In the terrestrial coat, about 2000 km deep near the border with the nucleus, they are found Two huge “islands” as big as continents, located one under Africa and the other under the Pacific Ocean and known as Llspv (Large Low Seismic Velocity Provinces or “great provinces at low seismic speed”). It is hottest and oldest rocky masses of the surrounding materials. A new study from the University of Utrecht, in the Netherlands, revealed that they have at least Half billion years. The discovery was made by studying the behavior of the seismic waves generated by earthquakes when they propagate inside the earth, which varies according to the materials they encounter. These masses do not participate in the movements that remit the terrestrial cloak, but remain isolated keeping the heat. The discovery is important because it shows that the terrestrial cloak is not homogeneous as it was believed in the past and why these regions could be at the origin of the activity of some volcanoes.
The discovery of the great provinces at low seismic speed in the terrestrial cloak
The LLSPVs have been uncovered in 90s. At the time, says the co -author of the study Arwen Deuss“Nobody knew what they were, and if they were only a temporary phenomenon, or if they remained there for millions or perhaps even billions of years”. Around these masses there are remains of lithospheric plates which over time are sunk in the cloak at the oceanic pits in the process called subduction. The remains of the plates have different characteristics than the LLSPV: they are colder and more recent. The LLSVP, on the other hand, were able to retain the heat for hundreds of millions of years remaining isolated compared to the rest of the cloak. This is an unusual fact, since the cloak behaves like a fluid and is affected by convective motions, triggered by the temperature differences, which remit it by redistributing the heat inside.
The new study on the LLSPV of the earth
The Large provinces at low seismic speed So they are called because the seismic waves that propagate inside the earth during earthquakes, when they meet them, slow down due to their high temperature. However, the new study found that in correspondence of these regions it dampening of seismic wavesthat is, the amount of energy that waves lose during their propagation, is lesser Compared to what you have where the remains of the sunk lithospheric plaques are found. Yet with high temperatures, the damping should be high: it is a bit about what happens going to run when it is hot and consequently not only slows down but you get tired even more.
To explain the low damping at the LLSPV, the researchers studied the size of granules (granulometry) minerals that make them up. The lithospheric plaques that have undergone the subduction present very small granulesbecause minerals recry up during sinking due to the increase in temperature and pressure. This means that there are many granules and consequently many boundaries between granules: the waves lose energy every time they cross one of these borders, therefore the damping is high. Conversely, the LLSPV granules are much larger and therefore the damping is less. Because the granules of the minerals develop to become so large takes a long time: this means that the LLSPVs are very ancient. Also these masses, with their large granules, are very rigid And do not participate in the convective motions of the cloak.
The importance of the study on the LLSPV
One might think that these phenomena do not concern us directly, but in reality the LLSPVs could be at the origin of the activity of the volcanoes located at a certain distance from the margins of the lithospheric plaques. In fact, researchers believe that from these masses date back to the surface “Pennacchi” of hot materiallike those who feed the Vulcans of Hawaii. In general, the discovery highlights how the terrestrial cloak is an uneven layer, unlike what was thought in the past.
