Antonio Cendrero

Volcanic evolution of the island of Tenerife (Canary Islands) in the light of new K-Ar data

New age determinations from Tenerife, together with those previously published (93 in all), provide a fairly comprehensive picture of the volcanic evolution of the island. The oldest volcanic series, with ages starting in the late Miocene, are formed mainly by basalts with some trachytes and phonolites which appear in Anaga, Teno and Roque del Conde massifs. In Anaga (NE), three volcanic cycles occurred: one older than 6.5 Ma, a second one between 6.5 and 4.5 Ma, with a possible gap between 5.4 and 4.8 Ma, and a late cycle around 3.6 Ma. In Teno (NW), after some undated units, the activity took place between 6.7 and 4.5 Ma, with two main series separated by a possible pause between 6.2 and 5.6 Ma. In the zone of Roque del Conde (S), the ages are scattered between 11.6 and 3.5 Ma. Between 3.3 and 1.9 Ma, the whole island underwent a period of volcanic quiescence and erosion. The large Canadas volcano, made up of basalts, trachytes and phonolites, was built essentially between 1.9 and 0.2 Ma. To the NE of this central volcano, linking it with Anaga, is a chain of basaltic emission centers, with a peak of activity around 0.8 Ma. The Canadas Caldera had several collapse phases, associated with large ignimbrite emissions. There were, at least, an older phase more than 1 Ma old, on the western part of the volcano, and a younger one, less than 0.6 Ma old, in the eastern side. The two large “valleys” of Guimar and la Orotava were formed by large landslides less than 0.8 Ma ago, and probably before 0.6 Ma ago. The present Canadas caldera was formed by another landslide, less than 0.2 Ma ago. This caldera was later filled by the huge Teide volcano, which has been active even in historic times. During the same period a series of small volcanoes erupted at scattered locations throughout the island. The average eruptive rate in Tenerife was 0.3 km3/ka, with relatively small variations for the different eruptive periods. This island and La Gomera represent a model of growth by discontinuous pulses of volcanic activity, separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years. The neighbouring Gran Canaria, on the other hand, had an initial, rapid “shield-building phase” during which more than 90% of the island was built, and a series of smaller pulses at a much later period. A comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation. Several gaps are present on each island, overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the Canaries.

Evolution of the eastern volcanic ridge of the Canary Islands based on new KAr data

The results of 64 new KAr age determinations, together with 32 previously published ages, show that after a period of erosion of the basal complex, Miocene volcanic activity started around 20 Ma in Fuerteventura and 15 Ma in Lanzarote, forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs. This series includes five separate volcanic edifices, each one with its own eruptive history. In Fuerteventura, several Miocene eruptive cycles have been identified: in the central edifice one around 20–17 Ma, followed by two others centred around 15 and 13 Ma; in the southern edifice the maximum of activity took place around 16–14 Ma, whereas in the northern one the main activity occurred between 14 and 12 Ma. In Lanzarote a first cycle of activity took place in the southern edifice between 15.5 and 14.1 Ma, followed by another between 13.6 and 12.3 Ma. In the northern edifice three pulses occurred: 10.2–8.3, 6.6–5.3 and 3.9–3.8 Ma. An important temporal gap, greater in Fuerteventura than in Lanzarote, separates Series I from the Plio-Quaternary Series II, III and IV, formed by multi-vent basaltic emissions. In Fuerteventura the following eruptive cycles have been identified: 5, 2.9–2.4, 1.8–1.7, 0.8–0.4 and <0.1 Ma. In Lanzarote, the activity was fairly continuous from 2.7 Ma to historic times, with a maximum in the Lower Pleistocene. Eruptive rates in the Series I edifices were on the average 0.1–0.01 km3/ka, comparable but slightly smaller than in similar edifices in Tenerife and La Gomera, but much lower than in Gran Canaria. Average post-Miocene eruptive rates were about 0.013–0.027 km3/ka in Lanzarote and 0.003–0.007 km3/ka in Fuerteventura. All these volcanic edifices show a similar general sequence (fissural eruptions, erosion, multi-vent volcanism), repeated at different periods in different parts of the eastern islands of the Canaries. The model of growth of the Series I edifices is comparable to those in Tenerife and La Gomera: long periods of activity, sometimes greater than 6 m.y., with pulses separated by gaps. However, salic and intermediate differentiates, frequent in Tenerife and La Gomera, are very scarce in these islands. The Fuerteventura-Lanzarote ridge shows a decrease in volcanic activity with time, and also a certain SSW-NNE polarity in the temporal development of volcanism.

Constructive and destructive episodes in the building of a young Oceanic Island, La Palma, Canary Islands, and genesis of the Caldera de Taburiente

The results of new field observations, 23 new KAr determinations and sixteen previously published determinations provide the basis for the reconstruction of the subaerial volcanic history of the island of La Palma, after the seamount activity represented by the materials of the Basal Complex. An eruptive phase between 2.0 and 1.3 Ma formed a large shield. A period of volcanic quiescence followed, until around 1 Ma, during which a large lateral collapse partly destroyed the former edifice. Between 1.05 and 0.7 Ma, activity was renewed in the shield and a N-S ridge was built in the southern part of the island. Around 0.7 Ma, two new large lateral collapses affected the western part of both edifices, and they were followed by eruptions between 0.71 and 0.65 Ma which built a new edifice that partly filled the depressions thus created. The Caldera de Taburiente constitutes the eroded remnants of the depression formed in the northern shield. From 0.65 Ma to present, activity has been restricted to the N-S ridge, which has continued to grow southwards. There was a general N-S migration of volcanic activity with time, but in the shield the trend was northwest to southeast. Eruptive rates seem to have been fairly constant during the different eruptive phases considered, between 0.15 and 0.37 km3/ka. A very similar succession of constructive and destructive episodes has been obtained for the neighboring island of Hierro, but in this case the activity started around 0.8 Ma and eruptive rates were about 0.5 km3/ka.

