ABSTRACT

The Azores Archipelago is located between 37° and 40°N latitude and 25° and 31°W longitude at the triple junction of the Eurasian, African and North American plates. The Azores islands emerge from an anomalously shallow and rough topography zone, the Azores Plateau, along WNW-ESE major tectonic lineaments.

The Azores islands can be divided into three groups. The western group (Flores and Corvo islands) is located west of the Mid-Atlantic Ridge (M.A.R.), while the central (Terceira, Graciosa, São Jorge, Pico and Faial islands) and the eastern groups (São Miguel, Santa Maria islands and Formigas islets) are located on the so-called Azores Microplate, an approximately triangular area with active volcanoes and high seismicity. The Azores Microplate is bordered to the west by the M.A.R, to the south by the East Azores Fracture Zone – GLORIA Fault and the Terceira Rift to the north.

While the north and south arms of the Azores triple junction are well established, the type and location of the third arm is still controversial in spite of several models proposed to explain the present kinematics of the Azores triple junction. Some of the available data indicate that the third arm, between the M.A.R. and the GLORIA Fault, is a leaky transform type structure, with oblique spreading and the present pattern of seismic activity agrees with the general idea that the Terceira Rift acts as the third arm of the Azores triple junction. However, another WNW-ESE major tectonic lineament, the Faial-Pico Fracture Zone, dis-plays also high seismicity and has clear bathymetric and magnetic expression, and probably defines at the intersection with the M.A.R. the present location of Azores triple point.

Absolute age dating of the stratigraphically lowest subaerial units from the three neighbouring islands of the central group, Pico, Faial and San Jorge, shows that the oldest and youngest dates are found on Faial (800,000 years) and Pico (300,000 years) respectively, while the age of rocks from San Jorge (600,000 years) lies in-between. These age datings show the generally young age of the subaerial part of all three islands but do not indicate any migration or shift in volcanic activity e.g. in relation to the Mid Atlantic Ridge, 130 to 180 km to the west. All three islands display recent volcanic activity without any favoured temporal or spatial distribution along WNW-ESE volcano-tectonic lineaments.

Pico island is located on the Faial-Pico Fracture Zone, a 350 km long leaky transform with a general WNW-ESE alignment between the M.A.R. and an area south of the Hirondelle basin. The elongated shape of the island along this tectonic alignment demonstrates the strong tectonic control of the volcanism.

Earthquakes on Pico island are fewer and lower in magnitude than on the other islands of the central group. The seismically active part are the Pico Mountain and the adjoining Faial-Pico channel. About 90 percent of felt earthquakes on Pico island have intensities less than IV and do not cause any damage. Earthquakes with intensity V or higher have occurred at intervals between 7 and 20 years. However, Pico island is occasionally hit by stronger earthquakes, with intensity from VI to VII, that have origin in nearby seismic zones such as the 1757 earthquake of San Jorge and the 1926 and 1998 earthquakes of Faial.

Three principal groups of tectonic lineaments were observed on Pico island: (1) a WNW-ESE lineament, (2) a NNW-SSE lineament, mostly as left strike slip faults, with a dip slip component, and (3) a NE-SW lineament, which is also the main trend of most of the dykes in the island. In terms of frequency and length of tectonic features the WNW-ESE to E-W directions dominates, some with clear displacements of the topography. The majority of these tectonic features are right strike slip faults.

Morphologically the island of Pico has three main units: (1) the high stratovolcano, Pico Mountain, that dominates the western part of the island, (2) a shield like volcanic structure, Topo Volcano, located on the south, central part of the island and (3) a 29 km long, WNW-ESE oriented plateau, Planalto da Achada, between the central part of the island and its eastern end, characterised by scoria and spatter cones and associated lava flows.

The elevation of the Pico Mountain is 2351 m.a.s.l. and it rises 3500 m above the surrounding ocean floor. It has a subaerial volume of 97 km³ as compared to the total subaerial volume of Pico island of 207 km³.

Pico Mountain is a basaltic stratovolcano. Below 200 m.a.s.l. the lava formation has a gentle slope of 5° or less. At 200 m.a.s.l. the slope increases to 10° to 16° while above 1200 m.a.s.l. the slope increases to 30° to 35°, occasionally rising to 60° in the vicinity of the top crater.

At 2250 m.a.s.l. the top of the mountain is occupied by a collapse crater 550 m in diameter and 25 m deep. The crater bottom is covered with pahoehoe lavas emerging from a nested, 125 m high lava cone, Piquinho. The rim of an older crater is visible at 2050 m.a.s.l. on the southern slope of the volcano. A number of flank eruptions have occurred, mainly on the western and eastern flanks.

