Urgent deployment of GPS in Afar (Ethiopia): determining the rheology of incipient oceanic lithosphere

点击翻译按钮获取中文摘要

When tectonic plates move apart, new oceanic crust is eventually formed. This crust grows episodically (every few hundred years or so for any given segment) by the intrusion of magma along vertical cracks (dykes) at mid-ocean ridges. Most of the world's oceanic crust is under several kilometres of water where it is impossible to directly observe the dyke intrusion process. On the 14 September a dyke intrusion event began on land at the northern end of the East African Rift system, in Afar (Ethiopia) - where continental crust has just been stretched sufficiently for 'oceanic' crust to be formed. A 60 km long segment of the rift opened by as much as 9 m, and more than 2 cubic kilometres of magma was intruded. There were many 10s of earthquakes and eruptions from long, open fissures at the surface, blanketed a wide area in ash, displacing some 25,000 pastoralists and their livestock. This is the first major dyke intrusion event on land since the 1970s, and the first to occur in the era of precision space-based measuring techniques. As well as wishing to understand the likelihood of any future activity, the event sets up a simple natural experiment that will help us understand the properties of the earth's outer layers. In essence, the intrusion has given the earth a sudden kick. Measuring how it responds to that kick tells us about how the rocks at depth behave. In particular, we are interested in whether we can see those rocks flow in response to the stress of the kick, and how quickly they flow. We plan to install GPS instruments on the ground that will monitor the movement of the crust continuously, and to use satellite radar images to get snapshots of the deformation. From the combination of these measurements, we will be able to learn about the processes by which continents split apart and oceanic plates grow.