Volcanic History and Hazards
Posed by the Basaltic Lava
Fields of Harrats Rahat and
Lunayyir, Saudi Arabia.
TERM PAPER
Suhail Alhejji
VOLCANOLOGY (GEO 527) | ANITA GRUNDER
Introduction:
The extended volcanic lava fields “Harrats” in the west of the Arabian need to be studied to determine the volcanic hazards. Although some volcanologists suggest that these volcanic lava fields to be monogenetic fields, they resulted single eruptive event, that does not mean these lands are free of any volcanic hazards (Runge et al., 2014). “Harrat” in Arabic means “the volcanic lava field” or “the black stony area” (Wehar, 1976). These Harrats cover a huge area in the Arabian plate, from Yemen in the south through Saudi Arabia and Jordan to northern Syria in in the north (Fig.1). Arabian Harrats cover a vast area of about 180,000 km2, however, the half of the Arabian Harrats take place in Saudi Arabia (~90,000 km2) (Camp et al., 1991). In this paper, the focus is on Harrats that show potential volcanic hazards because they have the most recent eruptive history. I focus on Harrats Rahat and Lunayyir in western Saudi Arabia, which are located about 8 km and 200 km from the ancient and holy city of Al-Madinah (also known as the city of prophet Muhammad), respectively. These two Harrats pose risk to more than one million people who live nearby. The paper focuses on two main topics: (1) the geological and volcanological background used to assess potential hazards and (2) a mitigation plan based on the assessment of hazards to the area.
Geological Setting:
The Arabian Harrats are Oligocene to Recent intra-continental basaltic provinces set on the stable Precambrian Arabian Shield, and are the only volcanism within the Arabian plate that remains potentially active. Camp and Roobol (1992) attributed the volcanism in western Arabia into two different distinct phases. The first phase (from 32 – 20 Ma) was associated with the opening of the Red Sea and a passive mantle upwelling, which produced basaltic lava fields of tholeiitic to transitional composition. The second phase (from 12 Ma – recent) has been associated with active mantle upwelling, producing transitional to alkalic lava compositions. The older volcanic activity has the same NW-SE trend as the Red Sea, whereas the younger period has the N-S trend like the volcanic axis of the Makkah-Madinah-Nafud (MMN) line (Fig. 1). Volcanism is associated with Plate extension and thinning of the lithosphere beneath the Arabian Harrats (Duncan and Al-Alamri, 2013). However, the current main control on the volcanism in western Arabia is the horizontal northward mantle flow that is coming from the Afar triple junction in east Africa as channels through the Gulf of Aden and Red Sea (Fig. 1) (Chang and Van der Lee, 2011).
A. Harrat Rahat:
The volcanic lava field of Harrat Rahat is one of the three main lava fields that constitute the MMN volcanic line. Harrat Rahat is divided into three main stratigraphic units:
the oldest unit Shawahit (10 – 2.5 Ma), Hammah (2.5 – 1.7 Ma), and Madinah (1.7 Ma – Present), these units can be easily identified on aerial photographs because they have not exposed to substantial erosion (Fig. 2) (Camp and Roobol, 1989).Some volcanologists (e.g.
Moufti et al., 2010 and Murica et al., 2015) suggest that the Madinah unit is a separate volcanic field; however, most of those who have worked in the area (e.g. Camp and Roobol, 1989; Murcia et al., 2016) keep the three units together based on their petrologic similarity.
Harrat Rahat consists of more than 644 scoria and other pyroclastic cones and has two volcanic eruptions at 641 AD and 1256 AD eruption (Camp and Roobol, 1989). The rock compositions of this volcanic field have a narrow range from alkali-basalt to hawaiite (Fig. 3), and the ratio among La/10, Nb/8, and Y/15 exhibits a typical continental basaltic composition (Murcia et al., 2016).
