This paper deals with a study of the physical and mechanical characteristics of volcanic tuff and ignimbrite from six quarries located at different areas in the western part of Yemen (Manakha, Jahran, Bakhran, Dar Al-Hanash, Abaser and Soraifa). In the region, volcanic tuffs and ignimbrite are locally known by their location names and have been used as solid masonry and cladding stones. All the investigated pyroclastic rocks belong to the Tertiary volcanic. The standard physical and mechanical tests (void ratio, porosity, density, specific gravity, water absorption, uniaxial compressive strength and tensile strength) were carried out on the tuff and ignimbrite samples collected from different parts of the region.Laboratory tests revealed that the void ratio average values range between 0.12 and 0.37, the porosity ranges between 10.57 and 27.12%, the dry density ranges between 1.66 and 2.25 gm/cm 3 , specific gravity ranges from 1.45 to 1.94, and water absorption ranges from 4.69 to 16.39%. The measured uniaxial compressive strength values range from 24 to 68 MPa, and the tensile strength values range between 4 and 10 MPa. These tuffs and ignimbrites generally are light green, gray, beige, or yellowish in color. With these colors they are favoured for building, coating and decorative stone. This paper concludes that the studied stones have acceptable to good properties as dimension stone. Jahrani and Manakhi tuffs are the best quality, whereas Hanashi ignimbrite is of poorer quality.

This paper presents the results of investigations on the potential industrial utilization of scoria, collected from Al-Jaif quarry, NW Sana'a, Yemen as a cement additive. Scoria was chosen as a cement additive material due to its availability and low cost from the Sana’a-Amran volcanic field in Yemen. The chemical composition of scoria was determined by X-ray fluorescence (XRF). The studied scoria is mainly composed of volcanic glass with a few zeolites (e.g. clinoptilolite) as revealed from petrographic investigation and X-ray diffraction (XRD). The scoria was added to the clinker in the range of 2, 4, 6, 8, 10 and 12% by weight. The fineness, surface area, water demand, setting time and compressive strength were conducted on scoria blended cement. According to experimental results, the high volcanic glass allows the addition of up to 12% scoria to the clinker to maintain a good potential of manufacturing blended cement. The results satisfy the European Standard EN requirements and confirm the viability of using scoria as a cement additive.

The distribution of inorganic nutrient salts (ammonium, nitrite, nitrate, phosphate and silicate) and phytoplankton chlorophyll-a (Chl-a) were investigated in the Gulf of Aden and along the Arabian Sea coast, Yemen. Seventy two surface water samples were collected during cruises in August 2014 and January 2015. The sampled area extends from As Suqayyah in the west of the Gulf of Aden to Hawf in the east. The study showed that the average values of nutrient salts (μg/l) in waters collected in August 2014 and January 2015 were 0.83 and 0.60, 10.98 and 10.03, 16.41 and 14.73, 10.36 and 8.76, and 29.72 and 22.67 for ammonium, nitrite, nitrate, phosphate and silicate, respectively. The average values of Chl-a (mg/m3) in August 2014 and January 2015 were 0.26 and 0.21, respectively. The results showed that nitrate levels were very high but those of ammonium very low. This may reflect the oxidation of ammonia to nitrite and then nitrate, leading to a very favorable ratio of ammonia and nitrate values. The low levels of dissolved inorganic nitrogen (DIN)compounds, (ammonium, nitrite, and nitrate), phosphate, silicate and Chl-a indicated that the southern coast of Yemen is not in a eutrophic condition. The highest nutrient salt values were in the eastern part of the study area, and may have resulted from water originating from the Indian Ocean and upwelling. Statistical analysis to seek correlations between nutrient salts and Chl-a show very good to excellent correlation, which may be due toconstant coastal environment.

Yemen has an appreciable amount of scoria aggregate, but a very small proportion is used as cement additions by local cement factories in blended Portland cement production and very rarely as aggregates into the production of lightweight blocks. Scoria aggregates are widespread in and around the volcanic cones present in Sana'a-Amran volcanic field. This paper presents the results of evaluation of engineering properties of natural scoria, collected from Al-Haweri volcanic cone, NW Sana'a, Yemen as lightweight aggregate. Chemical composition and petrographical characteristics of scoria were determined. The physical properties of the scoria aggregate such as flakiness and elongation indices, specific gravity, water absorption, unit weight, clay lumps and materials finer than 75µm gave acceptable results, but the gradation indicated it would need to be processed before use in a mix. The porosity is high, 60.79%. The Los Angeles abrasion and the aggregate crushing values are higher than the limits of ASTM and BS specification, but the aggregate impact value is close to the upper limits of BS specification. The soundness is within the ASTM specification limits. Therefore, it can be concluded that scoria aggregates can be used to produce structural concrete and masonry blocks with a relatively low density. The scoria aggregates satisfied the requirements of lightweight aggregate for structural concrete and masonry blocks purposes.

Mantle xenoliths in basalt and tuffaceous volcanoes at the Shawran Crater, Bir Ali area, along the southeastern coast of Yemen are very useful to elucidate the nature of the lithospheric mantle beneath the area, which is characterised by rift tectonic activity and volcanism during the Pliocene–Quaternary period. The studied mantle xenoliths are mostly fresh showing igneous textures and composed mainly of spinel harzburgite and lherzolite, in addition to, less prominently, dunite. Spinel harzburgite and lherzolite are holocrystalline with abundant granular spinel that occurs interstitially to olivine, orthopyroxene, and clinopyroxene. The forsterite content of olivine is high (Fo89–91) attesting its mantle residue origin. Orthopyroxene is mostly enstatite (En89–92), and clinopyroxene is diopsidic in composition (Wo24–47En49–52 Fs2–4). Spinel Cr# (Cr/(Cr + Al) is distinguishable between lherzolites (Cr# = 0.12–0.15) and harzburgites (Cr# = 0.20–0.31). The calculated oxygen fugacity (logfO2) of the studied lherzolites (−1.76 to −0.20) and harzburgite (−2.7 to −0.98) xenoliths, and their equilibration temperature (~ 820–1,145°C) at about 1.5 GPa pressure, along with their mineralogical and petrological characteristics, are all support their evolution by mixed enrichment from a fertile and depleted asthenospheric mantle beneath the Bir Ali area.