Marine Investigations of Greece's Santorini Volcanic Field Haraldur Sigurdsson University of Rhode Island, [email protected]
Total Page：16
File Type：pdf, Size：1020Kb
University of Rhode Island DigitalCommons@URI Graduate School of Oceanography Faculty Graduate School of Oceanography Publications 2006 Marine Investigations of Greece's Santorini Volcanic Field Haraldur Sigurdsson University of Rhode Island, [email protected] Steven Carey University of Rhode Island, [email protected] See next page for additional authors Follow this and additional works at: https://digitalcommons.uri.edu/gsofacpubs Part of the Oceanography Commons, and the Volcanology Commons Terms of Use All rights reserved under copyright. Citation/Publisher Attribution Sigurdsson, H., et al. (2006), Marine investigations of Greece's Santorini Volcanic Field, Eos Trans. AGU, 87(34), 337-342. Available at: http://dx.doi.org/10.1029/2006EO340001 This Article is brought to you for free and open access by the Graduate School of Oceanography at DigitalCommons@URI. It has been accepted for inclusion in Graduate School of Oceanography Faculty Publications by an authorized administrator of DigitalCommons@URI. For more information, please contact [email protected]. Authors Haraldur Sigurdsson, Steven Carey, Matina Alexandri, Georges Vougioukalakis, Katherine Croff, Chris Roman, Dimitris Sakellariou, Christos Anagnostou, Grigoris Rousakis, Chrysanti Ioakim, Aleka Goguo, Dionysis Ballas, Thanassis Misaridis, and Paraskevi Nomikou This article is available at DigitalCommons@URI: https://digitalcommons.uri.edu/gsofacpubs/5 Eos, Vol. 87, No. 34, 22 August 2006 VOLUME 87 NUMBER 34 22 AUGUST 2006 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 337-348 tions), as is typical of submarine pyroclastic Marine Investigations of Greece's flow deposits. This sequence has a mean thickness of about 29 meters, but ranges up to about 80 meters in areas close to the Santorini Volcanic Field coast of Santorini. The distribution of the sequence is greatly affected by seafloor PAGES 337,342 volume of ash fallout from the Minoan erup topography, and mainly follows pre-existing tion has been estimated previously to be 19 submarine valleys (Figure 1). The unit has a The most recent major explosive eruption cubic kilometers dense-rock equivalent total area of 1378 square kilometers inside of the Santorini volcano in Greece—around (DRE) [Sigurdsson et al, 1990]. This result the 10-meter thickness contour line, and the 3600 years before present (B.P.), often implied that the volume of submarine pyro total bulk volume around Santorini is 54.5 referred to as the Minoan eruption—is one clastic flows should be of the order of 20 cubic kilometers, or about 41 cubic kilome of the largest volcanic events known in his cubic kilometers DRE, based on mass bal ters DRE. torical time and has been the subject of ance assumptions [Sparks and Huang, The top of the sequence often is terraced intense volcanological and archeological 1980]. Thus, the early estimate was a total of or step-like, especially on the steep subma studies [Druitt et al, 1999]. The submarine 39 cubic kilometers DRE for the eruption as rine slopes of the volcano, with steps that volcano Kolumbo, located seven kilometers a whole. have a lateral extent of kilometers or more northeast of Santorini and associated with The recent marine seismic survey reveals and heights of tens of meters. These mor Santorini's tectonic system, erupted explo a sediment sequence on the ocean floor phological 'wrinkles' most likely are a result sively in 1650 A.D., resulting in fatalities on around Santorini that is generally massive or of downslope creep and slumping during or the island of Thera [Fouque, 1879]. A large chaotic (with highly irregular internal reflec just after deposition. On some steep slopes, fraction of the erupted products from the Minoan eruption has been deposited in the sea but, up to now, only has been studied in distal marine sediments. As part of a collaborative project between the University of Rhode Island (Narragan- sett), the Hellenic Centre of Marine Research 36.6CN (Athens, Greece), and the Institute of Geol ogy and Mineral Exploration (Athens), a marine geological survey was conducted around Santorini from April to June 2006. The new work now shows that the volume of the Minoan eruption may be comparable to that of the largest known historical erup tion, the 1815 eruption of Tambora in Indo nesia [Sigurdsson and Carey, 1989]; provides insights into the depositional processes and size of the Minoan eruption; and led to the discovery of important submarine hydrother- mal vents with active mineralization. Submarine Pyroclastic Deposits The Minoan eruption of the Santorini vol cano has remained of great interest to geolo gists, historians, and archeologists because of its possible impact on the Minoan civiliza tion in Crete and on the Cycladic islands. The eruption generated pyroclastic flows as well as widespread volcanic ash fallout. The 25.4°E 25.5°E 25°E 25.1°E 25.2°E 25i3°E Fig. 