Melting during late-stage rifting in Afar is hot and deepMelting during late-stage rifting in Afar is hot and deep by D.Ferguson et al, 2013

Non-technical summary

Evidence from the margins of some oceans, such as the Atlantic, show that when these ocean basin were created by the breaking apart of old continents this process (called rifting) involved large volumes of magma (molten rock). In Afar, Ethiopia, the current breaking apart of the African continent has also involved a large amount of magma, erupted as thick piles of lava flows and from the numerous volcanoes along the East African rift system. Recent volcanic eruptions and earthquakes in Afar have shown that this magma currently plays an important role in breaking up the African continent when it is forced upwards into the Earth's crust. In this study the chemical composition of the lavas that are erupted at the surface are analyzed to produce a model for how the magmas that fed the eruptions are created in the Earth's mantle, the hot region that lies beneath the rigid rocky outer layer of the planet. The results of this model show that the magmas that percolate upwards to erupt at the surface in Afar are created at high pressures and temperatures, between depths of around 100-80 km. One of the important implications of this deep melting is that the mantle beneath Afar is hot, probably around 100°C above the normal temperature range. This high temperature drives melting at high pressures, providing a sustained supply of molten rock to feed the widespread magmatic and volcanic activity observed in this region. A further implication of these new results is that the tectonic plate beneath Afar is still quite thick, probably around 60-80 km deep. This thickness is unexpected because the African continent has been undergoing extension or stretching for several million of years. To try and understand why this has happened the authors used a mathematical model to track how the tectonic plate changes as it is pulled apart during several million years of rifting. The results of this model show that the thick tectonic plate in Afar is a consequence of the slow rate of rifting here (around 20 mm per year). Like toffee or putty, when a tectonic plate is stretched it gets thinner and normally this thinning is expected to be proportional to the degree of stretching. However, because the plate in Afar is being stretched so slowly it has time to cool at the same time. This cooling causes some of the thickness of the plate to be maintained and means that melting beneath the plate is restricted to occurring at high pressures. If a new ocean basin is to open in the future in Afar then this thick plate must undergo a significant thinning event at a late stage in the formation of the rift system. The hot temperature of the Afar mantle means that the magma supply here can be sustained, even though the plate remains quite thick.

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