Extensive lava flows have completely covered the base of the volcano, but with reference to figures for the productivity of Pico island volcanism (see below), the age of the subaerial part of the volcano is assumed to be in the order of 240,000 years BP. The more recent (Holocene?) history of the mountain can be divided into three phases. The first two were terminated by the formation of the older top crater at 2050 m.a.s.l., and the present top crater, respectively. Absolute (¹⁴C) ages for lavas erupted from the present top crater give an age of 1700 years BP. The third phase includes eruptions from the nested lava cone (Piquinho) and from an ENE-WSW eruptive fissure on the top of the stratovolcano. The eruption from Piquinho has been dated (¹⁴C) at 1300 years BP. After the settlement, volcanic eruptions occurred on Pico Mountain in the period 1718-1720.

Topo volcano rises 1022 m.a.s.l. and 2500 m above the ocean bottom. It is a basaltic shield volcano consisting of fluid lava flows with 5-10% pyroclastic material. Its volume above sea level is 8 km³ and the exposed surface area on the south central coast of the island is 18 km². Several tectonic escarpments, with a general N-S to NE-SW trend, cut through the volcano. It has two subsidence structures (Terras Chãs and Santa Bárbara) that some authors heave interpreted as volcanic calderas. While Santa Bárbara subsided area can correspond to the remains of an old collapse crater, or caldera, an alternative explanation of the Terras Chãs depression, favoured by the present author, is lateral movement of the flank of the volcano driven by gravity and tectonic instability, rather than caldera collapse at the summit. The Terras Chãs subsidence is 3.5 km long and 1-1.5 km wide, open to the sea and similar to e.g. the Grand Brulé on the island of Reunion. The oldest subaerial formation of Topo Volcano has been dated at 250,000 years BP and the youngest lavas extend into the Holocene.

The Planalto da Achada is an elongated plateau occupying the central part of the island from its eastern termination to Pico Mountain in the west. Numerous volcanic cones are located along the axis of the plateau with a general WNW-ESE orientation. Lavas issuing from these craters are mostly aa-flows that have cascaded down the steep slopes of the plateau either towards the north or south coast. A total of 170 volcanic edifices have been identified on the Planalto da Achada, mostly as scoria cones, with a wide range in size and shape. The largest cone has a diameter of 900 m at the base and a height of 190 m, while the smallest represent fissure eruptions building small spatter ramparts. The oldest subaerial formation has an age of 230,000 years and the youngest volcanic event was the 1562-1564 eruption of Mistério da Praínha. While the volcanic activity has shifted between different parts of the Planalto da Achada for short periods of time, there is no indication of temporal or spatial migration of activity when averaged over a longer period such as the Holocene.

The volcanic eruptions that have occurred on the island of Pico after its settlement around 1482 AD behaved generally in the same way as earlier eruptions during late Pleistocene and Holocene: (1) Strombolian to Hawaiian eruptions with dominating lava effusion and low explosivity, (2) relatively small areal coverage by lavas and (3) migration of craters from higher to lower altitude and compositional and mineralogical heterogeneity (applies in particular to the 1718 AD eruption).

The volcanic products of Pico island are grouped on the basis of petrographic features that can be identified and applied in field mapping. As a result 11 groups of lavas were defi-ned on the basis of phenocryst, glomerocryst (xenocryst) occurrence, absolute and relative amounts, size distribution and crystal habit (see Table 3.2). Geographic distribution of these groups (Appendix C) shows that while some, like aphyric lavas (less than 5-10% crystals) and porphyritic lavas with olivin and clinopyroxene, are found in all volcanic formations on the island, other groups are largely restricted to one of the three volcanic complexes established for Pico island (Pico Mountain, Topo-Lajes and São Roque-Piedade volcanic complexes). That is the case for porphyritic lavas with radiating plagioclase glomerocrysts restricted to Pico Mountain and Topo or, in reverse, for porphyritic lavas with big tabular plagioclase phenocrysts/xenocrysts, present only in São Roque-Piedade volcanic complex. Mafic and ultramafic xenoliths occur in some lavas and could be used as an additional guide in the field mapping.

The rocks occupy a compositionally narrow part of the alkaline series without any strongly evolved components. Of about 150 new chemical analysis 50% are alkaline basalts, 35% transitional basalts and 15% subalkaline basalts. The silica content of the majority of these undersaturated rocks varies between 45% and 49% and they have a high sodium/potassium ratio. In fact 80% of the analysed samples are basalts and 18% hawaiites. A lava with benmoreitic composition was only found in the 1718 AD lava at Misté-rio de Santa Luzia. One sample of mugearite composition was also found at this location.