1. Santorini region showing bathymetry (collected with SeaBeam instrumentation), locations BY H. SIGURDSSON, S. CAREY, M. ALEXANDRI, G. of seismic lines (black), and the distribution of inferred massive pyroclastic deposits from the VOUGIOUKALAKIS, K. CROFF, C. ROMAN, D. SAKEL- Minoan eruption. The thickness of the surface sequence derived from the volcano is shown with LARIOU, C. ANAGNOSTOU, G. ROUSAKIS, C. IOAKIM, 10-meter-spaced contours (red), and the outer limit of the deposit is shown by the red dashed lines. A. GOGOU, D. BALLAS,T. MISARIDIS, AND PNOMIKOU Original color image appears at the back of this volume. Eos, Vol. 87, No. 34, 22 August 2006 as 60 cubic kilometers DRE, nearly twice the previous estimate. Hydrothermal Vents The Santorini volcanic field extends 20 kilometers to the northeast, as a line of more than 20 submarine cones. The line of craters lies within a rift that dissects the northern caldera wall of Thera island (Figure 1). By far, the largest of these submarine craters is Kolumbo, a three-kilometer-diameter cone with a 1500-meter-wide crater, a crater rim as shallow as 10 meters, and a crater floor 505 meters in depth below sea level. Kolumbo last was active in 1650 A.D., when an explo sive eruption produced hot surges across the ocean surface that caused 70 deaths on Thera [Fouque, 1879]. Seismic monitoring by the Institute for the Study and Monitoring of the Santorini Volcano shows that seismic- ity in the region today almost is exclusively limited to Kolumbo and the northeast trend ing Kolumbo volcanic line. A widespread hydrothermal vent field was discovered on the floor of the Kolumbo sub marine crater during the recent marine sur vey (Figure 2). High-temperature venting occurs in the northern part of the crater, with vigorous gas emission plumes more than 10 meters above the crater floor, and fluid temperatures up to 220°C from vent chimneys up to four meters in height that are constructed of polymetallic massive sul fides and sulfates. The exterior of most chimneys is densely covered with white, gray, and reddish filamentous bacteria. Some of the chimneys exhibit branching, trunk like structures that have continuous gas and fluid streaming from multiple openings. Other chimneys consist of a series of tightly clustered, tapered spires that are up to one 25.484 25,486 25.488 25.490 meter in height. Lower-temperature chimneys and vents Longitude along the northern and eastern margins of the crater floor discharge clear fluids up to Fig. 2. Map showing the distribution of hydrothermal vents in the crater of Kolumbo submarine 70°C and minor gas bubbles. The larger volcano and the track of the remotely operated vehicle (dashed/stippled black tracks). Shown chimneys in this area are covered by red are high-temperature chimneys (white circles), low-temperature chimneys (red circles), and areas dish bacteria and sea squirts (glassy tuni- of bacterial streams (yellow circles). Inset map shows Kolumbo volcano and the area depicted cates). The margins of the crater wall in the in the larger figure. The scale bar at the bottom right of the inset map is 1.5 kilometers. Original northeast have streams of white bacterial color image appears at the back of this volume. mats that are the colonization of fluid seeps moving downward toward the crater floor. the terraces detach and the sequence is sep eruption of Santorini and may have been In some of these areas, there are patches of arated into distinct hummocks or blocks. generated as a result of massive pyroclastic gray, hydrothermally altered clays emitting The origin of this extensive sequence, which flow discharge into the sea. If the sequence shimmering water at temperatures up to clearly is derived from Santorini, can be consists of dominantly juvenile material about 70°C. The entire crater floor of interpreted as submarine pyroclastic gravity from the Minoan eruption, then this would Kolumbo is covered by a reddish orange flow deposits by comparing this sequence with increase the total volume of erupted mate bacterial mat a few centimeters in thickness. the character of the submarine deposits rial estimated for this event. Volumetrically, In addition, the water column in the crater formed during the 1883 Krakatau eruption the Minoan eruption deposit consists of four deeper than 250 meters is turbid with abun [Sigurdsson et al., 1991]. The sequence is components: plinian ash fall from the main dant bacterial filaments and other sus likely to contain several layers, as shown by eruption column (2 cubic kilometers DRE), pended particles. some internal reflectors. These layers may con ash fall associated with pyroclastic flows (17 In contrast to the high-temperature vent sist of primary submarine pyroclastic flow cubic kilometers [Watkins et al., 1978]), ing in Kolumbo, only low-temperature vent deposits emplaced at high temperature or pyroclastic flow deposits on land on San ing was observed within the Santorini cal water-supported flows generated by transfor torini (1.5 cubic kilometers), and the newly dera.