The Madinah eruption, Saudi Arabia: Magma mixing ... - Springer Link

Volian°ology

Bull Volcanot (1987) 49:489--508

© Springer-Vertag 1987

The Madinah eruption, Saudi Arabia: Magma mixing and simultaneous extrusion of three basaltic chemical types Victor E Camp ~, Peter R Hooper ~, M John Roobol 1, and D L White ~ Saudi Arabian Deputy Ministry for Mineral Resources, Jiddah, Saudi Arabia 2 Department of Geology, Washington State University, Pullman, WA 99 164, USA

Abstract. During a 52-day eruption in 1256 A.D., 0.5 km 3 of alkali-olivine basalt was extruded from a 2.25-km-long fissure at the north end of the Harrat Rahat lava field, Saudi Arabia. The eruption produced 6 scoria cones and a lava flow 23 km long that approached the ancient and holy city of Madinah to within 8 km. Three chemical types of basalt are defined by data point clusters on variation diagrams, i.e. the low-K, high-K, and hybrid types. All three erupted simultaneously. Their distribution is delineated in both scoria cones and lava flow units from detailed mapping and a petrochemical study of 135 samples. Six flow units, defined by distinct flow fronts, represent extrusive pulses. The high-K type erupted during all six pulses, the low-K type during the first three, and the hybrid type during the first two. Three mineral assemblages occur out of equilibrium in all three chemical types. Assembla~]e 1 contains resorbed olivine and clinopyroxene megacrysts and ultramafic microxenoliths (Fo9o+ Cr spinel + Cr endiopside) which fractionated within the spinel zone of the mantle. Assemblage 2 contains resorbed plagioclase megacrysts (An6o) with olivine inclusions (Fo78) which fractionated in the crust. Assemblage 3 contains microphenocrysts of plagioclase and olivine in a groundmass of the same minerals with late-crystallizing titansalite and titanomagnetite; assemblage 3 crystallized at the surface and/or in the upper crust. Each assemblage represents a distinct range in PTX environment, suggesting that their coexistence in each chemical type may be a function of magma mixing. Such a process is confirmed by variable ratios of incompatible element pairs in a range of analyses. Offprint requests to: PR Hooper

All three chemical types are products of mixing. Some of the hybrid types may have developed from surface mixing of the low-K and highK lavas; however, the occurrence of all three types at the vent system suggests that subsurface mixing was the dominant process. We suggest that the Madinah flow was extruded from a heterogeneous magma chamber containing vertically stacked sections equivalent to the six eruptive pulses. This chamber may have developed contemporaneously with magma mixing when a crustal reservoir containing a magma in equilibrium with assemblage 2 was invaded by a more primitive magma containing cognate microxenoliths and megacrysts of assemblage 1.

Introduction The Arabian Peninsula contains extensive Cenozoic lava fields (harrats) dominated by alkali-olivine basalt and hawaiite. The harrats cover about 180 000 km 2, forming one of the largest alkali basalt provinces in the world (Fig. 1). The Arabian plate, upon which these intracontinental basalts occur, is rifting away from the African plate along the Red Sea rift system and colliding with the Iranian plate along the Zagros thrust zone. However, tectonic control on the evolution of the harrats is poorly defined. They are not clearly related to continental rift valleys as are their East African counterparts, nor are they associated with aulacogen volcanism along a failed rift junction. They are largely contemporaneous with Red Sea extension (Gregory et al. 1982), but their linear vent systems do not generally parallel Red Sea normal faults. The stress regime during harrat volcanism

490

Camp et al.: The Madinah eruption, Saudi Arabia 33"

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Fig. 1. Distribution of alkali basalt fields (harrats) on the Arabian Peninsula and location of the Madinah flow and other historical eruptions. Modified after Coleman et al. (1983) with historical information from Simkin et al. (1981), Van Padang (1963), and Richard and Van Padang (1957)

that the Arabian plate is in a dynamic intracontinental tectonic environment. Within-plate volcanism has resulted in at least 21 eruptions on the Arabian Peninsula during the past 1500 years; the latest eruption was in 1937 at Dhamar in North

