Hydrogeology of Volcanic Highlands Affects Prioritization of Land Management Practices

Volcanic highlands supply water to 40% of the world’s population. Soil degradation threatens this water supply. Studies on geohydrology that affect the effectiveness of land and water management (LWM) practices in reducing soil degradations are limited. To aid in the effectiveness of LWM practices, we conducted a field experiment in the Gomit watershed in the semihumid Ethiopian Highlands on the interaction of hydrogeology and LWM practices. We found that in a watershed with strongly faulted tertiary basalt, 30% of the rainfall was drained through faults to another basin. Consequently, the discharge at the outlet was less than half of that of other watersheds with quaternary basalts. Despite the high sediment concentration, i.e., around 15 g L−1, in the Gomit watershed, the sediment yield of less than 4 Mg ha−1 a−1 was below average for the agricultural watershed in Ethiopia because of the low runoff response. While some faults facilitated drainage, others acted as a barrier. Groundwater stored behind the barriers was used as a municipal potable water source. Since the effectiveness of LWM practices depends on the amount of erosion that can be prevented, considerations of country-wide prioritizing of investments in land and water management practices should include the geology of the watersheds.

The origin of glacial alpine landscape in Tröllaskagi Peninsula (North Iceland)

The Tröllaskagi peninsula is located in north central Iceland, between meridians 19º30’W and 18º10’W , limited by Skagafjödur fiord to the west and the Eyjafjödur fiord to the east, jutting out into the North Atlantic to latitude 66º12’N and linked to the central Icelandic highlands to the south. The peninsula is a Tertiary basalt plateau topped by flat summits with altitudes of 1000-1500 m, intensely dissected by the drainage network. The aim of this present study is to synthesize the recent advances in our understanding of the landscape and its dynamics in the Tröllaskagi peninsula and find the origin of its significant difference from the rest of Iceland. Results of the most recent research suggest the situation of Tröllaskagi as ice-free, delimited by the two great glacial outlets flowing down from the Icelandic Ice Sheet through the Skagafjödur and Eyjafjödur fiords, from at least the Oldest Dryas to the end of the Early Preboreal. Inland in Tröllaskagi, the glaciers formed in the north-facing cirques without losing their alpine characteristics during the Late Pleistocene and Holocene. The advances of these glaciers during the Oldest, Older and Youngest Dryas and the Early Preboreal were only a few hundred metres greater than the most important advance in the second half of the Holocene, during the Little Ice Age. Only a few of these glaciers remained debris-free and are sensitive to the minor climate oscillations. The rest, due to the important geomorphological activity on their walls, developed into debris-covered and rock glaciers and lost this significant dynamism.

The dykes and sills of the Early Tertiary Faeroe Island basalt plateau

ABSTRACTThe Early Tertiary basalt plateau of the Faeroe Islands is cut by dykes and sills. Chemical analyses show a two-fold division of the intrusive rocks into a group of low-TiO2 (0·73–1·93%), MORB-type tholeiitic basalts and a group of high-TiO2 (2·09–3·90%) tholeiitic basalts. The low-TiO2 group comprises about 15% picrites and olivine-phyric basalts and 85% plagioclase-phyric basalts, and shows a chemical range largely explicable in terms of low-pressure fractional crystallisation of olivine ± plagioclase ± clinopyroxene. The high-TiO2 group is strongly dominated by plagioclase-phyric basalts with only few olivine-phyric compositions. The chemical trends are less regular than those formed by the low-TiO2 basalt dykes and a number of subgroups may be identified on the basis of bulk rock chemistry. Dykes belonging to a specific subgroup were probably fed from the same magma chamber.Petrographically and chemically the dykes and sills are clearly related to the upper 2·5 km of the lava sequence. Field evidence suggests that some of the dykes were contemporaneous with the exposed lavas, while other dykes and the sills were intruded in response to a slight doming of the plateau during the final stages of volcanic activity. Our investigations demonstrate that high-TiO2 and low-TiO2 magmas were both emplaced until the very end of magmatism, with the latter being mainly concentrated in the northern part of the archipelago. We briefly sketch a possible relationship between the supposed NE-Atlantic mantle plume, the distribution of the various magma types and the location of the Early Tertiary continental splitting zone north of the islands.

Indication of migrated hydrocarbons in Tertiary volcanic rocks from western Nûgssuaq, central West Greenland

During the field season of 1985 the author briefly visited Marrait kitdlit at the southern part of the entrance to the Itivdle valley on Nugssuaq (fig. 1). This paper briefly describes the occurrence of possibly migrated organic material in a vein in Tertiary basalt and presents it in a regional context.

Cupriferous pseudobrookite in a Tertiary basalt from the Faeroe Islands

Forty-five electron microprobe analyses have been carried out on pseudobrookite occurring in a basalt from the Faeroe Islands. It is shown that pseudobrookite formed after ilmenite contains between 1 and 3% CuO, whereas pseudobrookite formed after titanomagnetite does not contain Cu. This difference in Cu content is not inherited from the original ilmenite and titanomagnetite, but arises during the formation of the pseudobrookite. The pseudobrookite in this basalt, regardless of whether it formed from ilmenite or from titanomagne­tite, is richer in Ti than in the formula Fe2TiO5, the surplus Ti4+ being balanced by the presence of divalent ions such as Mg, Mn, Fe and Cu. Mg and Cu dominate in pseudobrookite after ilmenite, Fe and Mg domi­nate in pseudobrookite after titanomagnetite. Pseudobrookite after titanomagnetite is richer in Ti than pseudobrookite after ilmenite. The pseudobrookite is not homogeneous. Both pseudobrookite formed from ilmenite and that formed from titanomagnetite contain small blebs of hematite and rutile, and furthermore pseudobrookite after ti­tanomagnetite is intergrown with larger coherent areas of hematite. The hematite blebs in pseudobrookite after ilmenite can contain up to more than 5% CuO, but there is virtually no copper in either type of he­matite in the pseudobrookite after titanomagnetite, nor do the rutile blebs contain copper.