Earth's Mantle
In the early 1900s, scientists were fairly certain that the Earth was made up of many layers, like an onion, but they did not know exactly where the layers started and ended. In 1909 Andrija Mohorovicic (1857-1936), a Croatian seismologist, helped reveal the existence of the shallowest of these layers, the crust, and the underlying layer, the Earth's mantle.
Mohorovicic studied Yugoslavian earthquake records that showed the same set of seismic waves arriving twice on a single earthquake record. Because the second set of waves exactly paralleled the first set, Mohorovicic concluded that the first set had traveled at a faster rate through a denser layer of rock deep inside the Earth. The second set had traveled at a slower rate, and a shorter distance, closer to the surface. This allowed Mohorovicic to interpret the existence of the crust and below it, the mantle. In tribute to that discovery, the boundary where the crust and the mantle meet is now referred to as the Mohorovicic Discontinuity, or Moho.
Even today, the study of earthquake waves continues to provide information about the composition of the mantle. Because waves travel at different speeds, depending upon the density of the rock, seismologists can estimate what materials are contained within the mantle. Three different types of rock are believed to compose the mantle, peridotite, eclogite, and kimberlite. Peridotite is by far the most common mantle rock. Coarse grained and dark in color, like all mantle rocks peridotite is composed chiefly of the mineral olivine (or one of its polymorphs). Eclogite is similar to peridotite in grain size, but contains a greater variety of minerals. Kimberlite, a source of diamonds, is more fine grained and so resembles basalt, the rock that people associate with volcanoes. It is possible that kimberlite forms by partial melting of eclogite when pressures decrease due to convection within the mantle.
Ever since the discovery of the mantle, scientists have wanted to probe into the physical nature of the Earth's deep interior. And because the mantle resides much closer to the surface in ocean basins, there were plans in the late 1950s to drill into the Moho from floating platforms out at sea. However, after a number of successful test drillings, Congress refused to continue funding, so the drilling program-- Project Mohole--was abandoned in the mid-1960s.
Seismic discontinuities are found within the mantle as well as at the Moho. The first of these is at a depth of about 250 mi (400 km). The second occurs at a depth somewhere between 375 and 440 mi (600 and 700 km). The interval between these discontinuities is called the transition zone; it separates the upper and lower mantle. These discontinuities probably result from variations (polymorphism) in the structure of the mineral olivine, the most common constituent of peridotite. At the first discontinuity, the olivine structure collapses to produce the mineral spinel. At the second discontinuity, the spinel structure collapses to perovskite.
In the 1920s Beno Gutenberg also discovered a seismic low velocity zone in the upper mantle at a depth from about 60-160 mi (100-250 km). This zone is due to a decrease in density resulting either from the rock being very near its melting point or actually containing a small amount of liquid (1-5 %), probably between individual grains. This low velocity zone was critical to the recognition of the lithosphere and the asthenosphere in the 1960s. The lithosphere is the layer of dense, brittle rock that lies directly above the low velocity zone. It includes the crust as well as part of the upper mantle. The asthenosphere roughly corresponds to the low velocity zone although the two are not considered one and the same. According to the theory of plate tectonics, the surface of the Earth is broken up into about a dozen lithospheric plates that move across the surface of the earth by sliding along on the less resistant asthenosphere below.
The mantle's density ranges from 3.3 gm/cc near the top of the mantle—at an average depth of about 15 mi (24.1 km)--to about 5.7 gm/cc at the base of the mantle--a depth of approximately 1800 mi (2,900 km). However, the depth of the Moho is highly variable. It averages about 3-6 mi (5-10 km) in the ocean basins, whereas below the continents it averages 20 mi (35 km)--and as much as 55 mi (90 km) beneath some mountain ranges.
The base of the mantle, the Gutenberg discontinuity, appears to the equally irregular according to seismic tomography studies. Seismic tomography works somewhat like a medical CAT scan. CAT scans are interior views of a person's body produced by computers that combine hundreds of digital x-ray cross sections of the body into a coherent picture. The density of the different tissues provides the contrast that allows recognition of medical conditions. Similarly, seismic tomographs are two- or three-dimensional views of the Earth's interior produced by computers using hundreds or thousands of seismic records as input. Differences in density, primarily due to temperature differences, reveal details about the structure of the Earth's interior. This technology only became possible in the mid to late 1990s as computers grew capable of processing huge sets of data at extremely rapid speed. Seismic tomography has shown that the base of the mantle has "mountains" and "valleys" much larger than anything at the Earth's surface. At these locations, low areas probably result from the cooler (denser) mantle sinking downward somewhat into the Earth's core and highs perhaps result as molten core material convects upwards into the mantle. Seismic tomography also reveals convection currents within the mantle itself, which may be the driving force behind plate tectonics.
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