Relatively few absolute age determinations on rocks from Pico island can be found in the literature. These include two K/Ar ages of hawaiite/mugearite from the Terra Alta-Arrife area, on the eastern part of the island, indicating ages less than 37,000 years. This age is too young for a precise determination by this method. Another three localities on the central and eastern part of the island give an age from 270,000 to 230,000 years and these represent the oldest dated rocks on the island. Earlier age determinations by the ¹⁴C method are in part inconsistent, difficult to correlate with the local stratigraphy.

Efforts were made to look for trachitic tephras from plinian eruptions on the nearby island of Faial. Five explosive eruptions from the Faial caldera have been dated by ¹⁴C with ages ranging from 10,000 to about 1660 y BP. These eruptions have left 0.7-4 m thick tephra deposits on Faial island. The apparent absence of any recognisable tephra layers on Pico island at a distance of about 20 km from the Faial caldera is noteworthy.

For the purpose of the present study 17 new ¹⁴C age determinations have been made (see Table 4.3). The range of ages reported is from 3520 years BP to 315 y BP for lavas from São Roque-Piedade volcanic complex and 1725 y BP to 365 y BP for lavas of Pico Mountain volcanic complex. Lavas from the top crater and Piquinho cone were dated at 1670 y BP and 1310 y BP respectively. These dates, in addition to the known volcanic activity after settlement of the island, in late 15^th century, demonstrate that substantial parts of Pico Mountain, and its flanks are a Holocene formation (see Appendix A).

On the basis of these absolute dates coupled with field relations determined in connection with the detailed geological mapping it is possible to provide a semi-quantitative assessment of volcanic productivity during the recent history of Pico island. The figures obtained for Pico Mountain over the past 1500 years give an average productivity of 0.040 km³/century. For São Roque-Piedade volcanic alignment in the past 2000 years the average productivity is 0.023 km³/century. This gives an overall figure for the productivity of the island of 0.063 km³/century.

Assuming that these productivity figures have remained approximately constant during the formation of the subaerial part of Pico Mountain with a total volume of 97 km³, the age of the subaerial basement of the volcano would be 240,000 years, in good agreement with K/Ar dates of the oldest exposed rocks in other parts of the island. Similarly the age of the oldest subaerial part of São Roque-Piedade volcanic alignment would be 300,000 years, well with the error limit of the actual K/Ar dates available for that area.

The higher volcanic productivity of Pico Mountain than that of São Roque-Piedade volcanic alignment can be compared with the observation that lavas emitted from Pico Mountain are smaller in volume than lavas erupted on São Roque-Piedade alignment, indicating a higher eruption frequency in Pico Mountain. The combination of high eruption frequency from a central crater, small volumes produced by individual eruptions and very fluid lavas with minor clastic material, has contributed to the steep slopes and stratovolcano morphology of Pico Mountain with its very low pryoclastics/lava flows ratio.

The pattern of crustal fractures dissecting the Pico Mountain differs from the adjoining areas of Planalto da Achada as well as the Faial-Pico channel and the island of Faial. In addition to the main WNW-ESE tectonic lineament of the Faial-Pico Fracture Zone the Pico Mountain is cut by NW-SE and NNE-SSW fractures that may provide improved connection with magma reservoirs at a deeper level in the crust or mantle.

Previous studies, including seismic and gravimetric data, do not give convincing proofs on the existence of a shallow magma chamber beneath Pico Mountain. It is, however, possible that small magma pockets do exist at higher levels in the crust where fractional crystallization may have produced some of the rarely observed evolved lava compositions.

The present study provides the basis for an evaluation of the volcanic hazard on Pico island. The hazards involved can result from lava flows, pyroclastic falls, volcanic gases, mass movements (slope failure), volcanic earthquakes and tsunamis.

The available data show that 14 eruptions have occurred on Pico island in the last 1000 years and more than 35 eruptions in the last 2000 years, with longer repose times on the Planalto da Achada than on Pico Mountain. Historic volcanism and the ¹⁴C datings of lavas allow to conclude that the eruptions are not evenly distributed in time. Volcanic episodes are separated by alternating short and long periods of repose. These ¹⁴C datings also suggest that the length of an average repose time for the whole island is 130 years. Areas of recent eruptive centres with higher probability of renewed activity are outlined and the need for state of the art volcano monitoring emphasised.