B. Harrat Lunayyir:
Harrat Lunayyir is a small volcanic field that is < ~100 km east of the Red Sea (Fig. 2c), and has both trends of the Red Sea (NW-SE) and the (MMN) volcanic line (N-S). Harrat Lunayyir consists of six volcanic untis; one late Tertiary unit and five Quaternary units (Al-Amri et al., 2012). The entire volcanic history of Harrat Lunayyir occurred within the last 600 Ka years, and the rate of eruptions has been decreasing with time according to 40Ar/39Ar increment heating dates of Duncan and Al-Amri (2013). Lava compositions at Harrat Lunayyir
exhibit a small range of variation, from basanites to alkali olivine basalts to trachy-basalts (Fig.
4) (Duncan and Al-Amri, 2013), also major and trace element measurements indicate that the magmas are primitive. The volcanism at Harrat Lunayyir produced more than 50 cinder cones (Al-Amri et al., 2012).
The History of Recent Volcanic Activities:
Western Arabia has been inhabited for a long time allowing for a well-documented eruptive history of the volcanoes in the region. The last eruption of Harrat Rahat was in 1256 AD, as confirmed by historical and Islamic documents which describe what happened and how people reacted to those events. Geological studies, however, are too few to tell the story in terms of the volcanic products or the number of volcanic events, particularly for areas that were not inhabited. Although the Arabian Harrats have been studied recently, yet more can be done to better assess for future eruptions with respect to likelihood, intensity and style.
Camp et al. (1987) determined that approximately twenty-one volcanic eruptions occurred in the western Arabian during the past 1500 years, including two volcanic events
Figure 4. Rock compositions of Harrat Lunayyir have a narrow range from basanites to trachy-basalts (after Duncan and Al-Amri, 2013).
within the lava field of Harrat Rahat, and one historical event in Harrat Lunayyir about 1000 years ago (Al-Amri et al., 2012) (Fig. 5). Even though the volcanism of these Harrats began from long time ago, ~10 Ma years for Harrat Rahat and 0.6 Ma years for Harrat Lunayyir, this does not mean that the volcanic eruptions have stopped.
Not only does historic volcanic activity indicate that the region at volcanic risk, but also there have been recent earthquake swarms thought to be related to magmas that are emplacement in dikes (Fig. 5). The latest one of these events was the seismic crisis in the city of Al-Ayis between the period of April to June 2009 when a swarm of more than 30,000 earthquakes struck the field of Harrat Lunayyir, which resulted in magmatic dyke intrusion.
Although that was failed eruption, the government of Saudi Arabia evacuated more than 40,000 people who live near this Harrat (Pallister et al., 2010). Pallister et al. (2010) attributed the continuous swarms of earthquakes in Harrat Lunayyir to the rising of magma to shallower levels underneath the volcanic field which increase the possibility of a future volcanic eruption. A similar, though less intense, event happened in 1999 AD at Harrat Rahat when a 500-earthquake swarm struck the region (Runge et al., 2013). These two volcano-seismic swarms are consistent with the model that the two volcanic lava fields of Harrat Rahat and Lunayyir are formed in response to crustal extension as a part of the Red Sea rift system (Hansen et al., 2013; Runge et al., 2013). Interpreting the eruptive history of Harrats Rahat and Lunayyir indicates that a low frequency of volcanic eruptions. Thus, if we scale the frequency of historic eruption in Harrats Rahat and Lunayyir, we can say that it is millennial frequency scale of eruptions; one to two volcanic eruption per thousand years.
Madinah Eruption:
1256 AD was the date of the last volcanic eruption near the holy city of Al-Madinah in region of Harrat Rahat (Fig. 2 (a)). The eruption is one of the few historic eruptions that has been geologically studied in the region of western Arabia. For 52 days, six pyroclastic cones and lava flows up to 23 km long erupted from a 2.25 km-long fissure with a total volume of alkali-olivine basalt produced estimated to be 0.5 km3 (Camp et al., 1987; Murcia et al., 2016).
