mantle

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Intermediate zone of the Earth between the crust and the core, accounting for 82% of the Earth's volume. The crust, made up of separate tectonic plates, floats on the mantle which is made of dark semi-liquid rock that is rich in magnesium and silicon. The temperature of the mantle can be as high as 3,700°C/6,692°F. Heat generated in the core causes convection currents in the semi-liquid mantle; rock rises and then slowly sinks again as it cools, causing the movements of the tectonic plates. The boundary (junction) between the mantle and the crust is called the Mohorovicic discontinuity, which lies at an average depth of 32 km/20 mi. The boundary between the mantle and the core is called the Gutenburg discontinuity, and lies at an average depth of 2,900 km/1,813 mi.

The mantle is subdivided into upper mantle, transition zone, and lower mantle, based upon the different velocities with which seismic waves travel through these regions. The upper mantle includes a zone characterized by low velocities of seismic waves, called the low-velocity zone, at 72 km/45 mi to 250 km/155 mi depth. This zone corresponds to the asthenosphere upon which the Earth's lithospheric plates glide. Seismic velocities in the upper mantle are overall less than those in the transition zone, and those of the transition zone are in turn less than those of the lower mantle. Faster propagation of seismic waves in the lower mantle implies that the lower mantle is more dense than the upper mantle.

The mantle is composed primarily of magnesium, silicon, and oxygen in the form of silicate minerals. In the upper mantle, the silicon in silicate minerals, such as olivine, is surrounded by four oxygen atoms. Deeper in the transition zone greater pressures promote denser packing of oxygen such that some silicon is surrounded by six oxygen atoms, resulting in magnesium silicates with garnet and pyroxene structures. Deeper still, all silicon is surrounded by six oxygen atoms so that the mineral perovskite MgSiO₃ predominates.

The number of stripes changed frequently until around 1800. Red, white, and blue became the colours of liberty and an inspiration for other revolutionary flags around the world. Effective date: 19 February 1937. >>