Studies of the Surtsey tephra deposits

Abstract

Similar to other tuf'fi-rings Surtur I, Surtsey, is characterized by a major unconforrnity interpreted as the result of large-scale slumping owing to possible fiormatiori of a ring fault at depth. The last deposits of the phreatomagmatic activity of Surtsey indicate that they are either the result of very good or of relatively little magmaisea- water contact. The first type of' deposit is characterized by base surge deposits that show phenomena like syn-sedimentary formation 3f ~nud-flowc hannels, gravity flowage ripples, and joints. The base surge deposits consist mainly ol' vesic~llated tuffs. Vesiculated accretionary lapilli are indicative of very rapid accretion in wet eruption clouds. The second type of deposit contains cauliflower bombs and lapilli that received their particulate size prior to contact with water. Dming subsequent contact with water the surface of' the particles was chilled and solidified to forrn sideromelane whereas the interior stayed fluid slightly longer and formed tachylite. Many cauliflower bombs impacted while still having a fluid interior and consequently they deformed.