on the surface of the Earth. This takes place during partial melting, which occurs
during the extraction of upper mantle material found in the region of the
asthenosphere. The magma that reaches the Earth’s surface produces about
2–3 km
3 of volcanic rocks (mainly basalts) and about 15–20 km
3 of intrusive
rocks (gabbros and various cumulates) per year along the planet’s 70,000 km of
spreading ridges. There is in equilibrium between the creation of new lithosphere
at ridge axes and the subduction of the old lithosphere in oceanic trenches (subduction zones). For this reason, the Atlantic Ocean, which is bounded by accreting
passive margins with no trenches or subduction zones, except for the region of the
Caribbean with the Lesser Antilles arc, is considered as being an ocean ‘‘in
expansion’’, while the Pacific Ocean, which is bordered by active margins (tectonically active subduction zones), is ‘‘decreasing its size’’ by about 0.5 km
3 per
year. In other words, the Pacific is an ocean closing in on itself.
The underwater structures covered by the oceans comprise mainly the deep
ocean basins ([4500 m depth) which represent about 30 % of the sea floor, the
spreading Mid-Ocean Ridge segments (\4000 m depth) which cover about 60 %
of the sea floor, and a smaller proportion (10 % of the total undersea provinces)
which is the location of the back-arc basins associated with subduction zones and
island arcs.
The exploration and description of these different oceanic provinces as presented in this book will help the reader to better understand the formation of our
planet. The upper portion of the lithosphere with a solid crust that is \10 km thick
Fig. 2.9 The cycle of sea floor accretion and subduction shows that the sea floor created at the
spreading ridge axis progressively ages as it is moved away from its point of origin (Hekinian and
Binard 2008). At the same time the lithosphere cools, thickens and sinks into the mantle in the
subduction zone. Convection cells (Fig. 2.8) drive the motions of matter and heat inside the
mantle
Sea Floor Renewal
47
during the extraction of upper mantle material found in the region of the
asthenosphere. The magma that reaches the Earth’s surface produces about
2–3 km
3 of volcanic rocks (mainly basalts) and about 15–20 km
3 of intrusive
rocks (gabbros and various cumulates) per year along the planet’s 70,000 km of
spreading ridges. There is in equilibrium between the creation of new lithosphere
at ridge axes and the subduction of the old lithosphere in oceanic trenches (subduction zones). For this reason, the Atlantic Ocean, which is bounded by accreting
passive margins with no trenches or subduction zones, except for the region of the
Caribbean with the Lesser Antilles arc, is considered as being an ocean ‘‘in
expansion’’, while the Pacific Ocean, which is bordered by active margins (tectonically active subduction zones), is ‘‘decreasing its size’’ by about 0.5 km
3 per
year. In other words, the Pacific is an ocean closing in on itself.
The underwater structures covered by the oceans comprise mainly the deep
ocean basins ([4500 m depth) which represent about 30 % of the sea floor, the
spreading Mid-Ocean Ridge segments (\4000 m depth) which cover about 60 %
of the sea floor, and a smaller proportion (10 % of the total undersea provinces)
which is the location of the back-arc basins associated with subduction zones and
island arcs.
The exploration and description of these different oceanic provinces as presented in this book will help the reader to better understand the formation of our
planet. The upper portion of the lithosphere with a solid crust that is \10 km thick
Fig. 2.9 The cycle of sea floor accretion and subduction shows that the sea floor created at the
spreading ridge axis progressively ages as it is moved away from its point of origin (Hekinian and
Binard 2008). At the same time the lithosphere cools, thickens and sinks into the mantle in the
subduction zone. Convection cells (Fig. 2.8) drive the motions of matter and heat inside the
mantle
Sea Floor Renewal
47