Int J Earth Sci (Geol Rundsch) (2002) 91:594–614 DOI 10.1007/s00531-001-0243-x ORIGINAL PAPER Ralf Schmidt · Hans-Ulrich Schmincke From seamount to oceanic island, Porto Santo, central East-Atlantic Received: 21 February 2001 / Accepted: 25 October 2001 / Published online: 21 June 2002 © Springer-Verlag 2002 Abstract The uplifted and deeply eroded volcanic suc- variably-shaped pillows, and thin sheet flows capped cession of Porto Santo (central East-Atlantic) is the by subhorizontal submarine to subaerial lava flows product of a wide spectrum of dynamic processes that along the eastern coast of Porto Santo. are active in shoaling to emergent seamounts. Two su- The facies architectures indicate emplacement: perimposed lapilli cones marking the base of the exposed section are interpreted as having formed from numerous 1. on a gently sloping platform in southwestern Porto submarine to subaerial phreatomagmatic explosions, Santo, and pyroclastic fragmentation being subordinate. The lower 2. on steep offshore slopes along high energy shorelines basaltic and the upper mugearitic to trachytic sections in eastern Porto Santo. are dominated by redeposited tephra and are called ‘la- Growth of the pillow-hyaloclastite breccia prism is domi- pilli cone aprons’. Vertical growth due to accumulation nated by the formation of foreset beds but various types of of tephra, voluminous intrusions, and minor pillowed syn-volcanic intrusions contributed significantly. Subma- lava flows produced ephemeral islands which were sub- rine flank eruptions occurred in very shallow water on the sequently leveled by wave erosion, as shown by con- flanks of the hyaloclastite prism in eastern Porto Santo. glomerate beds. Periods of volcanic quiescence are rep- The island became consolidated by intrusion of numerous resented by abundant biocalcarenite lenses at several dikes and by emplacement of prominent intrusions that stratigraphic levels. The loose tephra piles became stabi- penetrate the entire volcanic succession. Volcanic sedi- lized by widespread syn-volcanic intrusions such as mentation ended with the emplacement of a debris ava- dikes and trachytic to rhyolitic domes welding the volca- lanche that postdates the last subaerial volcanic activity. nic and volcaniclastic ensemble into a solid edifice. Shattering of a submarine extrusive trachytic dome by Keywords Eastern Atlantic · Madeira archipelago · pyroclastic and phreatomagmatic explosions, accentuat- Shoaling oceanic island · Seamount · Submarine ed by quench fragmentation, resulted in pumice- and volcanism · Volcanic processes · Tuff cone · Felsic crystal-rich deposits emplaced in a prominent submarine dome · Lava delta erosional channel. The dome must have produced an island as indicated by a collapse breccia comprising surf- rounded boulders of dome material. Subaerial explosive Introduction activity is represented by scoria cones and tuff cones. Basaltic lava flows built a resistant cap that protected the In July 1831, a small volcanic islet was born in the Medi- island from wave erosion. Some lava flows entered the terranean Sea about halfway between Sicily and Pantel- sea and formed two distinct types of lava delta: leria. It was immediately claimed by several European 1. closely-packed pillow lava and massive tabular lava powers because of the strategically important gateway flows along the southwestern coast of Porto Santo, to the eastern Mediterranean. The British name Graham and Island is most commonly used, although it is also known 2. a steeply inclined pillow-hyaloclastite breccia prism as Ferdinanda in Italy. The tephra cone had reached its composed of foreset-bedded hydroclastic breccia, maximum dimensions of almost 0.5 km in diameter and about 60 m in height after the climax of volcanic activity in the middle of August. The loose tephra pile was quick- R. Schmidt · H.-U. Schmincke (✉) GEOMAR Forschungszentrum, Wischhofstr. 1–3, 24148 Kiel, ly eroded by wave action and after only three months, the Germany island was almost leveled and finally disappeared by Jan- e-mail: [email protected] uary 1832 (Poulet Scrope 1862; Francis 1995). This sce- 595 Fig. 1 a Madeira island group, contour interval 1 km. Inset shows transitional stage is preserved (Sohn 1995). Several the Madeira archipelago in the central eastern Atlantic. b Bathy- lacustrine englacial successions reveal the internal struc- metry of the vicinity of Porto Santo. Contour interval is 0.1 km. Data from the TOPEX data set (Smith and Sandwell 1997) ture of shoaling to emergent volcanoes particularly well (Smellie and Hole 1997; Werner and Schmincke 1999), because wave energy in restricted basins is low com- nario illustrates the normal fate of emergent seamounts pared with those in open marine environments and sud- trying to establish themselves as a volcanic island. den drainage of glacial lakes can result in abrupt transi- Little is known about the volcanic processes of shoaling tion from subaqueous to subaerial conditions. Studies of to emergent seamounts, because deposits of this stage tran- uplifted submarine shoaling volcanoes and their