Camp et al.: The Madinah eruption, Saudi Arabia

Yemen (Fig. 1). Many historical eruptions have probably gone unrecorded. Several flows in the remote lava plains lack erosion and wind-blown dust which generally ponds on older flows. Some of these flows disrupt Neolithic sites; others are barren of such sites although surrounded by them. The eruption near the holy city of Madinah, Saudi Arabia, in 1256 A.D. (654 A.H.) is of particular historical importance. The eruption occurred on the northern part of Harrat Rahat (Fig. 1) and resulted in a basaltic lava flow that approached to within 8 km of the ancient city. The scoria cones and lava flow units of this eruption are typical of the most common type of Arabian basaltic effusive activity (Coleman et al. 1983), in contrast to the less common intermediate and silicic activity which mainly produces domes and pyroclastic deposits (Gass and Mallick 1968; Baker et al. 1973). We present here the first comprehensive description of a basaltic eruption on the Arabian Peninsula. Our intent is to describe the Madinah eruption and its products in detail and to demonstrate its petrogenetic implications for harrat evolution. Eyewitness accounts Two eyewitness accounts of the eruption were written by A1-Kistlani, a resident of Madinah, and Kadi Sinaan, the judge at Makkah. These manuscripts are no longer extant, but their descriptions are paraphrased in two later manuscripts; one, Wafa A1-Wafa, was written in 1568 A.D. (976 A.H.) (reprinted in A1-Samhoudy 1954) and the other, Jazb Al-Kulub, was recorded by an early European traveler, JL Burkhardt, and published

491

as a footnote in Burton (1893). These two sources show remarkably good agreement in detail and dates which, together with our field observations, permit a reconstruction of events (Table 1). Extracts from Wafa AI- Wafa " F o r days the volcanic eruption was preceded by many great earthquakes which occurred in Madinah at the beginning of Jumad AI-Thani (Monday, 1 June 654 A.H., i.e., 1256 A.D.). At first the movements were slight and not all of the residents of Madinah town felt them. On Tuesday, the second day of Jumad A1-Thani (2 June 654 A.H.), the earthquakes became stronger. On Wednesday (3 June 654 A.H.), in the third part of the night (the third of 3 parts between sunset and sunrise, ke., between 2--6 a.m.), the greatest earthquake occurred, which frightened the residents. The earth tremors continued through the rest of the night. On Friday (5 June 654 A.H.) a major event occurred, when the ground and ceilings of the houses were shaken. Eighteen earthquakes were recorded during this day .... After the main earthquake was felt in Madinah at midday, fire appeared associated with black smoke clouds which accumulated in the atmosphere. The greatest fire covered the horizon to the east of Madinah .... The lava flow carried along its way gravels, stones and trees. It was like a red-blue boiling river, with thundering noises. The lava flow moved toward the north of Wadi Eheline. The light of the fire was seen in Makkah, Busra, and Taima. The historians wrote that the fire continued for three months ... The lava flow descended on the rocky ground and was as high as a long spear above the ground level . . . . When the lava flow came to a complete stop ... it created a dam that formed a great lake in the rainy seasons (Lake A1 Habas)."

Products of the eruption The vent system and flow features were mapped on high-resolution aerial photographs enlarged to a scale of 1 : 10000 and complemented by oblique aerial photographs. Stratigraphic and field petrographic relationships were resolved by helicopter-

Table 1. Chronology of the 1256 A.D. (654 A.H.) eruption

Monday Tuesday Wednesday

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1st. 2nd. 3rd.

Thursday Friday

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Saturday

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Sunday

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Weak earth tremors felt by some residents of Madinah Earth tremors become stronger A strong earthquake between 2 and 6 a.m. frightened residents of Madinah. Other earth tremors followed Earth tremors continue 18 earthquakes were recorded culminating in the strongest one at midday which shook the ceilings of houses of Madinah. After midday the shaking ceased and fire fountaining of basaltic lava began on Harrat Rahat 19 km SE of Madinah. Thundering noises were heard. A pahoehoe basalt flow advanced toward the city. Black ash/ gas clouds rose into the atmosphere. At night the country was a bright as day After the entire population of Madinah, including women and children, prayed through Thursday and Friday nights at the Prophet's tomb, the lava flow stopped approaching the city (at a distance of 12 km from it) and the lava turned northward The lava ceased to flow after 52 days of activity having reached a NS length of 23 km


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supported field work and a program of detailed sampling.