The main type of lava flows in this eruption was a’a lava flow, but there were some other types
of lava flows such as blocky and pahoehoe (Table. 1) (Murcia et al., 2016). The pyroclastic products of this eruption are vesicular, lapilli and bombs (Murcia et al., 2016). The dominant volcanic eruption in Madinah was hawaiian-style. The eruption became more explosive and turned to strombolian-style at the end of the volcanic event, especially in the northern part of the fissure (Murcia et al., 2016). Pyroclastic cones consist of a wide range of spatter that shows variations in fluidity, from brittle to fluidal, especially in the beginning of the hawaiian- style phase (Murcia et al., 2016). Some volcanologists believe that this type of eruption that might represent the future eruption in the Arabian Harrats (Moufti et al., 2010). Documents have not shown any casualties for this eruption; however, many building destroyed in this event due to the earthquakes that were associated with the eruptions (Camp et al., 1987).
The Future Eruption Scenario:
Based on what I already mentioned, I make three reasonable predications for the future eruption scenarios in the two volcanic lava fields, Harrats Rahat and Lunayyir. The first scenario is the most likely scenario that could happen, and the other two scenarios are less and less likely, ranging from probable to not impossible to happen. The dominantly effusive
Figure 5. A timeline illustrates the documented eruptive and volcanic history on Harrats Rahat and Lunayyir. The last two volcanic events were earthquake swarms associated with rising of magmas and failed eruptions (probable dyke intrusions).
eruption style will remain. If there is magma differentiation over a period of time due to some processes such as crustal contaminations or changing in mechanism of the magmatic differentiation process, then more explosive eruptions might occur.
1. The most likely scenario:
Harrat Lunnayyir is showing the most recent volcanic hazards, which are the earthquake swarms and crustal deformation in 2009. Therefore, I think that is more likely to see a volcanic eruption in Harrat Lunayyir rather than in Harrat Rahat. Moreover, based on the past volcanic style on both Harrats Rahat and Lunayyir, I assume that the future volcanic eruption style will be the same, long fissures that produce alkali-basaltic lava composition. At the end of the eruption, there may be a change in the eruption style to stromoblian, as it happened in the last historic eruption in Harrat Rahat. The most hazardous aspects of this future eruption scenario are the very extended lava flows that could flow to more than 30 km long, and the earthquake swarms that precede the volcanic eruption. Tephra hazard was associated with the previous eruption of Harrat Rahat and I expect that the future eruption will have tephra fallout hazard too. I will discuss the characteristics of volcanic hazards in the hazard assessment part.
2. Possible to probable scenario:
I think what it happened in 2009 in harrat Lunayyir of failed eruption (dyke intrusion) (Pallister et al., 2010; Lovett, 2010) could be repeated many times in both harrats, without getting any magma up to the surface. In many cases, magma can get arrested beneath the earth surface, and make some forms of intrusions like dykes or sills. This prediction needs to be supported to make sure that there is no magma chamber is forming beneath these volcanic lava fields.
3. Not likely, not impossible scenario:
The lest possible scenario that could happen in Harrats Rahat and Lunayyir is the dramatic change in the magma composition which makes the volcanic eruptions to be more explosive and produce more pyroclastic deposits such as bombs, lapilli, or ashes. Changing in magma compositions happened in the Arabian Harrats, especially in the distinct lava field of Harrat Khaybar, when the processes of crystal fractionation played its role and modified the magmatic composition from mafic to felsic, representing by the comenditic lavas (Camp and Robool, 1991). I believe that this case is unlikely to occur because the current tectonic setting for harrats Rahat and Lunayyir tends to make the lithosphere of these volcanic fields much thinner by the extension forces, which makes the possibility of something like this to happen too small.
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Figure 2. (A) Distribution of youngest volcanic units of the Madinah unit with flow direction shown in dashed white arrows in white-bordered lavas(B) map for Harrat Rahat, the red unit is Madinah unit. (C). ages and distributions of the Arabian harrats. Harrat Rahat follows the N-S trend (from: Murcia et al., 2016).
