Formation of the lava flow field of the Enclos: an exceptional effusive eruption at Piton de la Fournaise volcano (Reunion) during the 18(th) century

A large lava flow field, essentially constituted of 'pahoehoe' flows, forms a relatively monotonous area on the north-west flank of the terminal cone of Piton de la Fournaise and on a large part of the: Enclos floor. We named this unit "champ de lave de l'Enclos Fouque" (CLEF). To the east, more recent lava flows almost completely cover the CLEF. Mappi ng of the CLEF has been attempted using satellite imagery. Radar (SIR-C) an d SPOT images have been used. Both types of images lead to comparable resul ts for the surface of the CLEF, estimated to 11 km(2). On satellite images, only a general map of the facies of the CLEF can be done. Air photos and g round observations allow to characterize more precisely the facies defined from satellite images. On the slopes of the Central Cone, the surface of th e CLEF is mostly composed of tumuli that may produce small lava flows at th eir downhill base. On the floor of the Enclos, the CLEF is constantly forme d by a multitude of intricate small flows. Except for small 'aa' patches, s urfaces are 'pahoehoe'. Tumuli, pressure ridges and collapsed plates are co mmon features of the CLEF. Near the NW border of Enclos, the surface become s less regular, with a higher proportion of 'aa' patches and tilted slabs. This is probably due to the increase of slope in this area. In the vicinity of Bory crater contemporaneous welded scoria cover a zone extending 150-20 0 m from the crater. Several elements, found in various historical documents collected by Lacroi x [1936 and 1938], bring evidence that the CLEF may result from a successio n of events between the years 1750's to the 1790's. During this period, the volcano has been virtually continuously in eruption. Bory crater was the o nly active crater until 1766, when a new summit crater was Formed by collap se, 400 m east of Bory crater. In the following years, this 1766 crater was filled by lava emissions which erected a gigantic tumulus, 50 m high, call ed Mamelon central. Several large lava flows, some of them reaching the sea , are unambiguously described as emitted from the summit craters. One of th e main event appears to be the phase that occurred in 1753 (or 1759 accordi ng to different authors). This phase was associated to felt earthquakes and widespread ash-falls in the island. Lava flows covered most the western pa rt of Enclos and reached the sea to the east. Whereas the activity of 1753 (1759?) appears as paroxysmal, extensive lava flows are described until 179 4. In 1791, a new crater formed south of the 1766 crater. In 1801, Bory de St-Vincent [1804] observed an active lava lake inside this 1791 crater. In summary, it appears that quasi-continuous activity took place during the se cond half of the 18(th) century at Piton de la Fournaise, with lava flows c overing almost all the Enclos and Grand Brule areas. It is obvious that the presently outcropping surface of the CLEF is only a fraction of its former extension. From the above-mentioned historic descrip tions, it is reasonable to assume the CLEF may have recovered the entire su rface of the Enclos Fouque and the Grand Brule depression. The surface exte nsion of the CLEF would thus lie between 11 km(2), its presently observed a rea, to ca. 90 km(2). The compounded thickness of the CLEF has not been dir ectly observed. In the rim of Dolomieu crater, a series of lava flows thoug ht to represent the CLEF is about 25 m thick, but this thickness is that of the shield built around the vents and not that of the lava field. Using st atistical laws for strombolian cones morphometry from Wood [1980], the thic kness of the CLEF may be approached considering the crater diameter of 3 co nes located in the north-western part of Enclos and partially flooded by th e CLEF. We obtain estimates of 5 to 15 m. If Enclos has been entirely cover ed by the CLEF and for a mean thickness of 5 to 10 m, then the volume of th e CLEF eruption is 450 to 900 x 10(6) m(3). Mineralogical and chemical compositions of the CLEF lavas are similar to th ose of the transitional olivine basalts of the historic period [steady-stat e basalts, Albarede ct al, 1997]. The chemical compositions vary somewhat d ue to variable modal olivine phenocrysts abundances and are characteristic of a rapid transfer of magma from deep reservoirs with virtually no low-pre ssure evolution. Major changes of the summit craters were associated to this eruption as a r esult of the unusually prolonged magmatic activity at Piton de la Fournaise during decades. The occurrence of similar phases at Piton de la Fournaise in the future can be appraised with reference to Kilauea. Holcomb [1987] re ports 7 long-lived eruptions at Kilauea in the last 3 centuries. Thus, alth ough not frequent, this type of event is not uncommon for Kilauea. By analo gy, it must be considered an probable that Piton rip In Fournaise will expe rience new long-lived eruptions in the future.