Structure of the Earth discontinuity (a second order discontinuity). The uppermost mantle together with the crust constitutes the lithosphere. The crust-mantle boundary occurs as two physically different events. First, there is a discontinuity in the seismic velocity, which is most commonly known as the Mohorovičić discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below). Second, in oceanic crust, there is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been
Tales from the Flat Earth of agate, the second is made of blue steel, and the third gate is made of black fire. There is only one city in the Underearth – Druhim Vanashta, the city of demons. It is made of precious minerals, gems, and metals and is beautiful, though its inhabitants are cruel and wicked. The Flat Earth appears to include temperate, sub-tropical, and tropical zones, including many deserts. Since "polar" regions would be impossible on a flat world, they are not mentioned in the series. There are many seas, but bodies of water large enough to be called "oceans" do not appear