Is Prediction of Future Landslides Possible with a GIS

This contribution explores a strategy for landslide hazard zonation inwhich layers of spatial data are used to represent typical settings inwhich given dynamic types of landslides are likely to occur. Theconcepts of assessment and prediction are defined to focus on therepresentation of future hazardous events and in particular on themyths that often provide obstacles in the application of quantitativemethods. The prediction rate curves for different applications describethe support provided by the different data layers in experiments inwhich the typical setting of hazardous events is approximated bystatistically integrating the spatial information.

K-Ar chronology of the volcanic eruptions in the Canarian archipelago: Island of La Gomera

A geochronological study of the Island of La Gomera (Canaries) has been carried out by the K-Ar method. The 26 new ages obtained, together with the 17 previous determinations, show that above the main unconformity of the island, separating the « basal complex » from the later volcanic series, there is a unit of « lower old basalts » more than 10 m.y. old. Polymictic volcanic breccias were emplaced between 10 and 9 m.y. ago. The « upper old basalts » above them were formed between 9 and 6 m.y. ago, with a peak of activity around 7 m.y. After a period of erosion (6-5 m.y.), a thick series of « young basalts » associated with trachytic and phonolitic domes and flows, were rapidly emplaced between 4.5-4 m.y. ago. Finally, local basaltic activity took place 2.8 m.y. ago. The age of the basal complex is not well known, although three ages (14.6, 15.5 and 19.3 m.y.) have been obtained for some alkaline intrusives which seem to represent the youngest events in the complex.

A methodological approach for the analysis of the temporal occurrence and triggering factors of landslides

The temporal occurrence of landslides in an area of the Cantabrian Range during the last 120,000 years is analyzed. An initial relative chronology was established on the basis of aging degree and spatial relationships between landslides and glacial and fluvial features. Ten landslide classes were thus identified and their chronological limits defined on the basis of 19 14C age determinations on fluvial and glacial deposits (including eight new ones). The chronology was tested with 14C dates obtained on landslide deposits, 10 from previous work and nine new ones. The chronological classes identified were compared with existing climate models for the region; the type and spatial distribution of landslides in each class were also analyzed. The approach made it possible to identify periods during which landslides were triggered mainly by channel incision, seismic activity and rainfall increase. Human activity played a significant role after 5000 BP and especially in the last few centuries. Mobilization of materials by slope movements has increased in the region by a factor of 10, compared to pre-Neolithic rates.

Late Quaternary climate changes and mass movement frequency and magnitude in the Cantabrian region, Spain

Abstract A reconstruction of Holocene climate changes in the Cantabrian region, based on pollen records and on complementary sedimentological, geomorphological, archaeological and oxygen isotope data is presented. The climate model thus proposed has been tested obtaining new sediment records and dating sixteen additional samples. The distribution and relative chronology of slope movements in a study area are presented. Eleven new dates have been obtained for landslide and flow deposits and a correlation between occurrence of mass movements and climate changes is proposed. The volumes affected by these movements in different periods have been calculated and rates have been obtained. Estimates of denudation rates in the study area during the Quaternary and in the Holocene are also presented and compared with the former.

The contribution of landslides to landscape evolution in Europe

Abstract After a review of some analytical and conceptual models of slope evolution with the intervention of mass movements, a proposal is made of a series of indicators for the quantitative description of landscape evolution in relation to mass movements. The ratio between landslide mobilisation rate and downwearing rate is proposed as a quantitative measure of the significance of mass movements in landscape evolution. A subdivision of European regions with respect to their susceptibility to landsliding as a process of landscape evolution is proposed. The role of landsliding for the evolution of landscape in some regions is described. The data presented show that landsliding may in some cases be the main contributor to landscape change.

Geomorphological indicators for environmental impact assessment: consumable and non-consumable geomorphological resources

Abstract A methodological approach is proposed for the incorporation of geomorphological features into environmental impact assessments. A series of quantitative indicators and indices are proposed, so that impacts on both consumable and non-consumable geomorphological resources can be objectively determined. Impact can thus be expressed by means of magnitudes with specific dimensions, or at least as fractions of a maximum theoretical value. Procedures for the integration of individual indices into general impact assessments are also proposed.

Mapping and evaluation of coastal areas for planning

Abstract A brief review of concepts about analysis and assessment of the earths surface for planning is presented. The relationships between planning levels, scales and types of maps, as well as the methodological approaches in the preparation of maps for planning are discussed. The application of those concepts to mapping and evaluation of coastal areas for different planning problems at various scales, is illustrated by means of a series of examples from different parts of the world. It is concluded that existing mapping and assessment techniques constitute flexible instruments for the description, diagnosis and zoning of coastal areas for very diverse planning problems.