Many people might not associate Saudi Arabia with volcanoes and earthquakes. A more common image might be miles upon miles of sandy desert. However, the Arabian region is home to some large fields of volcanoes that have erupted in the last thousand years or so. In May 2009, 40,000 people were evacuated from the Harrat Lunayyir province in northwest Saudi Arabia in response to a series of earthquakes, some over magnitude 5, in case larger, more damaging quakes were to come. In response to this, the Saudi Geological Survey invited the US Geological Survey to help investigate not only whether more, larger quakes could occur, but also if this could signal volcanic eruptions in the region. A recent paper in Nature Geoscience documents the scientific investigations that followed (Pallister, et al., 2010).
The Harrat Lunayyir province, adjacent to the Red Sea, contains numerous volcanoes and lava fields, which were formed around 30 million years ago as the Red Sea opened up, releasing hot magma from deep within the earth. The Red Sea is what is known as a ‘rift zone’, which means that it is a place on the Earth’s surface (crust) where two adjacent pieces of crust are being forced apart. A similar thing takes place at the more famous Mid-Atlantic Ridge, part of which effectively forms Iceland. The volcanoes in Saudi Arabia are of a similar type to those seen in Hawaii (“Strombolian to Hawaiian-type”), and result in large flows of lava, but are generally not explosive (so its fairly easy to get out of the way). These Saudi volcanoes have erupted numerous times in the last several thousands of years, and there is at least one historical record of an eruption. The visual result of all these eruptions can be seen clearly in Google Maps.
In the hope of anticipating a volcanic eruption, scientists install a series of seismic monitoring stations. These are effectively ‘ears on the ground’, listening to the pops and cracks made as rocks move deep in the Earth. In the lead up to an eruption, it is common to get a series of minor earthquakes (many not big enough for a person to detect, but easily detected by the seismic stations) that suggest that magma (which will become lava if it pours out onto the surface like you see in Hawaii) is on the move, and that it may erupt soon. This pattern of quakes is known as an “earthquake swarm” and it’s recognised by having more small earth tremors compared to larger ones, and that they happening quite close to the surface (shallow here being less than 5 km depth below ground surface). The problem that scientists face is how to distinguish an unusually active group of ‘normal’ earthquakes from an earthquake swarm that could foreshadow a volcanic eruption.
At Harrat Lunayyir, the April-June 2009 earthquake swarm of more than 30,000 quakes convinced scientists from the Saudi Geological Survey, and the US Geological Survey, that there may be an eruption for three main reasons. Firstly, these earthquakes were happening beneath volcanoes that have erupted relatively recently. Secondly, detailed measurements of the shape of the land surface in the area suggested that magma had pushed its way up, and actually deformed the surface of the earth, which is quite common before volcanic eruptions (though again, often the human eye would fail to see it). Thirdly, the pattern of earthquakes was the type of pattern that has been seen in the lead up to eruptions from similar types of volcanoes elsewhere. The team monitored the area for several months.
By August of 2009, the earthquake swarm had abated and surface measurements suggested that there was no more movement of magma beneath the surface. There had not been any eruption, and it was now considered unlikely. Subject to engineering tests on buildings and so forth, the evacuees were allowed to return.
The Nature Geoscience paper notes, however, that in other parts of the world, such events have been followed by similar swarms over coming years. Furthermore, historical and geological records suggest that there have been at some 21 different volcanic eruptions on the Arabian Peninsular in the last 1,500 years, with the most recent being in 1937 in North Yemen. Given the young age of the volcanoes, it remains a possibility that they will erupt again, with hazards posed by lava flows, as well as by the associated earthquake activity. As a result, the level of seismic monitoring in the Harrat Lunayyir region has been stepped up, so that local authorities can react quickly and in an informed way should a threat arise again. As the authors note, this is a reminder that even though quakes and volcanoes are infrequent in this part of the world, they still have to be monitored.
Pallister, J., McCausland, W., Jónsson, S., Lu, Z., Zahran, H., Hadidy, S., Aburukbah, A., Stewart, I., Lundgren, P., White, R., & Moufti, M. (2010). Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia Nature Geoscience, 3 (10), 705-712 DOI: 10.1038/ngeo966