Layers of the Earth
Scientists define and describe the interior of the Earth through deep drilling and seismic tomography. These techniques have allowed researchers to learn about Earth’s internal chemical and physical structure.
Layers based on chemical composition
During Earth’s early formation, the planet underwent a period of differentiation that allowed the heaviest elements to sink to the center and lighter ones to rise to the surface. Earth’s internal layering can be defined by this resulting chemical composition. The three main layers of Earth include the crust (1 percent of Earth’s volume), the mantle (84 percent), and the core (inner and outer combined, 15 percent). 
The solid crust is the outermost and thinnest layer of our planet. The crust averages 25 miles (40 kilometers) in thickness and is divided in to fifteen major tectonic plates that are rigid in the center and have geologic activity at the boundaries, such as earthquakes and volcanism.
The most abundant elements in the Earth’s crust include (listed here by weight percent) oxygen, silicon, aluminum, iron, and calcium. These elements combine to form the most abundant minerals in the Earth’s crust, members of the silicate family – plagioclase and alkali feldspars, quartz, pyroxenes, amphiboles, micas, and clay minerals.
All three rock types (igneous, sedimentary, and metamorphic) can be found in Earth’s crust. Crustal material is classified as oceanic crust or continental crust. Oceanic crust underlies our ocean basins, is thin, approximately 4 miles (7 kilometers) in thickness, and is composed of dense rocks, primarily the igneous rock basalt. Continental crust is thicker, ranging from 6 to 47 miles (10 to 75 kilometers), and has a high abundance of the less dense igneous rock granite. The oldest rocks on our planet are part of the continental crust and date back approximately 4 billion years in age. Ocean crust is constantly recycled through our planet’s system of plate tectonics and only dates back to approximately 200 million years ago.
The Integrated Ocean Drilling Program (IODP) has drilled deep in to the ocean crust (4,644 feet below the seafloor) but has not yet broken through to the next layer, the mantle.  The boundary between the crust and underlying mantle is termed the Mohorovicic discontinuity, often referred to as the Moho.
Mantle material is hot (932 to 1,652 degrees Fahrenheit, 500 to 900 degrees Celsius) and dense and moves as semi-solid rock. The mantle is 1,802 miles (2,900 km) thick and is composed of silicate minerals that are similar to ones found in the crust, except with more magnesium and iron and less silicon and aluminum.
The base of the mantle, at the boundary with the outer core, is termed the Gutenberg discontinuity. It is at this depth (1,802 miles, 2,900 km) where secondary earthquake waves, or S waves, disappear, as S waves cannot travel through liquid.
The outer core is composed mostly of iron and nickel, with these metals found in liquid form. The outer core reaches between 7,200 and 9,000 degrees Fahrenheit (4,000 and 5,000 degrees Celsius) and is estimated to be 1,430 miles (2,300 km) thick. It is the movement of the liquid within the outer core that generates Earth’s magnetic field.
The inner core is the hottest part of our planet, at temperatures between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius). This solid layer is smaller than our Moon at 750 miles (1,200 km) thick and is composed mostly of iron. The iron is under so much pressure from the overlying planet that it cannot melt and stays in a solid state.
The solid inner core is believed to have formed relatively recently, around half a billion years ago.  In February 2015, scientists reported in the journal Nature Geoscience their discovery that the inner core may in fact be two distinct cores with complex structural properties, where iron crystals in the outer layer of the inner core are oriented north-south, and iron crystals in the inner-inner core are aligned east-west.  This new discovery may help scientists learn more about the history and formation of planet Earth.
Layers based on physical properties
The Earth is separated into layers based on mechanical properties in addition to the composition layers described above.
The lithosphere is the outermost layer of the Earth ~100 km thick and is defined by its mechanical properties. This rigid layer includes the brittle upper portion of the mantle and the crust. The lithosphere is divided into 15 major tectonic plates, and it is at the boundary of these plates where major tectonic occurs, such as earthquakes and volcanoes. The lithosphere contains oceanic and continental crust that varies in age and thickness across locations and geologic time. The lithosphere is the coolest layer of the Earth in terms of temperature, with the heat from the lower layers generating the plate movements. The term "lithosphere" should not be confused with the use of "geosphere," which is used to indicate all of Earth's systems, including the atmosphere, hydrosphere, and biosphere.
The asthenosphere includes the upper part of the mantle that is highly viscous and mechanically weak. The lithosphere-asthenosphere boundary (LAB) is where geophysicists mark the difference in ductility (a measures a solid material’s ability to deform or stretch under stress) between the two layers. This boundary in the upper mantle is marked at the 1300oC isotherm. Above the isotherm marks where the mantle behaves in a rigid fashion and below which it behaves in a ductile fashion. It is the ductile rocks in the upper part of the asthenosphere that are believed to be in the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Seismic waves travel relatively slowly through the asthenosphere.
The mesosphere refers to the mantle in the region under the lithosphere and the asthenosphere, but above the outer core. The upper boundary is defined as the sharp increase in seismic wave velocities and density at a depth of 660 kilometers (410 mi). This layer should not be confused with the atmospheric mesosphere.
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