جستجوی مقالات مرتبط با کلیدواژه « Pyroclastic » در نشریات گروه « زمین شناسی »

Four small scoria cones in the western outskirts of Al Madinah City, the Kingdom of Saudi Arabia, form a distinct young volcanic landmark. These volcanoes, despite their very small size, provide one of the most fundamental sources of information about the early eruption mechanism of rising mafic magma in the context of an active volcanic field located next to a city with over one million people. An initial study of the area in 2012–2013 confirmed that these sites had a significant phreatomagmatic phase in their opening stage, leaving behind characteristic pyroclastic successions likely to be covered by subsequent eruptive products. The fact that in this location, unequivocal evidence emerged to show that explosive magma-water interaction driven eruptions occurred in the largely magmatic (“dry”) explosive style, and volcanic field evolution confirmed that this site has high geoheritage value. Since the first research was 20 years ago, a restudy of the present-day condition of the scoria cone was conducted. Applying satellite imagery, remote sensing and direct site visit, we find that about a third of the cone surface area has been modified and at least a quarter of its volume has vanished. Further excavation for cone material however opened the entire western side of the cone, exposing a nearly 5-m-thick succession of accidental lithic pyroclast-dominated lapilli tuff and tuff breccia, confirming that this location had a significant phreatomagmatic phase in its opening eruptions. This location shows graphically the need for geoconservation to preserve such sites that are potentially the only, or best locations to show the potential eruptive styles and scenarios of future eruptions if they occur in similar environments.

The area under study is located in northern Iran at 36° 47’ latitude and 50° 12'' longitude. It is 15 km from southeastern Deylaman. Petrological study of the kaolin mother rock (Eocene age) under a microscope shows that the rock is vitric tuff with a rhyolitic combination containing falcated and embowed glass shards. The thickness of the layer is approximately 20 meters, formed unconformably on a layer of conglomerate with a thickness of approximately 400 meters. Because the shards have a falcated and embowed shape and the percentage of reworked material within them is low, the tuff can be considered a type of pyroclastic airfall. The results of the study prove that ash falling occurred in the water. The stratabound layers show syngenetic deposits to be the main factor in the formation of the clay, bound as a result of alterations.