Basaltic volcanism occurs at all the active volcanic systems of Sao Miguel. For the last 30 ka, the eruption of basaltic magma has been particularly significant in the areas between the three polygenetic volcanoes - Sete Cidades, Fogo and Furnas - and two basaltic fields have developed: the Picos Fissural Volcanic System (PFVS) and the Congro Fissural Volcanic System (CFVS). About 5 ka ago volcanic activity at CFVS ended abruptly and since then almost all basaltic activity has been concentrated at the PFVS, where about 30 eruptions have taken place. Despite the absence of eruptive activity, the Congro system represents one of the most active seismic areas in the archipelago. It shows episodes of volcanic deformation that are indicative of magma ascent, which halted beneath the volcanic system. Two historical eruptions are associated with the PFVS. The 1563 eruption at Pico do Sapateiro was of basaltic composition, whereas that of the 1652 eruption was, for almost two centuries, also considered to be basaltic. This event had, however, a Vulcanian style and involved the production of large amounts of fine ash and the growth of three trachyte domes with associated coulees. It was accurately described in contemporary historical accounts.

The Azores is an archipelago in the Atlantic Ocean composed of nine volcanic islands. The islands are dispersed along a general WNW–ESE trend crossing the Mid-Atlantic Ridge in the area where the Eurasian, African and North American lithospheric plates meet. While Corvo and Flores lie to the west of the Mid-Atlantic Ridge and emerge from a present-day relatively stable geological setting, the other islands are located in an important seismic and volcanically active zone corresponding to the boundary between the Eurasian and African tectonic plates. Volcanic activity has occurred in historic times on the islands of Faial, Pico, Sao Jorge, Sao Miguel and Terceira, and there have also been several recorded submarine eruptions (Weston 1964). Sao Miguel is the largest and most volcanically active island of the archipelago. In the last 5 ka several eruptions took place on the three active central volcanoes – Sete Cidades, Fogo and Furnas – and in the basaltic fissure systems of Picos and Congro. Although details of the first settlement of the island in the fifteenth century are uncertain, there is evidence that Furnas was in eruption at the time the first settlers arrived, some time between 1439 and 1443 (Queiroz et al. 1995). The historian Gaspar Frutuoso (1522–1591?) provided the first geological descriptions of the Azores in his work ‘Saudades da Terra’, giving important details about the earliest volcanic and seismic events. In the sixteenth and seventeenth centuries there were two explosive trachytic eruptions of sub-Plinian scale on Sao Miguel: Fogo 1563 and Furnas 1630. The last eruption on land occurred in the Picos Fissural Volcanic System, in 1652, and involved the extrusion of lava domes. The …

have horizontal footprint sizes of about 10 km near periapsis and a vertical resolution on the order of 100 m.The Fresnel reflectivity data provide a comparison to the emissivity maps, and the rms slope parameter is an indicator of the surface tilts, which contribute to the quasi-specular scattering component.

We describe and compare two types of pahoehoe on Mount Etna: one similar to an Hawaiian inflated flow field consisting of tumuli, lava rises and lava rise pits; and the other a coalescing complex of rootless centres of ephemeral boccas fed from lava tubes and superimposed on an initial 'a'a flow field. The former we define as a primary pahoehoe field that results from relatively slow advance rate of lava over an almost horizontal surface. The latter is a result of an eruption of long enough duration to allow tube formation over a substantial portion of the flow field over steeper slopes; at breaks in slope in the underlying topography centres of ephemeral boccas develop giving rise to low effusion rate pahoehoe lavas covering the original channelled 'a'a surface. These we refer to as secondary pahoehoe flow fields.

