Evidence has been found for a mantle plume beneath Oman, on the Arabian Peninsula. Unlike all the confirmed mantle plumes found before, however, this one is not associated with current or recent volcanic activity, earning the title “ghost” plume.
Mantle plumes rise from the boundary of the Earth’s outer core, bringing extra heat to the underside of the Earth’s crust. In familiar cases, that heat drives magma through the crust, triggering volcanic activity at the surface. Hawai’i and Iceland are probably the most famous examples, but at least eight are known to exist, and around 50 more are still debated. Only one of the confirmed plumes, the Afar plume, lies beneath a continent.
The fact that most confirmed plumes lie under the ocean partially reflects the blueness of our planet, but may also be because it is harder for a plume to make its influence felt through thicker continental crust.
Geologists have considered the possibility that in some cases “ghost” plumes could exist without volcanic activity, and some potential examples have been proposed. However, confirming something that, by definition, doesn’t reach the surface is hard. Now, the behavior of seismic waves from distant earthquakes has led geologists to claim an example they say is clear, and they have named it the Dani plume.
Although the Dani plume has caused no modern volcanism, it may still be the reason eastern Oman is uplifted compared to surrounding areas. Moreover, the authors argue, in the late Eocene (40-34 million years ago), the plume was the likely cause of a previously unexplained rotation of the Indian plate, influencing its eventual collision with Asia.
Researchers studied the major types of seismic waveforms received from large earthquakes planet-wide at 30 permanent seismic stations near the border of Oman and the United Arab Emirates. They supplemented the results with data from 85 temporary stations.
The waves traveled through a relatively low-velocity region beneath the surface, which the authors estimate was around 200 kilometers (120 miles) across, widening to 350 km (210 miles) at the top. Discontinuities in density are found at depths of 410 km and 660 km beneath the surface as one type of rock changes to another, but in the low velocity zone, the upper boundary was shifted downwards and the lower one up.
These observations are consistent with a plume of material 100°C to 300°C hotter than the rocks around it rising through the mantle until it encounters the crust. The crust forms a lid, causing the hot material to take on a shape like a mushroom cloud. The authors think it is possible that some material initially reached high enough to produce volcanic activity, but if so, the record has been erased by the subduction as Arabia ran into Asia.
Among the questions the authors have about their own work, one stands out: “Is the Dani plume unique, or do similar ghost plumes exist elsewhere on Earth?” they write. Although unable to answer their own question, the authors point to several anomalies that might reveal further ghost plumes with more research. These include several locations beneath Africa and the geologically older parts of the Pacific Ocean, particularly the Coral Sea.
If ghost plumes exist today, it is likely others did in previous times, and the authors point to evidence from North America and Australia of ancient ghost plumes – ghosts of ghosts, one might say.
If ghost plumes are common, the authors note that estimates of heat escaping the Earth’s core to its crust may be too low.
The study is open access in Earth and Planetary Science Letters.
Source Link: First Clear Example Of A “Ghost” Mantle Plume Discovered Beneath Arabia