Haruj

Haruj (Template:Langx, also known as HaroudjTemplate:Sfn) is a large volcanic field spread across in central Libya. It is one of several volcanic fields in Libya along with Tibesti, and its origin has been attributed to the effects of geologic lineaments in the crust.

It contains about 150 volcanoes, including numerous basaltic scoria cones and about 30 small shield volcanoes, along with craters and lava flows. Most of the field is covered by lava flows that originated in fissure vents; the rest of the flows originated within small shield volcanoes, stratovolcanoes and scoria cones. Some of these vents have large craters. Volcanism in Haruj blocked ancient rivers and led to the formation of Lake Megafezzan.

Volcanic activity in Haruj began about 6 million years ago and continued into the late Pleistocene. A number of individual lava flow generations were emplaced in the Haruj volcanic field, the most recent ones in the Holocene 2,310 ± 810 years ago. There are reports of solfataric activity.

Geography and geomorphology

Haruj lies in central LibyaTemplate:Sfn and its highest summit is Garet es Sebaa, above sea level. It was first identified as volcanic in 1797 and had a reputation for being challenging to accessTemplate:Sfn owing to its remoteness and the hostile terrain and was thus avoided by explorers.Template:Sfn The town of Al-Foqaha is located northwest of the margin of Haruj,Template:Sfn and oil fields can be found north of the field.

The field is a low-relief expanse of volcanic rocks occasionally interrupted by volcanic conesTemplate:Sfn which covers an area of Template:Sfn-, making it the largest of the basaltic volcanic fields of Northern Africa. Its eruption products reach a thickness of in the central sectorTemplate:Sfn in the form of stackedTemplate:Sfn lava flows,Template:Sfn the total volume of volcanic rocks has been estimated to be about .Template:Sfn The Al Haruj al Aswad ("Black mountain") field in the northern part of Haruj and Al Haruj al Abyad ("White mountain") south are considered to be subdivisions of the main Haruj volcanic fieldTemplate:Sfn with Aswad covering a much larger surface than Abyad,Template:Sfn or even two separate volcanoesTemplate:Sfn that started overlapping each other during the Pliocene.Template:Sfn

Older lava flows have been completely flattened by erosion, while more recent ones still display fresh surface structuresTemplate:Sfn and some of the recent flows flowed out of the mountains into the surrounding landscapes.Template:Sfn Surface features include both aa lava traits and pahoehoe lava traits,Template:Sfn and there are lava channels,Template:Sfn skylights and tumuli.Template:Sfn The volcanic rocks are usually not very thick, their thickness decreasing from in the central sector to only a few metres at the margins,Template:Sfn and thus the underlying sedimentary rocks often crop out between lava flows.Template:Sfn

Vents

Most of the lavas appear to originate in fissure vents,Template:Sfn under the influence of dykesTemplate:Sfn and tectonic faults. In addition, there are about 150 individual volcanic massifs and more smaller volcanic cones, many of which form rows of cones and sometimes have large cratersTemplate:Sfn and which occur mainly in the Al Haruj al Abyad part of Haruj.Template:Sfn Craters range from deep pits with steep inward walls to wide and shallow depressions,Template:Sfn and the craters are often filled with material eroded from their flanks.Template:Sfn Phreatomagmatic processes triggered by groundwater interacting with rising magma have generated some of these large craters, while others formed when lava lakesTemplate:Sfn drained through gaps in their rims.Template:Sfn Like the fissure vents, the position of individual cones and massifs is controlled by ground fractures and often reflect the activity of dykes,Template:Sfn and some cones appear to have been active more than once.Template:Sfn

There are about 30 shield volcanoes with heights of , such as Um el Garanigh and Um el Glaa, and smaller stratovolcanoes with heights of Template:Sfn such as Garet el Graabia in the field; some stratovolcanoes are located on shield volcanoes.Template:Sfn Scoria cones consist of lapilli, lava bombs and tuffs,Template:Sfn with pyroclastic material exposed in the crater rims.Template:Sfn The formation of scoria cones was at times accompanied by subplinian eruptions that deposited tephra over large areas.Template:Sfn

Hydrology

Small depressions in the lava fields contain clay-filled ephemeral lakes, and a drainage network has developed in parts of the fieldTemplate:Sfn which sometimes carries water during spring.Template:Sfn Some craters show evidence of former and ephemeral crater lakes.Template:Sfn Beginning in the Messinian, growth of the volcanic field blocked pre-existing drainages, forming a closed basin southwest of HarujTemplate:Sfn that was filled by Lake Megafezzan, although it is possible that the lake at times overflowed across the volcanic field.Template:Sfn

Geology

Haruj is not located close to a plate boundary. Rather, volcanism there and in other African volcanic fields which are located on top of crustal domes, has been explained by the presence of hotspots,Template:Sfn but in the case of Haruj a mantle plume is considered unlikely.Template:Sfn Alternatively, volcanism at Haruj may be the consequence of the intersection of three geological structures of Paleozoic to Tertiary ageTemplate:Sfn and melting of the shallow mantle,Template:Sfn or of the rifting process of the Sirte Basin.Template:Sfn Wau an Namus is sometimes considered to be part of the field,Template:Sfn other volcanic fields in Libya are Gharyan, Gabal as Sawada, Gabal Nuqay and TibestiTemplate:Sfn some of which belong to a long line known as the Tibesti lineament.Template:Sfn Volcanism in general has shifted southward over time,Template:Sfn although more recent radiometric dating efforts indicate that volcanic activity in the fields was more contemporaneous than thought.Template:Sfn

