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
The Menace from Earth The Menace from Earth "The Menace From Earth" is a science fiction short story by American writer Robert A. Heinlein, first published in the August 1957 issue of "The Magazine of Fantasy & Science Fiction". The story is set in the near future, when the Moon is colonized with people living in underground cities. The "menace" of the title is a glamorous woman tourist who visits the Moon colony. She is assigned a young guide named Holly, a 15-year-old girl and aspiring starship designer who is the first-person narrator. Holly's best friend Jeff develops a crush on the "groundhog" visitor,