dissipated during its ascent in the mantle so it is not the same everywhere. In other
words, thermal flux (heat flow) is irregularly distributed within the mantle before
being dissipated through the lithosphere and the continental crust.
The ascent of deep-seated energy and matter is accompanied by a phenomenon
of decompression. This decrease in pressure will increase the melting point temperature of the matter (mineral phases). Depending on the mineral structure and
the surrounding temperature gradient, even just a small variation of only 10–30 °C
is sufficient for enabling a rock to melt and become a liquid magma. The density
decrease caused by heating will enhance the melted material’s ability to rise to
shallower depths and eventually to the surface. The melting takes place at the
junction between mineral grains. The ability of a melt to ascend will depend on the
porosity and the degree of fissuring of the surrounding area. Near the surface, a
rising melt will accumulate in pockets forming large reservoirs (called magma
chambers) at shallow depths of \500 m under spreading centers (see Chap. 5).
Probably not all the mantle’s thermal energy is liberated at the surface of the
Earth. Thermal energy and matter transfer could also flow laterally along major
physical discontinuities marked by the boundary between the lithosphere and
upper mantle. This generates weaknesses underneath the lithosphere, which could
subsequently break and give rise to the observed major oceanic provinces such as
fracture zones, spreading ridges and hotspots in oceanic basins.
Sea Floor Renewal
The major oceanic provinces are created along diverging plate boundaries at the
spreading ridge axes, but also, to a lesser extent, in marginal basins formed behind
island arcs, also called back-arc basins (see Chap. 10) (Figs. 2.8 and 2.9). The
lateral dissipation of energy and matter driven by the mantle convective currents
can move the lithosphere (tectonic plates) away from the heated region at
spreading ridge axes towards a margin of colder and lighter density continents and/
or older lithosphere at convergent margins where the heavier plates will sink back
into the mantle to begin another new cycle of magma creation and subsequent
volcanism.
The lithosphere (crust) moves away from its site of formation at a speed of a
few centimeters per year (at a rate of \1 to more than 9 cm/year). The plates will
cool progressively and will become thicker by incorporating a portion of the
underlying asthenosphere. As the lithosphere moves and ages, it will become
denser and eventually sink into the mantle at the subduction zones.
There is a geodynamic cycle of sea floor renewal. This cycle is relatively fast,
over a period of about 346 million years. That is why the oceanic rocks are
relatively young. The oldest oceanic sea floor located in the western Pacific is
about 173 million years old, which is the geological Jurassic period. The renewal
of the oceanic lithosphere occurs when a portion of the subsided lithosphere sinks
into the Earth’s mantle, regenerates itself and later reappears forming new material
46
2 Our Haven, Planet Earth
words, thermal flux (heat flow) is irregularly distributed within the mantle before
being dissipated through the lithosphere and the continental crust.
The ascent of deep-seated energy and matter is accompanied by a phenomenon
of decompression. This decrease in pressure will increase the melting point temperature of the matter (mineral phases). Depending on the mineral structure and
the surrounding temperature gradient, even just a small variation of only 10–30 °C
is sufficient for enabling a rock to melt and become a liquid magma. The density
decrease caused by heating will enhance the melted material’s ability to rise to
shallower depths and eventually to the surface. The melting takes place at the
junction between mineral grains. The ability of a melt to ascend will depend on the
porosity and the degree of fissuring of the surrounding area. Near the surface, a
rising melt will accumulate in pockets forming large reservoirs (called magma
chambers) at shallow depths of \500 m under spreading centers (see Chap. 5).
Probably not all the mantle’s thermal energy is liberated at the surface of the
Earth. Thermal energy and matter transfer could also flow laterally along major
physical discontinuities marked by the boundary between the lithosphere and
upper mantle. This generates weaknesses underneath the lithosphere, which could
subsequently break and give rise to the observed major oceanic provinces such as
fracture zones, spreading ridges and hotspots in oceanic basins.
Sea Floor Renewal
The major oceanic provinces are created along diverging plate boundaries at the
spreading ridge axes, but also, to a lesser extent, in marginal basins formed behind
island arcs, also called back-arc basins (see Chap. 10) (Figs. 2.8 and 2.9). The
lateral dissipation of energy and matter driven by the mantle convective currents
can move the lithosphere (tectonic plates) away from the heated region at
spreading ridge axes towards a margin of colder and lighter density continents and/
or older lithosphere at convergent margins where the heavier plates will sink back
into the mantle to begin another new cycle of magma creation and subsequent
volcanism.
The lithosphere (crust) moves away from its site of formation at a speed of a
few centimeters per year (at a rate of \1 to more than 9 cm/year). The plates will
cool progressively and will become thicker by incorporating a portion of the
underlying asthenosphere. As the lithosphere moves and ages, it will become
denser and eventually sink into the mantle at the subduction zones.
There is a geodynamic cycle of sea floor renewal. This cycle is relatively fast,
over a period of about 346 million years. That is why the oceanic rocks are
relatively young. The oldest oceanic sea floor located in the western Pacific is
about 173 million years old, which is the geological Jurassic period. The renewal
of the oceanic lithosphere occurs when a portion of the subsided lithosphere sinks
into the Earth’s mantle, regenerates itself and later reappears forming new material
46
2 Our Haven, Planet Earth