The volcanic field overlies a Tertiary surface between the Paleozoic to Tertiary Murzuk and Sirte Basins;Template:Sfn the Syrte embayment during the Miocene reached into the Haruj mountains.Template:Sfn A number of swells and tectonic lineaments, some of which are located at the margins between geologic blocks, characterize the basement beneath Haruj and have influenced the location of volcanic vents.Template:Sfn The basement is of Eocene to Oligocene age and consists of conglomerate, dolomite, limestone, marl and sandstone,Template:Sfn known as the Bishimah Formation;Template:Sfn where the lavas of Haruj are thinner, it often forms white outcrops.Template:Sfn

Composition

Eruptions at Haruj have produced relatively uniform volcanic rocks consisting of olivine basaltTemplate:Sfn that forms a tholeiitic to alkali basalt suite;Template:Sfn the alkaline basalts were originally interpreted as hawaiite.Template:Sfn Minerals contained within the volcanic rocks include clinopyroxene, olivine, plagioclase and titanomagnetite, with secondary calcite, iddingsite, serpentine and zeolite.Template:Sfn Based on compositional differences, the volcanic rocks have been subdivided into an older and a younger family.Template:Sfn

In some places in the northern Haruj a modified basalt has been found, which is dense and whose olivine has been transformed into iddingsite.Template:Sfn The lavas contain inclusions of lithic material as well as pyroxene and peridotite lherzolite.Template:Sfn Phonolite and trachyte are absent.Template:Sfn The magmas ultimately originated at depths of .Template:Sfn

Eruption history

The oldest volcanic rocks in Haruj appear to be not older than Pliocene, although the presence of buried Miocene age flows in the northern sector of the field has been suggested.Template:Sfn The oldest eruptions have been dated to be either 6.4 million years oldTemplate:Sfn or of Late Pliocene ageTemplate:Sfn and activity was originally thought to have continued to the Late Pleistocene;Template:Sfn Wau an Namus may be 200,000 years old.Template:Sfn Most of the field is younger than 2.2 million years ago and output appears to have decreased over time.Template:Sfn Some eruptions may have been large enough to impact the regional environment.Template:Sfn

Volcanic activity in Haruj has been subdivided into a variable number of phases, including one six generation scheme and a four class scheme based on composition and age.Template:Sfn Radiometric dating has yielded a Late Pliocene age for the oldest lava flow generation,Template:Sfn and ages established by paleomagnetic analysis are coherent with those established on the basis of the degree of erosion of flows.Template:Sfn The oldest generation of lava flows makes up the bulk of the field and has been completely flattened by erosion, save some exceptions.Template:Sfn Pre-existent valleys have influenced the emplacement of the oldest generation lava flows, and also that of the second oldest albeit to a lesser degree.Template:Sfn

An intermediary lava flow generation has most likely been emplaced during the Pleistocene.Template:Sfn Lava flows of intermediate age crop out mainly in the central part of the Haruj mountains and have recognizable flow forms. Their surfaces have lost the original microstructures and are often covered with large blocks.Template:Sfn

The youngest generations of lava flows are little eroded, although they can still be subdivided into an older generation that has lost most of its surface features and a younger generation with fresh surfaces. This younger generation has been inferred to post-date a wet period that commenced 4000 BCETemplate:Sfn and the Neolithic; the youngest dates obtained on lava flows are 2,310 ± 810 years BP.Template:Sfn Prior to the discovery of these youngest dates, volcanic activity was believed to have ended 100,000 years ago.Template:Sfn

Haruj may still be active,Template:Sfn considering the presence of partial melt at the bottom of the crust and the seismic activity of the Hun Graben.Template:Sfn Some toponyms such as Garet Kibrit ("sulfur mountain") refer to volcanic activity, and solfataric activity has been reported in the field.Template:Sfn

Climate, animal life and vegetation

Temperatures in Haruj fluctuate between in January and July respectively. The volcanic field lies within an arid climate with annual precipitation of ,Template:Sfn but the higher parts of the mountains are wetter than the surroundings.Template:Sfn 6,000 years ago, the region was much wetter and the extent of the Sahara was approximately half the size of today.Template:Sfn

Vegetation occurs in dry valleys. Barbary sheep, birds, foxes, gazelles and rabbits live in the valleys, and the Haruj is used as a pasture by Arabs and Tibbus.Template:Sfn 4,000 year old petroglyphs in the field show antelopes and cattle.Template:Sfn Neolithic stone weapons made out of Haruj rocks have been foundTemplate:Sfn and several millstones discovered in the Roman cities of Leptis Magna and Cyrene did originate in the volcanic field.

See also

• List of volcanoes in Libya
• List of volcanic fields

External links

• Template:Sister-inline