engla- sitional in the growth of volcanic ocean islands commonly cial counterparts have led to a general model of se- lie far below sea level, owing to subsidence of the islands amounts growing above sea level to form an island. Vol- grown on oceanic lithosphere (Watts and ten Brink 1989). canoes start in deep water with effusive pillow lava-sheet Studies of modern marine shoaling to emergent volcanoes, flow complexes intruded by dikes and sills with minor e.g. Surtsey (Iceland), Myojinsho (Japan), and Metis Shoal intercalated hyaloclastites (Schmincke and Bednarz (Tonga Islands), and of numerous seamounts, are common- 1990). Although vesiculation and explosive fragmenta- ly based on the interpretation of surface topography and tion of volatile-rich magmas can start at water depths ex- limited sampling using submersibles (Lonsdale and Batiza ceeding 1,000 m (Gill et al. 1990; Clague et al. 2000), 1980), bathymetric data (Fornari et al. 1984), dredging the amounts of clastic material and clast vesicularity (Moore and Fiske 1969), drill-hole data (Fleet and generally increase during shoaling and culminate in shal- McKelvey 1978), and on observations of the succession of low-water to emergent explosive tuff cones (Sohn and events that occurred during eruptions (Kokelaar 1983; Chough 1992). The subaerial stage typically produces Moore 1985; Fiske et al. 1998). The internal structure of subhorizontal lava flows and associated lava deltas seamounts, however, remains poorly known. that originate from lava flows that enter the sea (Jones Uplifted and dissected deep-water to shoaling volca- and Nelson 1970; Moore et al. 1973; Peterson 1976; noes (Staudigel and Schmincke 1984; Schmincke and Porebski and Gradzinski 1990; Schmincke et al. 1997). Bednarz 1990; Kano et al. 1993; McPhie 1995) provide Mass wasting processes accompany all growth stages insights into the internal structure of seamounts. Such (Schmidt and Schmincke 2000). sections generally lack the transitional (shoaling to emer- Here we focus on the constructive and destructive gent) stage, however, because this stage is small in volcanic and sedimentary processes that occur in shal- volume compared with the entire edifice and is easily low-water to emergent volcanoes. Porto Santo (Madeira eroded by wave action (Schmidt and Schmincke 2000). Archipelago, central East-Atlantic) offers the unique op- The uplifted Tok Islands (Korea) are, as far as we portunity to study this transitional stage of volcanic are aware, the only example of a shoaling to emergent ocean island evolution in its complexity owing to uplift island volcano located in a marine setting in which the of this Miocene island and its subsequent erosion. 596 Fig. 3 Generalized stratigraphic section of Porto Santo. The ex- trusive dome, part of the Seamount Stage, is schematically illus- trated to represent numerous trachytic to rhyolitic domes Submarine rocks occur at least as high as 300 m above sea level, indicative of uplift of the island, lowering of sea level, or both (Schmincke and Staudigel 1976; Fig. 2 Simplified geological map of eastern Porto Santo. Based Feraud et al. 1981). Fossiliferous limestones intercalated on reconnaissance work by HUS and later studies by Jocelyn with volcanic deposits are of mid-Miocene (ca. 14 Ma) McPhie and RS age (Lietz and Schwarzbach 1970; Rodrigues et al. 1998). K/Ar ages indicate that most of the exposed vol- canic rocks were emplaced between 12.5 and 13.8 Ma Porto Santo (Macedo et al. 1974; Feraud et al. 1981). Our new age data from subaerially exposed rocks of Porto Santo, dis- Porto Santo is located ca. 700 km off NW-Africa cussed more fully elsewhere, indicate that this part of the (Fig. 1a). Bathymetry shows that the island represents volcano was formed between ca. 14 and 10 Ma (see also the partly eroded subaerial portion of a much larger sub- Geldmacher et al.; 2000). First emergence of the volcano merged edifice (ca. 80 km×40 km) that rises from water occurred around 14 Ma, whereas younger rocks are sub- depths of more than 3,000 m. The edifice has a flat top at aerially erupted and submarine and/or subaerially em- ca. 100 m below sea level (b.s.l) and is star-shaped in placed. A simplified geological map of eastern Porto plan view with a prominent NW–SE oriented rift zone Santo and a generalized stratigraphic section are given in (Fig. 1b). Volcanic rocks comprise dominantly alkali ba- Figs. 2 and 3. salts, with minor mugearites, benmoreites, trachytes, and rhyolites (Schmincke and Weibel 1972). Most rocks ex- posed on the island are submarine in origin. The subma- Deposits resulting from explosive processes rine series can be divided into an older, trachytic series (intrusions, flows, and related hyaloclastites) unconform- Submarine lapilli cone aprons ably overlain by a basaltic-hawaiitic submarine to sub- aerial series (pillow lavas, associated hyaloclastite brec- Two submarine to subaerially formed hydroclastic tephra cias, and intercalated fossil-bearing calcarenites grading cones form the base of the succession exposed in eastern into subaerial lava flows towards the top of the section).