We examine the importance of localized volcanism in resurfacing on Venus by analyzing the results of geologic mapping of a 12° × 12° area at the full resolution of Magellan SAR data. Resurfacing due to corona‐, ridge‐, and small volcano‐related volcanism accounts for 27%, 6%, and 10% respectively of the mapped area. Mapping at the resolution of Magellan data, rather than a regional scale, gives corona‐related flow unit areas that can differ individually by almost an order of magnitude, with a total increase of 28%, and more than three times as many identifiable units. A total of 2919 small volcanoes or vents less than 10 km in diameter were identified in the F‐Map, with a mean diameter of 1.59 (s.d. = 1.08) km and densities of up to 36 small volcanoes per 50 km2. Taken together, coronae, ridge eruptions, and small volcanoes probably make a significant contribution to resurfacing on Venus.

1100 (CD-ROM). [9] Stofan, E.R. et al. (2001) Icarus, 152, 75-95. Table 1. Statistics relating to each morphological class in the catalogue. Volcano class Frequency Mean diameter

This paper presents an updated stratigraphical and compositional study of the exposed maria within the Imbrium basin on the Moon. Clementine multispectral data were employed to derive TiO2 and FeO wt% abundance estimates of potentially distinct basaltic flows. Additionally, NASA Lunar Orbiter images were used to estimate flow ages using crater count statistics. Mare Imbrium shows evidence of a complex suite of low to high-Ti basaltic lava units infilling the basin over an 800 million year timescale. More than a third (37%) of identified mare basalts were found to contain 1–3 wt% TiO2. Two other major mare lithological units (representing about 25% of the surface each) show TiO2 values between 3–5 and 7–9 wt%. The dominant fraction (55%) of the sampled maria contain FeO between 16 and 18 wt%, followed by 27% of maria having 18–20 wt% and the remaining 18%, 14–16 wt% FeO. A crater frequency count (for diameters >500 m) shows that in three quarters of the sampled mare crater counts range between 3.5 and 5.5×10−2 per km2, which translates, according to a lunar cratering model chronology, into estimated emplacement ages between ∼3.3 and 2.5 Ga. A compositional convergence trend between the variations of iron and titanium oxides was identified, in particular for materials with TiO2 and FeO content broadly above 5 and 17 wt%, respectively, suggesting a related petrogenesis and evolution. According to these findings, three major periods of mare infill are exposed in the Imbrium basin; despite each period showing a range of basaltic compositions (classified according to their TiO2 content), it is apparent that, at least within these local geological settings, the igneous petrogenesis generally evolved through time towards more TiO2- and FeO-rich melts.

Three major periods of basaltic activity characterize the infill of the basins. Each of these periods was itself punctuated by discrete phases of widespread magma eruptions: three during both the Late Imbrian Epoch and the early Eratosthenian Period and then two in the late Eratosthenian Period. We found the Youngest lavas off the eastern border of the Fra Mauro peninsula and, mantling a much larger area, over most of the central western Nubium basin.Our results place the Nubium/Cognitum basalts in the low-Ti category (1-5 wt% TiO(2)).The data indicate that the majority (similar to 90%) of the mare terrain has iron content between 18 and 22 wt%. In particular, FeO contents tend to concentrate toward two compositional poles, each or similar to 20 wt%, and a much smaller one of similar to 15 wt%. These Values are typical of nearside lunar maria.To complement Our compositional data, we present a census of craters larger than 500 Ill using Orbiter IV images. The result was a crater count average with frequency 5.6 x 10(-2) km(-2), translating into ail inferred mean age of 3300 Ma for the exposed lava flows.By combining lava chemistry with age, we find a possible correlation between the ages of the Most prominent flow units and their estimated titanium content, with younger basalts becoming progressively Ti-richer with time (from 2-3 to 4-5 wt% TiO(2)).

A geological map of the Aglaonice region, Venus, is presented. It is the first published attempt at mapping at the full limit of resolution of the available NASA Magellan mission data. It is primarily based on geological interpretation of a 1:1.5,000,000 scale Synthetic Aperture Radar (SAR) image and altimetry data of the same region. The map covers an area approximately 1.6 x 10(6) km(2) - roughly 1% of the venusian surface - and demonstrates the stratigraphic history revealed by mapping at this scale. We do not observe a regional plains unit, but rather five tectonically and texturally distinct volcanic plains material units. We have found that almost two-thirds of the mapped units originate from identifiable source regions, suggesting that the origin of most units on Venus can be identified if mapped at the resolution of Magellan data.

