 kin' (1998) due LO ~llarille abrasion.The geological formations on Surtsey have, howevei-, respoilded quite variably to erosion.The tephra pile was easily eroded, but inarine abrasion has also caused rapid cliff recessioil of the lava field, and longshore currents have deposited a sand-gravel spit on the north shore.The palagonite tuff, however, is very resistant to lnariile abrasion.The central core of palagollite tuff is estimated to be 2 0.39 km'.Statistical estimation of models of the decrease of Surtsey indicate that it will last for a long time.The numerical experiments indicate that it will take over 100 years until only the palagonite tuff core is left.It is postulated that the final remnant of Surtsey before complete destruction will be a palagonite t ~~f f crag, coinparable to those of the other islailds in the Vestlnanllaeyjar archipelago.'