Lava was extruded from a fissure 2.25 km long and oriented 343 °. The lava flow covers a total NS distance of 23 km; it has a total area of 56 km 2 and a volume of approximately 0.5 km 3 (uncorrected for vesicularity, which is < 20% and inside the error limits of calculation). The flow descends 250 m from an elevation of 870 m at the base of the vent system to 620 m at the northwestern flow terminus. The highest point on the vent system is 970 m at its northern end, where the system has a relief of 100 m.

Vent system A linear system of six scoria cones was built along the fissure (Fig. 2). Cones 1, 2, and 3 were relatively minor vents during the eruption. Cone 1 at the south end of the fissure is asymmetric and largely flooded by a short aa flow (Figs. 2, 3 a). Two small, open, late vents occur inside the higher eastern rim. Immediately to the north, a small pahoehoe driblet cone (diameter of 100 m) contains thin flow units and open lava tubes up to 2 m high. Cone 2 is small, symmetric, and composed of weakly agglutinated spatter; it coincides with an earlier, wider scoria cone now preserved as a single low rim west of cone 2. Cone 3 is corn-

Fig. 3. Linear vent system of Madinah flow. a Vents l to 6 from south-southeast; vent 1 in foreground, b Vents 5, 6A, and 6B, from southeast; vent 5 in foreground

Camp et al,: The Madinah eruption, Saudi Arabia

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494 Most of the basalt lava, as tephra and lava flows, issued from cones 4, 5, and 6. Cone 4 is symmetric and contains four nested crater rims, the innermost of which surrounds a small scoria cone; this morphology indicates progressive diminishing of fountaining as the eruption waned. White anhydrite from late fumarolic activity is present between scoria fragments in some parts of cone 4. Cone 5 is also symmetric and is composed of three nested cones (Fig. 3b). Cone 6 is the largest and, like cone 1, is asymmetric with the higher rim on the east side (Fig. 3 b). Cone 6 is complex with younger cones nested inside an early elongate cone with a crater 700 m long and 300 m wide. This large, early, asymmetric cone rises 60 m above the flows on its west side and 100 m on its east side. Nested inside the early cone 6 is a younger cone with an elongate crater measuring 400 m by 200 m. Further inside are two deep craters, 6A and 6B (Figs. 2, 3b), separated by a wall of scoria. In the bottom of crater 6A is a low bank of gray vesicular bombs and lapilli, the youngest tephra of the Madinah eruption. Crater 6B was occupied by a lava lake that drained back into the vent leaving the crater sides and bottom draped with basalt rafts. Lava welled from beneath the outer flanks of the cones at many points (Fig. 2). The western flanks of cones 4 and 5 were broken and distended where scoria was rafted on top of the upwelling basalt. Cone 5 is further modified by a fault that strikes 045 °, parallel to structures in the Precambrian basement. The fault scarp is blanketed by coarse air-fall scoria; thus, movement along the fault began and ceased during the eruption. Ejected blocks of Precambrian basement and large ultramafic xenoliths are absent, although they are common in vent areas of other Arabian harrats (Ghent et al. 1980; Coleman et al. 1983; Pallister 1984). The asymmetry of scoria cones 1 and 6 indicates that westerly winds occurred at some time during fountaining. However, the symmetry of the other cones indicates that for much of the time winds were weak or absent. This windless period is also recorded in the distribution of air-fall ash, which has a circular dispersal around the cones. Air-fall lapilli and ash are well preserved as far as 2.5 km from the cones, although they have been washed into hollows by rainfall. The tephra contains ash-size, elongate, and curved ribbon clasts; Walker and Croasdale (1972) describe similar clasts, or achneliths, elsewhere as having formed during flight by ejection of low-viscosity basalt lava. Spindle bombs were not observed on the

Camp et al.: The Madinah eruption, Saudi Arabia scoria cones but may be buried under younger scoria. The air-fall tephra is more than 1 m thick along the east side of the cones; here it has been used to divide lavas into older flow units with a tephra cover and younger flow units lacking such cover. The flow units west of the cones carry rafts of tephra that indicate disruption by flow.

