128 research outputs found
Repeat photography for studies on environmental changes and geomorphological processes in northern Ethiopia
Dryland vulnerability and ephemeral streams response to global change : example from the Raya graben, northern Ethiopia
Robertsite: un nuovo fosfato di grotta scoperto nella Tagusan Cave(Palawan – Filippine)
La Tagusan Cave è una cavità di attraversamento che si
apre nel massiccio calcareo che ospita la famosa grotta
Porto Pricesa Undergroud River nell’isola di Palawan (Filippine).
Al suo interno sono state rivenute alcune pisoliti di
colore nero che sono state campionate e quindi analizzate
dal punto di vista tessiturale e mineralogico. Le analisi al
microscopio ottico hanno evidenziato come queste pisoliti
non siano omogenee ma siano costituite da un nucleo
spesso costituito da una aggregazione di ooliti giallastre
su cui si è depositata un’alternanza di sottili strati picei e
rossastri. L’analisi mineralogica delle bande di accrescimento
ha evidenziato la presenza, oltre a fosfati di calcio
praticamente amorfi, di robertsite [Ca2Mn3(PO4)3O2•3H2O],
un fosfato che non era mai stato segnalato prima in grotta.
Sempre in queste bande, sulla base degli esami röntgenografici
e delle analisi EDAX, si è ipotizzata la presenza
anche di janggunite [Mn5-x(Mn,Fe)1-xO8(OH)6], di cui però
non si è raggiunta la certezza. Nel nucleo invece è stato
possibile identificare la presenza di strengite-Al [(Fe,Al)
(PO4) •2H2O].
La genesi di queste particolari pisoliti è da mettere in relazione
con i processi di digestione biologica del guano che
all’inizio della loro evoluzione doveva essere abbondante
nelle acque che fluivano nella grotta, mentre ora, l’apporto
idrico è molto più scarso e proviene da stillicidi che hanno
scarsa relazione con il guano medesimo.The Tagusan cave is a hydrologic tunnel in the karst massif
hosting the famous Porto Pricesa Underground River
in the Palawan Island (Philippines). It hosts some speleothems
among which are worth to be cited some black
pearls. A few of them have been sampled to be analyzed
from the textural and mineralogical point of view.
The optical microscope analyses put in evidence that the
inner structure of these pearls is un-homogeneous, consisting
of a nucleus often made by aggregates of yellowish
oolites covered by alternated pitch-dark and reddish layers.
The mineralogical analyses find out that the growing
layers consist of amorphous phosphates and robertsite
[Ca6Mn9(PO4)9O6(H2O)6•3(H2O)], which is here recognized,
for the first time, as cave mineral. The x-ray patterns and
the EDAX analyses suggest also the presence of janggunite
[Mn5-x(Mn,Fe)1-xO8(OH)6], but no definitive proof of it
has been yet achieved. Strengite-Al, [(Fe,Al)PO4•2H2O)] is
the main component of the yellowish oolites in the pearl’s
nucleus.
The genesis of these peculiar cave pearls is strictly related
to the biological digestion processes of the guano deposits.
In fact at the beginning of their evolution the feeding
water directly crossed guano deposits, while now the
few still active dripping practically has no contact with it
A new early warning drought index for Ethiopia
Abstract
This study investigates the occurrence of droughts in the Dire Dawa area of eastern Ethiopia. A new index based on the rainfall delay (Rd) with respect to the expected onset (and traditional) seeding time and other indices, i.e., the aridity index and the Z-score, alternatives to the Standard Precipitation Index (SPI), are used to test the validity of the new Rd index in identifying severe droughts extending back to 1955. Although only data of rain gauges located in the district of Dire Dawa were used, they proved, albeit with different accuracies, able to identify nation-wide droughts
Human impact on sediment yield and channel dynamics in the Arno River basin (central Italy)
Abstract The Arno River basin has been subjected to human disturbance and modification since Roman times. Until 1800 the main aims of such modifications were to provide flood protection for adjacent towns and to acquire new land for cultivation. During the nineteenth and twentieth centuries the Arno River basin underwent additional significant modifications, including reforestation, upland sediment retention, a huge increase in bed material exploitation, the construction of two reservoirs and bank protection works. The combination of reduced sediment supply and increased sediment transport capacity resulted in extensive streambed degradation that threatened the stability of several bridges and other structures. In order to identify the dominant channel changes and the relationship between vertical and lateral adjustments, a comparison of many cross-sections spanning a period of more than one century was made. The relative importance of human activity in causing channel adjustment compared with natural changes, such as the progressively lower runoff recorded during the last seven decades, is discussed
Spatio-temporal sedimentation patterns in beaver ponds along the Chevral river, Ardennes, Belgium
Impact of beaver ponds on river discharge and sediment deposition along the Chevral River, Ardennes, Belgium
Stream dynamics related land changes and implications to land management in a marginal graben along the Northern Ethiopian Rift Valley
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