The structure of Earth can be defined in two ways: by mechanical properties such as
rheology, or chemically. Mechanically, it can be divided into
lithosphere,
asthenosphere,
mesospheric mantle,
outer core, and the
inner core. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.
Crust and lithosphere , which are: Earth's crust ranges from in depth and is the outermost layer. The thin parts are the
oceanic crust, which underlies the ocean basins (5–10 km) and is
mafic-rich (dense iron-magnesium
silicate mineral or
igneous rock). The thicker crust is the
continental crust, which is less dense and is
felsic-rich (igneous rocks rich in elements that form
feldspar and
quartz). The rocks of the crust fall into two major categories –
sial (aluminium silicate) and
sima (magnesium silicate). It is estimated that sima starts about 11 km below the
Conrad discontinuity, though the discontinuity is not distinct and can be absent in some continental regions. Earth's lithosphere consists of the crust and the uppermost
mantle. The crust-mantle boundary occurs as two physically different phenomena. The
Mohorovičić discontinuity is a distinct change of
seismic wave velocity. This is caused by a change in the rock's density – immediately above the Moho, the velocities of primary seismic waves (
P wave) are consistent with those through
basalt (6.7–7.2 km/s), and below they are similar to those through
peridotite or
dunite (7.6–8.6 km/s). 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
obducted onto the continental crust and preserved as
ophiolite sequences. Many rocks making up Earth's crust formed less than 100
million years ago; however, the oldest known mineral grains are about 4.4
billion years old, indicating that Earth has had a solid crust for at least 4.4 billion years.
Mantle between bottom of crust and solid uppermost mantle Earth's mantle extends to a depth of , making it the planet's thickest layer. [This is 45% of the radius, and 83.7% of the volume - 0.6% of the volume is the crust]. The mantle is divided into
upper and
lower mantle separated by a
transition zone. The lowest part of the mantle next to the
core-mantle boundary is known as the D″ (D-double-prime) layer. The
pressure at the bottom of the mantle is ≈140 G
Pa (1.4 M
atm). The mantle is composed of
silicate rocks richer in iron and magnesium than the overlying crust. Although solid, the mantle's extremely hot silicate material can
flow over very long timescales.
Convection of the mantle propels the
motion of the tectonic plates in the crust. The
source of heat that drives this motion is the decay of
radioactive isotopes in Earth's crust and mantle combined with the initial heat from the planet's formation (from the
potential energy released by collapsing a large amount of matter into a
gravity well, and the
kinetic energy of accreted matter). Due to increasing pressure deeper in the mantle, the lower part flows less easily, though chemical changes within the mantle may also be important. The viscosity of the mantle ranges between 1021 and 1024
pascal-second, increasing with depth. In comparison, the viscosity of water at is 0.89 millipascal-second and
pitch is (2.3 ± 0.5) × 108 pascal-second.
Core ,
silicon dioxide, and
iron(II) oxide Earth's outer core is a fluid layer about in height (i.e. distance from the highest point to the lowest point at the edge of the inner core) [36% of the Earth's radius, 15.6% of the volume] and composed of mostly
iron and
nickel that lies above Earth's solid
inner core and below its
mantle. Its outer boundary lies beneath Earth's surface. The transition between the inner core and outer core is located approximately beneath Earth's surface. Earth's inner core is the innermost
geologic layer of the planet
Earth. It is primarily a solid ball with a radius of about , which is about 19% of
Earth's radius [0.7% of volume] or 70% of the
Moon's radius. The inner core was discovered in 1936 by
Inge Lehmann and is composed primarily of iron and some nickel. Since this layer is able to transmit shear waves (transverse seismic waves), it must be solid. Experimental evidence has at times been inconsistent with current crystal models of the core. Other experimental studies show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield melting temperatures that are approximately 2000 K below those from shock laser (dynamic) studies. The laser studies create plasma, and the results are suggestive that constraining inner core conditions will depend on whether the inner core is a solid or is a plasma with the density of a solid. This is an area of active research. In early stages of Earth's formation about 4.6 billion years ago, melting would have caused denser substances to sink toward the center in a process called
planetary differentiation (see also the
iron catastrophe), while less-dense materials would have migrated to the
crust. The core is thus believed to largely be composed of iron (80%), along with
nickel and one or more light elements, whereas other dense elements, such as
lead and
uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see
felsic materials). Some have argued that the inner core may be in the form of a single iron
crystal. Under laboratory conditions a sample of iron–nickel alloy was subjected to the core-like pressure by gripping it in a vise between 2 diamond tips (
diamond anvil cell), and then heating to approximately 4000 K. The sample was observed with x-rays, and strongly supported the theory that Earth's inner core was made of giant crystals running north to south. The composition of Earth bears strong similarities to that of certain
chondrite meteorites, and even to some elements in the outer portion of the Sun. Beginning as early as 1940, scientists, including
Francis Birch, built geophysics upon the premise that Earth is like ordinary chondrites, the most common type of meteorite observed impacting Earth. This ignores the less abundant
enstatite chondrites, which formed under extremely limited available oxygen, leading to certain normally oxyphile elements existing either partially or wholly in the alloy portion that corresponds to the core of Earth.
Dynamo theory suggests that convection in the outer core, combined with the
Coriolis effect, gives rise to
Earth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field (see
Curie temperature) but probably acts to stabilize the magnetic field generated by the liquid outer core. The average magnetic field in Earth's outer core is estimated to measure , 50 times stronger than the magnetic field at the surface. The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in the
solar wind. The rate of cooling by conduction and convection is uncertain, but one estimate is that the core would not be expected to freeze up for approximately 91 billion years, which is well after the Sun is expected to expand, sterilize the surface of the planet, and then burn out. == Seismology ==