Lava flow features The lava flow followed stream channels between older flows and comprises six N- to NW-directed flow units that advanced toward Madinah. The flow units are thinnest near the scoria cones, where flow fronts are 2--3 m high and systematically thicken away from the cones; at 10 kin, flow fronts are 4--8 m high, and at the flow terminus they are 7--8 m high. Most of the flow units have aa surfaces. The only pahoehoe flows are the earliest NW- and Ndirected units, together with the oldest exposed flow units in the vent area which are overlain by air-fall ash (Figs. 2 and 4). The pahoehoe flows range from massive to slabby. Around the margins of the aa flows are many lobed breakthroughs of thin ( < 1 m) late flows that are dense with large rounded vesicles; they have a surface structure transitional between aa and pahoehoe. These late-stage breakthroughs (Fig. 5 a) also occur on top of some aa flows, where they fill depressions in the rugged surface. Such breakthroughs occurred after the aa flow ceased moving. Similar structures have been described from other historical basalts, such as the Hekla 1947-1948 eruption (Einarsson 1949), and explained by the release of relatively fluid residual lava following exothermic crystallization. Lava channels are well developed. They are banked by lava levees and commonly contain arcuate pressure ridges of aa (Fig. 4). Near the northern end of the flow the basalt was channeled through a series of basement hills and spilled over lava falls up to 30 m high with a slope of about 30 ° (Fig. 5b). At the distal end of the flow north of the falls juxtaposed flow units moved onto an almost flat plain, where individual units can only be recognized near flow margins. Irregularities in the preeruption surface produced long arcuate troughs in the flow surfaces that indicate the direction of movement (Fig. 4). The large NW-directed lobe of aa southwest of the lava falls (Fig. 4) contains a number of rounded, smooth, accretionary lava balls up to 10


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tionated assemblage 2 in the crust. Neither parental magma was extruded at the surface, but the low-K and high-K types dominate the surface products. Also present, both at the vents and flow terminus, are relatively small volumes o f the hybrid type. These field relations are consistent with both major subsurface and minor surface mixing as described below. Subsurface processes. The petrochemical data require a model in which two bodies of parental magma, one in the mantle and the other in the lower crust (Fig. 16 a), interact to produce a third, heterogeneous magma body. A major assumption in the model is that the six eruptive pulses are regarded as vertically stacked sections through this heterogenous body (Fig. 16d). The upper 90% (Figs. 8, 9) of the heterogeneous body (pulses I, II, and III) contains all three chemical types with megacrysts of assemblages 1 and 2; the lower 10% (pulses IV, V, and VI) contains only the high-K chemical type (HKA) lacking megacrysts as a probable result of crystal settling before extrusion. The lower crustal magma body (Fig. 16b) behaved as an open system, simultaneously undergoing gabbro fractionation (assemblage 2) and assimilation of crust along its upper margins (O'Hara and Mathews 1981). The mantle magma body fractionated cumulate peridotite and megacrysts of assemblage 1 (Fig. 16b). We propose that this mantle parent rapidly ascended through the crust and "bulldozed" its way through the crustal reservoir where it mixed with the crustal parent (Fig. 16b and c). The low-K and high-K chemical types, and at least part of the hybrid chemical type, are products of this mixing; each represents a mix of variable proportions of the parental magmas. As the body of now heterogeneous magma continued upward the lower crustal reservoir was drained and the distribution of magma types was controled by drag along conduit walls and by internal flow influenced by compositional and thermal (i.e., density) variations. When the heterogenous magma reached the region of brittle fracture in the upper crust, the chamber was tapped, and the chemical types were extruded in six pulses along a 2°25-kmlong fissure. Surface mixing. The presence of some of the hybrid chemical type at the vent system (Fig. 7) indicates that it is a product of subsurface mixing. However, its presence at the flow terminus and its absence throughout the remaining part of the flow suggest that the bulk of the hybrid type may have been produced by surface mixing. The average


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