a mineral’s formation. Of the major mantle minerals, pyroxenes are the most
hydrous, containing from 200 to 500 parts per million (ppm) H 2 O by weight (Bell
and Rossman 1992) while garnet and olivine contain only 1–70 ppm (written as
H 2 O = 1–70 ppm).
The Lithosphere-Crust
The interior of the Earth is further subdivided into the lithosphere, which is about
60–100 km thick. The lithosphere (from the Greek words lithos meaning rock and
sphere, meaning ball) includes the outside layer, called the crust, which is a more
rigid layer and the underlying asthenosphere, which is more ductile (See Chap. 4)
(Fig. 2.3). In the outer part of the asthenosphere, the rocks are colder (\1000 °C)
and more rigid therefore they deform elastically under loads and eventually break
due to brittle failure. The continents consist essentially of granitic crust. Granite
consists of quartz (a pure silica oxide), plus grey feldspar (K and Na aluminum
silicate) and white colored plagioclase (Ca, Na, aluminum silicate) as well as
darker iron-magnesium bearing minerals, which could include pyroxene and mica
or amphibole. The granitic crust under the continents could be as much as 60 km
thick. Underneath the oceans, the outside layer of crust is \6–10 km thick and
consists of extrusives (volcanic rocks) or intrusives (dykes or sills).
The lower part of the lithosphere that intrudes into the upper mantle at about
100 km deep consists of peridotite (see Chap. 4). The lower boundary of the
lithosphere (called the asthenosphere) is composed of a less rigid and more plastic
(ductile) material than the upper-crust lithospheric layer.
In 1909, a Croatian seismologist named Andrija Mohorovicic discovered a
seismic discontinuity located at about 35 km under the Balkan mountains, during
his study of earthquakes. This has been called the Mohorovicic discontinuity and is
commonly thought to be the lower level of the crust. The difference between the
crust and the deeper lithosphere is observed by differences in seismic wave traveltimes, which are related to the composition of the rocks.
Underneath the ocean, the difference between the lithosphere and the crust is
arbitrarily defined. Generally the crust is defined as the portion of the most outer
layer of the Earth, about 6–10 km thick, that is created during the upwelling of
magma originating from the partial melting of the mantle. The magma transiting
through conduits and magma reservoirs will undergo crystal-liquid fractionation
and give rise to a solidified volcanic shell (see Chap. 5).
Crustal thickness varies between what is beneath the continents (estimated at
about 60–80 km thick) to underneath the ocean where it could vary from 6 to
25 km depending on the constructional features. In the oceans, the thicker crust is
located under islands, which have been built during repeated volcanic eruptions,
enabling seamounts to rise to the ocean’s surface. The lithosphere is really two
different zones: a more ductile asthenosphere and the solid crust. In geology, the
two zones are often combined together to represent the outermost shell of our
The Mantle
31
hydrous, containing from 200 to 500 parts per million (ppm) H 2 O by weight (Bell
and Rossman 1992) while garnet and olivine contain only 1–70 ppm (written as
H 2 O = 1–70 ppm).
The Lithosphere-Crust
The interior of the Earth is further subdivided into the lithosphere, which is about
60–100 km thick. The lithosphere (from the Greek words lithos meaning rock and
sphere, meaning ball) includes the outside layer, called the crust, which is a more
rigid layer and the underlying asthenosphere, which is more ductile (See Chap. 4)
(Fig. 2.3). In the outer part of the asthenosphere, the rocks are colder (\1000 °C)
and more rigid therefore they deform elastically under loads and eventually break
due to brittle failure. The continents consist essentially of granitic crust. Granite
consists of quartz (a pure silica oxide), plus grey feldspar (K and Na aluminum
silicate) and white colored plagioclase (Ca, Na, aluminum silicate) as well as
darker iron-magnesium bearing minerals, which could include pyroxene and mica
or amphibole. The granitic crust under the continents could be as much as 60 km
thick. Underneath the oceans, the outside layer of crust is \6–10 km thick and
consists of extrusives (volcanic rocks) or intrusives (dykes or sills).
The lower part of the lithosphere that intrudes into the upper mantle at about
100 km deep consists of peridotite (see Chap. 4). The lower boundary of the
lithosphere (called the asthenosphere) is composed of a less rigid and more plastic
(ductile) material than the upper-crust lithospheric layer.
In 1909, a Croatian seismologist named Andrija Mohorovicic discovered a
seismic discontinuity located at about 35 km under the Balkan mountains, during
his study of earthquakes. This has been called the Mohorovicic discontinuity and is
commonly thought to be the lower level of the crust. The difference between the
crust and the deeper lithosphere is observed by differences in seismic wave traveltimes, which are related to the composition of the rocks.
Underneath the ocean, the difference between the lithosphere and the crust is
arbitrarily defined. Generally the crust is defined as the portion of the most outer
layer of the Earth, about 6–10 km thick, that is created during the upwelling of
magma originating from the partial melting of the mantle. The magma transiting
through conduits and magma reservoirs will undergo crystal-liquid fractionation
and give rise to a solidified volcanic shell (see Chap. 5).
Crustal thickness varies between what is beneath the continents (estimated at
about 60–80 km thick) to underneath the ocean where it could vary from 6 to
25 km depending on the constructional features. In the oceans, the thicker crust is
located under islands, which have been built during repeated volcanic eruptions,
enabling seamounts to rise to the ocean’s surface. The lithosphere is really two
different zones: a more ductile asthenosphere and the solid crust. In geology, the
two zones are often combined together to represent the outermost shell of our
The Mantle
31