Abstract— We have studied the mare basalts of Mare Humorum and southeastern Procellarum (30°W–50°W, 0°–40°S). One hundred and nine basaltic units have been identified from differences in their FeO wt% and TiO2 wt% content, and variations in crater densities. Crater counting and reference to isotopically dated Apollo samples have provided an age for 33 major units. Some evidence for three distinct periods of volcanic activity has been found. We found that the large unit in the middle of Mare Humorum is the oldest in the basin. This supports the suggestion that the oldest central unit sank causing the lithosphere to bend and create dykes through which lava flowed to produce the outer units. No evidence of a trend in FeO wt% and TiO2 wt% content against time is found within Mare Humorum. There appears to be no lateral trend of basalts in terms of FeO and TiO2 wt% over the entire area with time. An increase in FeO content with time is found in the 33 major units and there is some evidence for an increase in TiO2 in the same units. A correlation between FeO wt% and TiO2 wt% content is evident when all 109 units are compared. A notable feature of this correlation is a sharp increase in gradient of TiO2 wt% content when the FeO wt% content rises above about 17%.

Introduction: Large volcanoes on Venus are defined as topographically positive, broadly domical structures with lava flow aprons larger than 100 km in diameter surrounding a central volcanic vent [1]. Several surveys have been carried out since the first return of data from the Magellan mission, investigating the location and distribution of volcanoes over the venusian surface [1,2], their structure [3] and variation in size with altitude [4]. The number of volcanoes included in each survey varies from 123 [4] to 167 [2], but to date, only the 1997 survey by Crumpler et al. [2] has been published. Large volcanoes on Venus show a diversity of morphologies with some displaying similar characteristics to large coronae. It is not surprising therefore that many features included in the 1997 database [2] have been classified by others as coronae and included in other relevant corona databases [5-7]. The first task of this new study was re-examine the large volcano population to eliminate the overlap with other catalogues and to try to accurately determine the number of features on the planet. In addition to their earlier surveys, Head et al. [8] and Crumpler et al. [9] described nine fundamental types of large volcano suggested to represent a morphological classification describing individual features and associations with each edifice. Large volcanoes are structurally diverse with many different morphologies. The 9 classes describe a mix of both summit and tectonic characteristics meaning that an individual volcano may not be described accurately by any individual field or fields. The second task was to therefore devise a new classification scheme that would better describe the major morphological features of each edifice. Survey Procedure and Design: The new survey located and classified the population of large volcanoes using full resolution F-Maps and C1-MIDR mosaics. Magellan digital altimetry data was used to take at least two cross-sections over each volcano in order to measure the average diameter of the main edifice to the outer breaks of slope, the maximum altitude and the level of the surrounding plains. The average diameter of the flow apron and the size of any summit caldera were measured directly from F-Maps. Synthetic stereo images were used extensively and were invaluable in determining the morphological properties of each feature. The new classification scheme (Figure 1, Table 1) describes four types of summit structure: Simple, Caldera, Elongate and Multiple Summit, and four types of tectonic structure: Radially fractured summit, Radially fractured flanks, Rift/fracture zone related, Concentric fractures. Any number of these categories can therefore be used to describe each feature. We also analysed the abundance and location of small edifices (shields, cones or domes) on each large volcano and any associated lava flows erupted from vents on its flanks. The small edifice and flank vents category may be noted as multiple entries if different styles and locations of flank activity are observed. Results: The survey located and recorded the morphological characteristics of 134 large volcanoes. Volcano flow aprons were found to have diameters ranging from 100 – 1000 km, and edifice diameters from 50 –